Essentials of Pericyclic and Photo (Biswanath Dinda) PDF

Lecture Notes in Chemistry 93
Biswanath Dinda
Essentials of
Pericyclic and
Photochemical
Reactions
Lecture Notes in Chemistry
Volume 93
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Biswanath Dinda
Essentials of Pericyclic
and Photochemical Reactions
123
Biswanath Dinda
Department of Chemistry
Tripura University
Agartala, Tripura
India
and
Department of Chemistry
NIT Agartala
Jirania
India
ISSN 0342-4901 ISSN 2192-6603 (electronic)

Lecture Notes in Chemistry
ISBN 978-3-319-45933-2 ISBN 978-3-319-45934-9 (eBook)
DOI 10.1007/978-3-319-45934-9
Library of Congress Control Number: 2016951666
© Springer International Publishing Switzerland 2017

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Dedicated
to
my
parents and teachers
Preface
The part of pericyclic and photochemical reactions is the cornerstone of organic

chemistry of the 20th century. Critical understanding of the principles of these
reactions will be useful to design the synthesis of enormous organic compounds
with high yields maintaining regio- and stereoselectivity. In this book, utilizing my
long teaching experience, I have aimed to present the basic principles of pericyclic
and photochemical reactions in the student’s comprehension by citing numerous
examples with references to develop a thorough and sound sense of actuality on the
subject. Literature citations throughout the text will be helpful to the students and
teachers, who want to get the access to the original work of the factual material.
This book is not designed to be comprehensive with respect to the experimental
details and evidences on which the reaction mechanisms are based. The main
objectives of this book are to develop a broad understanding and scientific thinking
of the students on the subject. The book will help teachers to motivate students in
their scientific imagination on the subject for new application in industrial fields
avoiding hazardous chemicals. A large number of excellent and representative
problems at the end of each chapter and their answers in Appendix-1 of the book
will help the students for their self-evaluation on the lessons of the chapter.
This book is basically designed for the students of postgraduate and M. Phil
levels. However, the students of upper undergraduate levels in chemistry may use it
for advancement of their knowledge on the subject. The book will also be useful for
students to compete for different qualifying examinations after postgraduation.
I have consulted three excellent books, Advanced Organic Chemistry by
F. A. Carey and R. T. Sundberg, Pericyclic Reactions by I. Fleming and Principles
and Applications of Photochemistry by B. Wardle at several points in writing this
book.
I wish to acknowledge the technical assistance of my students, Dr. Saikat Das
Sarma, Dr. Rajarsi Banik, Dr. Indrajit Sil Sarma, Dr. Prasenjit Rudrapaul, Smt.
Ankita Chakraborty, Sri Sukhen Bhowmik, Sk. Nayim Sepay, Sri Subhadip Roy,
Sri Arnab Bhattacharya and my son, Dr. Subhajit Dinda for typing of the major part
of the manuscript.
vii
viii Preface
I would appreciate to receive the letters from teachers and students on errors,
questions, criticisms and suggestions on this book so that I may improve this book
in the forthcoming edition.
Finally, I like to acknowledge to my wife, Chitralekha, and our children,
Subhajit and Manikarna, and son-in-law Shekhar for their constant encouragement
and patient endurance. I am grateful to my publishers for their support and interest
in this endeavour.
Agartala, Tripura, India Biswanath Dinda

January 2016
Contents
Part I Pericyclic Reactions

1 General Aspects of Pericyclic Reactions . . . . . . . . . . . . . . . . . . . . . . 3
1.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2 Molecular Orbitals and Their Symmetry Properties. . . . . . . . . . 4
1.3 Classification of Pericyclic Reactions . . . . . . . . . . . . . . . . . . . . 6
1.4 Concertedness of Pericyclic Reactions . . . . . . . . . . . . . . . . . . . 9
1.5 Orbital Symmetry Property of Pericyclic Reactions . . . . . . . . . 9
1.6 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
2 Electrocyclic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... 13
2.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... 13
2.2 Orbital Symmetry Basis for Stereospecificity . . . . . ......... 14
2.3 The Orbital Correlation Diagrams of Reactants
and Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ......... 15
2.4 Applications of Neutral Conjugated Systems
in Electrocyclic Reactions . . . . . . . . . . . . . . . . . . . . ......... 19
2.5 Applications of Ionic Conjugated Systems
in Electrocyclic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 27
2.6 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 32
2.7 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 34
3 Cycloaddition Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 37
3.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 37
3.2 [2+2]-Cycloaddition Reactions . . . . . . . . . . . . . . . . . . . . . .... 38
3.2.1 Overview of Thermal and Photochemical
[2+2]-Cycloaddition Reactions. . . . . . . . . . . . . . . . . . . 38
3.2.2 Applications of [2+2]-Cycloaddition Reactions . . . . . . 39
3.3 [4+2]-Cycloaddition Reactions . . . . . . . . . . . . . . . . . . . . . . . . . 43
3.3.1 The Diels–Alder Reactions . . . . . . . . . . . . . . . . . . . . . 44
ix
x Contents
3.4 Cycloaddition Reactions of More Than Six Electrons

Systems: [4+4]-, [6+6]-, [6+4]-, [8+2]-, [12+2]-,
and [14+2]-Cycloadditions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
3.5 Cheletropic Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 95
3.5.1 Overview of Cheletropic Reactions . . . . . . . . . . . . . . . 95
3.5.2 Applications of Cheletropic Reactions . . . . . . . . . . . . . 96
3.6 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 99
3.7 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 102
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 103
4 Sigmatropic Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 107
4.2 Orbital Symmetry Basis for Allowed and Forbidden
Sigmatropic Rearrangements and Their Stereochemistry . . . . . . 108
4.2.1 Orbital Symmetry Analysis of [1,3]-, [1,5]-,
and [1,7]-Sigmatropic Shifts of Hydrogen
and Alkyl Groups . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.2.2 Orbital Symmetry Analysis of [3,3]-
and [2,3]-Sigmatropic Rearrangements . . . . . . . . . . . . 110
4.3 [1,3]-, [1,5]-, and [1,7]-Sigmatropic Hydrogen
and Alkyl Shifts and Their Applications . . . . . . . . . . . . . . . . . . 112
4.3.1 [1,3]-Sigmatropic Hydrogen and Alkyl Shifts . . . . . . . 112
4.4 [3,3]-Sigmatropic Rearrangements . . . . . . . . . . . . . . . . . . . . . . 119
4.4.1 The Cope Rearrangements . . . . . . . . . . . . . . . . . . . . . 119
4.4.2 The Oxy-Cope and the Anionic Oxy-Cope
Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 123
4.4.3 The Amino- and Aza-Cope Rearrangements . . . . . . . . 126
4.4.4 The Claisen Rearrangements and Their Modified
Versions: The Carroll, Eschenmoser, Ireland,
Johnson, Gosteli, Bellus, and Enzymatic Claisen
Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 129
4.4.5 The Thio- and Aza-Claisen Rearrangements . . . . . . . . 137
4.5 [2,3]-Sigmatropic Rearrangements . . . . . . . . . . . . . . . . . . . . . . 140
4.5.1 Overview of Different Types of [2,3]-Sigmatropic
Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 140
4.5.2 [2,3]-Sigmatropic Rearrangements of Allyl
Ammonium Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . 141
4.5.3 [2,3]-Sigmatropic Rearrangements of Benzyl
Ammonium Ylides: The Sommelet–Hauser
Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 143
4.5.4 [2,3]-Sigmatropic Rearrangement of Allyl
Sulfonium Ylides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 144
Contents xi

Sulfoxides: The Mislow–Evans Rearrangements . .... 145
Selenoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 146
4.5.7 [2,3]-Sigmatropic Rearrangements of Anions of
Allyl Ethers: The Wittig and Aza-Wittig
Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . . .... 146
4.5.8 [2,3]-Sigmatropic Rearrangements
of Allyl Amine Oxides: The Meisenheimer
Rearrangement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 148
4.6 [3,5]-Sigmatropic Rearrangement . . . . . . . . . . . . . . . . . . . . . . . 148
4.10 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 151
4.11 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 156
5 Group Transfer Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
5.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
5.2 The Ene Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 161
5.2.1 Overview of the Ene Reactions . . . . . . . . . . . . . . . . . . 161
5.2.2 Stereochemistry and Regioselectivity . . . . . . . . . . . . . . 163
5.2.3 Applications of Intermolecular-, Intramolecular-,
and Enantioselective-Ene Reactions . . . . . . . . . . . . . . . 165
5.3 The Metallo-Ene Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . 169
5.4 The Retro-Ene Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 170
5.5 Diimide and Related Reductions . . . . . . . . . . . . . . . . . . . . . . . . 171
5.6 Thermal Elimination Reactions of Xanthates, N-Oxides,
Sulfoxides, and Selenoxides . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
5.7 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 174
5.8 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
Part II Photochemical Reactions

6 Principles of Photochemical Reactions . . . . . . . . . . . . . . . . . . . . . . . . 181
6.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 181
6.2 Light Sources Used in Photochemical Reactions. . . . . . . . . . . . 182
6.3 Laws of Photochemistry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 182
6.4 The Beer–Lambert’s Law of Light Absorption . . . . . . . . . . . . . 183
6.5 Physical Basis of Light Absorption by Molecules:
The Franck–Condon Principle . . . . . . . . . . . . . . . . ......... 184
6.6 Electronic Transitions and Their Nomenclature . . . . ......... 185
6.7 Spin Multiplicity of Electronic States . . . . . . . . . . . ......... 186
xii Contents
6.8 The HOMO and LUMO Concept of Electronic Transitions . . . 187

6.9 The Selection Rules for Electronic Transitions . . . . . . . . . . . . . 187
6.10 Physical Properties of Excited States: Jablonski Diagram . . . . . 188
6.11 Lifetimes of Electronic Excited States. . . . . . . . . . . . . . . . . . . . 190
6.12 Efficiency of Photochemical Processes: Quantum Yield
of Photochemical Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
6.13 Intramolecular Process of Excited States: Fluorescence
and Phosphorescence . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 191
6.13.1 Fluorescence and Its Measurement . . . . . . . . . . . . . . . 191
6.13.2 Kasha’s Rule for Fluorescence . . . . . . . . . . . . . . . . . . 193
6.13.3 Vavilov’s Rule for Fluorescence . . . . . . . . . . . . . . . . . 193
6.13.4 Phosphorescence and Its Measurement . . . . . . . . . . . . 194
6.14 Intermolecular Physical Processes of Excited States:
Photosensitization Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
6.14.1 Photosensitization/Quenching
and Excimer/Exciplex Formation . . . . . . . . . . . . . . . . . 195
6.14.2 The Stern–Volmer Equation for Determination
of Quenching Rate . . . . . . . . . . . . . . . . . . . . . . . . . . . . 196
6.14.3 Deviation from Stern–Volmer Kinetics . . . . . . . . . . . . 197
6.14.4 The Excimers and Exciplexes . . . . . . . . . . . . . . . . . . . 198
6.14.5 Long-Range Energy Transfer Process: The FRET
Process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 199
6.14.6 Short-Range Energy Transfer Process: The Dexter
Theory of Energy Transfer . . . . . . . . . . . . . . . . . . . . . 201
6.14.7 Photodynamic Tumor Therapy Using Singlet
Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
6.14.8 Photo-induced Electron Transfer (PET) Process. . . . . . 205
6.14.9 The Marcus Theory of Electron Transfer . . . . . . . . . . . 207
6.15 Photochemical Reactions and Their Kinetics . . . . . . . . . . . . . . 210
6.15.1 Determination of the Excited State Configuration . . . . 211
6.15.2 Determination of the Yield of Products . . . . . . . . . . . . 211
6.15.3 Determination of the Lifetime of Intermediates . . . . . . 212
6.15.4 Low-Temperature Matrix Studies. . . . . . . . . . . . . . . . . 212
6.16 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 213
7 Photochemistry of Alkenes, Dienes, and Polyenes . . . . . . . . . . . . . . . 215
7.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.2 Cis–Trans-Isomerizations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 215
7.2.1 Cis–Trans-Isomerizations of Alkenes. . . . . . . . . . . . . . 215
7.2.2 Cis–Trans-Isomerization of Dienes . . . . . . . . . . . . . . . 218
7.3 Photochemical Electrocyclic and Addition Reactions . . . . . . . . 219
Contents xiii
7.4 Photochemical [2+2]-Cycloaddition and Dimerization

Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 225
7.5 Photochemical Rearrangements . . . . . . . . . . . . . . . . . . . . . . . . . 226
7.5.1 The di-p-Methane Rearrangements . . . . . . . . . . . . . . . 227
7.5.2 The aza-di-p-Methane Rearrangements . . . . . . . . . . . . 233
7.5.3 The tri-p-Methane Rearrangements . . . . . . . . . . . . . . . 234
7.6 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 236
7.7 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 237
8 Photochemistry of Carbonyl Compounds . . . . . . . . . . . . . . . . . .... 241
8.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 241
8.2 Hydrogen Abstraction and Fragmentation Reactions . . . . . .... 242
8.3 Cycloaddition and Rearrangement Reactions
of Unsaturated Carbonyl Compounds . . . . . . . . . . . . . . . . .... 251
8.4 Isomerization of Unsaturated Carbonyl Compounds . . . . . .... 260
8.5 Cycloaddition Reactions of Carbonyl Compounds
with Alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 261
8.5.1 Limitations. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 268
8.6 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 271
8.7 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 272
9 Photochemistry of Aromatic Compounds . . . . . . . . . . . . . . . . . .... 277
9.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 277
9.2 Photoisomerization Reactions of Aromatic Compounds . . .... 277
9.3 Photocycloaddition Reactions of Aromatic Compounds
with Unsaturated Compounds . . . . . . . . . . . . . . . . . . . . . . .... 278
9.3.1 Photo-Diels–Alder Cycloaddition Reactions
of Aromatic Compounds . . . . . . . . . . . . . . . . . . . .... 287
9.4 Photo-Induced Hydrogen Abstraction and Addition
Reactions of Aromatic Compounds . . . . . . . . . . . . . . . . . . . . . . 288
9.5 Photocyclization Reactions of Aromatic Compounds . . . . . . . . 289
9.6 Photorearrangement Reactions of Aromatic Compounds . . . . . . 290
9.7 Photooxidation Reactions of Aromatic Compounds . . . . . . . . . 292
9.8 Photodimerization Reactions of Aromatic Compounds . . . . . . . 292
9.9 Photosubstitution Reactions of Aromatic Compounds . . . . . . . . 294
9.10 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 296
9.11 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 297
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 298
10 Photofragmentation Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
10.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 301
10.2 The Barton Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 302
10.3 The Hypohalite Reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 304
10.4 The Hofmann-Löffler-Freytag Reaction . . . . . . . . . . . . . . . . . . . 307
xiv Contents
10.5 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

10.6 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 312
11 Photochemistry in Nature and Applied Photochemistry . . . . . . .... 315
11.1 Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .... 315
11.2 Depletion of Stratospheric Ozone Layer
from Photochemical Degradation . . . . . . . . . . . . . . . . . . . .... 315
11.3 Photochemical Smog in Polluted Zones of Troposphere . . .... 316
11.4 Photochemistry of Vision: Geometrical Isomerisation
of Retinal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 317
11.5 Phototherapy of Neonatal Jaundice . . . . . . . . . . . . . . . . . . . . . . 318
11.6 Photosynthesis of Plants and Bacteria . . . . . . . . . . . . . . . . . . . . 319
11.6.1 Artificial Photosynthesis . . . . . . . . . . . . . . . . . . . . . . . 323
11.7 Photo-Induced DNA-Damage and Its Repair . . . . . . . . . . . . . . 323
11.8 Conservation of Solar Energy as Electrical Energy:
Photovoltaic Solar Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 323
11.9 Photo-Induced Supramolecular Devices . . . . . . . . . . . . . . . . . . 328
11.10 Further Reading . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 330
Appendix . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 333
Index . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
Abbreviations
acac Acetylacetonate
BINAP Bis-(2,2’-diphenylphosphinyl)-1,1’-binaphthalene
BINOL Binaphthol
Boc Tertiary-butoxycarbonyl [Me3COCO]
BOX Bisoxazoline
Bz Benzyl [PhCH2]
DBMP 6-di-tert-butyl-4-methyl phenol
DBP Dibutyl phthalate
DBU Diazabicycloundecane
DMA Dimethylallene
DPM Di-p-methane
ee Enantiomeric excess
Et Ethyl [C2H5]
FVP Flash vacuum pyrolysis
HMPA Hexamethylphosphoramide
HOMO Highest occupied molecular orbital
hv Ultraviolet or visible irradiation
IL Ionic liquid
i-Pr Iso-propyl[Me2CH]
KHMDS Potassium hexamethyldisilazane or potassium bis(trimethylsilyl)
amide [(Me3Si)2NK]
LDA Lithium diisopropylamide [LiNi-Pr2]
LUMO Lowest unoccupied molecular orbital
Me Methyl [CH3]
MTAD N-methylthiazolinedione
N,N-DEA N, N-diethanolamine [NH(CH2CH2OH)2]
n-Pr Normal-propyl [MeCH2CH2]
ODPM Oxa-di-p-methane
PET Photo-induced electron transfer
Ph Phenyl [C6H5]
xv
xvi Abbreviations
PhH Benzene
Pi Phosphate, inorganic
Py Pyridine
rt Room temperature
sens Sensitizer
SOMO Singly occupied molecular orbital
TADDOL a,a,a,a-tetraaryl-1,3-dioxolane-4,5-dimethanol
TBDPS Tert-butyldiphenylsilyl
TBS Tert-butylmethyl silyl
t-Bu Tertiary-butyl [Me3C]
TCB Tetracyanobenzene
THF Tetrahydrofuran
TMS Trimethylsilyl[Me3Si]
Ts Tosyl [4-MeC6H4]
TS Transition structure
List of Figures
Figure 1.1 Formation of bonding and antibonding orbitals . . . . . . . . . .. 5

Figure 1.2 Molecular orbitals formation in allyl systems . . . . . . . . . . .. 6
Figure 1.3 Molecular orbitals of 1,3-butadiene and their symmetry
properties. (S means symmetric and A means
antisymmetric) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 6
Figure 1.4 Molecular orbitals of 1,3,5-hexatriene and their wave
functions and symmetry properties . . . . . . . . . . . . . . . . . . .. 7
Figure 1.5 Huckel TS for thermal cycloaddition reactions . . . . . . . . . .. 11
Figure 2.1 a Thermal electrocyclization of 4npe conjugated
system; b photochemical electrocyclization
of 4npe conjugated system . . . . . . . . . . . . . . . . . . . . . . . . .. 15
Figure 2.2 a Thermal electrocyclization of 4n+2 pe conjugated
system; b photochemical electrocyclization of 4n+2
pe conjugated system . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 16
Figure 2.3 a C2-axis of symmetry is maintained in thermal
conversion of cyclobutene to butadiene; b mirror
plane symmetry is maintained in photochemical
conversion of cyclobutene to butadiene . . . . . . . . . . . . . . . .. 17
Figure 2.4 a Mirror plane (m) symmetry is maintained in thermal
conversion of 1,3,5-hexatriene into 1,3-cyclohexadiene;
b C2-axis of symmetry is maintained in photochemical
conversion of 1,3-cyclohexadiene into 1,3,5-hexatriene
or vice versa . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 18
Figure 3.1 Frontier orbital interactions of a thermally forbidden
[p2s+p2s]-cycloaddition reaction, b photochemically
allowed [p2s+p2s]-reaction of alkenes . . . . . . . . . . . . . . . . .. 38
Figure 3.2 Frontier orbital interactions of thermally allowed
antarafacial interaction of a ketene (LUMO)
and an olefin (HOMO) . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 39
Figure 3.3 Frontier orbital interactions in Diels–Alder reactions . . . . . .. 48
xvii
xviii List of Figures
Figure 3.4 Orbital interactions of HOMO of diene and LUMO

of dienophile and vice versa in a Diels–Alder reaction . . . .. 49
Figure 3.5 Symmetry properties of butadiene, ethylene, and
cyclohexene orbitals with respect to plane of symmetry.
m-sym means mirror, S means symmetric, and A means
antisymmetric . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 50
Figure 3.6 Symmetry correlation diagram for ethylene, butadiene,
and cyclohexene orbitals . . . . . . . . . . . . . . . . . . . . . . . . . . .. 51
Figure 3.7 The orbitals set for supra-, supra-[p4+p2]-cycloaddition
in Huckel and Mobius TSs . . . . . . . . . . . . . . . . . . . . . . . . .. 51
Figure 3.8 The orbital interactions in endo- and exo-transition
states (TSs) in a Diels–Alder reaction . . . . . . . . . . . . . . . . .. 53
Figure 3.9 The figure illustrates the HOMO–LUMO energy gap
in terms of FMO theory on the reactivity of diene
and dienophile in normal electron demand Diels–Alder
reaction. The narrower the gap the higher
will be the TS stability and faster will be the reactivity . . .. 56
Figure 3.10 a LUMO energy of dienophile is lowered by Lewis
acid catalyst in NED D–A reactions and b LUMO
energy of diene is lowered by Lewis acid catalyst
in IED D–A reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 68
Figure 3.11 Frontier orbital interactions in a 1,3-dipolar
cycloaddition reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 81
Figure 3.12 Orbital coefficients of the HOMO and LUMO
of some 1, 3-dipoles. Adapted with permission
from (Houk et al. 1973 J Am Chem Soc, 95:7287).
Copyright (1973) American Chemical Society . . . . . . . . . .. 82
Figure 3.13 The orbital interactions of HOMO and LUMO
in the TS in the reaction of nitrone 125
with ortho-hydroxyl styrene 137 . . . . . . . . . . . . . . . . . . . . .. 85
Figure 3.14 Orbital interactions in the TS for cheletropic addition
reactions in (4n+2) and 4n electron systems . . . . . . . . . . . .. 96
Figure 4.1 Orbital interactions in thermal and photochemical
reactions of [1,3]-sigmatropic hydrogen shift . . . . . . . . . . . . 109
Figure 4.2 Orbital interactions in thermal and photochemical
reactions of [1,5]-sigmatropic hydrogen shift . . . . . . . . . . . . 109
Figure 4.3 Orbital interactions in Huckel-type TSs for thermal
[1,5]-, and [1,3]-sigmatropic hydrogen shifts . . . . . . . . . . . . . 110
Figure 4.4 Suprafacial orbital interactions in thermal and
photochemical reactions of [1,7]-sigmatropic
hydrogen shift . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 110
Figure 4.5 Orbital interactions in the TSs of thermal reactions
of [1,3]- and [1,5]-sigmatropic suprafacial alkyl shifts . . . . . 111
List of Figures xix
Figure 4.6 Suprafacial orbital interactions in chair- and boat-like

TSs in thermal [3,3]-sigmatropic rearrangements . . . . . . . . . . 111
Figure 4.7 Suprafacial orbital interactions in the TS (Huckel type)
of [2,3]-sigmatropic rearrangements . . . . . . . . . . . . . . . . . . . 111
Figure 5.1 Orbital interactions of ene and enophile in the TS
of an ene reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
Figure 6.1 Schematic diagram of the electronic ground state
and the first excited electronic state of a diatomic
molecule. The vertical arrows show vibronic
transitions due to absorption of photons . . . . . . . . . . . . . . . . 184
Figure 6.2 Generalized ordering of molecular orbital energies
of organic molecules and electronic transitions
that occur by excitation with light . . . . . . . . . . . . . . . . . . . . . 185
Figure 6.3 Electronic states of molecular orbitals of an organic
compound . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 186
Figure 6.4 Modified Jablonski diagram for an organic molecule
showing ground and excited states and intramolecular
photophysical processes from excited states. Radiative
processes—fluorescence (hmf) and phosphorescence (hmp)
are shown in straight lines, radiationless processes—
internal conversion (IC), inter system crossing (ISC),
and vibrational cascade (vc) are shown in wavy lines.
Adapted with permission from (Smith MB and March
J 2006 March’s Advanced Organic Chemistry: Reactions,
Mechanisms and Structures, 6th Ed., John Wiley,
New York). Copyright (2007) John Wiley & Sons . . . . . . . . 189
Figure 6.5 Intramolecular energy transfer of
dimethylaminobenzonitrile by TICT process . . . . . . . . . . . . . 190
Figure 6.6 Basic components of a spectrofluorometer . . . . . . . . . . . . . . 192
Figure 6.7 Schematic diagram of a rotating can phosphoroscope
with shutter system . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 195
Figure 6.8 Stern–Volmer plot of fluorescence quenching . . . . . . . . . . . . 198
Figure 6.9 Electronic movements occurring in the long-range
singlet–singlet energy transfer process . . . . . . . . . . . . . . . . . . 199
Figure 6.10 The dependence of efficiency of energy transfer
ET on donor–acceptor distance R, as per Forster
theory in a FRET process . . . . . . . . . . . . . . . . . . . . . . . . . . . 200
Figure 6.11 Conformational change occurs in green fluorescent
protein (GFP) of jellyfish during fluorescence emission.
Adapted with permission from (Wardle B 2009 Principles
and applications of photochemistry, Wiley, p. 102).
Copyright (2009) John Wiley & Sons . . . . . . . . . . . . . . . . . . 200
Figure 6.12 Electron movements in Dexter short-range
(triplet–triplet) energy transfer process . . . . . . . . . . . . . . . . . 202
xx List of Figures
Figure 6.13 Electron movement in a triplet–triplet annihilation

process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 204
Figure 6.14 Generalized structure of porphyrin. The R groups
represent different side groups attached
to the porphyrin ring . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 205
Figure 6.15 Molecular orbital representation of electron transfer
in a PET process. a Oxidative electron transfer,
where B is electron poor acceptor molecule,
and b reductive electron transfer, where B
is electron-rich donor molecule . . . . . . . . . . . . . . . . . . . . . . . 206
Figure 6.16 Potassium cation sensor as a molecular fluorescence
switch in a PET process of anthracene fluorophore
having a macrocyclic donor unit . . . . . . . . . . . . . . . . . . . . . . 206
Figure 6.17 Principle of PET process in K+ bound sensor . . . . . . . . . . . . 206
Figure 6.18 Potential energy (PE) description of an electron transfer
reaction. The parabolic curves intersect at the transition
state (#) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 207
Figure 6.19 Reorganization of polar solvent dipoles during
PET process . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Figure 6.20 Free energy change, ΔG0 dependence of electron
transfer rate, KET according to Marcus theory
of electron transfer process . . . . . . . . . . . . . . . . . . . . . . . . . . 208
Figure 6.21 Normal and inverted regions of Marcus equation
for electron transfer process in a Zinc porphyrin—C60
dyad . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 209
Figure 6.22 Change of potential energy surfaces for excited-state
and ground-state molecules. Adapted with permission from
(Turro NJ 1991 Modern Molecular Photochemistry,
University Science Books). Copyright (1991) University
Science Books . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 211
Figure 7.1 Mechanism of photochemical cis–trans-isomerization
of alkenes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 216
Figure 7.2 The orbital array of di-p-methane rearrangement
through singlet excited state . . . . . . . . . . . . . . . . . . . . . . . . . 228
Figure 11.1 Photochemical reaction in the vision process. . . . . . . . . . . . . 318
Figure 11.2 Cis–trans-isomerisation of bilirubin . . . . . . . . . . . . . . . . . . . . 319
Figure 11.3 Structures of chlorophyll a and chlorophyll b . . . . . . . . . . . . 320
Figure 11.4 Structures of b-carotene and phycoerythrobilin . . . . . . . . . . . 321
Figure 11.5 Photochemical electron transport chain in a Z-scheme
during light-dependent reactions of photosynthesis.
EA and ED refer to the electron acceptor and electron
List of Figures xxi
donor of the two photosystems. Adapted with permission

from (Wardle B, 2009 Principles and Applications
of Photochemistry, Wiley, p. 226). Copyright (2009)
John Wiley & Sons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 321
Figure 11.6 The working mechanism of a silicon p–n junction solar
cell. Adapted with permission from (Wardle B, 2009
Principles and Applications of Photochemistry, Wiley,
p. 217). Copyright (2009) John Wiley & Sons . . . . . . . . . . . 324
Figure 11.7 Schematic diagram of a dye-sensitized solar cell where
semiconductor TiO2 nanoparticles are coated
with Ru(II)-based dye. Adapted with permission from
(Wardle B, 2009 Principles and Applications of
Photochemistry, Wiley, p. 202). Copyright (2009)
John Wiley & Sons . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
Figure 11.8 Photo-induced electron transfer from excited
MDMO-doped PPV to PCBM. . . . . . . . . . . . . . . . . . . . . . . . 327
Figure 11.9 Schematic device structure for polymer/fullerene bulk
heterojunction solar cells. Adapted with permission from
(Gunes et al. 2007 Chem Rev 107:1324). Copyright (2007)
American Chemical Society . . . . . . . . . . . . . . . . . . . . . . . . . 328
Figure 11.10 Molecular structures of the components for a light-driven
molecular scale machine. Adapted with permission from
(Bolzani et al. 2006 Aust J Chem 59:193). Copyright
(2006) CSIRO Publishing . . . . . . . . . . . . . . . . . . . . . . . . . . . 329
List of Tables
Table 1.1 Symmetry properties of the orbital wn of a linear

conjugated polyene . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 8
Table 2.1 Woodward–Hoffmann rules for electrocyclic reactions . . . . . .. 16
Table 3.1 Woodward–Hoffmann rules for [m+n]-cycloaddition
reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 49
Table 3.2 Global electrophilicity of some dienophiles in D–A
reactions with 1,3-butadiene (Dx = 1.05 eV) . . . . . . . . . . . . .. 55
Table 3.3 Relative rates of reactivity of some substituted butadienes
in D–A reactions with maleic anhydride . . . . . . . . . . . . . . . . .. 56
Table 3.4 Representative dienes and dienophiles used in Diels–Alder
reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .. 57
Table 3.5 List of common 1, 3-dipoles with resonating structures . . . . . .. 79
Table 4.1 Woodward-Hoffmann rules for sigmatropic rearrangements . . . . 112
Table 6.1 Comparison of light absorptions due to p ! p*
and n ! p* electronic transitions . . . . . . . . . . . . . . . . . . . . . . . 186
xxiii
List of Schemes
Scheme 3.1 Regioselectivity of Diels–Alder reaction . . . . . . . . . . . . . . .. 46

Scheme 4.1 Major types of sigmatropic rearrangements . . . . . . . . . . . . . . 108
Scheme 9.1 Mechanism for formation of photochemical adducts
from the reaction of aromatic compounds with alkenes . . . . . 279
Scheme 10.1 Generalized pathway for photofragmentation reaction . . . . . . 302
xxv
Part I
Pericyclic Reactions
Chapter 1
General Aspects of Pericyclic Reactions
1.1 Introduction
Reactions in Organic Chemistry are broadly classified into three major categories—
ionic, radical, and pericyclic. Ionic reactions involve the formation of ionic inter-
mediates by movement of pair of electrons in one direction of a covalent bond. In a
unimolecular reaction, it occurs by ionization process and in a bimolecular reaction,
it occurs when one component acts as a nucleophile (or electron pair donor) and
another component as electrophile (or electron pair acceptor). For example,
R3C X R3C + X
Nu + R X Nu R + X
E + R X E R + X
Radical reaction involves the homolytic cleavage of a covalent bond by

movement of single electrons in opposite directions. The movement of a single
electron is represented by fish hook arrow. For example,
Cl Cl Cl + Cl
H
(CH3)3C C H + Cl H Cl
(CH3)3C C + HCl (CH3)3C CH2Cl
H H
2,2-dimethyl propane 1-chloro-2,2-dimethyl propane
© Springer International Publishing Switzerland 2017 3

B. Dinda, Essentials of Pericyclic and Photochemical Reactions,
Lecture Notes in Chemistry 93, DOI 10.1007/978-3-319-45934-9_1
4 1 General Aspects of Pericyclic Reactions
Pericyclic reactions involve the continuous flow of electrons in cyclic transition

states (TS) by breaking and making of bonds in a concerted process, without
formation of an intermediate. Hence, these reactions are known as concerted
reactions [1]. These reactions are insensitive to solvent polarity and free radical
initiators or inhibitors. These reactions are activated by heat (thermal) or light
(photochemical). Detailed study of the mechanisms of these reactions by
Woodward and Hoffmann [2] predicted that these reactions occur by the mainte-
nance of symmetry properties of the orbitals of reactant(s) and product(s). The
Diels–Alder reaction is a typical example.
+
CN CN
A Diels-Alder reaction
1.2 Molecular Orbitals and Their Symmetry Properties
In pericyclic reactions, the reactivity of the reactions can be explained on the basis
of Perturbational Molecular Orbital (PMO) theory [3]. The basic postulate of PMO
theory is that a chemical reaction takes place by the perturbation of molecular
orbitals (MOs) of reactants on heating and on irradiation with light. The degree of
perturbation is a function of degree of overlapping interactions of the atomic
orbitals in an MO. These interactions are strongest among the orbitals close in
energies. These orbital overlapping interactions produce degenerate MOs of dif-
ferent energies. The suitable degenerate MOs take part in the reactions to give
products through cyclic TSs in a concerted process. The interactions of two atomic
orbitals will produce two MOs, one of them will be stabilized and other will move
to higher energy. The linear combination of atomic orbitals is known as LCAO
theory or PMO theory. The shapes of the MOs that are formed by the linear
combination of atomic orbitals (LCAO) are related to the shapes of atomic orbitals.
The MOs are denoted by ψ (psi) and atomic orbitals by Φ (phi). Dewar and
Hoffmann first developed a general PMO method to explain the reactivity in
organic chemistry. To illustrate the idea, let us consider a diatomic molecule where
an MO formed by the combination of atomic orbitals of A and B is represented as
w ¼ UA þ UB
Quantum mechanics shows that the linear combination of two wave functions
gives two combinations and hence two MOs are generated from two-component
1.2 Molecular Orbitals and Their Symmetry Properties 5
atomic orbitals. One MO is bonding orbital, more stable than the component atomic
orbitals and other one is an antibonding orbital, less stable than the component
orbitals.
w þ ¼UA þ UB ðbonding MOÞ

w ¼UA UB ðantibonding MOÞ
The MOs that have resulted from overlap of various kinds of atomic orbitals are
shown in Fig. 1.1.
Similarly, the linear combination of three p-orbitals in allyl system will give rise
to three new MOs w1, w2 and w3. The antibonding interactions increase the energy
of the MO. Thus, the energy of w2 is higher than that of w1 and w3 is of higher
energy than w2. The wave functions and their symmetry in relation to the mirror
plane are shown in Fig. 1.2.
Similarly, the linear combination of four atomic π orbitals in 1, 3-butadiene will
generate four MOs w1, w2, w3, and w4 (Fig. 1.3).
Next, the linear combination of six atomic p-orbitals of 1,3,5-hexatriene will
give rise to six MOs. The wave functions and symmetry properties of these MOs
are shown in Fig. 1.4.
On the basis of the above examples of polyene systems, the symmetry properties
of MO, wn of a linear conjugated polyene are summarized in Table 1.1.
node
σ∗ (antibonding)
A s B
s
σ (bonding)
node
π∗ (antibonding)
A B
p p π (bonding)
Presented + and - are mathematical signs
Fig. 1.1 Formation of bonding and antibonding orbitals

Fig. 1.2 Molecular orbitals formation in allyl systems
Fig. 1.3 Molecular orbitals of 1,3-butadiene and their symmetry properties. (S means symmetric
and A means antisymmetric)
1.3 Classification of Pericyclic Reactions
Pericyclic reactions are classified into four classes. These are electrocyclic reac-
tions, cycloadditions, sigmatropic rearrangements, and group transfer reactions.
Electrocyclic reactions are characterized by the creation of a ring from an
open-chain conjugated system with the formation of a new σ bond at the ends of the
conjugated system or its reverse process.
1.3 Classification of Pericyclic Reactions 7
Fig. 1.4 Molecular orbitals of 1,3,5-hexatriene and their wave functions and symmetry properties
Table 1.1 Symmetry Wave function Nodes m-symmetry C2-symmetry

properties of the orbital ψn of
a linear conjugated polyene wodd: w1, w3, w5 0 or even S A
weven: w2, w4, w6 odd A S
Cycloaddition reactions are characterized by the addition of two π-systems by

the formation of two new σ bonds, at the ends of both components, with the
reduction of one π-bond from each component, e.g., Diels–Alder reactions.
CO2Me CO2Me
+
CO2Me CO2Me
1, 3-Dipolar cycloadditions are another family member of cycloaddition reac-

tions, e.g.,
Me
CH2 Me CO2Me CO2Me
N
N +
N Me CO2Me N CO Me
Me 2
Diazomethane
Cheletropic reactions are a special group of cycloadditions or cycloreversions in

which two σ bonds are made or broken from the same atom.
O O
+ S
O
S
O , N N
-N2
Cheletropic addition Cheletropic extrusion
Sigmatropic rearrangements are characterized by the movement of a σ bond to a

more distant terminus of an adjacent π system followed by movement of the π
system to accommodate this new σ bond, e.g.,
1.3 Classification of Pericyclic Reactions 9
CH3
H3C H
Only a few reactions belong to the class of group transfer reactions. Most
common is the ene reaction, where allylic hydrogens are transferred to an elec-
trophilic alkene, referred as an enophile. For example,
H O O
H 200 oC H
H
+
+ O O
EWG EWG
ene enophile O O
An Ene reaction
Another type of group transfer reaction is the transfer of hydrogens from diimide
to an alkene or alkyne.
H
N
+ + N2
N
H
1.4 Concertedness of Pericyclic Reactions
High-level MO calculations suggest that these reactions proceed in concerted

pathways through the lowest energy TSs. Isotope effect studies also support the
concertedness of pericyclic reactions.
1.5 Orbital Symmetry Property of Pericyclic Reactions
All pericyclic reactions conserve a definite orbital symmetry property throughout

the course of their reactions, i.e., the reactants, TS, and products of a reaction have
the same orbital symmetry.
The allowedness and forbiddenness of a pericyclic reaction toward heat or light
depends on this symmetry property. Usually two types of element of symmetry,
namely mirror (m) plane symmetry and C2-axis of symmetry are considered to
correlate the orbital symmetry properties of reactants, TSs, and products of a per-
icyclic reaction. There are three ways of applying the orbital symmetry principle in
these reactions—the frontier molecular orbital (FMO) method, the Mobius–Huckel
aromatic TS method, and the orbital correlation diagram method.
The Frontier Molecular Orbital Method
The FMO method was introduced by Fukui, Woodward and Hoffmann [1, 2, 4].
According to this method, an electrocyclic reaction will be allowed when the ter-
mini orbitals of the highest occupied MO (HOMO) of a conjugated π-system (re-
actant) overlaps between them in such a way that a positive (shaded) lobe overlaps
with another positive lobe or a negative lobe with another negative lobe. Similarly,
a cycloaddition reaction will be allowed when the HOMO of one reactant and the
lowest unoccupied MO (LUMO) of the other overlap among themselves in such a
way that a positive lobe overlaps with another positive lobe and a negative lobe
with another negative lobe. A sigmatropic reaction will be allowed when a
σ-bonded atom or group moves from a positive lobe to another positive lobe or
from a negative lobe to another negative lobe. Woodward and Hoffmann predicted
some orbital symmetry rules for these pericyclic reactions for concertedness of
these reactions.
The Mobius–Huckel aromatic transition state method
The Mobius–Huckel method is based on the PMO method and is applicable for
prediction of an allowed or forbidden pericyclic reaction. It was developed by
Dewar and Zimmerman on the basis of Huckel aromaticity rule for a cyclic con-
jugated system of π-electrons [5]. In this method, the allowedness of a pericyclic
reaction is considered on the basis of arrangement of p-orbitals in the TS. When the
p-orbitals are arranged in the TS with zero or an even number of sign inversions
(node) of positive or negative lobes, the system is called the Huckel system.
A system of arrangement of p-orbitals in the TS with an odd number of sign
inversions is known as Mobius system. A thermal pericyclic reaction involving
Huckel system is allowed for a total number of 4n+2 π electrons, whereas a thermal
pericyclic reaction involving a Mobius system is allowed for a total number of 4n π
electrons. For photochemical pericyclic reactions, these rules are reversed for their
allowedness. For example, a thermal [2+2]-cycloaddition is forbidden and a thermal
[4+2]-cycloaddition is allowed as per Huckel system of TS (Fig. 1.5).
The Correlation Diagram Method
The orbital correlation diagram was introduced by Longuet-Higgins and
Abrahamson to predict the allowedness of a pericyclic reaction [6]. In this method,
the orbital symmetry properties of both reactants and products are considered. The
symmetry elements of the MOs are evaluated and the MOs of reactants and
products are arranged in a diagram in two columns. In an allowed pericyclic
reaction, the ground-state MO of the reactants and the products has the same
element of symmetry.
1.6 Further Reading 11
Fig. 1.5 Huckel TS for

thermal cycloaddition
reactions
(a) Huckel TS of (4n)e (b) Huckel TS of (4n+2)e

antiaromatic (forbidden) aromatic (allowed)
1.6 Further Reading
1. Gilchrist L, Storr RC (1979) Organic reactions and orbital symmetry, 2nd edn.
Cambridge University Press, Cambridge
2. Marchand AP, Lehr RE (1972) Orbital symmetry. Academic Press, New York
3. Woodward RB, Hoffmann R (1970) The conservation of orbital symmetry.
Academic Press, New York
References
1. Woodward RB, Hoffmann R (1970) Conservation of orbital symmetry. Academic Press, New
York
2. Woodward RB, Hoffmann R (1965) J Am Chem Soc 87: 395
3. Coulson CA, Longuet-Higgins HC (1947) Proc R Soc London Ser A 192: 16; Hoffmann R
(1963) J Chem Phys 39:1397; Dewar MJS, Dougherty RC (1975) The PMO theory of organic
chemistry. Plenum, New York
4. Fukui K, Yonezama T, Shingu H (1952) J Chem Phys 20:722; Fukui K, Fujimoto (1967) Bull
Chem Soc Jpn 40: 2018, (1969) 42: 3399; Fukui K (1971) Acc Chem Res 4: 57; Fukui K
(1982) Angew Chem Int Ed Engl 21:801; Fleming I (1978) Frontier orbitals and organic
chemical reactions. Wiley, London, pp 29–109
5. Zimmerman HE (1966) J Am Chem Soc 88:1566; idem (1971) Acc Chem Res 4:272; Dewar
MJS(1971) Angew Chem Int Ed Engl 10: 761
6. Longuet- Higgins HC, Abrahamson EW (1965) J Am Chem Soc 87:2045
Chapter 2
Electrocyclic Reactions
2.1 Introduction
An electrocyclic reaction is defined as the thermal or photochemical conversion of

an acyclic conjugated system into a ring system by formation of a σ bond between
the ends of the conjugated system in a concerted process, or the reverse of this
reaction. These reactions are reversible in nature.
6
CH3
2 3 5
CH3 4 H
175 oC Ref.1, 2
H 3 1
1 CH3
4 CH3 2 H
H
99.9%
cis-3,4-dimethyl cyclobutene cis, trans-2,4-hexadiene
CH3
CH3
175 oC H
H Ref.1, 2
H H
CH3
CH3
trans-3,4-dimethyl cyclobutene trans, trans-2,4-hexadiene
8
6 CH3
7 4 5 CH3
5
H Δ 3
H Ref.3
4 H CH3
2 6
3 2 1 H
CH3
1 hν cis-5,6-dimethyl-1,3-cyclohexadiene
trans,cis,trans-2,4,6-
octatriene
CH3
H Ref.4
H
Δ CH3
CH3
trans-5,6-dimethyl-1,3-cyclohexadiene
CH3
trans,cis,cis-2,4,6-
octatriene

14 2 Electrocyclic Reactions
2.2 Orbital Symmetry Basis for Stereospecificity
Woodward and Hoffmann [5] suggested a complete description of the mechanisms

of these reactions by introducing the terms ‘conrotatory’ and ‘disrotatory’ motions
of the groups at the termini of acyclic conjugated system or of the groups at the sp3
carbons of a ring system. The motion of the substituents in the same direction
clockwise or anticlockwise is known as conrotatory mode of motion, while the
motion of the substituents in opposite direction is known as disrotatory mode of
motion. They suggested that the stereochemistry of these reactions is controlled by
the symmetry properties of HOMO (highest occupied molecular orbital) of the
open-chain conjugated system. It was also supported by the frontier molecular
orbital (FMO) theory [6].
For thermal reactions of 4nπe conjugated systems, ψ2 would be HOMO because
it contains lowest number of nodes (one node) and provides a transition state of
lowest energy similar to Mobius topology as per perturbation molecular orbital
(PMO) theory. While for photochemical reactions of 4nπe systems, ψ3 would be
HOMO because it is the first excited state of ground-state ψ2. Therefore, in thermal
reactions of 4nπ systems, conrotatory motion of the groups in the terminal carbons
of the open-chain π system brings the lobes of the same phase for bonding with a
Mobius type TS and is orbital symmetry allowed process, while disrotatory motion
brings the lobes of the opposite phase for antibonding formation and is said to be
orbital symmetry forbidden process. While for photochemical reactions of 4nπe
systems, disrotatory motion brings the lobes of same phase, and hence, the reaction
will proceed with low activation energy and is said to be orbital symmetry allowed
process. On the other hand, conrotatory motion brings the lobes of opposite phase,
and hence, reaction is unfavourable for its high activation energy and is referred as
symmetry forbidden reaction path (Fig. 2.1).
In case of thermal reactions of 4n+2 πe conjugated system, ψ3 would be HOMO
as it has minimum even nodes (two nodes) and is related to Huckel topology (as per
PMO theory). Hence, the disrotatory motion of the groups of terminal carbons
brings the lobes of same phase for bonding, involving a Huckel-type transition state
and is said to be symmetry allowed path, while conrotatory motion would be
symmetry forbidden path as it leads to a TS of high activation energy. For their
photochemical reactions, ψ4 would be HOMO and hence conrotatory motion would
be symmetry allowed path (Fig. 2.2).
Woodward–Hoffmann rules for electrocyclic reactions are summarized in
Table 2.1.
2.3 The Orbital Correlation Diagrams of Reactants and Products 15
(a)
con motion
ψ2 bonding (low energy T.S) (one node)

allowed
dis motion
ψ2 antibonding (high energy T.S)

forbidden
(b)
dis motion
TS (Zero node)
ψ3 bonding
allowed
con motion
ψ3 antibonding
forbidden
Fig. 2.1 a Thermal electrocyclization of 4nπe conjugated system; b photochemical electrocy-

clization of 4nπe conjugated system
2.3 The Orbital Correlation Diagrams of Reactants

and Products
Longuet-Higgins and Abrahamson [6] suggested that in any concerted process, the
orbitals of the starting material and product have the same symmetry. This is also
supported by Woodward and Hoffmann [5]. The cyclobutene–butadiene intercon-
version may be considered as an example to verify the fact by construction of a
correlation diagram. For cyclobutene, the bonding orbitals are σ and π, while the
Fig. 2.2 a Thermal (a)

electrocyclization of 4n+2 πe
conjugated system;
b photochemical dis motion
electrocyclization of 4n+2 πe
conjugated system
Ψ3 bonding
allowed
con motion
Ψ3 antibonding
forbidden
(b)
con motion
Ψ4 bonding
allowed
dis motion
Ψ4 antibonding
forbidden
Table 2.1 Woodward– Acylic conjugated Reaction Motion

Hoffmann rules for system allowed
electrocyclic reactions
4n πe Thermal Conrotatory
Photochemical Disrotatory
(4n+2) πe Thermal Disrotatory
Photochemical Conrotatory
2.3 The Orbital Correlation Diagrams of Reactants and Products 17
σ∗ A A ψ4 A S
π∗ A S ψ3 S A
π S A ψ2 A S
σ S S
ψ1 S A
Mirror (m) C2-axis

Mirror (m) C2-axis
sym sym
sym sym
Cyclobutene butadiene Cyclobutene butadiene
σ∗ A S Ψ4 σ∗ A A Ψ4
π∗ S A Ψ3 π∗ A S Ψ3
π A S Ψ2 π S A Ψ2
σ S A Ψ1 σ S S Ψ1
(a) C2- axis of symmetry for (b) Mirror plane symmetry
conrotatory motion for disrotatory motion
Fig. 2.3 a C2-axis of symmetry is maintained in thermal conversion of cyclobutene to butadiene;

b mirror plane symmetry is maintained in photochemical conversion of cyclobutene to butadiene
antibonding orbitals are σ* and π* (Fig. 2.3). For butadiene, the bonding orbitals
are ψ1 and ψ2, and antibonding orbitals are ψ3 and ψ4. In thermal reaction, con-
rotatory ring opening of cyclobutene to butadiene, C2 (twofold) axis of symmetry is
maintained throughout the reaction, while for photochemical reaction, disrotatory
ring opening, a mirror plane (m) symmetry is maintained throughout the reaction
(Fig. 2.3).
Next, consider the thermal conversion of a 1,3,5-hexatriene to a
1,3-cyclohexadiene by the disrotatory motion where mirror (m)-symmetry is
maintained in the orbitals of the reactant and product (Fig. 2.4). In photochemical
conversion of 1,3-cyclohexadiene into 1,3,5-hexatriene or vice versa, the C2-axis of
symmetry is maintained in conrotatory motion of the termini groups (Fig. 2.4).
Ψ6 A S σ∗ A A
S A ψ4
Ψ5 A S
(π4∗)
Ψ4 A S
Ψ3 S A
(π3∗)
S A ψ2 A S
Ψ3
(π2)
Ψ1 S A
A S
ψ2 (π1)
S S
σ
S A
ψ1
C2- axis m plane C2-axis

m plane
Ψ6 (A) (A) σ∗ σ∗ (A) (S) ψ6
ψ5 (S) (A) π4∗ π4 *(S) (A) ψ5
ψ4 (A) (S) π3∗ π3 *(A) (S) Ψ4
(A) π2 π2 (S) (A) Ψ3

Ψ3 (S)
ψ2 (A) (S) π1 π1 (A) (S) Ψ2
ψ1(S) (S) σ σ (S) (A) ψ1
(a) m plane classification (b) C2 axis classification
Fig. 2.4 a Mirror plane (m) symmetry is maintained in thermal conversion of 1,3,5-hexatriene
into 1,3-cyclohexadiene; b C2-axis of symmetry is maintained in photochemical conversion of
1,3-cyclohexadiene into 1,3,5-hexatriene or vice versa
2.4 Applications of Neutral Conjugated Systems in Electrocyclic Reactions 19
2.4 Applications of Neutral Conjugated Systems

in Electrocyclic Reactions
Electrocyclic reaction of E, Z-1, 3-cyclooctadiene leads to cis-bicyclo[4.2.0]-

oct-7-ene because of strain associated with trans double bond.
H
6H 7
con
Ref.7
H
80 oC
1H
PhH
Although cyclobutenes are converted into butadienes on heating to get relief of

ring strain, cis-bicyclo[3.2.0]-hept-6-ene on heating gave Z,Z-1,3-cycloheptadiene
by forbidden disrotatory motion. This anomaly of the Woodward–Hoffmann rules
can be accounted for by the stability of the product formed. In this case, allowed
conrotatory motion gives the strained E,Z-1,3-cycloheptadiene, which is less stable
due to ring strain and hence rapidly isomerizes to Z,Z-isomer at the reaction tem-
perature in low yield.
2 H
con 1 400 oC
Z,Z-isomer 3 Ref.8
5 6 dis
4 H Z,Z-isomer
It was supported by the thermal conversion of cis-1,6-dideuterio-bicyclo[4.2.0]-

oct-2,7-diene to cis-3,8-dideuterio-isomer via trans-isomer.
D D D 3
D
6
180 oC H H
H H
D H H
1 con 1
D H
D D 8
Cyclobutene-3-carboxylic acid on heating in acidic and basic solutions gives

isomeric cis- and trans-pentadienoic acids [9]. In thermal ring opening of
cyclobutenes, electron-donating substituents tend to move outward of the butadiene
chains to minimize the repulsive interaction with π system of butadiene in the TS,
while the π electron-accepting substituents tend to move inward of butadiene chains
to stabilize the HOMO TS by interaction with the donor lobes of p-orbitals of
breaking C(3)–C(4) σ bond. This preferential direction of movement of C(3) sub-
stituent in cyclobutene ring opening is called torquoselectivity and it works in
ranges far beyond the 4πe-electrocyclic system [9].
D
O
preferred unfavourable
preferred
TS TS
TS
D = Donor group CHO substituent
O O- OH
O OH
C C OH C
C O- NaOH COOH
H2SO4 OH
Study of the electrocyclic ring opening of cis- and trans-

3,4-dichlorocyclobutenes indicated that trans-isomer reacts at lower temperature.
This is due to ring opening by outward conrotatory motion of donor chlorine
substituents while in case of cis-isomer, activation energy is higher as one of the
chlorines rotates inward.
Cl Cl
Cl
Cl Δ Δ Ref.10
; Cl
Cl
Cl
Cl
When a cyclobutene ring contains both electron-donating and electron-accepting

substituents, conrotatory outward or inward motion of the substituents depends on
the size of the substituents. Thus, 1 gives 2.
CHO CHO
20 oC
CH2OCH2 OMe Ref.11
Con CH2OCH2 OMe
1 86% 2
When a cyclobutene ring contains two electron-donating substituents at C-3

position, the substituent with greater donor ability will move outward [12]. For
example, 3 gives 4.
OMe
3 OMe Δ t
t Bu
Bu
outward
3 4 > 99%
3-Hydroxy-2-methylcyclobutene 5 on electrocyclic ring opening undergoes

keto-enol tautomerization to afford the product 6.
O H
Me Me CHO Me CHO
OH o
100 C Me
Ref. 13
CDCl3 Me Me H
5
H+ 6
When a cyclobutene ring contains both donor hydroxyl and olefinic substituents
at C-3, the inward con-motion of the olefinic substituent occurs preferentially to
increase the stability of the TS. For example, 7 gives 8.
OH OH O
OH
Δ ring closure
dis
con,outward
7 8
Cyclobutene 9 having olefinic function at C-3 or C-4 position undergoes inward

ring opening from the olefinic substituent site followed by ring closure to give the
product. This inward motion of the olefinic substituent stabilizes the HOMO of the
TS by π-orbital interaction of the substituent with the donor lobes of p-orbitals of
the breaking σ bond of the ring carbons [14].
H Me
2 3 C C Me
H Δ dis
Ref. 14
1 Me con
4 Me
Me
9 Me 10 TS
Similarly, when both electron-withdrawing substituents are present at C-3,

inward conrotatory motion of the less bulky substituent takes place. When alkyl and
olefinic substituents are present at C-3 or C-4 position, the olefinic substituent will
move inward to provide more stable TS by closer interaction with the π-orbitals of
C-1 and C-2. The following examples are illustrative [12].
CO2Me
CO2Me CO2Me
CHO H
55 oC CHO
CO2Me ≡ O O
C6D6
CO2Me
Me Δ CO2Me ring closure
CO2Me
CO2Me dis CO2Me
CO2Me
Cyclobutene 11 containing two ortho-alkoxy groups at C-3 and C-4 positions

gives major product 12 by inward movement of smaller alkoxy group [15].
Me
Me
OMe OMe
Me PhMe, 110 C o Me
Me OMe
OR 2h
Me
11
OR OR
R=TBDMS
OMe Ref. 15
ring closure Me
6e process Me
dis 12 OR
94%
Cyclobutene fused with a carbocyclic ring gives isomeric product by more than
one electrocyclic processes. For example, cyclobutene 13 gives 14 and cyclobutene
15 gives 16.
D
D D
D
180 oC Z D ≡ H
H H
con
E con
D D
13 D 14
CO2Me
CO2Me
CO2Me H
H CO2Me electrocyclic CO2Me
Δ ring closure
H Ref. 16
4e system dis
CO2Me H
H con,inward H 16
15
Dewar benzene having two cyclobutene rings on heating gives benzene rather
than the expected product 17 from an allowed conrotatory opening. This is due to
the presence of strained E-double bond in the expected product, which rapidly
isomerizes to benzene.
H H
H
con isomerizes
H Ref.17
Δ
E
17
Strained ring
Heptatriene 18 having electron-withdrawing substituents, undergoes rapid con-

version into bicyclo[4.1.0]-hepta-2,4-diene 19 and both the isomers remain in
equilibrium because of low activation energy Ea < 10 kcal/mol. This transforma-
tion is known as valence tautomerism [18].
H
dis CF3
CF3
CN CN
18 19 H
Triphenylhexatrienes 20 and 21 undergo ring closures by dis-motion to give 22

and 23, respectively. Similarly, cyclic trienes 24 and 25 undergo allowed disrota-
tory reactions to give products.
Ph Ph Ph Ph Ph
80 oC Ph 110 oC Ph
Ph
;
dis Ph dis
Ph
inward 22 23
20 Ph 21 Ph > 90%
92%
H H
25 oC 25 oC
O Ref. 16
dis O dis
inward H inward H
24 25
Substituted 1,3,5-hexatriene 26 on electrocyclization, followed by isomerization

gives 27.
215 oC Ref.19
H
decalin CO2CMe3
CO2CMe3 dis CO2CMe3
27
26 H
Bicyclic conjugated dienes 28 and 29 undergo ring opening by electrocyclic

process.
H H
hν Δ Ref.20
con H
28 H H H
Δ
dis
29
Some photochemical electrocyclic reactions take place to yield products of much

higher energy than the starting materials [21]. For example, pyrone 30, oxocin 31,
cyclic ketone 32 and cis-stilbene 33 undergo photo-induced allowed electrocy-
clization to give products of higher energy.
H
O
O Δ
hν
O dis O
H
30
H
hν
O
O dis H
31
OMe
OMe
hν
dis
H O
32 O
hν
con
H H
33
Light-induced electrocyclization of bicyclo-[4.2.1]-nona-2,4-dienes, 34 and 35
gives both endo- and exo-isomers. Direct irradiation of the unsubstituted diene
gives endo-isomer as major product through a singlet excited state. The presence of
heavy atom, such as chlorine in the diene system facilitates the ISC by spin-orbit
coupling and increases the percentage of exo-isomer through triplet excited state.
Use of photosensitizer gives exo-isomer as the major product [22].
hν
34 70 : 30
hν Cl
Cl Cl
Cl
Cl
35 60 : 40 Cl
1, 3, 5-Trienes, 36, 37 and 37a in a ring system undergo double electrocyclic
processes to yield stable products.
1 Me H Me H Me
H Et Et
Me 170 C o
Et dis
+
3 Ref. 16
Et con H
H H
less stable
36 for one E-double bond
9 7 H 9
1 10 8 10
Δ dis 10
con 1 Ref. 23
8 9 1
H 2
37 less stable
cis-Bicyclo [ 6. 2.0.]- trans-9,10-dihydro-naphthalene
deca-2, 4, 6, 9-tetraene
154 oC dis
Ref. 23
con
(8π process)
37a
The presence of methyl substituents in cyclic 1, 3, 5-triene 38 causes cyclization

involving 4π electrons. Due to steric interaction, one of the olefinic double bonds
remains out of plane of other olefinic double bonds.
Me Me Me MeH
hν
Ref. 16
Me
dis
38 Me H
2, 4, 6, 8-Decatetraenes 39 and 40 undergo electrocyclic reactions near room

temperature and maintain an equilibrium favouring the cyclooctatriene products. At
slightly more elevated temperatures, the cyclooctatriene system undergoes another
cyclization to produce bicyclo[4.2.0]-octa-2,4-diene.
-10 oC, 30h 20 oC, 8h

Ref.24
con dis
39
9 oC, 155h 40 oC
con dis
40
Conjugated decapentaene 41 undergoes electrocyclic ring closure using 6πe

because of nonplanarity of 10πe.
H
Δ
dis H
hν H
con
41
≡ H
Oxonin 42 and azonine 43 having a tetraene system undergo cyclization using

their triene system to bicyclo[4.3.0] systems due to interaction of the lone pair of
oxygen and nitrogen, respectively, with one double bond [25].
H
Δ , 30 oC 15 oC
H
O N COOEt
dis O
H N
42 43 H COOEt
Oxonin
Acyclic conjugated enyne 44 in singlet excited state undergoes skeletal rear-

rangement via photoelectrocyclization to a highly strained 1,2-cyclobutadiene,
followed by ground-state ring opening [26]. Calculation of energies of
1,2-cyclobutadiene suggested its planar geometry of C2-symmetry with the vinylic
hydrogens twisted 6° out of plane.
hν (254 nm) Me Bu Bu
Me Bu Me
pentane
cyclization H H only product
44
2.5 Applications of Ionic Conjugated Systems in Electrocyclic Reactions 27
2.5 Applications of Ionic Conjugated Systems

in Electrocyclic Reactions
The acetolysis of isomeric cyclopropyl tosylates (45–47) at 100 °C takes place by

concerted electrocyclic ring opening and ionization. The loss of tosyloxy group in
ionization step is assisted by the electron density of the developing p-orbitals that
are trans to the leaving group. Hence, the isomer 47 undergoes much faster ace-
tolysis than the other isomers [27].
Me Me
H Me OTs Me H
OTs H H H
H H OTs
45 46 47
rel. rate: 1 4 41,000
Me Me Me Me
HOAc
Me H outward H Me H
H dis OTs
OTs 2e system Me OAc
47 rate determining TS
preferred TS for
ionization process
because of maximum
interaction of p orbitals
for elimination of leaving group
in a E2 like process
Me
Me
inward Me OTs Me
Me H Me OAc
dis Me H
Me H
OTs
T.S
46 less favoured T.S
because of steric
crowding of methyl groups
The rate of acetolysis of bicyclic tosylates 48 and 49 with acetic acid at 150 °C
depends on the geometry of the generated allyl cation. Isomer 49 reacts about
2 × 106 times faster than 48 because in the former the reaction proceeds via the
formation of stable cis-cyclohexenyl cation [28].
H
OTs
OTs
H
48 49
OTs
H
inward
dis
2π system cis-cyclohexyl cation
49
OTs
outward
dis
2π system
48 trans- cyclohexyl cation
less stable due to highly strained ring
The similar electrocyclic reaction of bicyclic bromide 50 in aqueous dioxane at
100 °C gives trans-cyclooctene-3-ol 51 [29]. The generated p-orbitals from the
breaking of a σ bond in cyclopropanone ring by inward dis-motion participate in the
removal of bromine atom in an E2-like process. The outward ring opening will
provide less stable trans-cyclooctenyl cation.
Br
5 4
H OH2
3
H H2O-dioxan OH
6 2
H 100 oC
1
dis, inward
50 51 7 8
Methanolysis of bicyclic dibromide 52 with MeOH in the presence of AgClO4

gives trans product 53 [30].
Br
Br
AgClO4
H
MeOH OMe
Br
52 53
Hydrolysis of endo-2-chloro-exo-2-bromo-bicyclo[3.1.0]-hexane 54 gives

2-bromo-3-cyclohexenol 55 [16]. The inward disrotatory motion of ring opening is
preferred because it provides the movement of endo-chlorine atom inside the
cyclopropane ring to acquire an antigeometry with respect to electron-rich lobes of
the p-orbitals generated by cleavage of a sigma bond of cyclopropane ring and
helps to participate in an E2-like elimination process for faster elimination.
Moreover, it provides a more stable cis-cyclohexenyl cation [16].
Cl
AgClO4 H2O
Br
Br H
H2O OH OH
54
Br 55 Br
Acid-catalyzed cyclization of divinyl ketone 56–57 occurs by conrotatory

cyclization of 3-hydroxy pentadienyl cation [31]. This type of cyclization reaction
is known as Nazarov cyclization reaction [31].
O OH O H O
H3PO4
HH H H H H
56 57
Divinyl ketone 58 undergoes Lewis acid-catalyzed electrocyclization to give 59

[21].
Me3Si O Me3Si OFeCl3 Me3Si OFeCl3

FeCl3 con H
58 H H
OH O
H
H H H H
59
72%
Similar electrocyclic ring closure occurs in aryl vinyl ketone 60 by strong acid.
OH2 OH2
O
Me con Me
Me CF3SO3H
H
60
OH O
Me Me
Ref.32
97%
Cyclooctadienyl lithium 61 having pentadienyl anion undergoes cyclization by

disrotatory motion at 35 °C [33].
H
H H
nBuLi 35 oC ROH
+
Li
dis
H H
H
61
Cyclooctadienyl anion generated from cyclopentene derivative 62 undergoes

ring opening in the presence of a strong base to give 63 and 64.
NaOAmt dis H+
+
or Ref. 21
BuLi Ph
Ph Ph Ph Ph Ph Ph Ph
Ph Ph
62 63 64
Cyclooctatetraene dianion 65 on treatment with acetyl chloride gives 3, 5, 7,

9-dodecatetraene-2,11-dione 66 by electrocyclic ring opening along with other
products [34].
O
HO
C CH
AcCl C CH3 AcCl 3
CH3
C
65 O 8e system
con
H
O O
CH3 O
O CH3 5
CH3 C 1
CH3
H 3 2
7
AcCl CH3
9 C
O
H 66
Ac
O
CH3
H
AcO CH3
Cyclooctadienyl anion obtained from carbolithiation of 3-methylene-1,4-

cyclooctadiene 67, on electrocyclization gives a cis-bicyclo-product 68, which on
trapping with an electrophile gives exo-isomer 69 as major product [33].
But
But
But
HO
t
Bu-Li, Et2O Ph2CO Ph
6e-system H H
Et2O H Ph
-78 oC dis
H H
67 68
69
65% dr > 20 :1
Butadienyl pyridinium ylide 70 on electrocyclization gives stereoselective 1,

2-annulated-2,3-dihydroazepine 71 as major product [35].
N KOBut N
8e system N
Ref. 35
MeCN, THF con H
Ph reflux, 2h Ph Ph
Me Me Me
70 71
Hydrobenzamide 72 on treatment with phenyl lithium gives a pentadienyl anion,

which on electrocyclization gives dihydroimidazole, amarine 73 [36].
Ph Ph Ph Ph
H
1 2
N N PhLi N N dis N N H+ HN N 3
Ph
o
Ph -130 C Ph Ph inward
72 Ph Ph Ph 5 4 Ph
73
Similarly, heterocyclic compound 74 in the presence of a strong base gives 75 by

disrotatory reaction [36].
Ph Ph
Ph Ph
B-, THF
N N
N H+
HN
Ph O -120 oC Ph O
Ph Ph O O
Ph Ph Ph Ph
74 antiaromatic 75
2.6 Problems
2.6.1. Predict the structure, including stereochemistry, of the product for each of the
following reactions.
CF3
Δ Δ
a. b.
Me Me
CH2OSiR3
c. H KOt Bu d. Δ
D
Br Δ
CH O
H Me
H
Δ hν
e. f.
CD2 275 oC
g. hν
h.
CD2
i.
hν
j. Δ
OMe
k. hν
O
2.6 Problems 33
2.6.2 Rationalize the following reactions.
Ph H CO2Et O
Ph H HO CO2Et CO2Et
(a) 1. PhLi N NH (b) O N2
N N N NH
+
-130 oC N N
Ph Ph Ph Ph LDA THF, -78 oC
2. H3O+ Ph
rt, 1h Ph Ph
Me Me
O O Me Me
(c) 80 oC (d) hν N OMe
N N
MeOH NH2
O O
Me
Me
H H F
(e) 150 oC (f) Cl HCOOH OCHO
H Cl
H
Ph
Ph Ag+
(g) N (h)
N Ph Me2CO
Ph 60 oC reflux, 40 min
Ph
Ph
>100 oC
(i) Δ (j)
NC NC
2.6.3. Offer a mechanistic explanation for each of the following observations.

a. It has been found that both compound A and B undergo ring opening about
104 times faster than C in the presence of lithium di-t-butylamide.
C N
C N C N
Ph
Ph Ph
Ph
A B C
b. 2-Vinylcyclopropanols undergo facile rearrangement to give cyclopent-3-
enols.
OH
Li salt CH2CH2Ph
H H
C 250C
CH2CH2Ph OH
H
H Me
H
c. H
H Me
Me
Me
d. Compound D undergoes solvolysis readily with AcOH at 125 °C, whereas

compound E remains unchanged on prolonged heating with AcOH at 210 °C.
Cl H
H Cl
D E
2.7 Further Reading
1. Marvell EN (1980) Thermal electrocyclic reactions. Academic Press, New York

2. Durst T, Breau L (1991) Cyclobutane ring opening. In: Trost BM, Fleming I
(eds) Comprehensive organic synthesis. Pergamon, Oxford (Chapter 6)
3. Marchand AP, Lehr RE (1977) Pericyclic reactions, vols I & II. Academic Press,
New York
4. Carey FA, Sundberg RJ (2007) Advanced organic chemistry, Parts A & B.
Springer, New York
References
1. Winter REK (1965) Tetrahedron Lett 1207

2. Brauman JI, Archie WC Jr (1972) J Am Chem Soc 94:4262
3. Marvell EN, Caple G, Schatz B (1965) Tetrahedron Lett 385
4. Vogel E, Grimme W, Dinne E (1965) Tetrahedron Lett 391
5. Woodward RB, Hoffmann R (1965) J Am Chem Soc 87:395
6. Fukui K, Fujimoto H (1968) In: Thygarajan BS (ed) Mechanisms of molecular migrations, vol
2. Interscience, New York, p. 113; Longuet-Higgins HC, Abrahamson EW (1965) J Am
Chem Soc 87:2045
7. Schumate KM, Neuman PN, Fonken GJ (1965) J Am Chem Soc 87:3996
8. Bronton R, Frey HM, Montaque DC, Stevens IDR (1966) Trans Faraday Soc 62:659
9. Buda AB, Wang Y, Houk KN (1989) J Org Chem 54:2264; Hayes R, Ingham S,
Saengchantrara ST, Walker TW (1991) Tetrahedron Lett 32:2953
10. Criegee R, Seebach D, Winter RE, Boerretzen B, Brune H (1963) Chem Ber 98:2339;
Maier G, Bothur A (1998) Eur J Org Chem 2063
11. Binns F, Hayes R, Ingham S, Saengchantara ST, Turner RW, Wallace TW (1992)
Tetrahedron 48:515
12. Dolbier WR Jr, Koroniak H, Houk KN, Sheu C (1996) Acc Chem Res 29:471
13. Jefford CW, Boschung AF, Rimboult CG (1974) Tetrahedron Lett 3387
14. Rondon NG, Houk KN (1985) J Am Chem Soc 107:2099
15. Hamura T, Miyamoto M, Imura K, Matsumoto T, Suzuki K (2002) Org Lett 4:1675
16. Morrison RT, Boyd RN (1989) Organic chemistry, 5th edn. Prentice-Hall, New Delhi; De
Puy CH (1968) Acc Chem Res 1:33
17. Goldstein MJ, Leight RS (1977) J Am Chem Soc 99:8112
References 35
18. Maier G (1967) Angew Chem Int Ed Engl 6:402

19. von Zezschwitz P, Petry F, de Meijre A (2001) Chem Eur J 7:4035
20. Eberbath W (1975) Chem Ber 108:1052; Mosamune S, Dubby N (1972) Acc Chem Res
5:272
21. Fleming I (2002) Pericyclic Reactions. Oxford University Press, New York
22. Jefford CW, Delay F (1975) J Am Chem Soc 97:2272
23. Staley SW, Henry TJ (1971) J Am Chem Soc 93:1292
24. Huisgen R, Dahmen A, Huber H (1969) Tetrahedron Lett 1461; (1967) J Am Chem Soc
89:7130; Dahmen A, Huisgen R (1969) Tetrahedron Lett 1465
25. Masamune S, Takeda S, Scidner RT (1969) J Am Chem Soc 91:7769; Anastassiou A,
Cellura RP (1969) Chem Commun 1521
26. Zhang M, DiRico Kj, Kirchhoff MM, Phillips KM, Cuff LM, Johnson RP (1993) J Am Chem
Soc 115:12167
27. de Puy CH (1968) Acc Chem Res 1:33
28. Schleyer PVR, Sliwinski WF, van Dine GW, Schollkopf U, Paust J, Fellenberger K (1972) J
Am Chem Soc 94(125):132
29. Whitham GH, Wright M (1971) J Chem Soc C 883
30. Reese CB, Shaw A (1970) J Am Chem Soc 92:2566
31. Woodward RB (1969) In: Aromaticity, chemical society special publication no. 21, p. 217;
Habermas KL, Denmark SE, Jones TK (1994) Org React 45:1
32. Bates RB, Gosselink DW, Kaczynski JA (1967) Tetrahedron Lett 199, 205; Bates RB,
McCombs DA (1969) Tetrahedron Lett 977
33. Williams DR, Reeves JT, Nag PP, Pitcock WH, Baik MH (2006) J Am Chem Soc 128:12339
34. Cantrell TS, Shechter H (1967) J Am Chem Soc 89:5868
35. Marx K, Eberbach W (1997) Tetrahedron 53:14687
36. Hunter DH, Sim SK (1969) J Am Chem Soc 91: 6202; ibid (1972) Can J Chem 50:669
Chapter 3
Cycloaddition Reactions
3.1 Introduction
Cycloaddition reactions are the most useful pericyclic reactions in organic syn-
thesis. These are the reactions of two π systems to form ring compounds by the
breaking of two π bonds and making of two σ bonds in a concerted process. The
reverse of cycloaddition reactions are known as retrocycloaddition reactions. Both
cycloadditions and cycloreversions proceed through cyclic transition states in which
continuous flow of electrons occur among the reacting molecules. These reactions
are classified according to the number of π electrons involved in each reacting
molecules. The major classes are [π2+π2], [π4+π2], [π6+π2], [π8+π2], and [π6+π4].
These are simply known as [2+2]-, [4+2]-, [6+2]-, [8+2]-, and [6+4]-cycloaddition
reactions and are illustrated below.
a) +
π + π2
2
σ2 + σ2
b) +
2
π4+ π2 σ2 + π + σ2
+
c)
π6+ π2 σ2 + π4 + σ2
d) +
π8+ π2 σ2 + π6 + σ2
e) +
π6+ π4 σ2 + π4+ σ2+π2

38 3 Cycloaddition Reactions
3.2 [2+2]-Cycloaddition Reactions
3.2.1 Overview of Thermal and Photochemical

[2+2]-Cycloaddition Reactions
Thermal [2+2]-cycloaddition reactions are less common, but photochemical [2+2]-

cycloaddition reactions are very common. This fact can be explained by analyzing
these cycloaddition reactions using Woodward–Hoffmann selection rules. In fron-
tier orbital approach, the thermal reaction of two ethene molecules (one is HOMO
and other is LUMO) is orbital symmetry forbidden process for its suprafacial–
suprafacial [π2s+π2s]-cycloaddition, but a suprafacial–antarafacial [π2s+π2a]-
cycloaddition reaction is symmetry allowed process (Fig. 3.1). It signifies that the
cycloaddition of one two-π electron system with another two-π electron system will
be a thermally allowed process when one set of orbitals is reacting in a suprafacial
mode and other set in an antarafacial mode (“s” means suprafacial and “a” means
antarafacial). Thermal [π2s+π2a]-reactions usually occur in the additions of alkenes
to ketenes, when alkene is in the ground state and ketene in the excited state
[1] (Fig. 3.2).
Fig. 3.1 Frontier orbital (a)

interactions of a thermally
forbidden [π2s+π2s]- π* (LUMO) m(A) σ* m(A)
cycloaddition reaction,
b photochemically allowed
[π2s+π2s]-reaction of alkenes π* (HOMO) m(S) σ m(S)
π2s + π2s m(S) and m(A) mean σ2s + σ2s

mirror symmetry and
mirror antisymmetry
(b)
SOMO σ m(S)
LUMO σ m(S)
π2s + π2s σ2s + σ2s
SOMO = Single occupied molecular orbital

LUMO= Lowest unoccupied molecular orbital
HOMO= Highest occupied molecular orbital
3.2 [2+2]-Cycloaddition Reactions 39
Fig. 3.2 Frontier orbital C=O

interactions of thermally HOMO
C=C LUMO
allowed antarafacial
interaction of a ketene (ketene)
(LUMO) and an olefin
(HOMO) O
LUMO
HOMO
(alkene)
3.2.2 Applications of [2+2]-Cycloaddition Reactions
Cycloaddition of ethoxy ketene 1 with cis- and trans-2-butene is a concerted

process and gives stereospecific products 2 and 3, respectively [2].
H
Me
Me Me H
EtOCH C O + C C C O
H H EtO
1
H Me
H
Me
Me Me
H
C O ≡ H H
EtO H
H EtO O
Me 2
Me Me
H Me
EtOCH C O + C C H H
Me H
H
EtO O
3
Similarly, ethylketene 4 and dimethylketene 5 react with cyclopentadiene to give

6 and 7, respectively.
H
H O
C C O +
Et Et
4
H
6 H Ref.3
H
Me O
C C O +
Me
5 Me
H
7 Me
Ketene 8 reacts with ethoxy acetylene to give 9 [3] and phenyl methyl ketene 10
reacts with Z-alkene 11 to give 12 [4].
O
EtOC CH + H2C C O Ref. 3
8 EtO
9 30%
Ph OEt O OEt
C O + Ref. 4
Ph Me
Me Me
10 11 Me
12
Ethoxyethylene reacts with diphenylketene 13 to give cycloadduct 14 in high

yield [5].
Ph O
r.t.
+ C O Ref. 5
EtO 16h Ph
Ph EtO
13 Ph
14 67%
Ketenes also undergo [π2+π2]-cycloadditions with ketones, in the presence of

Lewis acids as catalysts, e.g., 8 reacts with ketone 15 to give 16 and with 17 to give
18 in high yields [6].
O
t- ZnCl2
Bu
CH2=C=O + O
t-
Me Bu O
8 15
Me 16
67%
O
Et2O+- BF3-
H2C=C=O + Me2C=O
O
17 -40 oC
18 90%
Intramolecular ketene cycloadditions are also observed in compounds having

both ketene and alkene functionalities in appropriate orientations. For example, 19
gives 20 [6] and 21 gives 22 [7].
Me CH2 Me Me
Me Et2NiPr
CH2COCl
Me -Cl- Me Ref. 6
19 20
O
CH2
43%
Ph Ph
Ph
O H
Ph
C
Ref. 7
H
21 22
O
Several non-concerted [2+2]-cycloadditions have been reported, where diradical

or zwitterionic intermediates are produced. The following examples are illustrative:
F F
F F F
o F
1. 82 C
Cl
Cl Cl Cl Cl
Cl
70%
CN H
NC CN NC C C OCH3
2. CH2 CH OCH3 + NC C CH2
NC CN CN
Zwitterion
CN H
NC OCH3 Ref. 8
NC
CN 90%
O O
H2C H2C
3. CH2 NR + CH2 C O
N CH2 N CH2
R R
α β CH2
CH2
Ref. 9
N
R O N C
O
β-lactam
R
Ph N MeO2C-CH2COCl
OH TBDPS-Cl Ph N
4. OTBDPS
MeO2C THF Et3N
MeO2C
TBDPS= tert-butyldiphenylsilyl
MeO2C
MeO2C Ph
CH C O
N
Ph O OTBDPS Ref: 10
N
MeO2C
OTBDPS
H
MeO2C 65%, 90% d.e
Photochemical [2+2]-cycloaddition in suprafacial mode is symmetry allowed

process and occurs in dimerization of alkenes, intermolecular additions of alkenes,
and intramolecular cycloadditions of dienes and alkynes. The following examples
are illustrative:
(a) Dimerization of alkenes
Me Me Me Me
hν
1. 2 Me Me
+ Ref: 11
Z-2-butene Me Me Me Me
hν Me Me Me Me
Me
2. 2 +
Me
Me Me Me
E-2-butene Me
(b) Intermolecular additions of alkenes

Me
Ph Ph
Me Me hν Me
3.
Ph + Ref: 12
Me Me Me
Ph Me
95%
(c) Intramolecular addition of dienes
hν
4. Ref: 13
H
H hν
5. Ref. 14
Cyclohexane
62%
hν
6.
CuCl Ref. 15
43%
O
H
[4+2] H hν H
+ O
7. Ref. 15
Δ H [2+2] O
O O O
(d) Intramolecular cycloaddition of enone and compounds bearing enolate and

yne functions
O O
8. hν
Ref: 16
hexane
77%
O O O
9. hν O O
O Ref: 16
Non-stereospecific photochemical [2+2]-cycloadditions occur in the dimeriza-

tion of phenyl cyclohexene 23 in the presence of a sensitizer to produce 24 and 25
[17], and in reactions of Z/E-2 butene with cyclohexenone 26 to give 27 and 28 [18]
through the formation of intermediate diradicals. The photoaddition of cyclohexene
to an enolised form of 1,3 diketone 29 gives 30 in a concerted process via the
formation of an unstable cycloadduct [18].
p y
Ph Ph Ph Ph Ph Ph
Ph
hν
+
Ref: 17
23 H H H H
24 25
O O O O
Me Me Me Me
hν Me
+ or
+
Me Me Me Me Me
26 27 28
Ref: 18
O
O hν O
+ Ref. 18
OH OH O
29
30
3.3 [4+2]-Cycloaddition Reactions
The Diels–Alder reactions and 1,3-dipolar cycloaddition reactions are known as [4

+2]-cycloaddition reactions because four electrons from diene or 1,3-dipole, and
two electrons from the dienophile or dipolarophile are involved in these reactions.
The 1,3-dipolar cycloaddition reactions are also called [3+2]-cycloaddition
reactions because three atoms of dipolar compound and two atoms of dipolarophile
are involved in the cyclization process.
3.3.1 The Diels–Alder Reactions
3.3.1.1 Overview of the Diels–Alder Reaction
The most important type of thermal [4+2]-cycloaddition reactions is known as the

Diels–Alder reaction, as this reaction was discovered by Otto Paul Herman Diels
and Kurt Alder in 1928 [19]. It may be noted that both of them awarded the Nobel
Prize in Chemistry in 1950 for their contributions on the development of the Diels–
Alder reaction. These reactions are defined as the concerted [4+2]-cycloaddition
reactions of conjugated dienes with an alkene or alkyne. The alkene or alkyne is
known as dienophile. Hence, these reactions are described as [π4+π2]-cycloaddition
reactions. These reactions are carried out by heating the compounds alone or in an
inert solvent or in the presence of a Lewis acid. An alkene or alkyne having
electron-withdrawing substituent acts as an effective dienophile. These reactions
proceed stereospecifically to syn-addition with respect to both diene and dienophile.
The following cycloaddition reactions of butadiene, furan, and cyclopentadiene
are illustrative examples:
H
CO2Et CO2Et
1. + H Ref. 15
CO2Et CO2Et
CO2Et
H CO2Et
H
2. + H
EtO2C H CO2Et
O H O
50-75 oC Ref. 20
+ O O
3.
2-2.5 h 93-97%
H O
O
O O
O
35 o C
4. O + O Ref. 15
PhH H O
O H
O
exo 100%
O
O
40 oC H
5. + H
PhH Ref. 21
O O
endo 97%
These reactions are broadly classified as normal and inverse electron demand
Diels–Alder reactions. In normal electron demand Diels–Alder reactions (NED D–
AR), diene system acts as electron-rich HOMO system and dienophile as
electron-seeking LUMO system, while in inverse electron demand Diels–Alder
reactions (IED D–AR), diene system serves as LUMO and dienophile as HOMO
[22]. For example, reaction of cyclopentadiene 31 with methyl acrylate gives 32 in
NED D–AR [23] and of ethoxyethylene with β-(2-cyclohexenone)-ethyl acrylate 33
gives 34 in NED D–AR [24].
MeOH
+ H Ref.23
CO2Me CO2Me
31 32
NED D-AR
90%
O
O
0 CO2Et
CO2Et 80-90 C
+ Ref.24
OEt OEt
34 H
33 IED D-AR
> 95%
3.3.1.2 Regioselectivity
Regioselectivity of Diels–Alder reaction depends on the position of substituents in

both diene and dienophile. Usually four types of D–A reactions are observed
(Scheme 3.1)
This regioselectivity of Diels–Alder reactions can be interpreted on the basis of
electron density (orbital coefficient) at C-4 of the diene and C-2 of the dienophile.
For dienes with electron-releasing groups at C-1, HOMO has its largest electron
density (largest HOMO orbital coefficient) at C-4. For dienophiles with
electron-accepting substituents, C-2 has minimum electron density (largest LUMO
orbital coefficient) in their LUMO. Hence, the strongest frontier orbital interaction
occurs between C-4 of the diene and C-2 of the dienophile in a normal electron
demand D–A reaction to give ‘ortho’-like product. For example, reaction of 35 with
36 gives cycloadduct 37 [5].
OMe O OMe
1000C, 3h CHO
+ H
Ref. 5
2 36
35 4 37
OMe O O
OMe
H H
;
Similarly, in other type of normal Diels–Alder reactions, dienes with

electron-releasing groups at C-2 have the largest electron density at C-1 in their
HOMO. Therefore, the strongest frontier orbital interaction occurs between C-1 of
Type A:
ERG ERG
EWG EGW
+
NEt2 NEt2 NEt2

eg.
CO2Et CO2Et CO2Et
200C
+
Ref 25
N,N-diethyl-1,3-butadiene "ortho"-like product

94%
Type B:
ERG ERG
+
EWG EWG
eg. EtO
EtO 160 oC
+
CO2Me Ref.25
CO2Me
2-ethoxy-1,3-butadiene "para"-like product
50%
MeO
MeO 160 oC
+ Ref.26
CHO 30 min. CHO
Type C: 75%
EWG EWG
ERG ERG
+
COOH COOH COOH

eg.
COOH 0 COOH Ref. 25
75 C
+ +
COOH
1-carboxy-1,3-butadiene " meta"
" ortho"
90% minor
COOH COOH COOH

Ph Ph
+ + Ref. 15
Ph
6 part 1 part
Scheme 3.1 Regioselectivity of Diels–Alder reaction

Type D:
EWG EWG EWG ERG
+ +
ERG ERG
para like
e.g.
Δ EtO2C OEt
CO2Me MeO2C EtO2C OEt
, Δ
+ +
CO2Me
CO2Me
only product
Ref.27
CO2Me 95 oC CO2Me
+
+ Ref.28
NC NC
NC CO2Me
[46]
84: 16
ERG, Electron- releasing group
EWG, Electron- withdrawing group
Scheme 3.1 (continued)
the diene and C-2 of the dienophile to give ‘para’-like product as major product,
e.g., reaction of 38 with 39 gives 40 and 41 as major and minor product, respec-
tively [29].
Me CO2Me 1200C Me Me CO2Me

+ +
6h Ref.29
CO2Me
38 39 41
40
70 : 30
δ δ
Me Me
The regioselectivity of inverse electron demand Diels–Alder reactions can be

rationalized on the basis of orbital coefficient rather than partial positive charge that
is expected at C-4 of the diene. This is because the positive charge at C-4 of the
diene will be repelled by the positive charge of the β-carbon of the dienophile
(acrylic acid). In such cases, the orbital coefficients of C-4 of the diene and C-2 of
the dienophile were comparable to that of normal electron demand Diels–Alder
reactions. The LUMO of the diene and HOMO of the dienophile are in lower
energy levels (Fig. 3.3).
O O OH
C OH C OH
O C
OH
O C
;
LUMO LUMO
HOMO HOMO
I. Normal electron demand II.Inverse electron demand

D-AR D-AR
Fig. 3.3 Frontier orbital interactions in Diels–Alder reactions
Hence, frontier orbital interaction takes place preferentially at these positions to

give “ortho”-like product as major product [30].
3.3.1.3 Stereochemistry
Woodward–Hoffmann selection rules predict the allowedness of thermal and

photochemical [4+2]-cycloaddition reactions (Table 3.1).
The Woodward–Hoffmann rules for cycloaddition reactions can be explained
from frontier orbital interactions, orbital correlation diagram and aromatic transition
state approaches.
(a) Explanation from the FMO approach
For a favorable TS structure of low energy for a Diels–Alder cycloaddition reaction,
the HOMO of diene and LUMO of dienophile or LUMO of diene and HOMO of
dienophile will approach face to face from the same side for maximum orbital
interactions (Fig. 3.4).
(b) Explanation from the orbital correlation diagram
Let us consider the orbital symmetry properties of the reactants and products for the
Diels–Alder cycloaddition reaction of butadiene and ethylene into cyclohexene
(Fig. 3.5). The addition of the diene and dienophile takes place face to face, where
diene assumes S-cis-conformation. The reactants, TS and product maintain sym-
metry (plane of symmetry) among their orbitals during the course of cycloaddition.
To understand this fact, an orbital correlation diagram [31] is constructed by
arranging the orbitals with respect to their energy content, or correlation lines are
Table 3.1 Woodward–Hoffmann rules for [m+n]-cycloaddition reactions

[m+n]πe Number of Aromaticity Thermal Photochemical
nodes
4n 0 Antiaromatic Forbidden Allowed (supra/supra
antara/antara)
4n 1 Aromatic Allowed (supra/antara Forbidden
antara/supra)
4n+2 0 Aromatic Allowed (supra/supra Forbidden
antara/antara)
4n+2 1 Antiaromatic Forbidden Allowed (supra/antara
antara/supra)
HOMO (Ψ2)
LUMO (Ψ3)
LUMO (π∗)
HOMO (π)
(a) Orbital symmetry allowed TS (π4S+ π2S) (b)Orbital symmetry allowed TS (π4S+ π2S)
for NED D-AR for IED D-AR
(c)Orbital symmetry forbidden TS (π4a+ π2s)
Fig. 3.4 Orbital interactions of HOMO of diene and LUMO of dienophile and vice versa in a
Diels–Alder reaction
drawn as in Fig. 3.6. From this diagram, it is reflected that thermal concerted
reaction between butadiene and ethylene is allowed process, because ψ2 and π
orbitals of butadiene and ethylene are correlated with σ1 and π orbitals of cyclo-
hexene in the ground state.
(c) Transition state stability
Now, we may consider the TS structure for a Diels–Alder cycloaddition reaction.
The Huckel TS structure of zero node is aromatic in nature. Thus, the reaction takes
place in suprafacial mode following the Huckel topology, whereas in Mobius
ψ4 A σ1∗ A
Ψ3 S
S
σ∗
Ψ2 A
π∗ A
Ψ1 S
π S
π∗ (Ψ2) A
σ1 A
π(Ψ1) S
S
σ
m-sym
m-sym
Fig. 3.5 Symmetry properties of butadiene, ethylene, and cyclohexene orbitals with respect to
plane of symmetry. m-sym means mirror, S means symmetric, and A means antisymmetric
topology, addition to opposite faces of the π system is required and is unfavorable

for its antiaromatic nature (Fig. 3.7) [32].
3.3.1.4 Stereochemical Features of the Products
The D–A reaction of a cyclic diene with a cyclic dienophile may give two
stereoisomeric products, endo- and exo-products depending on the conformation of
the transition states. The product in which the unsaturated substituents of the die-
nophile are cis to the double bond of the newly formed cyclohexene ring is called
σ1∗ (A)
Ψ4 (A) σ∗(S)
π∗ (A) π∗ (A)
Ψ3 (S)
Ψ2 (A)
π (S) π (S)
Ψ1 (S) σ1(A)
σ (S)
Fig. 3.6 Symmetry correlation diagram for ethylene, butadiene, and cyclohexene orbitals
LUMO
1 node
zero node
HOMO
Huckel TS (aromatic) Mobius TS (antiaromatic)
Fig. 3.7 The orbitals set for supra-, supra-[π4+π2]-cycloaddition in Huckel and Mobius TSs
the endo-product, whereas in the product, where these substituents are trans to the
double bond is called the exo-product. For example, furan 42 on reaction with
succinimide 43 gives endo- and exo-products 44 and 45, respectively, at different
reaction conditions.
O O
O O
o H O o
25 C 90 C
O + NH H H NH
NH
44 45 H O
42 43 O
endo O exo
90 oC
The D–A reactions of acyclic dienes with acyclic dienophiles also give endo-
and exo-products. For example, D–A reaction of 1-deuterio-1,3-pentadiene 46 with
trans-3-penten-2-one 47 gives endo 48 and exo 49 products as shown.
CH3 O H3 C H H3 C H
H COCH3
H CH3 COCH3 H
+ +
D CH3 H
H3 C H CH3
46 47 H D H D
H
48 49
endo exo
The relative orientations of the diene and dienophile in a favorable TS for a Diels–
Alder reaction is predicted by Alder’s endo-rule [33]. The Alder’s endo-rule states that
for Diels–Alder reactions of substituted butadiene derivatives with dienophiles having
an electron-withdrawing substituent, kinetically controlled endo-TS will be preferred
over exo-TS because of secondary orbital interactions of the electron-withdrawing
substituent with the butadiene π system. The endo-TS has lower activation energy than
that of exo-TS. The product derived from endo-TS is called kinetically controlled
product and the product derived from exo-TS is called thermodynamically controlled
product. Frequently a mixture of both stereoisomers is formed and sometimes the
thermodynamically controlled exo-product predominates. It has been observed that
reaction of butadiene with maleic anhydride using deuterium-labeled butadiene gives
85 % of the endo-product 50 from endo-TS [33]. The reaction of cyclopentadiene with
maleic anhydride also gives 97.5 % endo-product 51. The secondary orbital inter-
actions in preferred endo-TS are shown in Fig. 3.8.
For reaction of cyclopentadiene with acyclic dienophile-like methyl acrylate, the
endo- and exo-TS would be
+
Z +
Z
Z Z
endo- product exo- product
Z= CO2Me
endo -TS exo -TS
3.3.1.5 Substituent Effects on the Reaction Rates
(a) Steric effects of substituents

Diels–Alder reactions are sensitive to steric effects. The presence of bulky sub-
stituents on the dienophile or on the diene hinders the approach of the components to
each other and decreases the rate of reaction. For example, 1-tert-butyl butadiene is
about 20 times less reactive than butadiene toward maleic anhydride [34].
R O R Krel
O H
0
25 C H O H 1
+ O
C(Me)3 < 0.05
O
O
R
H O
HO
O
O D
H O
D H
+ O O
D
D
O
50 O
85%
X X X
Z H H
H Z
+ Z ≡
H H Y H
Y endo addition Y
endo TS preferred cis-cis product
X X X
H Z H
H Z Z
+
H H ≡
Y H
Y exo addition Y
exo TS trans-trans product
O
O H
H O
+ O
O
O endo addition product
(97.5 %)
TS
≡
HOMO
secondary orbital interactions
O O
O LUM
O
preferred TS for endo addition
Fig. 3.8 The orbital interactions in endo- and exo-transition states (TSs) in a Diels–Alder reaction
O
O
HO
+ O
H O
O exo addition product
TS
≡ (1.5 %)
HOMO
O
LUM
O
O
O
less preferred TS for exo addition
Fig. 3.8 (continued)
2,3-Dimethylbutadiene reacts with maleic anhydride about ten times faster than
butadiene due to electron-releasing effect of the methyl group.
δ+ δ−
Me Me
Me Me δ-
δ+
2-tert-Butyl-1,3-butadiene is 27 times more reactive than butadiene toward
maleic anhydride because the tert-butyl group favors the s-cis conformation due to
high 1,3-steric interaction in the s-trans conformation [35].
Me H
Me
C Me
Me Me
H Me C
H
H s-trans s-cis
The presence of two bulky substituents at C-1 position of a diene prevents the
adoption of s-cis conformation of the diene and decreases the reaction rate. For
example, 4-methyl-1,3-pentadiene is about 1000 times less reactive than trans-
1,3-pentadiene toward tetracyanoethene [36].
CH3 CH3 R Krel

CH3 R
H 1
H H CH3 10-3
Usually cyclic dienes such as cyclopentadiene and ortho-quinodimethane are

more reactive than open-chain dienes because of their s-cis conformations. The
rates of DA reactions are also affected by the nature of dienes and dienophiles.
(b) Electronic effects of substituents
In a Diels–Alder (D–A) reaction, TS is formed by a process of charge transfer. The
electron-rich reactant diene in normal electron demand D–A(NED D–A) reaction
and dienophile in inverse electron demand (IEDD–A reaction) acts as an electron
donor component (nucleophile) and the electron-poor reactant (dienophile in NED
D–A reaction and diene in IEDD–A reaction) acts electron acceptor component
(electrophile). The greater the extent of charge transfer, strongest is the interaction
between the reactant components and faster is the rate of the reaction. The reactivity
of 1,3-butadiene increases with increasing the electron acceptor capacity of the
dienophile. Tetracyanoethylene is a very strong dienophile having global elec-
trophilic power, Δω, 4.91 eV (where ω is the global electrophilic parameter) and is
highly reactive toward 1,3-butadiene. The global electrophilic powers of some
dienophiles are provided in Table 3.2 [37].
Similarly, the reactivity of some dienes in Diels–Alder reactions increases by
increasing the electron donor ability of the dienes. The relative reactivity of some
substituted butadienes with maleic anhydride is provided in Table 3.3 [38].
This electronic effect of the substituents in a Diels–Alder reaction can be
explained from the FMO theory with respect to the energy gap between the energy
levels of HOMO of the diene and LUMO of the dienophile for a normal electron
demand D–A reaction. The stronger the electron donor ability of diene and the
greater the electron acceptor ability of dienophile, the closer will be the energy gap
between the energy levels of HOMO of diene and LUMO of dienophile, and thus
the activation energy of the reaction will be lowered and the reaction rate will be
faster (Fig. 3.9).
Table 3.2 Global Dienophile Δω

electrophilicity of some
dienophiles in D–A reactions Tetracyanoethylene 4.91
with 1,3-butadiene 1,1-Dicyanoethylene 1.77
(Δω = 1.05 eV) Acrolein 0.79
Acrylonitrile 0.69
Ethylene −0.32
Table 3.3 Relative rates of reactivity of some substituted butadienes in D–A reactions with
maleic anhydride
Diene Relative rate
Cyclopentadiene 1350
1-methoxy-1,3-butadiene 12.4
2-phenyl-1,3-butadiene 8.8
1-phenyl-1,3-butadiene 1.65
1-Methyl-1,3-butadiene 3.3
2-Methyl-1,3-butadiene 2.3
LUMO
(unsubstituted dienophile)
LUMO
(substituted dienophile)
HOMO
(substituted diene)
HOMO
(unsubstituted diene)
EWG
EWG
Fig. 3.9 The figure illustrates the HOMO–LUMO energy gap in terms of FMO theory on the
reactivity of diene and dienophile in normal electron demand Diels–Alder reaction. The narrower
the gap the higher will be the TS stability and faster will be the reactivity
3.3.1.6 The Dienes and Dienophiles
Different kinds of diene and dienophile are used in the Diels–Alder reactions.
Dienes and dienophiles with a heteroatom such as N, O, or S in their π systems are
known as heterodienes and heterodienophiles, and their cycloaddition reactions are
called the hetero-Diels–Alder reactions. Some highly reactive dienes and dieno-
philes used in Diels–Alder reactions are listed in Table 3.4.
Table 3.4 Representative dienes and dienophiles used in Diels–Alder reactions
A. Dienes
1. Butadiene,
R
2. 1-Substituted butadiene R= Me, OMe, Ph, COOH
R
3. 2-Substituted butadiene R= Me, OR, CN, COOR, OSiMe3
4. 1,4-Diacetoxy butadiene OAc OAc
5. 1,2-Dimethylene cyclohexane
R
O
6. α-Pyrones R= OH, CO2 Me
O
7. 1- Vinyl cyclohexene
8. ortho-Quinodimethane
9. Cyclopentadiene
10. 1,3-Cyclohediene
11. O
Furan
B. Heterodienes
12. Acrolein
O
13. 1-Azabutadiene
N NMe2
14. 2-Azabutadiene
N
C. Dienophiles O
1. Maleic anhydride O
O O
2. Benzoquinone
(continued)
Table 3.4 (continued)
3. α,β-Unsaturated aldehydes, RCH CH Z

ketones,esters,nitriles Z= CHO, COR, CO2R,CN, NO2
and nitro compounds
R= H, Me, Ph, CO2Me
Br O
4. 2-Bromo 2-cyclobutenone
Alkyl and aryl vinyl sulfones CH2 CH SO2R R=Me, Et, Ph

5.
O
6. α,β-Unsaturated phosphonates RCH CH P(OR)2
7. Tetracyanoethylene (CN)2C C(CN)2
8. Esters of acetylene dicarboxylic acids RO2C C C CO2R

O O
9. DIbenzoyl ethylene Ph C C C CPh
H H
10. Dicyanoacetylene NC C C CN
11. Ethynyl sulfones SO2R R = Ph, p-tolyl
D. Heterodieneophiles
12. Esters of azodicarboxylic acid RO2C N N CO2R ; R= Me, Et
O
3
2 N
13. 4-Phenyl 1,2,4- triazoline-3,5-dione N Ph
1N 4
5
O
14. Imino urethanes CH2 N CO2R ; R = Me, Et
15. Nitrosobenzene N Ph
Some Diels–Alder reactions with less commonly used dienes or dienophiles are
illustrated:
H
NO2 NO2
150 OC Ref. 15
1. +
Ph H 70%
Ph
CN
NC
Me CN
NC CN
N Et CN
2. + Me
O Et Ref. 39
Me N
Me NC CN
Me O
Me
H H
CN CN
CN
180 OC
3. + 2 NC
CN CN Ref. 40
CN
NC
H
conjugated allene
Ph
Ph
Δ O
4. O + C (CH2)6 (CH2)6 Ref. 41
C
Ph 91% Ph
diphenyliso Cyclo octyne
benzofuran
O
H
Ph C Ph
H
COPh O
5. + Ref. 42
COPh
56%
CN CN
AlCl3 Ref. 43
6. +
CN
63%
CN
6
Ph 1
EtOH 5 N Ph
7. N Ref. 44
+
O 00C 4 O2
95%
3
N-phenyl-3,6- dihydrooxazine
3.3.1.7 Lewis Acid-Catalyzed Diels–Alder Reactions
Lewis acids such as ZnCl2, SnCl4, AlCl3, derivatives of AlCl3, Me2AlCl, and
Et2AlCl act as effective catalysts to accelerate the rates of Diels–Alder reactions by
increasing the electron-withdrawing capacity of the dienophiles via the formation of
Lewis acid complex. For example, the reaction of 2-methyl-1,3-butadiene 38 with
methyl acrylate takes place at room temperature and in the shorter time (3 h) in the
presence of AlCl3 compared to uncatalyzed reaction, which occurs on heating at
120 °C for 6 h [29].
o
120 C
Me 1
+ 6h Me Me CO2Me
o
20 C +
CO2Me 2 CO2Me Ref. 29
38 3h, AlCl3
1 - 70% : 30%
2 - 95% : 5%
C O AlCl3
OMe
Lewis acid complex of the dienophile
Similarly, the cycloaddition reaction of cyclopentadiene with 2-pyridyl styryl

ketone 52 gives 53 in the presence of Cu(NO3)2 takes place much faster than the
uncatalyzed reaction [45].
NO2 NO2
Solvent Rel. Rate

CH3CN 1
+
H2O + 0.01M Cu(NO3)2 250,000
O
R O N
N Ref. 45
53
O
52
N Cu
Lewis acid complex of the dienophile
Anthracene reacts with methyl fumarate 54 at room temperature in the presence

of AlCl3, whereas without catalyst, the reaction occurs at high temperature, 101 °C,
and in longer time (2–3 days) [46].
AlCl3, rt
MeO2C CO2 Me
CO2Me
+ 2h
MeO2C
54 o
101 C, 2-3 days
3.3.1.8 Applications of Neutral Dienes and Dienophiles in Diels–Alder

Reactions
Diels–Alder reactions using neutral dienes and dienophiles have been utilized in the
synthesis of various types of organic compounds. For example, in the synthesis of
steroids, the angular methyl group may be introduced by the reaction of
1,3-butadiene with 2-methoxy-5-methylbenzoquinone 55. 5,6-Double bond of the
quinone 55 is more reactive as dienophile because the electron donating effect of

methoxy group at C-2 position weakens the dienophilic character of 2,3-double
bond by the delocalisation with C-4 carbonyl group [47].
O O
4 Me
Me 5 3
PhH
+ 2
Ref.47
6 1 OMe 100 oC OMe
55 O H
O
86%
Dienophiles such as nitroethene 56, α-chloroacrylonitrile 57, and vinyl sulfoxide

58 can be used as ketene equivalent in the synthesis of organic compounds to
improve the yield of the products. The cycloadducts of cyclopentadiene derivatives
obtained from these dienophiles are used in the synthesis of prostaglandins.
ether MeO 1. NaOMe MeO

CH2OMe + Ref. 48
NO2 25 oC 2. TiCl3 , NH4Cl
56 NO2 O
56%
MeO MeO
+ H2O
Ref. 49
OMe NC Cl Cl
57 CN 50-55% O
CH(OEt)2 CH(OEt)2
NO2 t-BuOOH
CH(OEt)2 Ref. 50
20 oC VO(acac)2
NO2 PhH, rt O
OBz
O OEt BzO
BzO
S S
Et2OBF4
-30 oC, DCM, 50 h
58 Me Me then MeOH 32% O
S
Me endo >96% de
p-tolyl vinyl sulfoxide BF4 O endo:exo >95:5
Ref. 51
Dienophiles, phenyl vinyl sulfone 59, and ethynyl sulfone 60 in Diels–Alder

reactions can serve as potential ethene and ethyne equivalents, respectively.
Me Me SO2Ph Na/Hg Me
135 oC
+ Ref. 52
Me Me
Me SO2Ph
59 94% 76%
phenyl vinyl sulfone
Me
+
Δ
Me Na/Hg Me
O2S Me Ref. 53
Me Me
60 Me
O2S Me
Similarly, dienophile, vinyl triphenyl phosphonium bromide 61 is used as an

allene equivalent.
PPh3Br i. LDA
+ Ref. 54
61 PPh3Br ii. CH2=O
50%
Furan, thiophene 62, fulvene 63, and aromatic hydrocarbons 64 are used as
efficient dienes in the synthesis of various heterocycles and carbocycles.
CO2Et O O
CO2Et CO2Et CO2 Et
O + C Δ H2/Pd
C O Ref. 55
-C2H4
CO2Et CO2Et CO2Et CO2Et
O O
O O
O H
H2 SO4 -H2O
+ O O HO O
OH O Ref. 56
-H+
H H
O O
O exo product O
Me CO2Me Me
S Me CO2Me
180 oC CO2Me -S
S + Ref. 56
Me CO2Me
62 Me CO2Me
CO2Me Me
CO2Et
Δ CO2Et
+
Ref. 57
63 CO2Et CO2Et
OMe MeO
CO2H CO2H
+
Ref. 57
64
100%
NO2 NO2
CO2H
+
major product CO2H

81%
Cyclopentadiene is a highly reactive diene and reacts with both strong and weak
dienophiles, cyclopropene 65 and chiral allenes, 66 and 67 to give cycloadducts in
high yields.
H CO2 Me
H
CH2Cl2, 0 oC H TiCl2(Oi-Pr)2
+ H C CO2 Me
Ref.58; +
- 20 oC
65 Me
96% H Me 90%
H
66
CHCO2H
COOH
+ C
Ref. 56
CH2
CH2
67 84%
Cyclobutene 68 and benzocyclobutenes 69 and 70 are used for generation of

dienes in situ.
PhS O O O
∇ PhS PhS H
+ O O H O Ref. 59
MeO MeO MeO
68 O
O O
+ Ref. 60
+
CO2Me CO2Me CO2Me
69
quinodimethane
OH O
O H
OH
OH
PhH Ref. 61
+
reflux
H
70 O
O
Quinodimethane may also be generated from cheletropic elimination of

cycloadduct 71.
Me δ+
δ-
CO2Me Me
O 800C
Ref. 62
S -SO2
O CO2Me
71 Ph Ph
Ph major product
Diphenlyisobenzofuran 72 can be used as highly reactive diene for reaction with

electron-rich alkene [63].
Ph
Ph
o
100 C
O+ O
Ref. 63
72 Ph Cycloheptene
69% Ph
Butadiene and its derivatives such as 1,3-pentadiene 73 and α-pyrone 74 and

1,3-cylohexadiene 75 are used as effective dienes.
O
O O
90 oC Bu3SnH, PhH
+ Ref. 64
NO2 reflux
73 O2N
80%
CO2Me MeO2C CO2Me

CO 2Me
CO2Me hν
+ Ref. 65
CO2Me [2+2]
CO 2Me
CO2Me
OH O O
3
O H+ O O-H Me
4 -CO2
2
+ Me
CO2Me Ref. 66
5 O1 CO2Me
6 Me CO2Me
74
3-hydroxy-α-pyrone 85%
CHO CHO -CH=CH2 CHO

+ Ref. 56
retro D-A
75
1-Methoxy-3-trimethylsilyloxy-1,3-butadiene known as Danishefsky’s diene 76

is used for the synthesis of cyclic α,β-unsaturated ketone.
OMe HO
TMSO H3O O
benzene Ref.67
+ Me
Me Me
TMSO Δ
Me CHO CHO CHO
OMe OMe CHO
76 72%
1,1-Dimethoxy-3-trimethylsilyloxy-1,3-butadiene 77 is used as a diene for

synthesis of resorcinol derivative 78.
OMe
OMe MeO OMe
CO2Me MeO OMe
OMe PhH CO2Me CO2Me -MeOH
CO2Me
+
Me3SiO Reflux HO
77 Me3SiO HO
H 78 Ref.68
Vinylcycloalkene 79 is used as a diene in the synthesis of cyclobutane derivative

80 [69].
y y y y
Br
O
Br O o
23 C
+ Ref. 69
MeO 2h H
79
MeO
80
Several heterodienes and heterodienophiles are used in hetero-Diels–Alder

reactions in the synthesis of heterocyclic compounds. For example, aza-butadiene
81 and 81a and oxazoles 82 and 83 are used as heterodienes for synthesis of
heterocycles.
Me Me
Me
+ 100 oC Me Zn/AcOH Me Ref. 70
N N reductive N
COMe H
81 O cleavage O
NMe2 NMe2
CO2Me H
Me
CH3CN Me CO2Me Me CO2Me
+ Ref. 71
N N N
-20 oC CO2Me CO2Me
81a NMe2 CO2Me NMe2 58%
Me Me
N N
Me CO2Et
N 110 oC EtOH / HCl
EtO O HO O
EtO +
H
O CO2Et H H
82 EtO2C CO2Et EtO2C 85% CO2Et
- H+ - H2O
CH2OH Me
HO CH2OH LAH, Et2O, rt N Ref. 72
HO
Me N EtO2C CO2Et
40%
Ph R Ph
N N
200 oC O
- PhCN O
+ Ref. 72
O CHO OHC R
83 OHC R
Nitroso compound 84, imine 85, azodicarboxylate 86, and carbonyl compound
87 are used as heterodienophiles in Diels–Alder reactions for the synthesis of
heterocycles.
Ph 6 1Ph
N EtOH N
+
Ref. 73
O 0 oC O
84
nitrosobenzene N-phenyl-3,6-dihydro-oxazine
95%
Me
Me
Ts Ts
N PhH N
+
Ref. 74
Δ CCl3
85 CCl3 tetrahydropyridine derivative
trichloromethyl
tosylimine 72%
Me CO2Et CO2Et
N Me i) H2/Pt Me
+ rt N NH
Ref. 75
N N ii) OH –
/ H O NH
Me CO2Et Me CO2Et 2 Me
86 cyclic hydrazine
ethyl azo dicarboxylate
OMe OMe
O CH2Cl2
+ O
Ref. 76
H CO2Bu 20 oC CO2Bu
87
butyl ester of glyoxalic acid
3.3.1.9 Applications of Ionic Dienes and Dienophiles

in Diels–Alder Reactions
Several allyl cations can serve as dienophiles and allyl anions and pentadienyl
cations as dienes in Diels–Alder cycloadditions; for example, cycloaddition of
2-methyl allyl cation 88 with cyclopentadiene.
CCl3COOAg
+
I CH2Cl2 / SO2
88
2-methyl allyl 2-methyl allyl Ref. 77
iodide cation
Trimethylsilyloxy-substituted allyl halides in the presence of silver perchlorate in

nitromethane generate allyl cations 89, which react efficiently with cyclopentadiene.
OSiMe3 OSiMe3
OSiMe3 R
R R AgClO4 R R R R R
R Cl OSiMe3
R R 89 R R R R
R
[H2O]
R
R Ref. 78
O
R R
Enamine of 2-chloro cyclohexanone in the presence of AgBF4 generates allyl

cation 90, which serves as reactive dienophile for synthesis of tricyclic ketone 91 [5].
O
N AgBF4 N N N H2O
Ref. 75
Cl
91
76%
90
2,2-Dimethylcyclopropanone remains in equilibrium with zwitterionic oxyallyl

cation 92 in situ and undergoes D–A reaction with furan. Similarly, allene oxide
92a generated from silyl epoxide gives cyclopropanone and oxyallyl cation, which
is trapped as furan adduct [79].
O O O
O O
o Ref. 79
CH2Cl2, 25 C O O
92
O O
O
O KF O O- Ph
Ph SiPh3 Ph Ph
Ph Ref.79
H CH2Cl - Ph3SiCl H CH2 H O
92a
α-Methyl styrene in the presence of a strong base generates allyl anion 93 in situ,
which undergoes D–A cycloaddition with an alkene [5].
y
Ph
Ph Ph Ph
Ph
+
LDA Ph +H
o
Ref. 5
Ph THF, 45 C
Ph 93
Ph 42%
Ph
2-Aza allyl anion 94 serves as a diene in D–A reaction [80].

Ph H H
H i) Ph Ph
H Ph Ph
Ph Ref. 80
Ph ii) H2O N H
Ph N H
H
94 Li
83%
3.3.1.10 Enantioselective Diels–Alder Reactions
In Diels–Alder (D–A) reactions, racemic products are obtained from enantiomeric

diene or dienophile. D–A reactions are widely applied in the synthesis of bioactive
asymmetric natural products and hence enantio- and diastereoselectivities of the D–
A reactions are very much needed to get the desired products as major products.
Several approaches have been developed in the last three decades in this respect by
the use of different Lewis acid catalysts as chiral auxiliary or asymmetric catalysts.
Catalytic D–A reactions have twofold benefits. On one side, it provides high
enantiomeric/ diastereomeric excess of product and on the other side, it affords high
yield of products by reducing the activation energy of the transition states (TSs).
In NED D–A reactions, it lowers the LUMO energy of the dienophile and in IED
D–A reactions, it lowers the energy of LUMO of the diene so that the reaction
occurs at low temperature with ease (Fig. 3.10). At higher temperature the stere-
oselectivity of the D–A reactions is lost.
The choice of Lewis acid is very important in D–A reactions as in few cases it
leads to non-concerted stepwise process. The calculation of energy of the TSs by
DFT methods is very useful in the study of steric, stereoelectronic and chelating
(a)
L LUMO dienophile (without catalyst)
ERG
EWG M
L LUMO
dienophile (with catalyst)
ΔE
ΔE'
LUMO
HOMO HOMO
diene
(b) L
EWG M diene
LUMO
L
ERG
ΔE
ΔE'
HOMO LUMO
HOMO
dienophile
Fig. 3.10 a LUMO energy of dienophile is lowered by Lewis acid catalyst in NED D–A reactions
and b LUMO energy of diene is lowered by Lewis acid catalyst in IED D–A reactions
interactions of the Lewis acid catalyst with dienophile or diene, as the case arises.
To achieve diastereoselectivity, a chiral auxiliary is installed in the reaction by the
use of chiral Lewis acid catalyst or chiral esters or amides of acrylic acids [81]. The
latter method is better because the chiral auxiliary can be recovered by hydrolysis of
the purified adduct.
Enantioselective D–A Reactions Using Chiral Auxiliary
Enantioselective D–A reactions of chiral esters and amides of acrylic acid can be
achieved using achiral Lewis acid such as TiCl4. After the reaction, enantiomeric
pure carboxylic acid can be recovered on hydrolysis. For example, the reaction of
acrylic acid with cyclopentadiene using chiral α-hydroxy ethyl propionate 95a as
chiral auxiliary in the presence of TiCl4 gives only one enantiomeric product in
large excess (93 %) [82]. The chiral auxiliary 95a reacts with dienophile to produce
a chiral ester 95, which participates in the reaction with cyclopentadiene.
HO +
+ CO2Et H
Me H O CO2Et
COOH
95a
O Me H
95
Me
chiral dienophile CO2Et
H
OEt Cl O
Cl
TiCl4 C O Ti H O
Me Cl Cl
+ O CO2 Et
H H O O
O Me
95 96 96b
TS
The β-chloride ligand of TiCl4 shields the top face of the dienophile in the TS 96
and only bottom face of the dienophile is able to react with the diene to produce
mostly one enantiomeric product 96b, which on hydrolysis affords the desired
product 96a.
OH-,H2O
Ref. 82
O C CO2Et
H 96a CO2H
O Me
96b
93%
Similarly, 2,4-dihydroxy-3,3-dimethylbutyrolactone known as D-(-)-panto-

lactone 97 is used as chiral auxiliary along with TiCl4 in several D–A reactions. For
example, the reaction of 2,3-dimethylbutadiene with α-cyanocinnamic acid affords
the product of 92 % diastereomeric excess.
Me Me
Me H
OH
HOH2C C C COOH H2C
H
Me OH O
O
97
Pantolactone (Pan)
Cl
Cl Cl
Ti
Cl Me
Ph Me Me O
OH TiCl4 O Me
+ Me CH H O
H
NC COOH O O
O Ph
Me
97 CN Me
α-cyanocinnamic acid approach from
si face
Me CO2Pan
OH-,H2O Me CO2H
CN Ref. 83
H CN
H
Me Ph Me Ph
92%, de
Enantioselective D–A Reactions Using Chiral Catalysts
Different types of chiral catalysts including nonmetal and metal complexes have
been introduced in enantioselective D–A reactions.
Among nonmetal chiral complexes, chiral oxazaborolidines have been found
effective in many D–A reactions. The adduct obtained from the D–A reactions of
5-benzyloxymethyl-1,3-cyclopentadiene 98a with α-bromoacrolein in the presence
of catalyst 98 (S-tryptophan-derived oxazaborolidine) is an important intermediate
in the synthesis of prostaglandins. The aldehyde group of the dienophile is bound to
the catalyst by coordination with boron by Lewis interaction and the Lewis complex
is stabilized by H-bonding. The upper face of the aldehyde is shielded by indole
moiety of the catalyst. The benzyloxymethyl substituent of the cyclopentadiene
produces a steric differentiation on the two faces of cyclopentadiene ring resulting
the approach of the diene preferably from one face.
OBz
OBz
Br CHO Cat. 98 (5mol%)
CHO Ref.84
+
Br
Bz= CH2Ph 95% (99% ee)
98a 96% exo
HN Br
O
H
O
O
B
N
Ts 98
CH2OBz
TS of the Lewis complex with catalyst 98

Chiral bis-oxazolines known as BOX–Cu2+ complexes are efficient catalysts for

asymmetric hetero D–A reactions. For example, BOX–Cu (II) complex 99 catalyzes
the reaction of pentane-2,3-dione with Danishefsky’s diene 76 efficiently with 76 %
yield of the product and of 97.8 % ee [85].
OMe
O
+
Cat. 99 (0.05 mol%) O
Et
Me Me
Me3SiO O
76 O C Et
O
O O 76%, 97.8% ee
N N
But t
Bu
Cu
OMe
O
Et O
Me OSiMe
TS of BOX-Cu(II) complex (Cat. 99)
Binaphthol (BINOL)–Ti complexes 100 are found to be effective catalysts in

NED D–A reactions. It catalyzes the reaction of 5-hydroxynaphthoquinone with
butadienyl acetate in 86 % yield and 96 % ee of endo-product. This intermediate is
useful in the synthesis of anthracycline antibiotics [86].
O OAc O
H
Cat. 100 (10 mol%)
+
CH2Cl2, r.t
H
OH O OH O OAc
86%, 96% ee
Cl O endo-adduct
O Ti O
HO
Cl
O
AcO
TS with Cat. 100
R(+)-BINOL–Yb triflate complex 101 catalyzes the D–A reaction of cyclopenta-

diene with 3(2-butenoyl)-1,3-oxazolidin-2-one 102 in the presence of some additives,
3-acetyl-1,3-oxazolidin-2-one, MS 4A and cis-1,2,6-trimethylpiperidine at room tem-
perature to yield enantiomeric excess of endo-adduct (2S, 3R) (93 % ee) with endo–exo
ratio of 96.5: 3.5, whereas this chiral Yb triflate complex prepared from Yb(OTf)3, R(+)
binaphthol, cis-1,2,6-trimethylpiperidine, MS 4A, and 3-phenylacetylacetone affords
endo-adduct (81 % ee) of other configuration (2R, 3S). Possibly 3-phenylacetyl acetone
reverses the enantiofacial selectivity of the dienophile [87].
NR'3 O
O
O
H N
O 101 (20 mol%)
Yb Me
O
O
H site B attack
O CO N O
O MS4A, CH2Cl2, 0 oC
O
TS with 101 NR'3 N (2S, 3R)
site A 102 93% ee
diene attack
site B
N O
NR'3
H O 101 (20 mol%)
O
O Me O
Yb site A attack
O MS4A, CH2Cl2, 0 oC OCN O
H
O O
(2R, 3S)
TS with 101 NR'3 81% ee
Ph
Organocatalysts such as TADDOLs (α, α, α, α-tetraaryl-1,3-dioxolane-4,5-

dimethanols) are effective in asymmetric oxa-D–A reactions. For example, TADDOL
103 catalyzes the HDA reaction of less reactive Danishefsky’s diene with ben-
zaldehyde to afford corresponding hydropyranone in moderate yield [88].
OMe
O Cat. 103 (20 mol%) O
O
+ o
H Ph 15 C, 72h O Ph
Me3SiO (S) Ph O
76 CF3CO2H
77% 76.3% ee
1 Ar Ar OMe OMe OMe
O 5 H O
O CF3CO2H
2 O O
4 O H
O Ph
H OSiMe3 Ph OSiMe3 Ph OH
3 Ar Ar
TS with 103
Ar= 1-naphthyl
Asymmetric aza-Diels–Alder reaction of chiral imines with Danishefsky’s diene

in chiral ionic liquids have been reported [89]. These reactions occur at room
temperature under green chemistry conditions without using Lewis acid catalyst and
organic solvent. For example, the reaction of Danishefsky’s diene 76 with imine
104 in the presence of chiral ionic liquid (IL) 105, gives pyridone derivative in
moderate diastereoselectivity [89]. In the TS, the chiral liquid 105 binds the diene
and the dienophile in such a way that the diene approaches from top face of
dienophile.
O
Ph OSiMe3
IL 105
+
N
30 oC, 3h
104 Ph Ph N
OMe 76
OSiMe3 Ph
Ph 48%, 51% ee
N
OMe -
Ph OTf
H Me
O N Me
C8H17
Ph Me
TS with 105
3.3.1.11 Intramolecular Diels–Alder Reactions
Intramolecular Diels Alder (IMDA) reactions are extensively applied in the syn-
thesis of polycyclic compounds. Most of these reactions require high temperatures
to occur and hence these are catalyzed by Lewis acids to occur at ordinary tem-
peratures. These reactions are classified into two types according to the connectivity
of alkyl chain to the diene part at C-1 and C-2. Type-1 is very common.
Type 1:
(CH2)n
(CH2)n
n = 1-4
Type 2:
(CH2)n
(CH2)n
n = 1-4
Study of the synchronicity, i.e., the formation of two sigma bonds to the same
extent of the reactions indicated that the formation of a bicyclo-[4.3.0]-non-2-ene
system (n = 3) is preferred kinetically than a bicyclo-[4.4.0]-dec-2-ene for both cis-
and trans-ring junctions. Steric and torsional strains are the important factors to
control the TS of the reactions. Usually a cis-ring junction is favored for n = 1−3
and a trans-ring junction is favored for n = 4. The following examples are
illustrative:
O O
H
1. 0 oC
Ref. 90
Me Me
CHMe2 H
CHMe2
cis-junction 87%
H
160 oC
2. Ref. 91
H
Me Me
trans-junction 95%
O
Me HC
Me
H
3. CH O Me3Al OMOM Ref. 92
CH2MOM
Me 0 C o
Me
H
MOM =-CH2OMe
trans-junction 75%
H
PhH
4. H Ref. 93
EtAlCl2, 23 oC CO2Me
MeO2C 60%
trans-junction
206 o C, 2h
5. Ref. 94
xylene
CO2Et
CO2 Et
91%
Et2AlCl
O Ref. 95
6. 21 oC, 1h
CH2Cl2
O
71%
The geometry of the TS of these reactions is helpful to understand the stereo-

chemistry of the ring junction. In entry 1, the preferred TS is endo-boat, which
gives the cis-fused product.
O
O H
H
H ≡
CHMe2
Me
H
CHMe2
major
Me more stable
O
H H
O
≡
Me2HC Me
H
peripheral bond CHMe2
internal bond Me minor

less stable
In entry 2, preferred TS is exo-chair, which gives the product of trans-ring

junction.
H H
H
H ≡
H
H
Me
Me
In entry 3, preferred TS is endo-chair, which gives trans-adduct.

O
HC
H Me
H
Me
Me OMOM
≡
H CH O Me
OMOM H
In entry 4, preferred TS is endo-boat, which gives trans-adduct.

H
H H
H ≡
CO2Me H
CO2Me
In entry 5, preferred TS is exo-chair and favors trans-ring junction.

H
CO2Me
CO2Me
In entry 6, preferred TS is exo, and gives exo-product.
O O
Lewis acid-catalyzed intramolecular D–A reactions prefer endo-TS to yield

kinetically controlled endo-products in preference. For example, triene 106 gives
mainly the endo-product 107 in the presence of Lewis catalyst, Et2AlCl [96].
CO2Me
MeO2C
CHMe2 H H
o
160 C Me2HC Me2HC
+ Ref. 96
CO2Me
H H
107
(CH2)4 endo exo
50% 50%
106
Et2AlCl, r.t.
88% 12%
IMDA reaction of benzocyclobutane 108 having a vinyl cyclopentane sub-

stituent is useful for construction of steroidal skeleton 109 [97]. Benzocyclobutane
generates an ortho-quinodimethane in situ.
MeOH
Me OH H
200 oC
Ref. 97
H H
7.5 h
MeO MeO
108 H 109
91%
Other typical examples are:
CH2 2
CO2Et
n-Bu2O EtO
1. 0
EtO2C
150 C , 2h
60 % Ref. 98
TS
O
CH2 2
CO2Et
n-Bu2 O EtO
2.
2 1500C , 7d Ref. 98
OEt
64%
Et Me
O
MeTs Et Me
3. TiCl4 Me
O O
CH2Cl2 , - 78 oC
Ref. 98
58%
3.3.1.12 The Retro-Diels–Alder Reactions
The reverse reactions of Diels–Alder reactions for thermal dissociations of

cycloadducts in to dienes and dienophiles at higher temperatures or in the presence
of Lewis acid or base are known as the retro-Diels–Alder (rDA) reactions. These
reactions in most cases proceed in a concerted process. These reactions are often
used for separation of diene or dienophile from their mixture with other com-
pounds. Proper selection of conditions of these reactions provides new dienes and
dienophiles, which are important synthons for synthesis of several bioactive natural
products and organic molecules of complex structures. For example, the D–A
adduct of 4-phenyl oxazole 110 with methyl acetylene dicarboxylate, on retro-D–A
reaction gives new compounds, benzonitrile, and furan 3,4-dicarboxylic acid
methyl ester 111 [65].
O
CO2 Me
Δ N CO2Me CO2Me
N Δ N
O + + O
Ph D-A r.D.A
Ph Ph CO2Me
110 CO2 Me CO2Me 111
Similarly, 1,3-dienic δ-sultone 112 can be used for the synthesis of highly
substituted aromatic compound 113 by a domino DA/rDA process [99].
P CO2Me O SO2
h SO2 Ph CO2Me
D-A Ph CO2Me rD-A
+ Ref. 99
O Δ CO2Me
112 Ph CO2Me Ph CO2Me
Ph 113
These reactions are very useful in the synthesis of highly substituted aromatic,
heterocyclic and bicyclic compounds. The following examples are illustrative:
MeO2C
CO2Me CO2Me
Δ CO2Me Δ
1. CO2Me + CO2Me +
D.A r.D.A CO2Me
CO2Me CO2Me Ref. 100
CO2 Me
Δ [H] Δ
2. Ref. 101
+ D-A rD-A
CO2Me CO2Me -C2H4 MeO2C
CO2Et Me EtO2C
H CO2Et
N N Me rD-A
+ N Me
3. N N R 25 oC N -N2 N
EtO2C N Ref. 102
D-A N
CO2Et EtO2C - HN EtO2C R
R
CO2Et 50% CO2Et
R=
Br
Δ
Δ
4. r. D. A Ref. 103
D.A D
D D
CO2Me CO2Me
400 oC CO2Me
5. + + Ref. 56
(vacuum) CO2Me
CO2Me CO2Me
3.3.1.13 1,3-Dipolar Cycloaddition Reactions
Overview of 1,3-Dipolar Cycloaddition Reactions
The cycloaddition reactions of 1,3-dipolar compounds with alkenes or alkynes or

heteroatom containing double or triple bonds are known as 1,3-Dipolar cycload-
dition reactions, or simply 3-DPCA reactions. These are another type of concerted
[π4s +π2s ]-cycloaddition reactions, analogous to D–A reactions [104]. 1,3-Dipolar
compounds are known as 1,3-dipoles and alkenes or alkynes are known as dipo-
larophiles. These reactions are represented as
c b
b b b a c
1,3-dipole a c a c a
d e
d e e
dipolarophile d e d
1,3-Dipoles are a class of organic molecules, analogous to allyl or propargyl

anions having a 4π elctron system, while dipolarophiles are alkenes or alkynes with
an electron-withdrawing or electron-releasing group. Other multiple-bonded groups
such as carbonyl, azo, imino, and nitroso groups can also serve as dipolarophiles.
Some common 1,3-dipoles with their resonating structures are listed in Table 3.5.
Methods for Generation of 1,3-Dipolar Compounds
Most of the 1,3-dipoles are short lived and are generated in situ. The common
methods for generation of 1,3-dipolar compounds in situ are:
Table 3.5 List of common 1, 3-dipoles with resonating structures

a. Allylic anion type
i) Nitrogen atom in the middle
R
Nitrones C N O R2C N O
R
R R
R
Azo methine imines
C N N R R2C N N R
R
R R
Azo methine ylides R R R
C N C R2C N C
R R R
R R
Azimines
R N N N R
R N N N R
R
R
ii) Oxygen atom in the middle
Ozone O O O O O O
R R R
C O C R2C O C
Carbonyl ylides R
R R
Nitroso imines R N O N R R N O N
iii) Propargyl anion type
Nitrile oxides R C N O R C N O
Nitrile imines R C N N R R C N N R
Nitrile ylides R C N C R R C N C R
R R
R R
Diazoalkanes
C N N C N N
R R
Azides R N N N R N N N
Nitrous oxide N N O N N O
Nitrile oxides:
R R
Cl2 Et3N
C N OH C N OH R C N O
H Cl (-HCl)
Nitrile ylides:
R
Et3N
C N CHR2 R C N CR2
Cl (-HCl)
Nitrones:
OH O
1. R N OH + H2C O R N R N
H CH2OH (-H2O) CH2
OH O
HgO
2. H3C N H3C N
CH2R [O], -H2O CHR
Nitrile oxides:
PhNCO R C N O
RH2C NO2
Et3N,PhH
(-H2O)
Regioselectivity and Stereoselectivity of 1,3-Dipolar Cycloaddition

Reactions
Most of the 1,3-DPCA reactions are highly stereospecific with respect to dipo-
larophiles and give syn-addition products. For example, diazomethane reacts with cis-
and trans-2-methyl-methyl-2-butenoate 114 and 115 separately to afford cis- and
trans-product 116 and 117, respectively, with more than 99.9 % stereospecificity
[105].
N
Me N
Me
Me CO2Me
114 Me CO2Me
116
> 99.99%
H2C N N
Me Me N
N Ref. 105
CO2Me
115 CO2Me
Me Me
117 > 99.99%
While the reaction of phenyl diazoalkane with unsymmetrical dipolarophile

α-methyl-methyl maleate 118 gives two diastereoisomeric products 119 and 120
from the endo- and exo-transition states [106].
H Ph
N H N
Me Ph N N
+
Ph C N N + Me Me Ref. 106
MeO2C CO2Me
H MeO2C CO2Me MeO2C CO2Me
118 119 120
When both 1,3-dipole and dipolarophile are unsymmetrical, two products are
possible. The formation of major product can be predicted by consideration of their
TSs. The most stable TS will provide the major product. The stability of the TS is
controlled by both electronic and steric factors. Therefore, the regioselectivity of a
1,3-DPCA reaction is determined by the steric and electronic properties of the
substituents attached to 1,3-dipole and dipolarophile. The FMO theory may also be
applied to analyze the regioselectivity of 1,3-DPCA reaction [107]. A relatively
stronger donor–acceptor interaction between HOMO and LUMO and lowest dipole
moment favors the TS. The HOMO and LUMO of a 1,3-dipole are similar to that of
a diene in a Diels–Alder reaction. The interactions of HOMO or LUMO of a dipole
with a LUMO or HOMO of a dipolarophile depend on their electron donor and
electron acceptor property. The orbital interactions of HOMO and LUMO of dipole
and dipolarophile are shown in Fig. 3.11.
To understand the strong interaction between HOMO and LUMO, the knowl-
edge about the orbital coefficients of the frontier orbitals of the 1,3-dipoles and
dipolarophiles is essential. The orbital coefficients of the common dipolarophiles at
C(1) and C(2) carbons depend on the nature of the substituents.
HOMO of 1,3- dipole HOMO of diene (Ψ2)
LUMO of 1,3- dipole LUMO of diene (Ψ3)
HOMO of 1,3- dipole LUMO of 1,3- dipole
LUMO of dipolarophile HOMO of dipolarophile
Fig. 3.11 Frontier orbital interactions in a 1,3-dipolar cycloaddition reaction

largest HOMO coefficient

largest HOMO coefficient
δ+ δ- δ-
EWG δ+
ERG
LUMO HOMO
largest LUMO largest LUMO
coefficient coefficient
R
N
O- largest
HOMO coefficient
largest
LUMO coefficient
The reported orbital coefficients of some 1,3-dipoles in their HOMO and LUMO
states are given in Fig. 3.12 [108].
The HOMO or LUMO of the 1,3-dipole will interact through the atom having
highest orbital coefficient to LUMO or HOMO of the dipolarophile. For example, in
1,3-Dipole HOMO LUMO

0.78 0.15 0.61
0.51 0.74 0.50
Diazoalkane R2C N N R2C N N
R2C N N
0.83 0.01 0.56 0.40 0.71 0.58
Azide R N N N
R N N N R N N N
0.65 0.15 0.74 0.62 0.67 0.41
Nitrone R2C N O
R2C N O R2C N O
0.68 0.67 0.30

0.56 0.21 0.80
Nitrile oxide R C N O R C N O
R C N O
0.52 0.70 0.49

0.64 0.07 0.76
Nitrile ylide R C N CR2

R C N CR2
R C N CR2
0.59 0.05 0.80 0.60 0.70 0.40
Nitrile imine R C N NR R C N NR R C N NR
Fig. 3.12 Orbital coefficients of the HOMO and LUMO of some 1, 3-dipoles. Adapted with
permission from (Houk et al. 1973 J Am Chem Soc, 95:7287). Copyright (1973) American
Chemical Society
the reaction of phenyl azide 121 with methyl acrylate, azide is HOMO and dipo-
larophile is LUMO. The dominant interaction gives 122 as major product [109]:
largest HOMO
N
Ph N N N Ph N
25 oC, 5d N Ref. 109
121
122 CO2 Me
CO2Me 77%
largest LUMO
In the reaction of p-nitrophenyl azide 123 with ethoxyethylene, the azide is

LUMO and dipolarophile is HOMO. Hence, their dominant interaction gives 124 as
only product [109].
O2N N N N O2N N
200C , 11d
N N Ref. 109
123
EtO 124
EtO
99%
highest HOMO
Similarly, in the reaction of C-phenyl-N-methylnitrone 125 with methyl acrylate,

nitrone is HOMO and methyl acrylate is LUMO. Hence, their dominant interaction
gives 126 because steric factor prevents the formation of 4-substituted isomer as per
electronic factor [5].
p p
Me
PhCH N O Ph N
110 oC O Ref. 5
Me 4 5
125 21h
CO2Me
126
CO2Me 99% exo : endo, 67:33
The reaction of nitrone 125 with nitro ethene, bearing strong

electron-withdrawing group, gives only 4-substituted products 125a and 125b as
per FMO-controlled electronic factor [108].
Me Me Me
Ph N - N N
+ O Δ Ph Ph
O O Ref. 108
125
O2 N 125a O2N 125b
O2N
cis trans
cis : trans, 2 : 1
The cycloaddition reaction rate of nitrone with α,β-unsaturated aldehydes in the

presence of a pinhole Lewis catalyst is enhanced dramatically and gives only
electronically controlled cycloadducts, isoxazolidine-4-carboxaldehydes. For

example, C,N-diphenyl nitrone 127 reacts with acrolein 127 at room temperature to
give a 20:80 mixture of regioisomeric cycloadducts, 2,3-diphenyl-isoxazolidine-
4-carboxaldehyde 128 and -5-caboxaldehyde 129 in only 5 % of total yield. When
the same reaction is carried out at 0 °C in the presence of a catalytic amount
(10 mol%) of aluminum–tris (2,6-diphenyl phenoxide) (ATDP) 130, a single
regioisomer (128:129, 99:1) is obtained, but in a poor diastereoselectivity (endo:exo,
77:23). On the other hand, crotonaldehyde 131, a 1,2-disubstituted alkene, reacts with
127 under both non-catalyzed and ATDP-catalyzed conditions to give only the
electronically controlled product 132. No 132a was found. It indicates that steric
factor is minimized in case of 1,2-disubstituted alkenes [110].
Ph Ph
Ph N Ph N
Ph CHO 1. CH2Cl2, rt, 8h O O
+ +
PhCH N
127 O 2. CH2Cl2, 0 oC, 8h, 130 OHC CHO
128 129
electronically sterically
controlled product controlled product
Ph
O without catalyst 20 : 80 (5%)

Al
Ph
with catalyst 130 >99 : 1 (100%)
3
130
Ph Ph
Ph Ph N N
CHO O O
PhCH N + +
O Me
131 OHC Me Me CHO
132 132a
without catalyst 100 : 0 (2%)
with catalyst 130 100 : 0 (100%)
The use of an auxiliary of α,β-unsaturated compound in the cycloaddition

reaction of acyclic nitrone improves the yield and diastereoselectivity of product.
Moreover, the use of bulky catalyst favors the exo-selectivity of the product. For
example, N-crotonoyl succinimide 133 reacts with C,N-diphenyl nitrone 127 in
toluene at rt to give exclusively endo-product 135, whereas in the presence of
5 mol% of TiCl2-TADDOLate 134 in toluene gives exclusively exo-product 136
with high (73 %) ee. The X-ray study of the TS indicates that the nitrone
approaches the alkene from an Re face to give the exo-product [111].
O O O O
Ph Me Ph Me
H Ph 1. PhMe, rt, 19h
+ +
N Me N
2. N2H4 (aq) Ph NH2 Ph NH2
Ph O-
127 133 135 O 136
O O
Ph Ph endo exo
Me O 94%
O TiCl without catalyst 95 : 5
2
O with catalyst 134 <5 : >95 >95%
Me O
Ph Ph
Catalyst 134
The cycloaddition of C-phenyl-N-methyl nitrone with a dipolarophile having an

ortho-hydroxy group is completely controlled by intermolecular hydrogen bonding
between dipolar compound and dipolarophile leading to an E-endo transition state
to give exclusively cis-cycloadduct.
For example, reaction of C-phenyl-N-methyl nitrone 125 with ortho-hydro-
xystyrene 137 gives only cis-adduct (100 %) in 92 % yield [112].
Ph
0 Ph HO
PhMe , 120 C
N 5d Me N O
Me O HO 138
125 137
100% exo
The reaction proceeds through an E-endo-TS in which intermolecular, H-bonding
and secondary orbital interactions make the TS tighter and gives only cis-product
from the interaction of LUMO nitrone and HOMO dipolarophile (Fig. 3.13).
In the case of styrene, such rigidity of the TS is not occurred and hence both cis-
and trans-adducts are formed from E-endo- and Z-endo-TS, respectively [112].
Ph
Ph Ph
PhMe
+
N Me N O
Me O Me N O
exo endo
2 : 1
Ph
highest LUMO
Me
N
LUMO
O
H
O
highest HOMO
E-endo
HOMO
Fig. 3.13 The orbital interactions of HOMO and LUMO in the TS in the reaction of nitrone 125
with ortho-hydroxyl styrene 137
Nitrone reacts with vinylethers, vinylalkyls and vinylnitrile to give major

5-substituted exo-products. The following examples are illustrative:
Ph
N
O Me3Al Ph O Ot-Bu Ph N O Ot-Bu
Ph + N
1. + Ref. 113
rt, 4h Ph Ph
t-
O Bu exo endo
73 : 27 (>90%)
O
N O N O
N
2.
+ Ref. 5
+ O O
O
exo endo
92 : 8
H
Me
toluene Me
LAH
Ref. 114
3.
N O NH
N O 110 oC H H OH
exo only
endo TS is difavored by steric
interaction with methylene protons
Me
Me Me
N CN Ph Ph
O N N
4. O O Ref. 115
+ +
PhH, reflux
Ph NC CN
trans cis
91% 3 : 1
Synthetic Applications of Intermolecular 1,3-Dipolar Cycloaddition

Reactions
1,3-DPCA reactions are very useful for the synthesis of five-membered heterocyclic
compounds. Sometimes the reaction products undergo hydrogen shifts to afford
stable heterocyclic molecules. Some of these reactions are illustrated to highlight
the yields and regioselectivity of the products.
Azides are used for the synthesis of triazoles, e.g.,
N N
δ O2N N H
1. O2N N N N Ref. 116
H
Me OPr Me OPr
Pr = n-propyl
(opposite regioselectivity
due to steric factor)
N
2. O2N N N N N N Ref. 117
92%
NO2
Nitrile oxides are used for synthesis of isoxazoles, e.g.,

OH Ph N
N Et3N, Et2O PhC CH O
Ph N O Ref. 118
Ph 20 oC Ph
Cl 97%
(-HCl)
sterically controlled
product
Diazoalkanes are used in the synthesis of pyrazolines.
N N Me COOMe N N
N + 0 OC Me COOMe
N Ref. 119
C C 12-16 h Me Me
Me Me Me Me LUMO
HOMO 98%
N
CO2Me
N 0 OC N N
+ Me
C t- t-
Bu Ref. 120
Me Me Bu Me
CO2Me
100%
(sterically controlled)
Nitrile ylides generated in situ are used for the synthesis of Δ1-pyrrolines.
Ar Ar
COOMe Ar
N N Ph N
C H Ref. 56
C H
Ph Ph COOMe
cis product
Azomethine ylides generated in situ are used in the synthesis of pyrrolidines by

trapping with dipolarophiles.
p p
Ar Ar MeO2C CO2Me CO2Me
MeO2C CO2Me MeO2C
1. N or N +
EtO2C CO2Et
EtO2C CO2Et EtO2C CO2Et CCl4 , 1000C EtO2C
N
CO2Et N
aziridine Ar Ar
major minor
Ar = OMe
mechanism:
Ar
Ar Ref. 121
N N
CO2Et EtO2C CO2Et

EtO2C
con
Ar Ar
MeO2C
N MeO2C CO2Me CO2Me
N CO2Et
CO2Et EtO2C
CO2Et CO2Et N CO2Et
major Ar
trans cis
(more stable) (less stable)
CO2Et COOH OHC OHC

2. Ph N + N
CHO CO2Et CO2E
H CO2Et H +
DMF, -300C Ph N CO2Et Ph N CO2E
H H
48h
cis trans
major
60% > 95 : 5 Ref.122
Nitrones are used in the synthesis of isoxazoles. For example,
Me
O Me Bu 2
Ph N
1. N O1 Ref. 123
3
5
Ph 4
Bu
93%
Ph
O Ph Me
2. N PhOC N
O
Ph Ref. 124
CO2Me
O MeO2C Me
90%
Me
O N
3. N + CO2Me MeO2C O
Me CO2Me Ref. 125
MeO2C
MeO2C 55%
CO2Me
Carbonyl ylides generated by reversible ring opening of epoxides undergo

cycloadditions with electrophilic alkenes. For example, tetracyanoethylene oxide
generates a carbonyl ylide, which is trapped as tetrahydrofuran.
NC NC
NC CN
CN 1100C CN
O + O
O CN Ref. 126
Ph
NC CN NC
CN CN Ph
50%
Carbonyl ylide generated by Cu (1) chloride gives regioselective product with

benzaldehyde, which is governed by dipole character of 1,3-dipole and dipo-
larophile [5].
+ CO2Et
Ph O CO2Et Ph O
CuCl
N2CHCO2Et + PhCHO - Ref. 5
O
80 oC Ph
O Ph
91%
Carbonyl ylides generated by the transition metal-catalyzed decomposition of

diazo compounds are trapped with dipolarophiles [127].
O Me CO2Me
O Me O
Me
O Cu(acac)2 O MeO2C CO2Me O CO2Me
CHN2
H
O O O
Similarly, carbonyl ylide generated from the diazo compound in the presence of
rhodium (II) acetate reacts with dimethyl acetylene dicarboxylate to give dihy-
drofuran derivative [127].
Me Me Me
CO2Me
O Rh2(OAc)4 O+ MeO2 C CO2Me
O Ref. 127
- CO2Me
N2 PhH, rt
O O O 88%
3.3.1.14 Synthetic Applications of Intramolecular 1,3-Dipolar

Cycloaddition Reactions
Several intramolecular 1,3-DPCA reactions are used in the synthesis of heterocyclic

compounds. The following reactions are illustrative:
A. Intramolecular additions of nitrones

O
O Me
O N i)H2, Pd/ C
N N
1.
N PhMe ≡ ii) CH2O, HCO2H
OH Ref. 128
pseudotropine
H
MeNHOH
2. O O Ref. 129
O
PhMe, Δ N N
H Me
O
MeNHOH.HCl, Δ
3. (Me)2C=CH-CH2-CH2–CH–CH2–CHO (Me)2C=CH-CH2-CH2–CH–CH2–CH=N–Me
Me NaOMe, PhMe Me
Me H Me H Me
Me Me Me
O + O Ref. 130
N N O N
Me Me Me
H H Me
Me Me
major minor
(64-67%)
Me
NHOH Pr Ref. 131

PhO2S H
4. PhO2S Me N
N O
O Pr
PhO2S 74%
O
B. Intramolecular addition of azomethine ylides
O O O O O
5. LDA LDA H
NMe2 H
OLi N
N CH2 N CH2 N CH2 Me
O
Me Me Me Ref. 132
S S S
S PhMe, Δ H
6. N N N Ref. 133
10 hr
N
EtO2C
EtOOC COOEt C-OEt EtOOC 30%
EtOOC COOEt O
O O O O
:CF2 - HF
7.
CH2F2 in Ref. 134
CH2Cl2 H N
N N N
Ph :CF2 Ph F
CF2 Ph F Ph
C. Intramolecular addition of nitrile ylide
H
KOtBu N
N HN
8. N C CH2 C CH
C Ref. 135
Ph C DMF, 25oC Ph CH
H2
Cl H2 C Ph H Ph (91%)
D. Intramolecular addition of nitrile oxides
NO2 p-ClC6H4NCO N O
9. N Ref. 136
O
Et3 N, PhH
CO2Et CO2 Et H
25OC EtO2C
O HO
N N O H2N
O2N H H
10. Et3N LAH
Ref. 137
S PhNCO S
(- H2O) S S
H H
1,3-amino alcohol
Br
OH N Mg Ph N
EtMgBr O O
11. Ph N O +
O Ref. 138
Me H Et Ph H OH
Me Et
Et H
Me H
TS
3.4 Cycloaddition Reactions of More Than Six Electrons

Systems: [4+4]-, [6+6]-, [6+4]-, [8+2]-, [12+2]-, and [14
+2]-Cycloadditions
Some cycloaddition reactions of more than

six π electron
systems
have been
reported. Thermal suprafacial p4S þ p4S , p6S þ p2S and p6S þ p6S -cycloadditions
are forbidden according to Woodward–Hoffmann rules. These cycloadditions
are
photochemically allowed processes. Thermal antarafacial p4S þ p4 a addition is
possible, but is rare. The following examples are illustrative for [4+4]- and [6+6]--
cycloadditions:
O
Ph Ph
Ph
Δ
1. 2 O Ref. 139
Ph O Ph
1,3-diphenylisoindenone
hν Ref. 5
2.
- 110οC 65%
9,10-dihydro-naphthalene
O
hν
3. O+O dil. H2SO4, rt, Ref. 140
140h
tropone
O
Thermal [6+4]- and [12+2]-cycloadditions are allowed processes. Among them,

[6+4]-additions are common. These additions are favored than D–A cycloaddition
reaction because of the largest coefficients of HOMO and LUMO. The following
examples are illustrative:
O
r.t., 3d
1. O
Ref. 141
tropone
exo, 100%
cyclopentadiene
(repulsive seconary orbital
interaction destabilizes endo TS)
O
Me Me
Ph
Ph THF, 60 oC
2. O O Ph
Ref. 142
Ph 8h Me
Me O
95%
2,5-dimethyl-3,4-
diphenylcyclopenta-
dienone
O O
COOMe Ph CO2 Me
COOMe Ph
Ph rt Ph
3. O O +
MeOOC O Ph Ref. 143
PhH MeO2C
Ph COOMe
12%
O
[4+4] [4+2] 65%
H
O O
O
4. 80 C
Ref. 144
H
80%
3.4 Cycloaddition Reactions of More Than … 93
N-Ethoxycarbonylazepine 139 dimerizes to 140 on heating, where one molecule

acts as a 6π system and other as a 4π electron component.
N COOEt COOEt
N
EtOOC
EtOOC 130οC, 2d
N
N
EtOOC N N
140 COOEt
139
85%
Ref. 145
1,3-Pentadiene reacts with tropone to give [6+4]-cycloadduct 141.
140οC O
O
H H Ref. 146
141
exo 60%
Conjugated 8π electron systems undergo thermal [8+2]-cycloaddition reactions

with 2π electron system. The following examples are illustrative:
20 οC N COOMe
N COOMe
1. N COOMe Ref.147
N COOMe
40%
dimethylazocarboxylate
COOMe
4 οC COOMe
2. 3d COOMe
COOMe
Pd/C
(dehydrogenation) Ref. 147
heptafulvene
COOMe
COOMe 30%
azulene derivative
COOMe Pd/C
3.
N PhMe, Δ N N
H Ref. 148
H
indolizine COOMe MeOOC COOMe COOMe
MeOOC cyclazine
O
O MeO
Me
OMe Me
4. Ref. 149
Me Me
O
O
8-methoxyheptafulvene
Ph Ph Ph Ph Ph
COOMe Ph Ph
Ph
electrocyclic ring opening
5. Ph Ph Ph
COOMe COOMe Ref. 150
Ph
Ph COOMe Ph Ph
Ph
COOMe Ph COOMe
Ph
CO2Et CO2Et
EtO2C H
O
H H
PhMe, 190 C [1,5]-H
6. Ref. 151
5h
H H
O O O
PhMe COOMeOMe COOMe
COOMe
MeO OMe Sealed tube H
O O O O
7. Me Me ο O
O 200 C, 24 h
Me
MeO
OMe OMe COOMe Ref. 152
MeO
-CO2
Me Me - MeOH
-MeOH
Me
Thermal suprafacial [12+2]-cycloaddition is an allowed process, e.g., 142

undergoes cycloaddition with tetracyanoethylene suprafacially to give 143.
But
But
t
Bu
Ref. 153
142 t
CN CN Bu
NC CN H
H
CN CN
NC CN
143
Thermal [14+2]-cycloaddition of heptafulvene 144 with tetracyanoethylene

takes place in an antarafacial manner, in which the heptafulvene acts as an antara
component.
3.4 Cycloaddition Reactions of More Than … 95
H Ref.154
H
144 NC CN
NC CN
NC CN
145
NC CN
3.5 Cheletropic Reactions
3.5.1 Overview of Cheletropic Reactions
The cycloaddition of a conjugated π system to an electrophilic molecule by the

formation of two new σ bonds to an atom of the electrophile in a concerted manner
is known as cheletropic addition reaction and its reverse process in which two σ
bonds are broken from the same atom of the adduct is known as cheletropic
elimination reaction. In cheletropic elimination, the driving force is often from the
entropic benefit of gaseous elimination of N2, CO, and SO2. For example, the
cycloaddition of 1,3-butadiene and its derivatives with SO2 and of alkene with a
carbene are cheletropic addition reactions.
6e system SO2 SO2

SO2 SO2
dis motion
4e system Cl Cl
CCl2
CCl2 con motion Cl Cl
The mechanism of this cycloaddition can be explained by FMO theory, in which

one component acts as a HOMO and other as LUMO in a favorable low-energy TS
to afford a stereoselective product. The Woodward–Hoffmann rules for electro-
cyclic reactions are also applied to this cycloaddition reaction. The reaction of an
alkene with a carbene is considered as a 4n electron process and of a conjugated
diene with an electrophilic molecule as a 4n+2 electron process. Therefore, for
thermal reaction of 4n electron process, conrotatory motion of the substituents from
the termini of the π system will favor a low-energy TS to afford the product and in
photochemical process, the reverse disrotatory mode of motion will be the favored
path. Similarly, for a 4n + 2 electron process, disrotatory mode is a symmetry
allowed process in thermal reaction and conrotatory mode for its photochemical
reaction.
(a)
For (4n+2)e process
Diene
Diene LUMO
HOMO
Electrophile S S Electrophile
LUMO O O O O HOMO
p- orbital perpendicular to the plane p- orbital containing a lone pair of electrons

of the electrophile molecule takes part in the plane of the electrophile molecule takes part
in orbital interaction in orbital interaction
For carbon monoxide HOMO

LUMO
C
O
(b)
For 4n e process
LUMO HOMO
LUMO (alkene)
(suprafacial) HOMO
O LUMO
HOMO (electrophile) Cl Cl Cl Cl
S
O (antarafacial)
p-orbital containing a lone pair
of electrons in the plane of the atoms takes part in orbital
interaction
Fig. 3.14 Orbital interactions in the TS for cheletropic addition reactions in (4n+2) and 4n
electron systems
The orbital symmetry allowed HOMO and LUMO interaction for the reactions
of 4n+2, and 4n processes can be explained as follows (Fig. 3.14)
3.5.2 Applications of Cheletropic Reactions
The cheletropic elimination reactions are widely used for generation of relatively
unstable dienes in situ for Diels–Alder reactions. For example, extrusion of nitrogen
from diazene 146, and sulfur dioxide from 2,5-dihydrothiophene-1,1-dioxide 147
3.5 Cheletropic Reactions 97
and 2,7-dihydrothiepin-1,1-dioxide 148 generate diene and triene for Diels–Alder

reactions.
4 3
H3 C 2 CH 6e process
3 H3 C CH3 + N2
H 5 dis motion (outward)of H H Ref. 155
N1 H C(2) and C(5) for ring
146 N opening -N2
H CH3 100-150OC
dis-motion H Me + SO2
H3 C S H
O2 Me H
outward
147 Ref. 156
hν
Me Me + SO2
con-motion
H H
3
2 CH3 Δ CH3
4 + SO2
8e process Ref. 14
5 SO2 CH3
1 con-motion
6 7 CH
3
148
Extrusion of carbon monoxide from Diels–Alder adducts 149 and 150 gives
benzene and cyclopentanone derivatives [157, 158].
O
Ph Ph
Ph COOMe Ph
Δ Ph
- CO Ph COOMe
O Ph COOMe Ref. 157
Ph Ph Ph COOMe
Ph COOMe COOMe
Ph
149
Δ - CO
2 O Ref. 158
150
O O
Cheletropic eliminations of SO2 from cycloadducts 151–155 generate dienes and

triene, which are usedin Diels–Alder and electrocyclic reactions.
O
COOMe O COOMe
105 C O
O
S O
O2 D-A
O
151
H O
MeOOC Ref. 159
O
H O
Ph Ph O
Ph O O H
SO2 O O O Ref. 160
O O H O
152 Ph Ph
Ph
Ph O
O
Ph
ο
250 C
SO2 air
153 Ph O Ph O
D-A
Ref.160
Ph O
Ph O
H
- H2O
Ph O H
Ph O
O O O
electocyclization H
DBP, 210οC
SO2
8h H H
154 Ref. 161
85%
Δ electrocyclization
SO2
Ref. 162
155
Cheletropic addition of SO2 to a polyene depends on the geometry of the

intrusion adduct. For example, cheletropic addition of SO2 to 1,3,5-cyclooctatriene
156 gives cycloadduct 157 using 4π e system in a disrotatory motion process [163].
3.5 Cheletropic Reactions 99
SO2 dis
SO2
157
156
Similarly, the reverse process of extrusion is more facile for 6e process. For
example, the adducts 158 and 159, on heating above 300 °C undergo extrusion
process; the rate of 158 is about 60,000 times faster than 159 [163].
O2 O2
S S
158 159
Cheletropic addition of SO2 to divinylallenes takes place preferably at the most

substituted vinyl allene and at the E-site of the vinyl groups. For example,
divinylallene 160 having both E-and Z-vinyl units selectively undergoes addition at
the E-site to give 161 [164].
Z OH OH
SO2 t
Bu
But H
hexane, 30 min
dis, outward Ref. 164
E O 2S
160
Ph Ph
161
79%
3.6 Problems
3.6.1 Suggest a mechanism for each of the following transformations. More than
one pericyclic step may be involved in each case.
MeO OMe
OMe COOMe COOMe
(a) OMe Δ
+
COOMe
O COOMe O
Ph Me Me CHO
N 200 oC
(b) +
O O
CHO
COOMe Me
Me Me
Δ MeOOC
(c)
O +
MeO OMe Me OMe
O
NC O Me NC CN
CN + Δ O
(d)
NC CN Me NC CN
Me Me
H COOMe COOMe
(e) Δ
+ +
COOMe
H COOMe
Ph Ph
i) LDA, THF
(f) PhCH2N=CHPh
ii) Ph Ph Ph
N
Ph
H
CH2
(g) CH2 Et3N
CH2COCl
O
(h) H H
PhMe OH OH
N N
N o O
HO 120 C O
O H H
major minor
COOMe
110 oC
(i) + COOMe
COOMe
COOMe
(j)
200 oC
O O
(k) COOMe
Δ
O
OAc
COOMe
(l) AcO COOMe
+ +
AcO COOMe
COOMe OAc
Me
O
Me 80 oC
(m)
O H
O
H
O O
O
Me Me O
Me
Me
(n) Et2O, rt
O
3 weeks
3.6 Problems 101
3.6.2 Predict the structure of expected product including stereochemistry in each

reaction and indicated the basis of your prediction
0 oC Me3C r.t.
(a) Me2C=C=O + EtO (b) C C O +
NC
COOMe OMe
0-5 oC EtAlCl2
SPh
(c) + (d)
THF +
COMe -78 oC, 45 h
COOMe
O O
Et2O, rt 420 oC
Me
(e) + O (f)
O O
O
OMe
Ph CO2Me CO2Me
xylene Δ
(g) N Me (h) +
+ o
O 0 C
O Ph
CO2Me
O O COOMe
N Δ
(i) + (j) O +
O Et3N
COOMe
(k) + (l) + CH3CH(Cl)COCl
N O 0-5 o C
CN N Cl CO2Me
(m)
PhH
(n) O PhMe, 200 oC
+ +
reflux, 2h
O N O in sealed tube, 16 h
MeO OMe
Me
Me O
(o) (p) OEt
o Me3N
420 C Cl
+
18 mm, 3 sec
Me
Et
Me
Δ PhH, sealed tube
(q) (r) O
SMe 135 oC, 6d
O CO2Et
C rt
(s)
+
(t ) Δ
+
BuO S
O O O CO2Et
o CN
150 C
(u) 2 CH2=C=CH2 (v) xylene
+ N
reflux, 15h
N CO2 Me
(w) dioxan Me
+
N N
Δ
CO2 Me
3.6.3 Predict the product(s) in the following reaction sequences and justify your
answer.
NC CN
O 340-360 oC NC CN
(a) [A] [B]
O OMe
COOH Δ KI/ I2
(b) + [C] [D]
NH2 NH2
COOMe COOMe
Δ
(c) O+ [E]
COOMe COOMe
o CO2Me
200 C DDQ
(d) [F] [G] [H]
CH2Cl2, Δ
(e) Me
HO
H
O
3.6.4 Outline the synthesis of the following compounds
hν 100 oC
(e) [I] Ph
N
a. O b.
N
O
CO2Me
Ph Ph
c. d. N N
N N N
N NMe2
H CO2Me
Me
H
e. f.
HO CO2Me
H Me
O H
O
g.
O
3.7 Further Reading
1. Padwa A (1984) 1,3-dipolar cycloaddition chemistry. Wiley, New York.

2. Taber DF (1984) Intramolecular Diels–Alder and alder ene reactions. Springer,
Berlin.
3. Hamer J (1967) 1,4-cycloaddition reactions: the Diels–Alder reactions in hete-

rocyclic syntheses. Academic Press, New York.
4. Marchand AP, Lehr RE (1977) Pericyclic reactions, vols 1 & 2. Academic
Press, New York.
5. Gothelf KV, Jorgensen KA (1998) Asymmetric 1,3-dipolar cycloaddition
reactions. Chem Rev 98:863.
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Chapter 4
Sigmatropic Rearrangements
4.1 Introduction
Sigmatropic rearrangements are another class of pericyclic reactions which are

governed by the orbital symmetry. Woodward and Hoffmann coined the term
‘sigmatropic shifts’ because one sigma-bonded atom or group is shifted from its
allylic position in these rearrangements [1, 2]. This rearrangement involves the shift
of one sigma-bonded atom or group from its allylic type position to the distant end
of the conjugated π system followed by simultaneous shift of π electrons. The
rearrangement is described by the order [i, j], where i specifies the number of atom
or atoms shifted and j specifies the number of atoms in the π system that are directly
involved in the bonding changes. If the sigma-bonded atom or group shifts from
one surface of the conjugated system and arrives at the other end of the same
surface, the rearrangement is described as suprafacial and is an orbital symmetry
allowed pathway for the total number of [4n+2] electrons [1]. Alternatively, if it
leaves from one surface of the π system and arrives at the opposite surface, the
rearrangement is called antarafacial, and this is an orbital symmetry allowed
pathway when the total number of electrons in the process is 4n, where n is a
positive integer. The topological properties of the interacting orbitals dictate the
facility of most of the sigmatropic rearrangements and their stereochemistry. The
sigmatropic rearrangements are classified on the basis of their orders. The major
types of sigmatropic rearrangements are illustrated by general examples in
Scheme 4.1.

108 4 Sigmatropic Rearrangements
H 2 H
a c [1, 3]-H a H a
a [1, 5]-H H
b 1 3 c b
4e process b 6e process b c
d c d
d
d
suprafacial [1,3]-hydrogen shift (forbidden)
suprafacial [1,5]- hydrogen shift (allowed)
H
H 2 a c a
a c [1, 3]-H a [1, 5]-H c
1 4e process
d b b
b 3 b 6e process H d
H c
d
d
antarafacial [1,3]-hydrogen shift (allowed)
antarafacial [1,5]-hydrogen shift (forbidden)
5 6 2
4 3 [2, 3]
1
c 7 [1, 7]-H
3 d c 6e process R2N O
H a d R2N O
8e process
2 1 a
b
H 1 2
b
[2,3]-sigmatropic rearrangement of an amine oxide
antarafacial [1,7]- hydrogen shift (allowed)
2
[3, 3] 1 3 [2, 3]
RS O 6e process RS O
6e process
1 2
[3,3]-sigmatropic rearrangement of 1,5-hexadiene
(Cope rearrangement) [2,3]-sigmatropic rearrangement of an allyl sulfoxide
[3, 3] [1, 3]-R

O O
6e process R 4e process R
[3,3]-sigmatropic rearrangement of allyl vinyl ether suprafacial [1,3]-alkyl shift

(Claisen rearrangement)
Scheme 4.1 Major types of sigmatropic rearrangements
4.2 Orbital Symmetry Basis for Allowed and Forbidden

Sigmatropic Rearrangements and Their
Stereochemistry
4.2.1 Orbital Symmetry Analysis of [1,3]-, [1,5]-, and [1,7]-

Sigmatropic Shifts of Hydrogen and Alkyl Groups
The orbital symmetry study of sigmatropic reactions will help us to understand the
stereochemistry of these reactions. An FMO analysis of this process indicates the
interaction between the frontier orbitals of the π system and orbitals of the migrating
atom or atoms. Let us consider the simplest case of 1,3-sigmatropic shift of
hydrogen. In this case, π system is an allyl radical and the migrating atom is
hydrogen. Their frontier orbitals are allyl ψ2 and hydrogen 1s for thermal reaction
4.2 Orbital Symmetry Basis for Allowed and Forbidden Sigmatropic … 109
allyl-system allyl-system
HOMO (ψ2) HOMO (ψ3)
suprafacial antarafacial suprafacial

Thermal reaction Photochemical reaction
Fig. 4.1 Orbital interactions in thermal and photochemical reactions of [1,3]-sigmatropic

hydrogen shift
and allyl orbital ψ3 in photochemical reaction. The interactions between these

orbitals are depicted in Fig. 4.1.
It is seen from the figure that in thermal reaction, a bonding interaction (in the
same phase) can be maintained only in the antarafacial mode of shift. Therefore,
thermal 1,3-suprafacial shift of hydrogen is forbidden from orbital symmetry
considerations. The antarafacial shift is orbital symmetry allowed process and will
be a concerted process. Photochemically, [1,3]-suprafacial shift of hydrogen is a
symmetry allowed process because the bonding interaction takes place in the same
phase of allyl group [1, 2].
A similar FMO approach for 1,5-shift of hydrogen may be considered for study
of symmetry property of the process. In a thermal reaction of [1,5]-
suprafacialsigmatropic hydrogen shift, the interaction of 1s orbital of hydrogen with
pentadienyl radical ψ3 orbital takes place maintaining the orbital symmetry [1, 2].
In thermal reaction, bonding interaction is maintained in the suprafacial mode of
1,5-shift and hence this process is symmetry allowed, while the antarafacial shift is
symmetry forbidden. The suprafacial shift also corresponds to a favorable
six-electron Huckel-type transition state in thermal reaction, whereas Huckel-type
TS for suprafacial [1,3]-sigmatropic hydrogen shift is antiaromatic and is a for-
bidden process (Fig. 4.2) [1, 2]. Photochemically, [1,5]-hydrogen shift in the
suprafacial mode is a symmetry forbidden process, but antarafacial shift is a
symmetry allowed process (Fig. 4.3).
Analysis of a 1,7-hydrogen shift process indicates that the suprafacial hydrogen
shift is symmetry forbidden in a thermal reaction. Photochemically, [1,7] suprafa-
cial shift of hydrogen is symmetry allowed (Fig. 4.4) [1, 2].
pentadienyl system
Ψ3 (HOMO) Ψ4 (HOMO)
H (1s)
H (1s)
Thermal [TS] - suprafacial Hshift Photochemical [1,5] antarafacial H - shift

is allowed is allowed
Fig. 4.2 Orbital interactions in thermal and photochemical reactions of [1,5]-sigmatropic

hydrogen shift
Huckel type TS of 6e system Huckel type TS of 4e system

for suprafacial [1,5]-H shift for suprafacial [1,3]-H shift
aromatic, allowed antiaromatic, forbidden
Fig. 4.3 Orbital interactions in Huckel-type TSs for thermal [1,5]-, and [1,3]-sigmatropic
hydrogen shifts
heptatrienyl system
Ψ4 (HOMO) Ψ5 (HOMO)
Thermal [1, 7]- suprafacial H- Photochemical [1, 7]- suprafacial H-

shift is forbidden shift is allowed
Fig. 4.4 Suprafacial orbital interactions in thermal and photochemical reactions of [1,7]-
sigmatropic hydrogen shift
When an allyl group migrates, an additional stereochemical feature arises

because of p-orbital of migrating carbon of alkyl group. In this case, the thermal
[1,3]-suprafacial alkyl shift is an allowed process with inversion of configuration of
migrating alkyl carbon and the thermal suprafacial [1,5]-alkyl shift is also allowed
process with retention of configuration of alkyl carbon (Fig. 4.5) [1, 2].
1,7-Alkyl shift is analogous to 1,3-alkyl shift as both are 4ne processes. Hence
1,7-suprafacial alkyl shift with inversion of configuration is symmetry allowed
process [1, 2].
4.2.2 Orbital Symmetry Analysis of [3,3]- and [2,3]-

Sigmatropic Rearrangements
In [3,3]-sigmatropic rearrangements, the suprafacial orbital interactions of two allyl

radicals take place. Suprafacial interactions of the orbitals of two allyl radicals
produce both chair-like and boat-like TS (depicted below). Both these TSs are
thermally allowed, but in most cases preferred TS is chair like (Fig. 4.6) [3]. [3,3]-
sigmatropic rearrangements of 1,5-hexadienes known as the Cope rearrangements
and of allyl vinyl ethers or allyl ethers of phenols known as the Claisen rear-
rangements have both chain-like and boat-like TS. The major product is derived
from chair-like TS having larger substituent in the pseudoequatorial position.
4.2 Orbital Symmetry Basis for Allowed and Forbidden Sigmatropic … 111
1,5-suprafacial alkyl shift with 1,5-suprafacial alkyl shift with

retention of configuration is allowed inversion of configuration is forbidden
6e process, zero node, Huckel topology 6e process, one node, Mobius topology
aromatic antiaromatic
Fig. 4.5 Orbital interactions in the TSs of thermal reactions of [1,3]- and [1,5]-sigmatropic
suprafacial alkyl shifts
Chair- like TS, 6e process Boat- like TS, 6e process

Huckel topology Huckel topology
aromatic aromatic
Fig. 4.6 Suprafacial orbital interactions in chair- and boat-like TSs in thermal [3,3]-sigmatropic
rearrangements
Fig. 4.7 Suprafacial orbital

interactions in the TS (Huckel
type) of [2,3]-sigmatropic
rearrangements
Huckel type TS, 6e, aromatic
The TS is stabilized by hydrogen bonding and the reaction rate increases with
increasing solvent polarity. Water is a favorable solvent in many cases.
In [2,3]-sigmatropic rearrangement, the interactions between the orbitals of
allylic radical and migrating group take place in a suprafacial manner to produce a
TS of Huckel-type topology consisting of 6e. TS is aromatic in nature and sym-
metry allowed process (Fig. 4.7).
Table 4.1 Woodward-Hoffmann rules for sigmatropic rearrangements

a. Order [i, j], i = 1 Thermal allowed Photochemical allowed
4n Supra/inversion, antara/retention Supra/retention, antara/inversion
4n+2 Supra/retention, antara/inversion Supra/inversion, antara/retention
b. Order [i, j], i > 1
4n Supra/antara Supra/supra, antara/antara
4n+2 Supra/supra, antara/antara Supra/antara
Analysis of these sigmatropic processes using Woodward–Hoffmann rules leads

to selection rules for sigmatropic rearrangements (Table 4.1) [1, 2]. For sigmatropic
rearrangements of order [i, j], where i > 1, the suprafacial or antarafacial nature of
migration for both the components should be specified.
4.3 [1,3]-, [1,5]-, and [1,7]-Sigmatropic Hydrogen

and Alkyl Shifts and Their Applications
4.3.1 [1,3]-Sigmatropic Hydrogen and Alkyl Shifts
Thermal [1,3]-suprafacial hydrogen shift is orbital symmetry forbidden process, but

[1,3]-suprafacial alkyl shift is symmetry allowed process with inversion of con-
figuration of migrating alkyl carbon. For example, the thermal rearrangement of
bicyclo-[3.2.0]-heptene 1 to bicyclo-[2.2.1]-heptene 2 [4].
4 2
7
2 1 D
o 3 1
H H 300 C 1 6 OAc
5 4 5
3 6 H
5 OAc H
2 3 7
D
4 6
H 7
H
1 2 D AcO
TS
In compound 1, deuterium is trans to the acetoxy group, while in the product 2,

it is cis. This indicates that the inversion of configuration at C-7 occurs during this
rearrangement via the TS.
Similarly, bicyclo[2.1.1]-hexenes 3 give bicyclo[3.1.0]-hexenes 4 on heating
through a favored TS [5].
4.3 [1,3]-, [1,5]-, and [1,7]-Sigmatropic Hydrogen and Alkyl Shifts … 113
X 6 Y X Y
6
X Y
1
2 150 - 200 oC 3 4
4 5 2 5
3
3 1
4
(a) X = H, Y = Me
(b) X = Me, Y = H TS
(c) X = H, Y = OAc
Although inversion of configuration of alkyl carbon is symmetry allowed process in

thermal [1,3]-suprafacial alkyl shift, the presence of strong electron-withdrawing or
electron-donating substituents results in the retention of configuration. For example,
thermal rearrangement of 5 to 6 proceeds with about 95 % retention of configu-
ration [6]. Possibly, the presence of strong EWG lowers the energy level of LUMO
and of ERG raises the energy level of HOMO causing closure of HOMO and
LUMO energy levels, respectively, to facilitate this conversion at ease.
H Me
Me 250 o C
Ph
H
Ph Me NC
NC CN Me
CN
5 6
1,3-Alkyl shift also occurs in cyclopropane derivative 6a.
6a
Photochemically, [1,3]-alkyl shift is symmetry allowed process with retention of
configuration of alkyl carbon. The photochemical rearrangement of 7 to 8 is an
illustrative example [7].
Ph Ph
hν CN
CN
Me CN Me
7 NC 8
Thermal suprafacial [1,5]-hydrogen shift is orbital symmetry allowed process. For

example, diene 9 is rearranged to stable diene 10 on heating [8].
CH3 CH3
CH2 550 oC CH3
H [1,5]-H-shift CH2
CH2
9 CH3 CH3
10
In cyclic systems, [1,5]-hydrogen shift is preferred than [1,5]-alkyl shift or [1,5]-

deuterium shift. For example, [1,5]-H-shift of diene 11 gives 12, and of indenes 13
and 14 gives 13a and 14a, respectively [9, 10].
H 3C H CH3
Δ
Ref. 9
11 12
D D D
H
150 oC H
H
[1,5]-H-shift [1,5]-H-shift
13 13a
H3C H CH3 CH3

100 oC H Ref. 10
H
14 14a
The suprafacial nature of thermal [1,5]-hydrogen and deuterium shifts is indicated
in the rearrangement of chiral diene 15 into its stereoisomers 16 to 18 [11].
H
Me
Et H 250οC H
D
Et Me Me
Et Me
Me D Me D Et
15 16 Me H
Me D
major
H Et
Me D
Me Me Me Me
Et D H Et
D Me H
18 17
minor
Thermal [1,5]--hydrogen shift occurs in cyclobutane 19 and arylallene 21 to give 20

and 22, respectively. The product 20 undergoes electrocyclic ring opening on
heating at slightly higher temperature to give 20a [12].
200 oC 260 oC
H
[1,5]-H-shift electrocyclic ring opening
19 20
20a
Δ Me
H
[1,5]-H-shift electrocyclic ring closure
21 22
2-Methylcyclohepta-1,3-diene 23 at room temperature gives a mixture of isomeric

methylcycloheptadienes by 1,5-hydrogen shifts [13].
CH3 CH3 CH3 CH3
H
H H
23
Similarly, 5-methyl-1,3-cyclopentadiene 24 rapidly rearranges at room temperature

to yield a mixture of 1-methyl-, 2-methyl-, and 5-methyl-substituted products by
[1,5]-hydrogen shifts [14].
CH3 CH3 CH3

H [1,5] H
H
rt H
24 H
Both [1,5]-alkyl and hydrogen shifts take place in the thermal conversion of the
spiro-dienes, 6,9-dimethylspiro-[4,4]-nona-1,3-diene 25 to 26 and of spiro-[4.2]-
heptadiene 27 to 28 [15].
Me H Me H Me H
230 - 280 oC
[1,5]-alkyl-shift [1,5]-H-shift
H H 26
25 Me Me
H
Me H
Δ
[1,5]-C-shift [1,5]-H-shift [1,5]-C-shift
27 H 28
In [1,5]-shifts, the migratory aptitude of carbonyl group is greater than methyl

group in indene skeleton 29 [16].
H3C CH3 H3C

COR H
140 oC COR
COR
H
29 CH3 CH3 CH3
Thermal conversion of cycloheptatriene 30 to 31 and 32 takes place by [1,5]-alkyl

shift followed by electrocyclic ring opening of norcaradienes. This phenomenon is
known as valence tautomerism of norcaradiene [17].
Δ [1,5]-C [1,5]-C
Me Me
Me Me Me
Me electrocyclic Me Me
30 ring closure nor-caradiene Me Me
Me Me
electrocyclic electrocyclic
ring opening ring opening
Me Me Me
31 Me Me Me 32
Thermal [1,5]--hydrogen shift also occurs in the conversion of cyclic enone 33 to

33a.
CH3 CH3 CH3 CH3

100 C
o D2O [1,5]-D
O D2O O
O O
[1,5]-H H D
H3C C H H3C CH2 H3C CH2 H3C CD3
H2 33
33a
7-Methyl-cycloheptatriene 34 on heating undergoes slow [1,5]-suprafacial hydro-

gen shift rather that [1,7]--antarafacial H-shift to yield a mixture of
methyl-substituted isomers [9].
H 7
5 6
Me H
4 > 146 oC
H
Me H Me H Me
3
H
1 2
H
34
Thermal [1,5]-sigmatropic hydrogen shift also occurs in vinyl allenes 35 to 37 to

give products, which in many cases undergo spontaneous [1,7]-sigmatropic
hydrogen shifts to give thermodynamically stable products of linearly conjugated
system [18, 19].
R o R
100 C
R
[1,5]-H R
C H CH2
35 H2
isooctane O
H [1,7] -H
o Me
H 100 C
H
[1,5] -H
O
36
O
R R
R
isooctane O
100 oC Me Me
+ O
H [1,5] -H and [1,7] -H
Me
O
37
R= C8H17
Photochemical [1,5]-hydrogen shift is rare as it is possible in antarafacial path.

Photochemical conversion of the diene 38 to 39 illustrates the case [20].
CH3
C H CH2
CH3
H3C CH3
H
CH2 H3 C
C
38
CH3
(antara-supra) hν
[1,5]- H
H2C CH(CH3)2 CH(CH3)2
electrocyclic
CH2 ring opening CH2
H2C
CH3 39 H3C
Similar to [1,5]-suprafacial H-shift, [1,6]-suprafacial H-shift occurs in cycloocta-

dienyl cation 40, which is generated from cyclooctadienyl epoxide 41 [21].
H+ OH
O OH O
HClO4 [1,6]-H +
H - H+
o
H
0 C, 5 min +
41 40 H
[1,7]-hydrogen shift occurs in antarafacial manner in a flexible π system. This type

of hydrogen shift is common when one part of the π system is in the ring and other
part in open chain. The most important reaction of this shift is the thermal con-
version of precalciferol (previtamin D3, 42) to calciferol (vitamin D3, 43) in our
body [22]. For this reason, it is advisable to expose our bodies to sunlight some-
times in a day for synthesis of vitamin D3 in natural process.
C8H17 C8H17
H
H2C above rt CH2
42 43
HO HO
Thermal conversion of ortho-butadienyl phenol 44 to 45 and of 46 to 47 involves

[1,7]-D and [1,7]-H-shift, respectively [23].
110 oC
OD [1,7]-D O CH2D O CH2D

44 45
Me
H Δ
Me [1,7]-H Me
46 47
A similar [1,7]-antarafacial hydrogen and deuterium shifts are observed in the

rearrangement of the steroid 48 on heating [24].
MeC8H17 MeC8H17 MeC8H17
o
78 - 98 C D H
H
D CH +
3 H3C H D CH3
OH OH OH
48
[1,7]-Suprafacial alkyl shift is observed in the thermal interconversion of bicyclo
[6.1.0]-nonatriene 49 into its isomers. The interconversion occurs much rapidly and
hence it is difficult to detect the endo- and exo-substituents. This rearrangement is

known as Walk rearrangement [25].
Me NC Me
9 CN
NC
7 8
6
[1,7]-alkyl shift Me
1
etc.
2 Me Me Me
5
3 4
49
Photochemical [1,7]-suprafacial alkyl and hydrogen shifts have been observed in

the conversion of cycloheptatriene derivative 50 into its isomers [26].
CH3
H3C CH3 H H3 C H
CH3
hν CH3 hν
H3C [1,7]-C [1,7]-H
H3C H3C
50
4.4 [3,3]-Sigmatropic Rearrangements
4.4.1 The Cope Rearrangements
The Cope rearrangement is the most important category of sigmatropic rear-

rangements from synthetic point of view. The thermal [3,3]-sigmatropic rear-
rangements of 1,5-hexadienes are called the Cope rearrangements. These
rearrangements are reversible in nature. The Cope rearrangement proceeds through
a chair-like or boat-like transition state. Usually, a chair-like transition state with
minimum steric interactions between the substituents provides the major product.
For a chair-like TS of a trans-3,4-disubstituted 1,5-hexadiene, the major product
would be either, E,Z- or Z,E-diene. For example, diene 51 on heating gives 53 and
54 as major and minor product, respectively, through chair-like TSs 52a and 52b
[27]. Enantiomerically pure compound 51 gives an optically pure product of >95 %
ee [27].
Ph CH3 Ph CH3
Ph CH3
CH3 52a H3C
CH3 53
51 favoured TS major product
(minimum steric interaction) (E-isomer)
Ph H Ph H
Ph
H3C H3 C
CH3 CH3
H3C
52b 54
maximum steric interaction minor product

( Z-isomer)
Conjugated substituents at C-1 (or C-6) and C-3 (or C-4) of 1,5-dienes accelerate
the rates of the reactions, since the substituents at these positions weaken the bonds
being broken in the reactions [28]. Donor substituents at C-2 and C-3 also accel-
erate the reaction [29]. The following examples support these facts.
CN CN CN
1. 150 C
EtO2C EtO2C EtO2C CH3
CN CH3 CH3
H3C
CO2Et
favoured TS Ref. 30
Ph
2. Ph Ph Ph
Ph Ph Ref. 31
Ph
Ph TS
CH3 CH3 CH3

H3C
225οC
3. H3C H3C H3C Ref. 32
H3C TS
major product
The reversible nature of Cope rearrangement is supported by the fact that optically
active compound 55 undergoes racemization on heating by reversible Cope process
[33].
H H H
H 2
1
3 50οC
1
3 [3,3]-shift H
2 H H
55 H
TS
Bicyclic compounds 56 and 57 on Cope rearrangement undergo skeletal rear-

rangements to give 58 and 58a, respectively [34].
4.4 [3,3]-Sigmatropic Rearrangements 121
H3C
o
98 C
CH3
O 56 CH3
O 58
80-90%
H H H
H
O 3 2
60 oC
Ph3P-MeBr 1
3
t
BuOK, DMSO 58a
1 2
TS
57
Analysis of the product ratio from chair and boat TS geometry from a Cope rear-
rangement of deuterated 1,5-hexadiene indicated that the boat TS is about 6 kcal/mol
less stable than the chair TS [35]. It is reflected in the Cope rearrangement of cyclic
dienes 59 and 60. Comparison of their reaction rates showed that diene 59 reacted
faster by a factor of 18,000. This fact can be rationalized by considering their TS.
Compound 59 reacts through a chair-like TS while 60 through a boat-like TS. The
chair-like TS has lower activation energy and hence 59 reacts much faster [36].
H
H
59 chair-like TS
H H
60 boat-like TS
When the C-3 and C-4 carbons of 1,5-dienes are connected to a cyclopropane ring,
the reaction rates are accelerated due to favorable interactions of the diene termini
resulting in the reduction of the enthalpy of activation. For example, the conversion
of cis-divinylcyclopropane 61 to 1,4-cycloheptadiene 62 occurs readily at tem-
peratures below −40 °C [37].
H 4 5
-40 o C 3
6
2
7
H 1
61 62
This reaction occurs at much lower temperature to relieve the ring strain. The
cis-geometry of the vinyl substituents plays one of the key roles to accelerate this
reaction. This is substantiated by the fact that the reactions of trans-
divinylcyclopropane 63 and vinylcyclopropane 64 take place at much higher

temperatures in a non-concerted process [38].
o
190 C
63
H 350 oC
H H + H +
D
H D H D D H
64 D H
47% 47% 6%
The ring strain is another important factor to increase the reaction rate of
cis-divinylcyclopropane. For instance, the rearrangements of cis-vinyloxirane 65
and cis-divinylthiirane 66 require relatively higher temperatures because of lesser
degree of ring strain in their rings [39]. Similarly, the change of ring size from
three-membered to four- and five-membered requires higher temperature for the
reaction. Thus, the rearrangement of cis-divinylcyclobutane 67 occurs at 120 °C
and of cis-divinylcyclopentane does not occur even at 250 °C [40].
O 60 oC
O S 100 oC
65 S
66
H
120 oC, 10 min
67 H
91%
Divinylcyclopropane rearrangements take place with even greater ease if the vinyl
groups are incorporated in another ring. This condition favors the entropy of
activation of the reaction to be less negative. This is found in the degenerate
rearrangement of homotropilidene 68 [41]. A degenerate rearrangement is a reac-
tion when the product of the rearrangement is structurally identical to the starting
material in terms of nature and types of bond order. The occurrence of a dynamic
equilibrium in the reaction of homotropilidene is evident from the NMR study of
the reaction. At low temperature, the rate of interconversion is slow and the NMR
vinyl protons
20 oC
cyclopropyl
protons
allylic protons
68
spectrum showed the presence of four vinyl protons, two allylic protons and four
cyclopropyl protons. When the temperature is raised, the rate of rearrangement
increases and the NMR spectrum recorded the signals of two vinyl protons and
signals of other two vinyl protons coalesce with two cyclopropyl protons and the
signals of two allylic protons coalesce with two cyclopropyl protons. This indicates
that the sets of protons undergoing rapid interchange with one another show an
averaged signal.
Several degenerate Cope rearrangements are known. One of the most interesting
cases is of bullvalene 69 [42]. At 10 °C, its 1H-NMR spectrum showed a single
signal at δH 4.22 ppm, indicating the ‘fluxional’ nature of its molecule and identical
environment of all the carbons. The first-order rate constant of the reaction is
3.4 × 103 s−1 at 25 °C with ΔG of 12.6 kcal/mol. The rearrangement of bullvalene
69 is shown by the change of environment of labeled carbons.
etc.
69
indicates labelled carbon
Among other degenerate rearrangements, the rearrangement of barbaralane 70 [43]

and semibullvalene 71 [44] are important. The free energy of activation ΔG for their
rearrangements is 7.6 kcal/mol at 25 °C and 5.5 kcal/mol at −143 °C, respectively.
It indicates that degenerate rearrangement of 71 is much more rapid than the
conformational inversion of cyclohexane.
a d
b c b a
b
b c
71
70
4.4.2 The Oxy-Cope and the Anionic Oxy-Cope

Rearrangements
The presence of a hydroxyl group at C-3 position of the 1,5-diene system drives the
rearrangement into the less stable enol product, which is converted into stable
carbonyl product. This version of this Cope rearrangement is termed the oxy-Cope
rearrangement [45]. A particular advantage of this process is that it proceeds under
mild conditions, so that many sensitive functional groups are tolerated and is useful
for construction of complex organic molecules. The simplest case is the conversion
of 3-hydroxy-1,5-hexadiene to 5-hexenal [46].
H
2 H-O O
HO 3 1 1
If the hydroxyl group at C-3 position of the 1,5-diene is converted into the
respective potassium alkoxide, the rearrangement is known as the anionic oxy-Cope
rearrangement. The conversion of C-3 hydroxyl group into an alkoxide ion
accelerates the reaction rate by a factor of 1017 [47]. The following examples [48–
53] of this reaction are illustrative:
H H
0 oC , K H, T HF H
1. R ef. 48
18-crown-6 O O
OH H H
O 67
MeO O OMe
HO
Me H R ef. 49
2. K H, 18-crown-6
ο
70 C , 0.5h Me
Me
Me 89%
Me O
HO K H, T HF Me R ef. 50
3.
85 οC , 9.5h
MeO MeO
85%
MeO MeO
K H, T HF H 2O R ef. 51
4.
OH
MeO Δ O O
MeO
O K
OH OH H
5. K HMDS , 18-crown-6
R ef. 52
OH T HF , r.t., 20h
OH
O C HO
H
6. K H, 18-crown-6 Ph
1M HC l
Ph OP ri OP ri
OP ri T HF , r.t.
OP ri
Ph Ph
O
HO K
Ph R ef. 53
O
69%
Acyclic 1,5-diene-3-ols 72 and 73 undergo anionic oxy-Cope rearrangements to

give major products from equatorial orientations of the oxyanions.
KH H2O Ref. 54
O
18-crown-6 O
HO THF O 72a
72
favoured TS 99%
KH H 2O R ef. 55
18-crown-6 O
HO T HF O O
73 73a
80%
The reaction rate of an anionic oxy-Cope rearrangement is accelerated by the

presence of an additional unsaturation function on the terminal position. This
additional unsaturation function probably stabilizes the transition state and helps the
reaction to occur at ease. For example, bicyclic allyl alcohol 74 on rearrangement
gives bicyclo-[5.3.1]-undecenone 75 in 88 % yield, while under the same condi-
tions, isomeric alcohol 76 gives 77 in 25 % yield [56].
[3, 3] O
5 4 3 NaH, THF O
6
OH
2 reflux, 22h
1 75
88%
74
O
NaH, THF
OH
reflux, 38h
76 77
25%
Base-catalyzed allenic oxy-Cope rearrangement of 78 to 78a is useful for synthesis

of bicyclic compound [57].
O O O
H
KH,18-C-6 [3,3]
O
THF, I2, rt, 2h
OH O
78
O O
OH O
78a
90%
The products from the anionic oxy-Cope reactions of norbornenyl derivatives 79

and 80 depend on the steric demands of the oxyanions. In both cases, the orien-
tation of the oxyanion favors chair-like TS structures for exo-bond formation to
give the major aldehyde 81 along with minor aldehyde 82 [58].
i. KH, 18-Cr-6 O
H
HO H THF, 50οC
ii. CH3OH H
O
79 equatorial oxyanion boat-like TS
chair-likeTS
CH3OH
CHO H
81:82 = >99:1 H CHO
81 CH2 82 CH2
HO H O
O H
H
80 axial oxyanion boat-like TS
81:82= 10:1 chair-likeTS
4.4.3 The Amino- and Aza-Cope Rearrangements
3-Amino group in the 1,5-diene system also accelerates the Cope rearrangement.
This version of the Cope rearrangement is known as the amino-Cope rearrange-
ment. The products of the reactions are useful in the synthesis of unsaturated
aldehydes. The following examples are illustrative [59, 60]:
CH2Ph
R2N ο R2N i) PhCH2Br Ref. 59
240 C OHC
1.
15 min ii) H2O
R Ph
N
Li ο i) PhBr
RHN N R THF, 25 C OHC
n-BuLi Ref. 60
ii) H2O
2.
When nitrogen atom belongs to a part of 1,5-diene skeleton, the Cope rearrange-
ment is known as the aza-Cope rearrangement. The conversion of the mesylate 83
to 84 is an example of 2-aza-Cope rearrangement [61]. Similarly, the reaction of
pyridine 3-aldehyde with N-methyl-2-hydroxy-2-methylbutenamine gives an imi-
nium salt as an intermediate 85, which undergoes 2-aza-Cope rearrangement to give
acetylnicotine derivative in high yield [62].
O SO2Me
H
- 5οC [3, 3] Ref. 61
N N N
H Me
Me Me
84
83
O−H
Me
Me
OH
CHO H HO Me camphor
N N
+ N
Me sulphonic acid Me Me
N PhH, reflux N
N
85 H
Ref. 62 COMe
N
Me
N
The aza-Cope rearrangement of cyclopentane derivative 86 gives a bicyclic

pyrrolidine 87 in which original ring is expanded by one carbon atom [63].
Ph O
Ph Ph Ph
HO H−O
HO
camphor
+ CH2 =O
sulphonic acid N
NHMe N N 87 H Me
H EtOH, 78οC, reflux H Me
Me 83%
86
When cyclic ketone was used to generate an iminium ion, the yield of the reaction
becomes low due to unfavorable steric strain in chair-like TS; for example, the
synthesis of 1-aza-spiro-[4,5]-decane ring system 88 from cyclohexanone via

iminium ion 88a [64].
O
NH2
PhH O H−O
+ HO
80 οC NH
NH NH
HO reflux 88a
O O
NH
88
The rearrangements of 4-hydroxy-2-aza-1,5-dienes 89 to 89a in anionic forms are

known as the anionic-4-oxy-2-aza-Cope rearrangements [65].
2
N 1 Ph N Ph N Ph N Ph
3 KH, THF NH4Cl
[3, 3] H−O
HO 4 O
6 18-crown-6, 25οC, O
Me 5 Me Me
24h Me
89
1H Ref. 65
2 N
Ph
Me 3
O 89a
3-acetyl-5-phenyl-pyrrolidine
The Cope rearrangements of 1-aza-1,5-dienes are known as the 1-aza Cope rear-
rangements (1-ACR). For example, N-acylimine 90 obtained from flash vacuum
pyrolysis (FVP) of N-acyl hydroxylamine derivative gives pyridine derivative 91
by 1-ACR [66].
O
1) FVP
R H 1) NaBH4.TFA
R R
2) NH2OH.HCl R 2) ClCO2Me CO2Me (- HOCO2Me)
N N
R = H, Me N OCO2Me CO2Me
OH 90
H
R
N N
H 91 MeO2C R
CO2Me
40%
The [3,3]-sigmatropic rearrangement of 4-nitroso-1-butene 92 and nitrosobicyclo-

[2.2.2]-octene 92a is known as the 1,2-oxaza-Cope rearrangements [67].
Δ
O O
N N
92
H
1. DBMP, TMS-Cl CO2Me
6
2. iAmNO2 /TiCl4, -45o to -15oC, 1.5 h 4 3 CO2Me N
5 O
CO2Me N H
O 2 92a
1
4.4.4 The Claisen Rearrangements and Their Modified

Versions: The Carroll, Eschenmoser, Ireland,
Johnson, Gosteli, Bellus, and Enzymatic Claisen
Rearrangements
Thermal [3,3]-sigmatropic rearrangements of allyl aryl ethers and allyl vinyl ethers
are known as the Claisen rearrangements [68]. These reactions are sensitive to
solvent polarity and the rates of the reactions are increased by increasing the solvent
polarity [69]. The simplest examples are the thermal conversion of allyl phenyl
ether to ortho-allyl phenol and of allyl vinyl ether to 4-pentenal [70].
O OH
O O
H
200οC
73%
2
3O 1 O
180οC O
4 6
5
Allyl vinyl ethers 93 and 94 are generated from the reaction of allyl alcohols with
alkyl vinyl ether in the presence of Hg(OAc)2 [71].
Hg(OAc)2
CH2=CH-CH2OH + CH2=CH-O-CH2-CH3 CH2=CH-CH2-O-CH=CH2
Δ
93
CH2=CH-CH2CH2-CH=O
OH O
o
ROCH=CH2 200 C, 12h
Hg(OAc)2
94 85% CHO
The Claisen rearrangement is intramolecular in nature. It was confirmed by a

crossover experiment in which two aromatic allyl ethers 95 and 96 were heated
together and found to yield same products 97 and 98 as when they were heated
separately. No crossover products 99 and 100 were found [72].
OH Ph
O
O OH
Ph + +
96 97 98
95
Ph
OH
OH
and
99 100
not found
The intramolecular mechanism of the Claisen rearrangement was also verified by

the use of 14C-labeled allyl phenyl ether 101 [73].
H2
C CH
O O OH H2
* CH2 H
C C CH2
* * H
C C CH2
H2 H
101
* = 14C label
The major product of the Claisen rearrangement is derived from a preferred

chair-like transition state in which the larger substituent occupies the pseudoe-
quatorial position. For example, in the Claisen rearrangement of 102, the major
product 103 was obtained [74].
CH3 CH3 H
O CH3
H3C (H3C)2CHCH2 O O
CH3 (H3C)2CHCH2 C
(H3C)2HCH2C H
102 preferred TS of minimum 3
steric interactions 103
In the rearrangement of aryl allyl ethers, when both the ortho-positions of the
aromatic ring are substituted, the migrating allyl group will shift to the para-
position. This rearrangement is known as the para-Claisen rearrangement.
For example, the rearrangement of the aromatic ether 104 gives the major product
105 [75].
O O OH
Me O
Me Me 186 oC MeMe Cope Me Me Me Me
Me
3.5h [3,3] Me
[3,3]
104 H Me
105 (91%)
The products of Claisen rearrangements undergo further rearrangements to yield

rearranged products, these rearrangements are known as the abnormal Claisen
rearrangements [76]. For example, vinyl ether 106 gives 107.
H CH3
H CH
O O O OH CH3
CH2CH3
CH2CH3
[3,3] CH3 CH3
H
106 250 oC 107
Ref. 76
Similarly, the phenyl allyl ether 108 gives abnormal product 109 on heating. But in
the presence of dimethylaniline gives normal Claisen rearrangement product 110.
H
1
1O
2
O OH O
3
2 3 Δ
H
108 [3,3]
109
OH O
PhNMe2
+ PhNHMe2
Δ
110
108
When an allyl aryl ether 111 contains a vinyl group at the ortho-position of aryl
group, the Claisen product undergoes further Cope rearrangement and [1,5]-H-shift
to give the product 111a [76].
O * O 1 2 3 O OH
* *
Δ * [3,3]
1' 3' H Ref. 76
[3,3] 2' [1,5]-H
111a
111
* = 14C label
Several modifications of Claisen rearrangement were developed to increase the

versatilities of this rearrangement in the synthesis of different classes of organic
compounds. For instance, thermal rearrangement of allylic β-keto esters 112 to
112a are known as the Carroll–Claisen rearrangements [77].
R R 2 1 R R
R' 3
R' R' R
Me O LDA LDA rt
Me O O1 R'
3 O
2 R'
O O [3,3]
O O O O O O CO2H
112
R,R' = H, alkyl or aryl O 112a
The Claisen rearrangements of amide acetals of allyl or crotyl alcohols are known
as the Eschenmoser–Claisen rearrangements [78]. For example, E- and Z-isomers
of 113 give 113a and 113b as major product, respectively [78].
Me-CH2-C(OMe)2 MeCH C OMe + MeOH

NMe2 NMe2
Me-CH CH-CH2-OH + MeCH C OMe Me-CH CH-CH2-O C NMe2 + MeOH

NMe2 CH-Me
Me Me
O Me
Me [3,3] H
Me Ref. 78
ο
~130−140 C COMe2 HO2C Me
113
NMe2
E-isomer erythro (major product) 113a
(95%)
Me Me
O Me
[3,3]
ο
COMe2
~130−140 C COMe2 Me
Me 113 NMe2 Me 113b
Z-isomer threo (major product)
(97%)
This version of Claisen rearrangement is not suitable for compounds having

acid-sensitive functional groups.
Trimethylsilyl derivatives of enol esters of allylic alcohols 114 undergo [3,3]
sigmatropic rearrangements on heating below 100 °C. This version of Claisen
rearrangement is known as the Ireland–Claisen rearrangement. The major product
of the reaction is derived from the geometry of the silyl derivative, which depends
on the condition of solvent used for the preparation of lithium enolate with LDA.
When THF is used, Z-lithium enolate gives the major product 114a, whereas use of
23 % HMPA (hexamethyl phosphoramide)—THF, gives the thermodynamic con-
trolled E-lithium enolate-derived product 114b as major product [79].
1. LDA, THF, -78οC 70οC O

O 2. Me3SiCl O then acidified
with AcOH H
O Me3SiO H
114 OSiMe3 [3,3]
Z-enolate
HO2C
114a HO
major product (erythro)
87%
1. LDA, 23% HMPA-THF 70οC O

H
O 2. Me3SiCl O then acidified

with AcOH
O Me3SiO
114 OSiMe3 [3,3]
E-enolate
HO2C
114b
major product (threo) 81%
Aromatic ketone 115 containing allyloxy group at the α-position undergoes

Ireland–Claisen rearrangement [80] to give 115a.
1) 71οC
2
3
O 1) THF, LDA 1
Ph Ph Ref. 80
O 2) AcOH O
Ph 1O
2) Me3SiCl 3 OSiMe3 [3,3] OSiMe3
2 115a
115
TBS (tert-butyldimethylsilyl) and TES (triethylsilyl) were also used instead of

TMS for preparation of silyl derivatives.
The rearrangements of orthoesters of allyl alcohols are known as the Johnson–
Claisen rearrangements. This modification of Claisen rearrangement improves the
yield of the reaction and transfers the chirality of the alcohols in high levels and
allows the introduction of carboalkoxyalkyl group. For example, orthoester 116 of
(2R,3E)-3-penten-2-ol gives ethyl ester of (3R,4E)-3-methyl-4 hexenoic acid 117 in
90 % optical yield [81] and methyl orthoester 118 gives 119 [82].
Me
Me OEt Me OEt Me
- EtOH
HO + MeC(OEt)3 EtO C H2C C
H O O
Me
Me H Me H
116
O O O
2 1 Me Me Me
Me 5 4 CH2CO2Et Me Me Me
OEt OEt
3 Me
H H H OEt H H
117 TS
Et Me
Et Me
CO2Me
CO2Me MeC(OMe)3
ο O H
OH H 110 C
118
OMe
Ref. 82
Et Me
CO2Me
MeO2C
H H
119
85%
The [3,3]-sigmatropic rearrangements of 2-alkoxycarbonyl-substituted allyl vinyl

ethers 120 are known as the Gosteli–Claisen rearrangements [83].
O OR O OR O OR
CH2Cl2 O
O O
o
80 C
120 TS
The catalyzed version of this rearrangement is useful for the synthesis of medium
and large-sized carbocycles [84]. For example, the enantioselective synthesis of
carbocyclic natural product, (−)-9,10-dihydroecklonialactone B 121 was done
successfully by catalytic asymmetric Claisen rearrangement of a Gosteli-type allyl
vinyl ether 122 in the presence of (S,S)-Cu (box)-catalyst A to produce a chiral
α-ketoester 123, as a building block unit [85].
Z O
CO2Me
Z Cat. A (0.1 eq) CO2Me O
O
(CH2Cl)2, rt, 16h O O
BnO
OBn 122
[Z,Z]-isomer 2 123(3S, 4R) 121
O O
96%, 98% ee
N N
t- Bu-t
Bu H Cu H
O O
F3C CF3
Cat. A
The ketene-Claisen rearrangement of a 1,3-dipolar allyl vinyl ether 124 is known as

the Bellus–Claisen rearrangement [86].
O O
Cl Me Cl
Cl 23 oC O [3,3]
O+ R OMe Cl OMe
Cl
Cl ether
R R
124
Subsequently, this version of Claisen rearrangement was extended to tertiary allyl

amines and acyl chlorides [87]. Using Lewis acid catalyst, excellent stereoselec-
tivity of the product was found. For example, 3,3-disubstituted allyl morpholine
125 with propionyl chloride in the presence of TiCl4 gives highly stereoselective
syn product 126 [87]. This method is useful for the synthesis of
α,β-disubstituted-γ,σ-unsaturated carbonyl compounds.
O O Et Me
Me Me
10 mol%, TiCl4, THF2 [3,3]
O N
N
Et + Me N O
O Me i-Pr2EtN, CH2Cl2, 23 o C O Me
Cl Et
125 126
72%, syn / anti, 99 : 1
In the rearrangement of allyl vinyl ethers in conformationally rigid cyclohexane

system 127, the major product 128 is derived from the preferred axial bond for-
mation in the TS [88].
t t
Bu Bu t
Bu
Py, reflux
H +
O HO H
HO
127 O 128 O O
86 : 14 89%
Ref. 88
H O
But But O
OH
O
Enzymatic Claisen rearrangement is observed in the rearrangement of chorismate

129 into prephenate 130 [89].
COO OOC COO
chorismate mutase O
O COO 130
OH 129 OH
In the Claisen rearrangements of chiral substrates, the chirality is maintained in the

products. The following examples are illustrative [90, 91]:
O O
1
3 O O
O 2
Δ
1. 3
1 2
T.S.
Δ, PhMe 3
CHO
2. 1
2
O trace of MeCH2COOH
O 2 O
3
1 T.S.
3. Δ 3
1
O 3
2 O
1 2 O O
O
boat-like Ref. 90
T.S.
H O 2
1 1
3 Δ
4. 2
H CHO
H 3 H
catalytic MeCH2CO2H
O
5. Ref. 91
O + o-xylene, reflux H
PhO2C O
EtO OEt PhO2C
OH
80%
The Claisen rearrangements are extended to allyl and propargyl esters 131 and 132
in the presence of catalysts [92, 93].
Me
Me
0.04 eq. PdCl2 (MeCN)2 O O
O O Ref. 92
H Ph
THF, rt, 2 h [3, 3]
Ph
131 96%, E/Z, 98/2
Ar Ar
O O 0.005 eq. AgBF4 O O
Et Ref. 93
PhCl, 37οC, 50 min Et
Et
132
Et
Ar = p-NO2-phenyl 68%
The Claisen rearrangements of aryl propargyl ethers 133 are used in the synthesis of
flavonoids [94].
Ar Ar Ar
Ar Ar
1
2
O O-H O
1O
O Ref. 94
2 3 Δ
3 H
133 1
TS O 2 Ar
flav-3-ene
4.4.5 The Thio- and Aza-Claisen Rearrangements
4.4.5.1 The Thio-Claisen Rearrangements
[3,3]-Sigmatroic rearrangements of phenyl allyl sulfides or vinyl allyl sulfides are

known as the thio-Claisen rearrangements. For example, 134 gives 135 [95].
S S
Δ S Ph Ref. 95
Ph
Me Me 135
Ph 134 Me
TS
Thio-Claisen rearrangements (TCRs) of vinyl allyl sulfides 136 are useful in the
synthesis of unsaturated aldehydes [96].
Ph H
S DME-H2O (3:1) [H2O] O
1) n-BuLi S Ph δ Ph
S
2) PhCH2Br CaCO3, reflux, 12h γ
136 -78οC TS 62%
TCR provides an efficient synthetic route of several sulfur heterocycles. The fol-
lowing examples are illustrative [97–100]:
5 5
4 4
6 H
3 Δ 6
3 Ref. 97
1. 7 7
1 2 8
8a S 2 Me
S S S
O O O H+ O 1
2-(Allylthio)-tropone 2,3-dihydro-(8H)-cyclohept
[b]-thiophene-2-methyl-8-one
Me CO2Et
Me
2. Me-C CH-CO2Et CH2=CH-CH2Br CH-CO2Et CH C CO2Et
Me-C quinolin
SH S S
S reflux, 6h,
N2 atm
EtO2C H
Me Me Ref. 98
S
Me Me S Me
Me S S
S quinoline Me Me Me
3. Me
H Ref. 99
O O O
O
O O
O O
Me SH Me S
Me 3 5 S Me S N N
N N,N-DEA N H Me
4.
Δ [3, 3] O N O N
O 1N 6 O N
H Me Me
Me Me
1,3-Dimethyl-5-
(prop-2-ynyl) thiouracil Ref. 100
4.4.5.2 The Aza-Claisen Rearrangements
[3,3]-Sigmatropic rearrangements of N-allyl-N-vinylamines or N-allyl-

N-arylamines are known as the aza-Claisen or 3-aza Cope rearrangements. The
rearrangements proceed through both chair and boat-like transition states, the chair
TS being the preferred one. For example, 137 gives 138 [101].
i-Bu Me
i-Bu Ph i-Bu Me
N PhMe, reflux N N Ref. 101
Ph Ph
Me ο
110 C [3, 3] n-Bu
138 n-Bu
n-Bu
major product
137 yield 54%
E:Z (95:5)
The presence of a methyl substituent at C-4 of N-allyl-N-vinyl/aryl amines 139 and

140 improves the yield of the reaction by assuming the chair TS in preference [101,
102]. The presence of an electron-releasing substituent in the p-position of aromatic
ring in amine 140a increases the yield [102].
2 i-Bu Me
i-Bu 3 1 Ph N
N PhMe, reflux Ph
Ref. 101
4 Me 110οC Me n-Bu
Me n-Bu 92%
139
Me
Me NHM e M e
N
Me EtOH−HCl Ref. 102
reflux, 12 h Me
140 90%
Me
Me NHMe Me
N
Me
EtOH−HCl Ref. 102
reflux, 12h Me
140a Me
Me
95%
The aza-Claisen rearrangement of chiral amine 141 gives diastereoselective product

in moderate yield [103].
R1 R'
R2
n-BuLi, THF R2 decalin N
N N Ref. 103
185οC, 5h R'
141 O - 78οC O R2
O
de 52−78%
R1 = H, R2= Me
R1 = Me, R2= H
The aza-Claisen rearrangement also occurs in N-allylic indoles 142 [104].

Me
AlCl3 (1eq)
Ref. 104
N
PhH, 80οC, 2 h N
H
142 43%
(work up with HCl)
Me
This aza-Claisen rearrangement is useful in the synthesis of medium-sized ring

compounds. For example, 143 gives 144 in high yield [105].
O OMe
O
CDCl3, p-TsOH [3, 3]
N Me + N O
N C OMe
rt, 41h
143 Me Me OMe
CO2Me CO2Me CO2Me
Ref. 105
N
CO2Me H
Me N O
CO2Me
71% 144 Me OMe
CO2Me
4.5 [2,3]-Sigmatropic Rearrangements
4.5.1 Overview of Different Types of [2,3]-Sigmatropic

Rearrangements
Concerted [2,3]-sigmatropic rearrangements of allyl and benzyl ammonium ylides,

allyl sulfonium ylides, allyl sulfoxides, selenoxides, amine oxides and anions of
allyl ethers have been reported. These are illustrated by the following general
examples:
a. Ammonium and sulfonium ylides
R
R N R'
S R
S R C H2 N
C H2 R'
b. Allyl sulfoxides, selenoxides, and amine oxides
R
R Se
S
O SeR
O S R O
O
NR 2
O NR 2
O
c. Anions of allyl ethers
O
C HR O
CH
R
All of these rearrangements occur through a five-membered ring TS following

Huckel-type topology of 6e process with an endo-orientation of the substituent of
the migrating fragment. For example, in the rearrangement of an allyl sulfoxide, the
preferred endo-TS 145 has the substituent on sulfur pointed toward allylic fragment.
For a branched allyl sulfoxide, the bulkier group prefers a pseudoequatorial position
in the TS 146 to give E-isomer as major product [106, 107].
R
O O
S S
R
preferred endo TS less preferred
145 exo TS
For example,
R4
R3 R3
S
O H S O O
R S R4 R1
4 X
X R1 X
R1 R2 H
R2 R3 2
R 146 H
R1>R2
endo TS

Ammonium Ylides
Allylic ammonium ylides generated in the presence of a strong base undergoes

[2,3]-sigmatropic rearrangement. The following reactions are the illustrative
examples [108–115]:
Me Me
N Me Me
1. K2CO3, DBU N Me [2,3] N Me
R ο
CO2Me DMF, 10 C R CO2Me R CO2Me
Br t
2. BuOK [2, 3] CO2Me
N CO2Me CO2Me
N N
THF Ref. 108
Ph Ph
Ph
93%, >90% ee
3. CsF [2, 3]
OTf Ref. 109
N N
N
Me3Si Ph Ph
Ph
54%
4. NaOH [2, 3] Zn/AcOH

N Br H H Ref. 110
Δ N
COPh
COPh N COPh
COPh 88% ee
CR2 CR2
Br
n-BuLi, THF [2, 3] N CR2
5.
N ο N Ref. 111
- 80 C, 30 min
CR2 CR2 R
SnBu3 Li R
R=Me, 71%, 94% ee
Ph Ph
R O
O R
Br NaOH R R PhH
6. Ref. 112,113
O N reflux N Ph
N N
O
Ph endo TS exo TS only product
R= H, Et, tBu
H
7. H DBU
N N Ref. 114
N 20οC Ph
Ph CO2Et Ph CO2Et
EtO2C
90%
Ph
Ph Me
N CO2Me MeI N CO2Me [2, 3] Me
N Ph
8. Ref. 115
DMF, K2CO3, DBU, CO2Me
ο
Me 40 C Me Me
63%
4.5.3 [2,3]-Sigmatropic Rearrangements of Benzyl

Ammonium Ylides: The Sommelet–Hauser
Rearrangement
[2,3]-Sigmatropic rearrangements of benzyl quaternary ammonium salts are known

as the Sommelet–Hauser rearrangements. The treatment of the benzyl quaternary
ammonium salt with sodium amide or other alkali metal amide generates benzyl
ammonium ylide. The following examples are illustrative [108, 116–119]:
H
CH3 I NaNH2
1. CH3 CH2 N CH3
N CH3
(- NaI, NH3) N CH3 N CH3 CH3
CH3
CH3 CH3
N CH3 Refs. 116,117

CH3
CH3
N,N-dimethyl-2-methyl benzyl amine
I
t
BuOK CO2 tBu
N CO2tBu
2. ο N Ref. 108
THF, -40 C, 6 h CO2tBu
t
CO2 Bu
96%, >99% ee
Br NMe2
t
NMe2 BuOK
3.
THF, r.t., 30 min. Ref. 118
99%, 100% ee
2' 1'
3'
CH3 [2,3]
N NMe product
1 CH2
2
[2, 3]
NaNH2/NH3 N
4. N Br N
- 33οC [2, 3] N
H Ref. 119
N N
N Br N
H
OH [2, 3]
5.
4.5.4 [2,3]-Sigmatropic Rearrangement of Allyl Sulfonium

Ylides
Allylic sulfonium ylides undergo [2,3]-sigmatropic shifts. The presence of car-

banion stabilizing substituent accelerates the reaction-rate by stabilization of the
ylide. These reactions are utilized in the synthesis of medium-sized ring com-
pounds. The following examples are illustrative [21, 120–123]:
[2,3]
K tBuO Ref. 120
1. S
- 40οC S
S
S CH2
85%
Me
[2,3]
2. n-BuLi, THF Ref. 120
S ο S
Ph - 78 C Ph S Ph
Ph Ph Ph
Me Me Me Me
Cl
K2CO3 S [2, 3] S
S S Ref. 21
3.
O O
O O 93%
Br
4. CsF
Ph S CO2Et Ph S CO2Et + Ph S CO2Et
MeCN
SiMe3
[2, 3]
[2, 3]
Ref. 121
CO2Me
S CO2Et Ph
Ph SMe
81% 9%
Me Me
Me
S
Ph S [2, 3] PhS
5. Me2Zn Ph
Ref. 122
CH2I2
Me Me
Me
78%
Cl
1. Me S CH CO Et, - οC [2, 3]
2 2 78
6. N N N
N 2. Et3N H
S Me S Me
H MeS OEt
EtO2C EtO2C O
N Ref. 123
MeS O
53%
H
N E t3N N
S S
7. [2, 3]
Ph
Ph
4.5.5 [2,3]-Sigmatropic Rearrangements of Allyl Sulfoxides:

The Mislow–Evans Rearrangements
[2,3]-Sigmatropic rearrangements of allyl sulfoxides to allyl sulfenates are known

as the Mislow–Evans rearrangements. Allylic sulfoxide with a bulky substituent at
the allylic position was converted to the corresponding E-allylic alcohols with high
stereoselectivity. Some illustrative examples are [124–127] as follows:
Ph O SPh OH
S
[2, 3] PhS
O
1. Ref.124
CMe3 CMe3
CMe3
95%
S-Ph
HO
70οC [2, 3] O
2. + Na2S, H2O
N N N N
Ph O S
Ph O
Ref. 125
3. mCPBA/CH2Cl2 [2, 3] Ph S [2, 3] OSPh

O
SPh S
O
Ph
OH
Et2NH/MeOH Ref. 126
59%, E,Z, > 99:1
O O
O [2, 3]
1) LDA, THF, - 60οC S S Me OSPh
4. S Ph Ph
Ph 2) MeI
Me Me
P(OMe)3 Ref. 127
Me OH
MeOH, 25οC
74%

Selenoxides
Several thermal [2,3]-sigmatropic rearrangements of allyl selenoxides have been

reported. The following examples are illustrative [128–130]:
[2, 3] Ph
Ph [O] Ph
1. Ph
OH
SePh Se O
Ph O PhSe Ref. 128
OSiMe2tBu OSiMe2tBu OSiMe2tBu

Me3SiO Me3SiO Me3SiO
O O O
SePh Se Ph [2, 3]
H2O2, Py O Ref. 129
2.
O O O
H H H
OH
70%
MeO MeO MeO
H2O2 [2, 3]
O O
3. O
NMe NMe NMe
Ph Se Ph Se OSePh
MeO O
Ref. 130
aq. KOH
O
NMe
OH
60%
4.5.7 [2,3]-Sigmatropic Rearrangements of Anions of Allyl

Ethers: The Wittig and Aza-Wittig Rearrangements
[2,3]-Sigmatropic rearrangements of the anions of allyl and benzyl ethers are

known as the Wittig rearrangements. The anions are generated by treatment of allyl
or benzyl ethers with a strong base. The following reactions are illustrative
examples [21, 131–135]:
n-BuLi [2, 3]
1.
O Pri
O OH
Pri Ref. 21
Pri
Ph O Li OH
n-BuLi Ph
2. [2, 3] H3O
O Ph Ph
THF, -78οC O
Ref.21
Et Et Me3Si
- O
MeLi OLi 1. silyl shift Si
3. O O Et
Et
[2, 3] - 2. H3O+ Ref. 131
SiMe3 SiMe3
Me Me [2, 3] OLi
n-BuLi -
4. Me Si O Me3Si O
3 Me3Si
Me
H3O+
OH Ref. 132
Me3Si
72% syn: anti = 97:3 Me
5. Me Me Me O
Bu KH, Bu3SnCH2I n-BuLi Bu
[2, 3]
Bu Bu
Me
OH O O
SnBu3
Li Ref. 133
OH H
H3O Bu
Bu O
Me H O Bu
E-isomer
.96%, 96% Z-isomer Z-isomer
Me Me
favoured TS dis favoured TS
R R
R R
6. O O O R' HO R'
R' R'
MeLi, -60οC [2, 3] ROH
Ref. 134
OTBS
7. OTBS OTBS OTBS

O
n-BuLi, THF [2, 3] H3O
TBSO ο O
-78 C, 1 h TMS TMS
TBSO TBSO H TBSO H
O HO
TMS
90% only product
TMS Ref.135
The [2,3]-sigmatropic rearrangements of the anions of vinyl aziridines are

known as the aza-Wittig rearrangements. For example, 147 gives 148 [136].
CO2t-Bu - CO2t-Bu H
t-
[2,3] Bu N CO2t-Bu
N 1. LDA N
Ref. 136
t- 2. H2O t-
Bu Bu 147
148
92%
4.5.8 [2,3]-Sigmatropic Rearrangements of Allyl Amine

Oxides: The Meisenheimer Rearrangement
[2,3]-Sigmatropic rearrangement of allyl amine oxides is known as the

Meisenheimer rearrangement. This rearrangement provides O-allyl hydroxylamine
derivatives. Some examples are [137, 138] as follows:
Ph Me
-20οC Ph Me
1.
O NMe2 24 d
O
[2,3]
NMe2
H
N Cl N O [2,3] N
K2CO3−Me2CO N mCPBA O
2. +
0−5οC, 8 h
83%
Ref. 137
O Me
Me
N [2,3] N
O
3.
Me O Ref. 138
O H H
4.6 [3,5]-Sigmatropic Rearrangement
3,5-Sigmatropic shifts are not pericyclic because suprafacial overlapping of cyclic

p-orbitals is not involved. In these shifts, the orbitals from the breaking bond and
the lone pair overlap with the p-orbitals in the TS. These reactions have planar
nonaromatic eight-centered cyclic transition states with orthogonal orbital
4.6 [3,5]-Sigmatropic Rearrangement 149
overlapping, where pz orbital of the nucleophilic site acts as HOMO and p-orbital of
cyclohexadienyl group acts as LUMO. Either chair or flattened boat TS is involved
with an antarafacial geometry. For example, acyloxycyclohexadienone 149 gives
the major product 150 from [3,5]-rearrangement and minor product 151 from [3,3]-
rearrangement [139].
H O O OH
4 5
O O
Δ
3 O OH O
1 [3, 5] O
2
O1 O
149 2 O 150
3
O OH
Δ O O O
O
[3, 3] H 151
[4,5]-Sigmatropic shift is thermally allowed process as it involves 10 e (4n+2)

process and TS is Huckel-type and aromatic. The ammonium salt 152a on treatment
with a base gives ammonium ylide 152, which on [4,5] -shift affords 153.
Ph NMe2
Ph NMe2 NaOMe Ph NMe2 [4, 5]
r.t., 12 h
153
152a 152
60% Ref 21
[5,5]-Sigmatropic shift is thermally allowed suprafacial process of 8πe and 2σe. The
thermal rearrangements of aryl ethers 154 to 155, 1,2-(1,3-butadienyl) cyclohexyl
enolates 156 to 157 and hydrazobenzene 158 to p-benzidine 159 are illustrative
examples [140–142]. N,N′-Diarylhydrazide 160 with substituents at the ortho-

position undergoes highly regioselective [5,5]-shift to give 161 in high yield [143].
O OH
O
[5,5] Ref. 140
185οC, 3.5 h
H
154
50% 155
OK OK O
[5,5] Ref. 141
r.t., 3.5 h
157
156
NH2
NH2
H2N NH2
HN NH
H2SO4 [5,5]
Ref. 142
H H
158
159
hydrazobenzene 70% NH2
NH2 p-benzidine
Me
Me NH2
OMe BOC HCl, EtOH
N o
80 C, 2h MeO Me Ref. 143
N
H Me
H2N 161
160
91%
[9,9]-Sigmatropic shift is thermally allowed process and it involves a Huckel-type

TS of 18e (16π and 2σe). For example, bis[4-(2-furyl)-phenyl] diazane 162 gives
hydrochloride salt of 5,5′-bis(4-amino phenyl)-2,2′-bifuryl 163 in high yield in
acidic solution [144].
4.9 [9,9]-Sigmatropic Rearrangement 151
HN NH H2N NH2 NH2
95% EtOH [9,9]

ο
0.1N HCl, 0 C
O O O O O
H
O
162 NH2 H NH2
-2H 5
O
2' O 5'
2
NH2
163 75%
4.10 Problems
4:10:1. Indicate which of the following reactions are orbital symmetry allowed
processes and which are forbidden. Also indicate the order of sigmatropic
shift for each reaction. Each reaction may take place in one or more steps.
Justify your answer.
CH3 H
Δ H
(a) H
D
H3C D CH CH3
3
H3C H3C
H Δ CH3
(b) H
H3C
O O
S 25οC
(c) R S
Ph Ph
R
190οC
(d) O
CHO
H Me
H H
Me
(e) Δ H
H H
Me H H
Me
4:10:2. Suggest a mechanism for each of the following transformations. More than
one step may be involved in each case. Predict the major and minor
products when more than one product is expected.
(a) Δ
O
OH H H
(b) KHMDS
18 crown 6
H
THF, 20οC
(Ref: Tetrahedron, 1988, 44, 3139)
CH3
(c) Δ
CH3
CH2
H
Et3N N
N
(d) S S
Ph
Ph
O OH CH3
(e) 0.01M HCl

CH3 25οC
H3C
CH3 H
Cl
(f) + HCl CH3
N
CH3 CH3
H
H3C
Sealed tube
(g) CO2Et CO2Et
150οC, PhMe O
O 5h
(Ref. Tetrahedron Lett., 1981, 28, 1367)
(h) Δ
O
(i) Δ
(j)
O Δ
CH2CHO
4.10 Problems 153
O H OH H CD3H
(k) Me Δ Me
H
CD3
H H
Δ
(l)
H
Me Me
Δ
(m) Me
4:10:3. Predict the expected product with structure and stereochemistry for each of
the following reactions:
OH
(a) 1. OEt Hg2+
2. Δ
Me
O t
KO Bu
(b) Ph
S Me
O
OMe
1. MoO5−Py−HMPA/CH2Cl2
(c)
SPh 2. Et2NH/MeOH
(d) Δ
H OH
H Me Δ
(e) O CMe2
D D
O
ο
BF3, -40 C
(f)
O Ph
Δ
(g) Me O KH / THF
O Δ
(h)
OH
KH/THF
(i)
Δ
i) LDA, TMSCl
(j) O ii) 150οC
iii) H3O
Ph Ph
N TiCl4
(k) Me
Me PhH, 50οC, 24 h
+
CH2S(Me)2
Cl
(l) BF4- NaOMe (excess)
MeOH, Δ
Cl
CH3
O
(m) 185 oC
H3C Ph
(n) Δ
H3C CH3
Ph
(o) Ar
N O PdCl2 L / AgBF4
n-
Pr 15h, CH2Cl2, reflux
Ar = 4-CF3C6 H4
L= O -
n-
Pr = n-Propyl
N
PPh2 Ph
( p) o
180 C
O
(q) BF3-OEt2
N n-
BuLi
Me
N
(r) Cu(acac)2
PhH
O
N2
n-
(s) BuLi, THF
O
Me3Si 5 oC
(t) O Δ
O
4.10 Problems 155
4:10:4. Suggest the mechanism for each of the following reactions, which may
occur in two or more pericyclic steps not restricted to sigmatropic shifts
(a) Δ
H + H2C CH2
H
300οC
(b)
O
Δ
(c)
O
HH
160 oC
(d)
CHO
CHO + 1) Dehydration
(e)
OH
2) Δ
Me
D 225οC Me
(f)
D Me
D HD
Me
375οC
(g)
Ph
Ph
Δ *
(h) Me*
Me
H
O O
(i)
Δ
(j) H
ο
300 C
Δ H
(k) H
4.11 Further Reading
1. Hill RK (1991). In: Paquette LA (ed) comprehensive organic synthesis, vol 5.

Pergamon, New York, p 827
2. Bruckner R (1991) [2,3]-Sigmatropic rearrangements. In: Winterfeldt W
(ed) Comprehensive organic synthesis, vol 6. Pergamon, New York
3. Castro AMM (2004) Claisen rearrangement over the past nine decades. Chem
Rev 104:2939
4. Nakai T, Mikami K (1986) [2,3]-Sigmatropic rearrangement of allyl ethers.
Chem Rev 86:885
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Chapter 5
Group Transfer Reactions
5.1 Introduction
The transfer of one or more groups from one molecule to another in a concerted
process is known as group transfer reaction. In most of the cases hydrogen is
transferred. Only a few reactions of this class are known. Among them, the most
common are ene reactions and diimide reduction.
5.2 The Ene Reactions
5.2.1 Overview of the Ene Reactions
The process of transfer of one hydrogen atom from an allylic alkene (an ene) to an
electrophilic alkene (an enophile) followed by formation of a new r-bond and
migration of allylic double bond is known as ene reaction or Alder ene reaction [1].
The ene may be an alkene, alkyne, allene, cyclopropane ring or arene, whereas an
enophile may be an alkene, carbonyl, thiocarbonyl, imino or diazo compound
having an electron withdrawing substituent. Oxygen may also serve as an enophile.
The reaction usually takes place from left to right, since a new r bond is formed at
the expense of the p bond of the enophile as depicted below.
H H H EWG H EWG
+ +
; ;
enophile: C=C, C=O, C=N
These reactions resemble Diels–Alder reactions, with one of the p bonds of the
diene is replaced by a r bond in the allylic alkene. These reactions are also con-
sidered as intermolecular 1,5-hydrogen shifts. The Lewis acid catalyzed ene

162 5 Group Transfer Reactions
reactions occur at lower temperatures with high stereoselectivity compared to

uncatalyzed ene reactions. The Lewis acid is attached to carbonyl function of the
electron withdrawing group of the enophile and increases the electrophilic character
of the enophile. The following examples are illustrative:
AlCl3 O
O
H ο H
OMe 25 C, 48 h OMe
1. Ref. 1
AlCl3
70%
CO2Me
2. H3C H AlCl3 H3C CO2Me
+ Ref. 1
ο
25 C
61%
3. O Ac2O
+ BF3
OAc Ref. 2
H H H AC2O, CH2Cl2 OH
84%
In entry 3, allylic function in the open chain takes part in ene reaction because
reaction with endocyclic double bond will produce less stable TS.
When an unsaturated ketone in enol form undergoes ene reaction, the reaction is
known as the Conia ene reaction [3]. The following examples [3–5] are illustrative.
1. O Δ
O O
Ref. 3
7-octen-2-one H
O H O
400 οC O
2. Ref. 4
20 h
55% camphor
dihydro carvone
H Me O
O O
350 οC Ref. 5
3.
Ph O Ph O H Ph O
ο
250 C
4. Ref. 3
90%
5.2 The Ene Reactions 163
Fig. 5.1 Orbital interactions

of ene and enophile in the TS HOMO
of an ene reaction
EWG
LUMO
TS
5.2.2 Stereochemistry and Regioselectivity
Most of the ene reactions are concerted and orbital symmetry allowed processes
involving all suprafacial transition states of 6e (4p and 2r) with endo orientations
of the electron withdrawing group as depicted in (Fig. 5.1). The addition of the ene
to the enophile is stereospecific syn. The TS requires higher activation energy
compared to that of Diels–Alder reaction because two r-electrons of the allylic
r-bond are involved instead of a p-bond of a diene.
Regioselectivity of the ene reaction of an unsymmetrical enophile is governed by
the orbital coefficient of carbons in allylic alkene and enophile. For example, in the
reaction of propylene with methyl acrylate, the major and minor products are
obtained as per orbital interactions of ene and enophile [4]:
H CO2Me
H H CO2Me
+ +
CO2Me
major minor
highest highest
HOMO LUMO 7:1
While in the ene reaction of 1-octene with methyl acrylate, steric interaction in
the TS reduces the yield of major product.
C5H11 H CO2Me CO2Me C5H11

C5H11
+
+
CO2Me
major minor
MeO2C 3:1
C5H11 H Ref. 4
TS
The presence of a germinal methyl group induces the methyl group at C-1 to
deliver the hydrogen atom rather than from the vicinal methyl group at C-4.
H MeO2C MeO2C H
1 Cl
EtAlCl2
2 + * Cl
H3C Ref. 4
3 rt H3C *
CH3
4
highest CH3
highest LUMO 40%
HOMO
In some cases, the Woodward–Hoffmann rules are not applicable to ascertain the
major product of ene reaction due to ring or steric strains. As for example, the trans-
octa-1,6-diene 1 gives mainly the cis disubstituted cyclopentane (14:1, cis:trans) on
the basis of favourable folded endo TS of lower energy, while cis-octa-1,6-diene 2
gives the major product through exo TS because endo TS is highly strained [6].
The endo TS of 1 is of lower activation energy as the hydrogens are on the same
side of the folded bicyclic transition structure.
H
H
ο
457 C
56 s H H
H
35 % major product
1 endo TS
H H
420 οC
H H
H
2 favoured exo TS major product
Similar folded TS is also observed in the gas phase intramolecular ene reaction
of the carbonyl compound 3.
140 οC, 4 h
O Ref. 4
H OH
3 O 60%
5.2.3 Applications of Intermolecular-, Intramolecular-

and Enantioselective-Ene Reactions
Both intermolecular and intramolecular ene reactions have been utilized in inter-
mediate steps for the synthesis of several bioactive organic compounds. Use of
Lewis acid catalysts with chiral ligands provides diastereoselective products in high
enantiomeric excess. Some of the ene reactions are illustrated.
(a) Intermolecular ene reactions
O
O
xylene O
1. + O Ref. 7
ο
135 C O
O
31%
CH2−H Ts SnCl4
N HN Ts
2. Ref. 8
CH2Cl2
CO2tBu
CO2tBu
H O 150 οC OH
3. + Ref. 9
MeO2C CO2Me CO2Me
CO2Me
CO2Me
CO2Me
Δ CO2Me
4. + CO2Me D.A. CO2Me
ene Ref. 10
H CO2Me CO2Me CO2Me
O O
Me Δ Ph
5. Ph + O O Ref. 11
H Me
H
O O
Me
O O CH3
O
6. H O O
+ O Ref. 12
H
O O
O
endo TS major product (85:15)
Me O
O CH3
O
O
H O
O
disfavoured exo TS
minor product
H SnCl4
O TBSO
+
7. -78 οC MeO2C Ref. 12
TBSO H CO2Me OH
H SnCl4 71%, > 99% anti

MeO2C H
O
TBSO
preferred TS
CN
S
S SMe
+ Ref. 13
8.
MeS CN
H CO2Et H CO2Et
N 80 οC N Ref.14
9. +
N
N CO2Et
EtO2C
MeS Bn MeS
H N AlCl3
10. + Ref. 15
NH-Bn
Ph H H
Bn = PhCH2 Ph
H OH
11.
O 18 oC S O
Me Me + S O Me Ref. 16
72 h
(b) Intramolecular ene reactions
MeO2C
CO2Me
280 οC
12. Ref. 17
LUMO HOMO 68%
H
mixture of stereoisomers
O O
H
Δ
13.
O
O H Me
14. H modhephene Ref. 18

(sesquiterpene)
H O H OH
H Me O H CO2Me
decane Me
15. OMe Me
180 οC
OTBS O TBSO
OMe OTBS
O
62%
TBS = tert-butyldimethyl silyl Prefered TS, 1,3-trans
annular interactions
minimized
O OH
OMe CO2Me
Me Ref. 19
H Me
TBSO O
OTBS
disfavoured TS
12%
(c) Enantioselective ene reactions
OH
Ph CH2−H catalyst A (1%) OEt
O Ph Ref. 20
16.
+ OEt CH2Cl2 O
H
97%, 93% S, ee
O
2+
-
O O 2SbF6
N N
Re face
Me3C CMe3 attack
Cu
O O
Preferred TS
with Catalyst A H OEt
Si face
attack
OH OMe
O H−H2C OMe
Catalyst B
H + Ref. 21
17. BaO, 4 οC
CMe3 82%, 88% ee
CMe3
N O
Cr
Cl
Catalyst B, Schiff base, Cr (III) complex
O
H (iPr)2CH-CH2NH2 OH
OEt
18. + H OEt
O Catalyst C, CH2Cl-CH2Cl
60 οC Ref. 22
O
97%, 86% ee
P NC-CH3
Pd P = BINAP, bis-(2,2'-diphenyl-phosphinyl)-1,1'-binaphthalene
P NC-CH3
P
Catalyst C
OH
O (i-Pro)2TiX2/(R)-BINOL
19. + CO2Me Ref. 22
H CO2Me MS 4A, CH2Cl2
(R)
H
X X = Cl (10 mol%) 72%, 95% ee (8h)
X O (1 mol%) 78%, 93% ee (8h)
Ti X = Br (10 mol %) 87%, 94% ee (3h)
CO2Me
H
O O
preferred TS with catalyst BINOL
5.3 The Metallo-Ene Reactions
The transfer of metal atoms such as lithium, magnesium, silicon, or palladium from
an allylic organometallic compound to strong electrophilic olefin is known as the
metallo-ene reaction [23].
MRn RnM
+
For example [24],

Me3Si
SiMe3 O AlCl3
O
+ CF3 Ref. 24
F3C CF3
CF3
51%
These reactions proceed through a concerted process of six-atom TS or an ionic

process, where the intermediate is stabilized in polar solvent or by the catalyst. The
intramolecular metallo-ene reactions are useful for synthesis of carbocyclic com-
pounds. Some of the examples [25–27] of these reactions are illustrated:
Et2O
1. H H Ref. 25
reflux
MgBr
67%
Boc Boc Boc Boc

N N N
N
Pd(PPh3)4 (S)
2. H Ref. 26
OAc AcOH, 70 oC
Pd
Pd
Boc = tert-butyloxycarbonyl 64%, > 99% ee
O O
MeO2C O MeO2C O
OH
3. OMe (PPh3)AuOTf (1 mol%) H H
Au H
CH2Cl2, rt, 15 min
MeO2C Au Ref. 27
95%
5.4 The Retro-Ene Reactions
The reversal of ene reactions are known as the retro-ene reactions. These reactions
are favored at higher temperatures.
H H H
+
α γ
β
The principal synthetic value of these reactions is for synthesis of allenes, dienes,
and other compounds, which would be difficult to obtain under normal conditions.
Entries 1–4, illustrate the use of the retro-ene reaction to produce different classes of
organic compounds [28–31].
O
1. H Δ O H 180 oC
+ Ref. 28 2. H OH O Ref. 29
CHO
OH
280 oC > 320 oC

3. 4.
H Ref. 30 Ref. 31
H
OHC
OH 100%
5.5 Diimide and Related Reductions 171
5.5 Diimide and Related Reductions
Diimide reduction of alkenes and alkynes are also group transfer reactions. Delivery
of two hydrogen atoms to an alkyne or alkene takes place in a concerted process
involving suprafacial delivery of two hydrogens in a TS. These reactions are per-
icyclic in nature.
π2
σ4
T.S.
Diimide is unstable species and is generated in situ by oxidation of hydrazine

and its derivatives. Common methods for generation of diimide are [32]:
diglyme
SO2−NH−NH2 boiling
1. H3C NH=NH
[O]
p-Toluene sulphonyl hydrazide
[O], CuSO4
2. NH2−NH2 ο
NH=NH
EtOH, 50−60 C
90−100 οC
HN NH=NH
3. NH
Anthracene 9,10-diimide
HOAc
4. KO2C−N=N−CO2K NH=NH
CH2Cl2
Pot. azido dicarboxylate
Diimide reduction is a metal-free catalytic hydrogenation process and does not

cleave the sensitive O–O and N–O bonds in the substrate. Diimide reduces sym-
metrical double bonds, e.g., C=C, N=N, O=O, etc.
Some of the important reactions of diimide are illustrated:
Ph H Ph
diglyme
1. + Me SO2NHNH2
Δ H Ph
Ph
cis -stilbene
diphenyl acetylene
N2H4, CuCl2 Me
2. +
Ref. 32
Me O2, EtOH, 55 οC Me
Me
Me Me
Me Me
92 : 8
O KO2C-N=N-CO2K O
3. O Ref. 33
HOAc, CH2Cl2 O
(−)
OH OH
4. N2H4, H2O2
HOAc, 4h, r.t
Me Me
74%
Ph Ph Me
5. TsNHNH2
Me base, MeOH Ref. 34
Me
28%
C5H11 I
KO2C-N=N-CO2K C5H11
6. I Ref. 35
MeOH, C5H5N
THPO
HOAc, rt THPO
81%
7. N2H4 (10 eqv.), Cat. D (5 mol%)

Ref. 36
Selectively reduces less substituted double bond 99%

R H
N N O
NH
N
Et
O
Catalyst D
D
KO2C-N=N-CO2K
8. D
CO2H MeCO2D, DMSO CO2H Ref. 37
SOPh SOPh
75%
Among other group transfer reactions, the reduction of 1,2-dimethylcyclohexene

with 9,10-dihydronaphthalene is used frequently [4].
H
H Me 150οC, 48 h Me
+ +
H Me Me
H
5.6 Thermal Elimination Reactions of Xanthates … 173
5.6 Thermal Elimination Reactions of Xanthates,

N-Oxides, Sulfoxides and Selenoxides
Thermal b-elimination reactions of acetates, benzoates, xanthates, sulfoxides, se-

lenoxides, and N-oxides are also group transfer reactions. All these elimination
reactions are syn-stereospecific and proceed through a cyclic six membered—or five
membered—ring transition state of 6e process by intramolecular transfer of
hydrogen atom, where all the participating orbitals have suprafacial interactions.
These reactions are fundamentally retro-group transfer reactions.
Pyrolysis of xanthate esters known as Chugaev reaction requires temperatures of
150–250 °C. For example, the pyrolysis of xanthates 4 and 5 gives more substituted
olefins (Saytzeff products) as major products.
MeS MeS
Me Me
S 180οC S
O H O H +
Ph Ref. 4
Me Ph
H Me H Me 45% 32%
Me Ph Me Ph
favoured
4
H
S
H
H
MeS O
Me
Ph
less favoured
H
S 100 - 180 oC
H +
O SMe S
O
5 SMe
(+)-neomenthyl xanthate ester 66% 34%
3-menthene 2-menthene
Pyrolysis of N-oxides known as Cope elimination takes place at lower tem-

peratures (100–150 °C). The pyrolysis of sulphoxides and selenoxides takes place
easily below 100 °C because of weaker C–S and C–Se bonds. As for example,
erythro-N-oxide 6 and erythro-sulfoxide 7 on pyrolysis give cis and trans olefins as
major product, respectively.
O O
Me2N H 120 οC Me2N H
Ref. 4
H H Ph
Ph Ph 2-phenyl-cis-2-butene
6 TS
erythro 93%
O O Ph
Ph-S H 80 οC Ph-S H Ref. 4
Ph
Ph Ph
H Ph H Ph trans-methylstilbene
7 TS
erythro ~90%
Similarly, syn-elimination of PhSeOH from selenoxide 8 and selenoxides derived

from selenides 9–11 in presence of H2O2 occurs below rt to produce enones in high
yields.
O
Ph
Se CO2Me O CO2Me
O
ο
- 20 C
Ref. 38
O O
H OMe
OMe
8
57%
O O O
1. Ph2CuLi SePh H2O2 Ref. 39
2. PhSeBr
10οC
9 Ph Ph
72%
Me SePh H
H
H2O2
O O Ref. 39
0οC O
O
H H
10
Me SePh Me H
H
H2O2
O O O
0 οC + Ref. 39
O O O
H H H
11 90%
10%
5.7 Problems
5:7:1. Suggest a mechanism of the following reactions: More than one step may be
involved.
O Me2AlOTf OH OH
a. + +
H CO2Me CO2Me CO2Me
-78 oC
Me Me
91 % 9%
b. O2 hν
+
H H OO
ο
c. 260 C
O OMe
2.5 h
D D H T
T D
O O
d. Δ
5.7 Problems 175
5:7:2. Predict the expected product with structure and stereochemistry.
O
AlCl3, CHCl3
a. Me2AlCl b. H
o
O -78 C, 8 h
H N
Ts AlCl3, CHCl3
o
61 C, reflux, 16 h
H
CHO 2 .1 eq. FeCl3
c. d. EtAlCl2
N o +
20 C Cl CO2Me PhH, 25 oC
AlCl3 Δ
e. + f.
F3C Me
-78 oC
OH
O2, TPP Me
g. h.
hν
CO2Me AlCl3
i. j.
+ 200 oC
25 oC
Bu
O
H CO2Me
Me2AlCl
k. Me3Si l.
+ O
-78 oC
SiR3
Δ
m. n. 130 - 140 oC
O
O
R'
Si FVP
o.
FVP = Flash Vacuum Pyrolysis

5.8 Further Reading
1. Hoffmann HMR (1969) The ene reaction. Angew Chem Int Ed Engl 8:556.
2. Oppolzer W, Snieckus V (1978) Intramolecular ene reactions in organic syn-
thesis. Angew Chem Int Ed Engl 17:476
References
1. Alder K, Pascher F, Schmitz A, (1943) Chem Ber 76:27; Snider BB, Rodini DJ, Conn RSE,
Sealfon S, (1979) J Am Chem Soc 101:5283
2. Blomquist AT, Himics RJ (1968) J Org Chem 33:156
3. Conia MJ, LePerchec P (1975) Synthesis 1
4. Fleming I (2002) Pericyclic reactions. Oxford University Press, New York, pp 84–87
5. Leyendecker F, Drouin J, Conia JM (1974) Tetrahedron Lett 2931
6. Huntsman WD, Solomon VC, Eros D (1958) J Am Chem Soc 80:5455
7. Arnold RT, Showell JS (1957) J Am Chem Soc 79:419
8. Achmatowicz O, Pietraszkiewicz P (1981) J Chem Soc Perkin Trans 2680
9. Salomon MF, Pardo SN, Salomon RG (1984) J Org Chem 49:2446
10. Giguere RJ, Namen AM, Lopez BO, Arepally A, Ramos DE, Majetich G, Defauw J (1987)
11. Mislow K (1964) Introduction to stereochemistry. Benjamin, New Jersey (Chapter 3)
12. Mikami K, Shimizu M (1992) Chem Rev 92:1021
13. Bachrach SM, Jiang S (1997) J Org Chem 62:8319
14. Hoffman HMR (1969) Angew Chem Int Ed Engl 8:556
15. Hayashi Y, Shibata T, Narasaka K (1990) Chem Lett 1693
16. Lucchi O, Filipuzzi F, Lucchini V (1984) Tetrahedron Lett 25:1407
17. Oppolzer W, Mahalanabis KK, Ballig K (1977) Helv Chim Acta 60:2388
18. Oppolzer W (1981) Pure Appl Chem 53:1181
19. Schnabel C, Sterz K, Muller H (2011) J Org Chem 76:512
20. Evans DA, Tregay SW, Burgey CS, Paras NA, Vojkovsky T (2000) J Am Chem Soc
122:7936
21. Ruck RT, Jacobsen EN (2002) J Am Chem Soc 124:2882
22. Mikami K, Terada K, Takeshi N (1990) 112:3949; Corey EJ, Barnes-Seeman D, Lee TW,
Goodman SN (1997) Tetrahedron Lett 37:6513
23. Oppolzer W, Pitteloud R, Strauss F (1982) J Am Chem Soc 104:6476
24. Abel EW, Rowley RJ (1975) J Organomet Chem 84:199
25. Felkin (1972) Tetrahedron Lett 22:2285
26. Oppolzer W (1990) Pure Appl Chem 62:1941
27. Kennedy-Smith JJ, Staben ST, Toste FD (2004) J Am Chem Soc 126:4526
28. Viola A, Collins J, Filipp N (1981) Tetrahedron 37:3772; Hoft H, Kirsch R (1985)
29. Conia JM, Barnier JP (1971) Tetrahedron Lett 4981
30. Joulian D, Rouessac F(1972) J Chem Soc Chem Commun 314; Ohloff G (1970) Angew
Chem Int Ed Engl 9:743
31. Hoffmann HMR (1969) Angew Chem Int Ed Engl 8:556
32. Norman ROC, Coxon JM (1992) Principles of organic synthesis, 3rd edn. ELBS with
Chapman and Hall, Oxford; Corey EJ, Pasto J, Mock L (1961) J Am Chem Soc 83:2967
33. Adam W, Eggelte J (1977) J Org Chem 42:3987
34. Moro K, Ohki M, Sato A, Matsui M (1972) Tetrahedron 28:3739
References 177
35. Luethy C, Konstantin P, Untch JG (1978) J Am Chem Soc 100:6211

36. Smit C, Fraaije M, Minnard A (2008) J Org Chem 73:9482
37. Annunziata R, Fornasier R, Montanan F (1974) J Org Chem 39:3196
38. Callant P, Ongena R, Vandewalle M (1981) Tetrahedron 37:2085
39. Reich HJ, Wollowitz S (1993) Org Reactions 44:1
Part II
Photochemical Reactions
Chapter 6
Principles of Photochemical Reactions
6.1 Introduction
Photochemical reactions of organic compounds have attracted much interest in the

recent times for its fascinating nature and wide applications in the synthesis of
organic compounds. There are two key features of photochemical reactions which
give them special importance over thermal reactions. First, the reactions take place
in the excited state of the molecules having a large excess of energy compared to
ground state, it is often possible to effect reactions which are thermodynamically
unfavorable due to their ground-state reactants. Second, the reactions are usually
carried out at low temperatures so that the products can be formed in cold. Hence, it
is often possible to make highly strained ring systems by pumping out excess
energy as light to overcome the activation energy barrier in their formations.
The photochemical reactions are usually carried out by irradiation with ultra-
violet (λ 200–350 nm) and less frequently by visible lights. Therefore, these
reactions are essentially limited to those in which at least one of the reactants is
unsaturated or aromatic.
Another important criterion for these reactions is to carry out these reactions in
pure fused quartz vessels, which can transmit UV radiations, whereas common
borosilicate pyrex glass or sodium silicate glass vessels are unsuitable to use
because they can transmit radiations only longer than 300 nm and absorb below this
wavelength.
Now we have to understand the basic elements of photochemical reactions and
the basic processes that are involved in light–matter (organic molecules)
interactions.
Photochemical reactions of organic compounds are the chemical reactions that
result from interactions between organic molecules and ultraviolet or visible light.
In a photochemical reaction, the light of a particular wavelength is irradiated to
excite the molecules. According to the quantum theory, both matter and light are
quantised, and only certain specific energies of light are absorbed by a specific

182 6 Principles of Photochemical Reactions
organic molecule for its excitation. The absorption or emission of light occurs by
the transfer of energy as photons. These photons have both wave and particle-like
properties and the energy E of a photon is given by Planck’s law,
E ¼ hm
where h is Planck constant and is equal to 6.63 × 10−34 Js and υ is the frequency of
oscillation of the photon in units of s−1 or Hertz (Hz).
m ¼ c=k
where c is the velocity of light and λ is the wavelength of oscillation of photon.

Thus,
E ¼ hm ¼ hc=k ð1Þ
Therefore, the energy of a photon is proportional to its frequency and inversely

proportional to its wavelength. The energy of one mole of photons (6.02 × 1023
photons) is called an Einstein and is measured in units in kJ mol−1. It is equal to
Nhc/λ.
6.2 Light Sources Used in Photochemical Reactions
For ultraviolet region, mercury vapor lamps are suitable. These lamps mainly emit
at 214, 254, 313 and 366 nm. Low-pressure mercury lamps strongly emit spectral
line at 254 nm. For the visible region, a tungsten lamp or more powerful xenon arc
is suitable. The composition of the radiation reaching the sample can be controlled
by the filters in a spectrophotometer. For the reaction of aromatic compounds, the
radiation at 254 nm is desired. At this wavelength, the energy of a photon is equal
2.86 × 104/254 = 112.6 kcal mol−1 is irradiated for excitation of a molecule. This
energy is sufficient to rupture most of the single covalent bonds in an aromatic
compound. Sometimes solid-state lasers (light amplification by stimulator emission
of radiation) such as ruby laser and neodymium-doped yttrium aluminum garnet
(Nd-YAG) laser, and gas lasers such as helium–neon laser and argon ion laser are
used for generation of light of strong intensity.
6.3 Laws of Photochemistry
Two fundamental principles relating to light absorption by the organic molecules

are the basis for understanding of their photochemical transformations.
6.3 Laws of Photochemistry 183
1. The Grotthuss–Draper law: The law states that the only the fraction of light
which is absorbed by a chemical entity can bring about the photochemical
change.
2. The Stark–Einstein law: The law states that each molecule or atom absorbs
one photon or one quantum of light for its excitation or activation, i.e., for a
molecule, AB,
AB þ hm ! AB
groundstate excited state
This law is obeyed in the majority of cases but exception occurs when very
intense light sources such as lasers are used for irradiation of a sample.
6.4 The Beer–Lambert’s Law of Light Absorption
The extent of light absorbed by a substance depends on its molar absorption

coefficient (ɛ). The fraction of light absorbed (I/I0) by a substance is given by the
Beer–Lambert law. The law states that the ratio of the intensity of the emergent
light (I) and incident light (I0) has an exponential relationship with the concen-
tration (c) and path length (l) of the absorbing substance, i.e.,
I=I0 ¼ 10ecl
Taking logarithm to the base 10 gives
logðI=I0 Þ ¼ ecl
or logðI0 =I Þ ¼ ecl
The left-hand quantity is the absorbance, A, and hence
A ¼ ecl
where c is the concentration of the substance in moles per liter, mol L−1, and l is the
path length in cm.
The higher the ε value, higher will be the intensity of the absorption. Usually the
intensity of light absorption is high for allowed electronic transition process and
low for forbidden transition process.
6.5 Physical Basis of Light Absorption by Molecules: The

Franck–Condon Principle
Chromophores or chromophoric groups present in the molecules are responsible for

the absorption of light. The absorption of UV or visible light by a molecule results
in the promotion of an electron from its ground-state orbital to a higher excited state
orbital. Normally the amount of energy necessary to make this transition depends
mostly on the nature of the two orbitals involved and much less on the rest part of
the molecule. When this electronic transition occurs, the absorbing chromophore
undergoes an electronic dipole transition. This transition dipole moment lasts for
the duration of the transition only and the intensity of the resulting absorption of
light is proportional to the square of the transition dipole moment.
The total energy of a molecule is made up of its electronic energy and energy of
its nuclear (vibrational and rotational) motion. It is expressed as
Et ¼ Ee þ Ev þ Er
where the subscripts refer to the total energy, electronic energy, vibrational energy,
and rotational energy, respectively. The energy gap between electronic states is
much greater than that between vibrational states, which in turn is much greater
than that between rotational states. Absorption of light by molecules causes tran-
sition of electrons from one electronic state to another much more rapidly than that
of the nuclei because of their heavier mass compared to the mass of electrons. The
electronic transition takes place so rapidly than that of the nuclei of the vibrating
molecule and hence nuclei can be assumed to be fixed during this electronic
transition period. This is called the Franck–Condon principle [1]. It states that
absorption of light by a molecule causes an electronic transition within a stationary
nuclear framework of the molecule.
Thus, the electronic transition by absorption of a photon is often referred to as a
vertical transition or Franck–Condon transition. The electronic transition in a
Fig. 6.1 Schematic diagram v3

of the electronic ground state v2
and the first excited electronic v1 S1
state of a diatomic molecule. v0
The vertical arrows show
vibronic transitions due to
absorption of photons
+ hν
v3
v2
v1
S0
v0
6.5 Physical Basis of Light Absorption … 185
molecule by absorption of light results in changes in both electronic and vibrational

states of electron. Hence, such electronic transition is called vibronic transition of
electron. Figure 6.1 shows the potential energy curve known as Morse potential
energy curve of a diatomic molecule for its electronic ground state (S0) and the first
excited electronic state (S1).
6.6 Electronic Transitions and Their Nomenclature
In principle, six types of electronic transitions, designated as σ → σ*, σ → π*,

π → π*, π → σ*, n → σ*, and n → π* are possible (Fig. 6.2). The σ → σ*
transition corresponds to absorption in the inaccessible far-UV (100–200 nm) and
both σ → π* and π → σ* are obscured by much stronger π → π* absorptions,
and n → σ* transition occurs by vacuum UV light (below 200 nm). So in pho-
tochemical reactions, only π → π* and n → π* transitions occur, which produce
(π, π*) and (n, π*) electronically excited states, respectively.
Absorptions due to π → π* and n → π* transitions differ from one another in
several important aspects as shown in Table 6.1.
In a molecule, the singlet electronic states are denoted by S0, S1, S2, etc., of
increasing energy and triplet electronic states as T1, T2, etc. S0 indicates the ground
state of singlet electron, whereas S1, S2, etc., and T1, T2, etc., are excited singlet and
triplet states, respectively. The n → π* transition is the lowest energy transition for
most of saturated carbonyl compounds and this transition is known as S0 → S1 and
T1. These transitions occur from the promotion of an electron from n molecular
orbital to the π* molecular orbital and these transitions are referred as 1(n, π*) and
3
(n, π*) states, respectively. Similarly, S2 and T2 excited states arise from the
Fig. 6.2 Generalized

ordering of molecular orbital σ∗
energies of organic molecules
and electronic transitions that
occur by excitation with light
π∗
n
E
σ
Table 6.1 Comparison of light absorptions due to π → π* and n → π* electronic transitions

Absorptions due to π → π* transitions Absorptions due to n → π* transitions
1 Occurs at shorter wavelengths (220– Occurs at longer wavelengths (270–
260 nm) 350 nm)
2 Substitution moves the absorption to longer Substitution moves the absorption to
wavelength shorter wavelengths
3 Shows relatively strong absorption with Shows relatively weak absorption with
εmax of *103–105 l mol−1 cm−1 εmax of *1–102 l mol−1 cm−1
4 Absorption band occurs at longer Absorption band occurs at shorter
wavelength in a polar solvent than in non wavelength in a polar solvent than in a
polar solvent (shows bathochromic shift) non polar solvent (shows hypsochromic
shift)
Fig. 6.3 Electronic states of

molecular orbitals of an
organic compound
S2
T2
E π π∗
S1
T1
n π∗
S0
promotion of an electron from its π MO to its π* MO and are referred to as 1(π, π*)
and 3(π, π*) states, respectively.
These transitions are represented in Fig. 6.3.
6.7 Spin Multiplicity of Electronic States
The electronic state of a molecular orbital is specified by its spin multiplicity. Spin
multiplicity is determined from the equation,
Spin multiplicity ¼ 2S þ 1
where S is the sum of the spin quantum numbers of the electrons present in an
orbital.
6.7 Spin Multiplicity of Electronic States 187
A ground-state helium atom has a pair of electrons of opposite spin in the 1 s

orbital (1S2). Hence, total spin S=½−½=0 and spin
multiplicity = (2S + 1) = 1.
Thus, the ground-state singlet S0 has spin multiplicity of 1.
In the lower excited state (S1 and T1) of helium atom, there are two possible spin
configurations:
In the S1 state, total spin S = ½ − ½ = 0.
So, its spin multiplicity is 1. In the T1 state, total spin S = ½ + ½ = 1 and hence
its spin multiplicity is 3.
E E
S1- state T1-state
6.8 The HOMO and LUMO Concept of Electronic

Transitions
The excitation of a molecule promotes an electron from a filled molecular orbital to

a vacant molecular orbital. The highest occupied molecular orbital (HOMO) is the
filled nonbonding n molecular orbital of a saturated carbonyl compound. For an
alkene, it is the bonding π molecular orbital, whereas for a diene, it is the bonding
ψ2 molecular orbital for thermal and ψ3 molecular orbital for photochemical
reaction. The lowest unoccupied molecular orbital (LUMO) is the antibonding π*
molecular orbital of a saturated carbonyl compound or alkene. For a diene, it is ψ3
and ψ4 molecular orbital, for thermal and photochemical reaction, respectively.
6.9 The Selection Rules for Electronic Transitions
Electronic transitions between energy levels in organic molecules are governed by

some compulsions, known as selection rules.
1. Spin-selection rule
An electronic transition in which spin multiplicity of the election remains the
same in both ground and excited states, i.e., ΔS = 0, the transition is allowed.
The change of spin multiplicity involves the change in angular momentum and
such a change violates the law of conservation of angular momentum.
Therefore, singlet–singlet and triplet–triplet transitions are allowed, whereas
singlet–triplet and triplet–singlet transitions are forbidden. However, this rule is
not obeyed in certain cases, most often when a heavy atom (such as iodine) is
present in the molecule. In such cases, singlet–triplet promotions take place [2].
2. Orbital symmetry selection rule

An electronic transition proceeds more rapidly when the wave functions of the
initial and final states closely resemble each other, i.e., the transition is allowed.
In the π → π* transition, π and π* orbitals occupy the same regions of space
and so the overlap between them is large. Thus, the π → π* is allowed process.
In n → π* transitions, the n and π* orbitals are perpendicular to each other and
so they overlap to much smaller extent. Therefore, this transition is forbidden. In
practice, this n → π* transition is weakly allowed due to coupling interaction
between vibrational and electronic motion in the molecule.
6.10 Physical Properties of Excited States: Jablonski

Diagram
Electronically excited states of the molecules are short-lived because of excess

energy content and try to deactivate through various photophysical and photo-
chemical processes for return to their original ground states.
Photophysical relaxation processes may be classified as:
1. Intramolecular processes:
a. Radiative transitions: These processes involve the emission of electro-
magnetic radiation from the excited state during return to ground state as
fluorescence and phosphorescence, collectively known as luminescence
process.
b. Radiationless transitions: These are the internal conversions without
emission of electromagnetic radiations such as from S2 to S1, intersystem
crossing (S1 to T1), and vibrational cascade (v4 → v3, v3 → v2, v2 → v1,
etc.) processes.
2. Intermolecular processes:
a. Vibrational relaxation: The excited molecules undergo rapid collision with
one another and with solvent molecules to produce molecules in the lowest
vibrational levels of a particular electronic energy level (say, S2, S1, T2, T1,
etc.)
b. Energy transfer: The excited molecule acts as donor or photosensitizer and
transfers excess energy to another molecule, known as acceptor or quencher.
c. Electron transfer: The photoexcited molecule donates its electron to a
ground-state acceptor molecule to form an ion pair. This ion pair results in
quenching of energy from the excited donor molecule.
Jablonski diagram
Jablonski diagram illustrates the electronic states, properties and relaxation pro-
cesses of an excited organic molecule (Fig. 6.4) [3]. The electronic states are
arranged vertically by energy and grouped horizontally by spin multiplicity.
6.10 Physical Properties of Excited States: Jablonski Diagram 189
Fig. 6.4 Modified Jablonski diagram for an organic molecule showing ground and excited states
and intramolecular photophysical processes from excited states. Radiative processes—fluores-
cence (hνf) and phosphorescence (hνp) are shown in straight lines, radiationless processes—
internal conversion (IC), inter system crossing (ISC), and vibrational cascade (vc) are shown in
wavy lines. Adapted with permission from (Smith MB, March J 2006 March’s Advanced Organic
Chemistry: Reactions, Mechanisms and Structures, 6th Ed., John Wiley, New York). Copyright
(2007) John Wiley & Sons
Nonradiative transitions are indicated by squiggly arrows and radiative transitions

by straight arrows.
The Jablonski diagram shows the following:
1. The electronic states of the molecule and their relative energies. Singlet elec-
tronic states are S0, S1, S2, etc., and triplet electronic states are T1, T2 etc.
2. A molecule may be excited to either S2 or S1 state. In liquids and solids, the
higher S2 state rapidly drops to S1 state by internal conversion (*10−13–
10−11 s). S1 state may undergo intersystem crossing to T1 state or return to S0
state by fluorescence. In addition, it may take part in chemical reaction or
photofragmentation.
Fig. 6.5 Intramolecular

energy transfer of
dimethylaminobenzonitrile by
TICT process
3. T1 state may take part in chemical reaction or may return to S0 state by

phosphorescence.
4. In each S2, S1 and T1 states, it undergoes vibrational relaxation by vibrational
cascade, e.g., S2 (v = 3) → S2 (v = 0)
In addition to these photophysical processes, twisted intramolecular charge
transfer (TICT) process (non-radiative process) also takes place to return to ground
state. Molecules of compounds such as dimethylaminobenzonitrile (DMABN) are
flexible. These are planar in the ground state, but are twisted in excited state. The
twisted conformation is able to transfer its full electronic charge from one part to
another in polar solvents and thus deactivated to ground state (Fig. 6.5) [4].
6.11 Lifetimes of Electronic Excited States
Lifetime of radiative S1 state is given by:
s0 ¼ 104 =emax ;
where τ0 has units of s and εmax has units of l mol−1 cm−1. Thus, for [1] (π, π*)
transitions, [1] τ0 is 10−9–10−6 s, whereas for [1] (n, π*) transitions, [1] τ0 is 10−6–
10−3 s.
Lifetime of radiative T1 state is relatively long. In general, [3] (π, π*) states have
long lifetimes (1–102 s), whereas [3] (n, π*) states have short lifetimes (10−4–
10−2 s).
6.12 Efficiency of Photochemical Processes … 191
6.12 Efficiency of Photochemical Processes: Quantum

Yield of Photochemical Reaction
After photon absorption, the excited molecules undergo several competing pro-
cesses including photophysical processes, and hence only a fraction of the excited
molecules undergo photochemical reaction. Therefore, the fraction of the molecules
that chemically react relative to those that are excited is called the quantum yield of
the photochemical reaction. It is denoted by φ.
φ = Number of molecules reacted in a given time/Number of photon absorbed
by the molecules in a given time.
According to the Stark–Einstein law, φ should be equal to 1. In practice, in most
cases, it is less than 1. For instance, if the quantum yield is 0.01, then only one
hundredth of the molecules that are excited undergo photochemical reaction. In
chain reactions, secondary processes occur and hence their φ is greater than 1. For
example, in the photo dissociation of acetone, quantum yield of the reaction may be
1 or 2 depending on the number of bonds broken.
hν
CH3COCH3 (CH3)2CO* *CH3 + CH3C*O
λ > 266 nm
φ=1
hν
CH3COCH3 (CH3)2CO* 2 *CH3 + CO
λ < 193 nm
φ=2
The quantum yield for some photophysical events such as fluorescence can also
be defined from an excited fluorophore [5].
φ = No of photons emitted by a fluorophore/No of photons absorbed by the
fluorophore.
6.13 Intramolecular Process of Excited States:

Fluorescence and Phosphorescence
6.13.1 Fluorescence and Its Measurement
Fluorescence is the radiative emission of light energy from an excited molecule for
its return to ground state of same spin multiplicity, i.e., from S1 to S0 state.
Fluorescence is a spin-allowed transition process and occurs strongly in a relatively
short time in the order of picoseconds to microseconds. The fluorescence emission
spectra are almost the mirror image of the absorption spectra. Only one peak is
Sample holder
light unabsorbed
source exciting light
excitation
monochromator
emission monochromator
detector
Fig. 6.6 Basic components of a spectrofluorometer
common among these spectra that occurs from transitions between the lower
vibrational levels of the two states, i.e., S1 (v0) to S0 (v0) and vice versa and this
peak is called the 0–0 band. In solution, this 0–0 band may appear at slightly
different wavelengths due to solvation effect in the two states. Because of the
possibility of fluorescence, any photochemical reaction in S1 state occurs very fast
before the occurrence of fluorescence.
The intensity of fluorescence is measured for quantitative analysis of fluorescent
compounds present in different clinical and industrial samples.
The intensity of fluorescence is directly proportional to the concentration of the
fluorescent compound. If the target compound is not fluorescent, then it is con-
verted into a fluorescent derivative by reaction with a suitable (nonfluorescent)
reagent. The fluorescence emitted by the fluorescent compound is measured using a
spectrofluorometer [6]. Most of the modern spectrofluorometers employ diffraction
grating monochromators to select the appropriate wavelengths for maximum
excitation and emission. The basic components of a fluorometer are: a light source,
an excitation monochromator, a sample holder, an emission monochromator, and a
fluorescence detector as shown in Fig. 6.6.
Light source
The most commonly employed lamps are medium- and high-pressure mercury lamp
or xenon arc lamp, having an output covering the whole UV–Visible spectrum
range. Xenon arc lamp operated stroboscopically is preferred for its continuous
output. The lamp is operated in a current of air to disperse the toxic ozone formed
from oxygen on exposure to UV radiation.
6.13 Intramolecular Process of Excited States … 193
Excitation monochromator
The slit width of the monochromator is adjusted to select the wavelength for
maximum absorption by the sample and allow its transmission for excitation of the
sample.
Sample holder
The majority of the fluorescence assays are carried out in solution contained in a
circular or square cuvette, made from quartz material. The cuvette is placed normal
to the incident beam. The resulting fluorescence is collected from the front surface
of the cuvette, at right angle to the incident beam.
Emission monochromator
The slit width of the monochromator is adjusted to get maximum emission of
fluorescence.
Detector
Photomultiplier tubes (two) are used to cover the complete UV–Visible range of
emission spectra. The output from the detector is amplified and displayed on a
readout digital device. A continuous sensitivity adjustment is useful in the mea-
surement of sample of widely differing concentrations.
Concentration range of sample
The concentration of the sample solution is adjusted to provide a solution of
absorbance <0.1 A to minimize reabsorption effects.
6.13.2 Kasha’s Rule for Fluorescence
Kasha rule states that the fluorescence from organic compounds usually originates
from the lowest vibrational levels (v = 0) of the lowest excited singlet state (S1).
The exception to this rule is the hydrocarbon azulene, where fluorescence originates
from S2 to S0 [7]. This is due to large S2 − S1 energy gap, which results in the
slowing down of S2 to S1 internal conversion.
Azulene
6.13.3 Vavilov’s Rule for Fluorescence
Vavilov rule states that the fluorescence quantum yield is independent of the
wavelength of the exciting light. If a molecule in S1 (v = 0) undergoes fluorescence
emission and has fluorescence, intersystem crossing and internal conversion having
rate quantum yields φf, φisc and φic, respectively, then
uf þ uisc þ uic ¼ 1
Since φic is much smaller than φf and φisc,
uf þ uisc 1
This mathematical expression is known as Ermolev rule.
6.13.4 Phosphorescence and Its Measurement
Phosphorescence is a radiative emission of light energy from an excited state of a

molecule during its return to ground state of different spin multiplicity, i.e., T1 to S0
state. This is a spin-forbidden transition process and hence occurs at a much smaller
rate than that of fluorescence and is less intense. Most of the organic molecules from
their S1 state can undergo an intersystem crossing (ISC) to their lowest triplet state
[8]. For example, almost 100 % of the molecules of excited benzophenone from
their S1 state cross over to T1 state [9]. For this reason, benzophenone is preferred as
effective photosensitizer. Because of the slow rate of phosphorescence, the lifetime
of T1 state is longer and hence T1 state is susceptible to between the molecules of
oxygen or other impurity molecule. To observe phosphorescence, it is necessary to
prevent the diffusion process of quenching.
T1 lies at lower energy level than S1 and hence phosphorescence spectrum is
always found at longer wavelengths than the fluorescence spectrum. The phos-
phorescence of a compound is usually measured in solid solution rather than fluid
solution to avoid quenching process, which may arise by diffusion of either two T1
molecules or the T1 molecule and a dissolved oxygen molecule or some impurity
molecule. The phosphorescence is determined using a rotating can phosphoroscope
(Fig. 6.7) [10]. It consists of a hollow cylinder having one or more slits which are
equally spaced in the circumference. The sample solution in a tube is inserted in the
cylinder and is allowed to freeze in liquid nitrogen (77 K) so that a clear glassy
solid is formed. The rotating can is then rotated by a variable-speed motor. During
the rotation of the rotating can, the sample is first illuminated by the light source
and then darkened. Whenever, there is darkness, the phosphorescence radiation
passes to the monochromator and is measured. The rotation of the rotating can is set
so that the path of the detector is blocked when the exciting light reaches the sample
and open when the exciting light is blocked and the decayed phosphorescence
reaches the detector.
Ethanol is an excellent solvent for polar molecules. Small quantities of acid or
base are added to produce a clear transparent solid. For nonpolar or less polar
6.13 Intramolecular Process of Excited States … 195
Fig. 6.7 Schematic diagram

of a rotating can
phosphoroscope with shutter
system
compounds, a mixture of diethyl ether, isopentane and ethanol in the ratio of 5:5:2,
commonly called EPA, is used as an excellent solvent.
Both fluorescence and phosphorescence are emitted by the compound. The two
forms of luminescence are separated by exploiting the fact that T1 states are much
longer lived than S1 states and so phosphorescence persists long after the fluores-
cence has decayed. Moreover, phosphorescence occurs at longer wavelengths. For
the study of phosphorescence of a very few compounds in room temperatures, the
compound is taken in a transparent polymer matrix such as perspex. A large number
of organic compounds with conjugated ring systems have the property of phos-
phorescence emission and thus the phosphorimetry provides an excellent method
for their analysis in trace amounts in clinical and industrial samples.
6.14 Intermolecular Physical Processes of Excited States:

Photosensitization Processes
6.14.1 Photosensitization/Quenching and Excimer/Exciplex

Formation
A molecule in an excited state (S1 or T1) may transfer its excess energy all at once to
another molecule of same or different type (in ground state) in the environment, in a
process of deactivation, called a quenching process and the acceptor molecule
which receives the energy from the excited molecule is called a quencher and
excited molecule is called a photosensitizer [11]. If the excited molecule on transfer

of energy excited the quencher molecule, the process is called a photosensitization
process. The intermolecular energy transfer from an excited molecule to another
acceptor molecule in a photosensitization process takes place generally in two
ways. These are: a triplet excited state generates another triplet state, and singlet
excited state generates another singlet state. Both these processes obey the Wigner
spin conservation rule, which states that the total electron spin does not change after
energy transfer process. These processes are represented as:
3
D þ 1 A ! 3
A þ 1 D ðtriplettriplet energy transferÞ
1
D þ 1 A ! 1
A þ 1 D ðsingletsinglet energy transferÞ
where D* is the excited donor molecule and A is the acceptor molecule in ground
S0 state
The triplet–triplet energy transfer normally requires a collision between the
molecules, whereas, singlet–singlet energy transfer takes place over a relatively
long distance (about 40 A). Triplet–triplet energy transfer is a very important
process of energy transfer because triplet states are usually much difficult to prepare
by direct irradiation than singlet states and lifetimes of triplet states are much longer
than singlet states.
6.14.2 The Stern–Volmer Equation for Determination

of Quenching Rate
The efficiency of a quencher can be evaluated by means of Stern–Volmer equation

by measuring the fluorescence intensity in the absence and presence of the
quencher.
Let us consider a quenching process of S1-excited state in presence of a quencher
Q.
S1 → S0 + hν: fluorescence
S1 ⇝ T1: intersystem crossing
S1 + Q ⇝ S0 + Q: quenching
In absence of quencher, fluorescence takes place unimolecularly.
So, rate of fluorescence emission,
1
Jtotal ¼ ðKf þ Kisc þ Kic Þ½S1
¼ 1 Ktotal ½S1 ;
where Kf, Kisc, and Kic are the rate constants of fluorescence, intersystem crossing,
and internal conversion, respectively.
6.14 Intermolecular Physical Processes of Excited States … 197
In presence of quencher, fluorescence takes place in bimolecular process.

So,
QJ ¼ Kq ½S1 ½Q;
where QJ is the rate of deactivation by quenching and Kq is the rate constant.

The overall rate of deactivation is given by the sum of the rates of unimolecular
and bimolecular processes
Q
Jtotal ¼ 1 Jtotal þ QJ ¼ 1 Ktotal ½S1 þ Kq ½S1 ½Q
If Qφf and φf are the fluorescence quantum yields in presence and absence of a
quencher, then
Q
uf ¼ J f =Q J total ¼ K f ½S1 =ð1 K total þ K q ½S1 ½Q

¼ K f = 1 K total þ K q ½Q ;
where Jf is the rate of fluorescence emission and is equal to Kf [S1]and

uf ¼ K f =1 K total
Therefore, uf =Q uf ¼ ð1 K total þ K q ½Q=1 K total ¼ 1 þ K q ½Q=1 K total :
The lifetime of S1 ¼ 1 s ¼ 1=1 Ktotal .
So,
uf =Q uf ¼ 1 þ K 1q s½Q
¼ 1 þ K q ½Q
where Kq is the Stern–Volmer quenching constant.

If the fluorescence quantum yields φf in the absence of quencher and Qφf at
different concentration of Q are measured, the Stern–Volmer plot of φf/Qφf against
[Q] will give a straight line of slope Kq and intercept 1 (Fig. 6.8)
The Stern–Volmer method may also be used for monitoring of phosphorescence
quenching.
6.14.3 Deviation from Stern–Volmer Kinetics
Stern–Volmer method for the study of the rate of decrease of fluorescence and
phosphorescence in the presence of a quencher does not provide satisfactory results
when the interactions between donor and acceptor are maximum at a certain con-
centration of the donor or acceptor. Further increase of donor or acceptor con-
centration shows a non-linear interaction between them. Possibly static quenching
has dominant role in the luminescence emission and binding of quencher to donor is
Fig. 6.8 Stern–Volmer plot

of fluorescence quenching φf / Qφf
[Q]
the major factor for this deviation. For example, the binding of dissolved organic
matter (DOM, humic acids) to phenanthrene shows a non-linear fluorescence
spectrum at higher concentration of phenanthrene or DOM [12]. The concentration
limit depends on the nature of donor and quencher.
6.14.4 The Excimers and Exciplexes
Excimer
The excited state dimer of a compound is called excimer. For example, a high
concentrated solution of pyrene in toluene is irradiated with UV light, an excimer is
formed.
1
P þP ! 1
½PP ðexcimerÞ
# #
P þ hm P þ P þ hm0
P denotes ground-state pyrene.

Exciplex
Exciplex is an excited complex formed by the reaction of an excited molecule of a
compound with a quencher molecule. For example, when a solution of anthracene
(A) in presence of diethylamine is irradiated with visible light, an exciplex is
formed. Ground-state N,N-diethyl aniline acts as a quencher (Q).
hν
A A*
A* + Q [AQ]*
(exciplex)
A + hν A + Q + hν '
6.14.5 Long-Range Energy Transfer Process: The FRET

Process
Forster proposed a theory for the transfer of energy between two fluorescent
chromophores, known as Forster resonance energy transfer (FRET) process [13]. In
this process, the energy transfer takes place by the dipole–dipole (Coulombic)
interactions between the transition dipoles created between the electrons of the
donor and acceptor molecules on absorption of light. This mechanism can only
occur where spin multiplicity is conserved in energy transfer process. Singlet–
singlet energy transfer occurs by this mechanism as the donor molecule (excited
singlet to singlet) and acceptor molecule (singlet to excited singlet) undergo no
change of spin multiplicity, resulting in the creation of large transition dipoles. This
process is equivalent to the energy transfer process in a transmitter-antenna system.
The relaxation of excited donor molecule to its ground state creates a transition
dipole, which simultaneously induces a transition dipole in electronic excitation of
singlet ground-state acceptor molecule into its excited singlet state. Thus, the
coupling of donor and acceptor transition dipoles requires an equal energy for this
long-range non-radiative energy transfer. This energy transfer process is sometimes
called resonance energy transfer because the energies of the coupled transitions are
identical, or in a state of resonance. The electronic movements in this energy
transfer process are shown in Fig. 6.9.
1
D þ 1 A ! 1 D þ A
According to the Forster theory, the probability of energy transfer falls off
inversely with the sixth power of the distance between the donor and the acceptor
Fig. 6.9 Electronic LUMO

movements occurring in the
long-range singlet–singlet
energy transfer process
HOMO
1 1 1 1
D* A D A*
Fig. 6.10 The dependence of

efficiency of energy transfer
ET on donor–acceptor
distance R, as per Forster
theory in a FRET process
molecules. The efficiency of resonance energy transfer, ET increases with

decreasing distance R, according to equation,

ET ¼ R60 = R60 þ R6
where R0 is the critical transfer distance, characteristic for a particular donor–

acceptor pair and R is the distance between D* and A. At R0 the efficiency of energy
transfer is 50 % (Fig. 6.10).
This type of energy transfer frequently occurs in biological macromolecules such
as proteins. The study of absorption and fluorescence spectra of green fluorescent
protein (GFP) and blue fluorescent protein (BFP) isolated from living cells of
jellyfish is useful to determine the donor–acceptor distances for FRET to occur.
The BFP absorbs light at wavelength 380 nm and acts as donor, while GFP absorbs
light and emits fluorescence at wavelength 510 nm and acts as acceptor. When
these donor and acceptor fluorophores are labeled and their dynamic protein
interactions in the living cells are visualized, it is found that in their normal con-
formation, the distance between them is 12 nm and no FRET occurs (Fig. 6.11).
Fig. 6.11 Conformational

change occurs in green
fluorescent protein (GFP) of
jellyfish during fluorescence
emission. Adapted with
permission from (Wardle B
2009 Principles and
applications of
photochemistry, Wiley,
p. 102). Copyright
Under certain circumstances of cellular functioning, they are brought closer toge-
ther within a distance of 4 nm, the excitation of donor (BFP) at 380 nm gives
emission fluorescence from acceptor at 510 nm.
6.14.5.1 Efficiency of Energy Transfer in FRET Process
The efficiency of energy transfer in FRET process depends on the following factors:
a. The relative fluorescence intensity of the donor in the absence and presence of
the acceptor. The higher the fluorescence intensity (FD) in the absence of
acceptor, relative to that (FDA) in the presence of acceptor, higher will be the
efficiency of energy transfer.
ET ¼ 1ðFDA FD Þ
where ET denotes the efficiency of energy transfer and FDA is the fluorescence
intensity in the presence of the acceptor.
b. Similarly, ET depends on relative fluorescence quantum yield of the donor in the
absence (φD) and the presence (φDA) of the acceptor.
E T ¼ 1ðuDA uD Þ
c. ET depends on relative fluorescence lifetime of the donor in the absence (τD) and
in the presence (τDA) of the acceptor.
ET ¼ 1ðsDA sD Þ
6.14.6 Short-Range Energy Transfer Process: The Dexter

Theory of Energy Transfer
David L. Dexter proposed a theory of energy transfer between donor and acceptor
molecules from their close approach (within 10 Å), so that their electron orbitals
can overlap to exchange the electrons between them. This theory is sometimes
called short-range electron exchange or collisional energy transfer theory [14]. The
distance that makes the energy transfer to occur between molecules D and A is
almost comparable to their collisional diameter. For this reason, this theory is
referred to collisional energy transfer theory.
The energy transfer by this exchange process occurs when the molecules have
spin conservation, that is, the total electron spin does not change after the energy
transfer.
Fig. 6.12 Electron movements in Dexter short-range (triplet–triplet) energy transfer process
1
D þ 1 A ! 1 D þ 1 A
3
D þ 1 A ! 1 D þ 3 A
It means that an excited singlet molecule will produce another excited singlet
molecule and an excited triplet molecule will produce another excited triplet
molecule after their energy transfer. The singlet–singlet energy transfer can occur
when the long-range Coulombic interaction takes place between the donor and
acceptor molecules. Thus, the Dexter theory of energy transfer is applicable to
triplet–triplet energy transfer process because this energy transfer process requires
orbital overlap for exchange of their electron. The electron movement in this
exchange process is shown in Fig. 6.12.
6.14.6.1 Rate of Dexter Energy Transfer
The rate constant for the Dexter exchange mechanism is given by:
KET ðexchangeÞ ¼ 4p2 =hðH en Þ2 JD
where Hen is the electronic coupling between donor and acceptor, exponentially
dependent on distance.
H en ¼ H en ð0Þ exp½ben ðrDA r0 Þ=2
where βen is the attenuation factor exchange energy transfer and rDA is the distance
between D and A.
JD is the overlap factor and h is Planck constant.
This equation is simplified as
KET ðexchangeÞ ¼ K expð2rDA Þ

Thus, the rate is dependent on the distance. It is observed that this mechanism
operates when the rDA is 5–10 A.
6.14.6.2 The Triplet–Triplet Energy Transfer in Photosensitization

Process
The excitation of benzophenone in solid solution at 77 K with light of wavelength

366 nm emits phosphorescence. When naphthalene is added to this solid solution,
the benzophenone phosphorescene is replaced by naphthalene phosphorescence
even naphthalene does not absorb photons from light of wavelength 366 nm. This
quenching process takes place by formation of exciplex between excited ben-
zophenone and ground-state naphthalene followed by triplet–triplet energy transfer
and emission of phosphorescence from the triplet naphthalene [15].
hv ISC
Ph2CO
1[Ph
2CO]* [Ph2CO]*
3
C10H8 (naphthalene)
benzophenone
3 3
1
[C10H8] + hv'
1
[Ph2CO] + [C10H8]* [Ph2CO. C10H8]*
ground state ground state
6.14.6.3 Applications of Triplet–Triplet Energy Transfer Process
The principle of Dexter energy transfer is frequently applied for commercial

manufacture of white organic light-emitting diodes with fluorescent tubes and
energy up conversion systems such as blue light emission [16] and while light
emission [17], by triplet–triplet annihilation.
6.14.6.4 Triplet–Triplet Annihilation
Triplet–triplet annihilation (TTA) is an important process of exchange energy

transfer. Both donor and acceptor molecules in their triplet states exchange their
energy to produce their singlet states (Fig. 6.13).
3
D þ 3 A ! 1 D þ 1 A
The fluorescence observed in triplet–triplet annihilation is known as P (pyrene)-

typed delayed fluorescence because it was first observed in pyrene. This mechanism
is utilized to produce high-energy light device simply using medium-energy light.
The accepted mechanism of this exchange process is:
Absorption: S0 + hν → S1
NWOQ
JQOQ
5 5 3 3
F, C, F C,
Fig. 6.13 Electron movement in a triplet–triplet annihilation process
ISC
Intersystem crossing: S1 !T1
Triplet–triplet annihilation: T1 + T1 → E + S1 + S0
Delayed fluorescence: S1 → S0 + hν
The higher energy S1 state is responsible for emission of high-energy light as
fluorescence [18].
6.14.7 Photodynamic Tumor Therapy Using Singlet Oxygen
The ground state of oxygen molecule is triplet (3O2) and its lowest excited state is
singlet state (1O2), which is difficult to generate by direct irradiation of the triplet
ground state. In such case, energy transfer occurs in the reverse direction involving
triplet excited state to singlet excited state in a spin-forbidden process. In photo-
dynamic therapy, a photosensitizer(S) such as s, chlorophylls and dyes is injected in
the blood, when it spreads out in different tissues including tumor cells. The tumor
cells are exposed to laser light at a longer wavelength (*700–850 nm) corre-
sponding to the absorption maximum of the sensitizer. It causes excitation of the
sensitizer to its excited singlet state. The excited singlet state of the sensitizer is
converted to excited triplet state by intersystem crossing. The excited triplet state of
the sensitizer undergoes energy transfer to triplet oxygen producing singlet oxygen.
The resulting singlet oxygen is toxic and oxidizes substances within the tumor cells,
destroying the tumor in the process. The triplet sensitizer also undergoes photo-
chemical hydrogen abstraction with organic molecules within tumor cells producing
a number of radical species for the destruction of tumor [19].
The most common porphyrins used in this tumor therapy are protofrin and
verteporfin having the generalized porphyrin structure (Fig. 6.14). It is extensively
used in the treatment of oesophageal and lung cancer.
Fig. 6.14 Generalized R1 R2

structure of porphyrin. The
R groups represent different
side groups attached to the R8 N R3
H
porphyrin ring N N
R7 H
N R4
R6 R5
The energy transfer process takes place in the following steps:
ISC
1
P þ hv ! 1 P !3 P
3
P þ 3 O2 ! 1 P þ 1 O2
1
P represents porphyrin sensitizer in the ground state.
6.14.8 Photo-induced Electron Transfer (PET) Process
Photo-induced electron transfer (PET) process occurs in nature in the photosyn-

thesis of bacteria and higher plants. The absorption of light by a molecule promotes
the molecule to its higher electronic state, which makes the molecule a better
electron donor or better electron acceptor state than its ground state. The transfer of
electron from donor excited molecule to another ground-state molecule or from
ground-state donor molecule to excited acceptor molecule is called the
photo-induced electron transfer (PET) process. The PET process changes the redox
properties of the molecules and involves a weak orbital overlapping and charge
separation among them. Thus, the excited molecule (A) becomes a better reducing
agent or oxidizing agent and interacts with a ground-state molecule B. It is a
primary photochemical process in supramolecules.
A þ B ! A: þ þ B: ðreductant nature of A and the process is oxidative electron transferÞ

A þ B ! A: þ B: þ ðoxidant nature of A and the process is reductive electron transferÞ
Both these oxidative and reductive electron transfer processes are represented in
molecular orbital interactions (Fig. 6.15)
6.14.8.1 Application of PET Process in Molecular Fluorescence Switch
This photo-induced electron transfer process among the molecules may be utilized
as fluorescence switching. A fluorophore having a macrocyclic unit, on irradiation
Fig. 6.15 Molecular orbital (a) LUMO

representation of electron LUMO
transfer in a PET process.
a Oxidative electron transfer, HOMO
where B is electron poor HOMO
A* B A.+
acceptor molecule, and B.-
b reductive electron transfer,
where B is electron-rich donor LUMO
molecule
(b) LUMO
HOMO
HOMO
A* B B.-
A.+
with light causes no emission of fluorescence. But on insertion of potassium cation

in the macrocyclic unit of the fluorophore, it results in emission of fluorescence.
Thus, potassium cation sensor acts as a molecular switch for the fluorescence [20].
The Fig. 6.16 illustrates the principle of the process using anthracene fluorophore.
The principle of PET process with potassium cation (K+) sensor can be
explained as follows. Excitation of the fluorophore causes the promotion of one
electron from the HOMO to the LUMO. It enables the flow of one electron from the
HOMO of donor macrocyclic unit to fluorophore, resulting in quenching of
fluorescence. On binding with K+ cation, the HOMO of the macrocyclic unit is of
lower energy than that of fluorophore anthracene. Hence, PET is no longer possible
and fluorescence quenching is stopped (Fig. 6.17).
Fig. 6.16 Potassium cation O O

sensor as a molecular O O O O
fluorescence switch in a PET K+
process of anthracene O O O O
..
fluorophore having a N K+ N
macrocyclic donor unit CH2 e- CH2 hν'
hν hν
Fig. 6.17 Principle of PET process in K+ bound sensor

The fluorescence switching devices are useful in the manufacture of optoelec-

tronic materials and in the study of biological dynamics, living cells imaging and
biomaterial sensors [21].
6.14.9 The Marcus Theory of Electron Transfer
The basic assumption of the Marcus theory of electron transfer process is that the
reactants needed a weak interaction between them for this process to operate. The
Marcus theory considers the reaction rate theory, potential energy surfaces and
reorganization of the system to explain the electron transfer process [22]. The
potential energy curves of an electron transfer reaction for the initial (i) and final
(f) states of the system are represented by parabolic curves (Fig. 6.18). These
curves quantitatively relate the rate of electron transfer to the reorganizational
energy (λ) and the free energy changes for the electron transfer process (ΔG0) and
activation (ΔG#).
In a polar solvent, the solvent dipoles are arranged around the molecules taking
part in PET. The solvent reorganization is required to accommodate and stabilize
the changed species (Fig. 6.19) and this reorganization process requires some
energy from the system.
The free energy change, ΔG0, of an electron transfer process is the driving force
of the process. The free energy of activation, ΔG#, is needed to reach the transition
state, #. It is related to the reorganizational energy, λ of the system. From the
geometry of the parabolas:
2
DG# ¼ DG0 k =4k
Fig. 6.18 Potential energy

(PE) description of an
electron transfer reaction. The
parabolic curves intersect at
the transition state (#)
vertical e - transfer solvent reorganisation

.+ .-- .+ .--
A* + B A +B A +B
Fig. 6.19 Reorganization of polar solvent dipoles during PET process
According to the collision theory, the rate constant KET is given by

KET ¼ Aexp DG# =RT
h 2 i
¼ Aexp DG0 k =4k =RT
Or,
h 2 i
ln KET ¼ ln A þ DG0 k =4k =RT
This equation based on Marcus model gives the relation between the kinetics
(KET) and thermodynamic driving force (ΔG0) of PET process. Analysis of this
equation gives three distinct kinetic regions, as shown in Fig. 6.20, depending on
ΔG0.
a. Normal region: The PET process rate increases with increase of ΔG0.
b. Activationless region: The change of ΔG0 has negligible effect on the rate
process.
c. Inverted region: The rate of PET process decreases with increase of ΔG0
Fig. 6.20 Free energy

change, ΔG0 dependence of
electron transfer rate, KET
according to Marcus theory of
electron transfer process
The equation for KET can be generalized to

2
log10 KET ¼ 130:6ðR3:6Þ3:1 DG0 k =k
where R is the distance between the redox centers, i.e., center to center reactant
distance.
The maximum rate of electron transfer occurs when λ = −ΔG0, and the equation
is
log10 KETðMaxÞ ¼ 130:6ðR3:6Þ:
6.14.9.1 Calculation of KET for an Electron Transfer System
Let us consider two groups A and B in a protein molecule having E0/mV of −100
and −90, respectively. Their internuclear distance is 11.8 Å. The protein on irra-
diation with light undergoes PET process from A to B.
Therefore, ΔG0 of the system = −100 − (−90) = −10 mV.
λ for intramolecular electron transfer is 750 mV.
Using the expression for KET,
log KETðA!BÞ ¼ 130:6ð11:83:6Þ3:1ð0:01 þ 0:75Þ2 =0:75

¼ 5:82
KETðA!BÞ ¼ 6:60 105 s1
6.14.9.2 Evidence of Inverted Region in a Dyad
A dyad is a supramolecular structure consisting of a donor and an acceptor com-

ponent. In a fullerene–porphyrin-based dyad, C60 is the acceptor component and
porphyrin is the donor component. On photoirradiation of this dyad, PET process
takes place in the normal region and back-electron transfer (BET) from C60− to Zn
Por+ in the inverted region (Fig. 6.21) [23].
Fig. 6.21 Normal and

inverted regions of Marcus
equation for electron transfer
process in a Zinc porphyrin—
C60 dyad
6.15 Photochemical Reactions and Their Kinetics
Organic molecules in their singlet and triplet excited states can undergo photo-
chemical reactions. Singlet excited states have very short lifetimes and triplet states
have relatively longer lifetimes and hence most of the photochemical reactions
occur through triplet excited states [24]. Excited molecules undergo unimolecular
or bimolecular reactions in a single step (concerted process) or in two or multistep
processes involving one or more intermediates. Most of the photochemical reac-
tions proceed through photolytic cleavage into radicals followed by radicals cou-
pling, isomerisation, dimerization, hydrogen abstraction, elimination and
rearrangements [25].
Absorption of a photon by an organic molecule, R, leads to the formation of an
electronically excited state, R* of the molecule.
R þ hm ! R
The excited state R* may react in any one of the two ways:
In a concerted process (i.e., in a single step) gives the product P:
R ! P
These concerted processes include a series of pericyclic reactions from S1 (π, π*)
via cyclic transition states, where σ or π bonds are cleaved and formed
simultaneously.
In two or multistep process, one or more intermediates I are formed:
R ! I ! P
or
R ! I1 ! I2 ! I3 ; etc ! P
These include free radical reactions involving diradical intermediate or inter-

mediates from either S1 (π, π*; n, π*) or T1 (π → π*, n, π*). The most common
photochemical reactions are the reactions of carbonyl compounds, alkenes, and
aromatic compounds as well as chain reactions of hydrocarbons.
All these photochemical reactions involve funnel-like conical intersections
(CIs) in their electronic excited and ground-state potential energy surfaces, where
the cones touch each other to give reactive intermediates and products (Fig. 6.21).
These conical intersections are analogous to transition state in thermal reactions
[26] (Fig 6.22).
6.15 Photochemical Reactions and Their Kinetics 211
Fig. 6.22 Change of potential energy surfaces for excited-state and ground-state molecules.
Adapted with permission from (Turro NJ 1991 Modern Molecular Photochemistry, University
Science Books). Copyright (1991) University Science Books
6.15.1 Determination of the Excited State Configuration
Luminescence spectra (fluorescence and phosphorescence) measurements provide

information about the configuration of excited states (S1 or T1) involved in the
reactions. Moreover, the lifetimes of the excited states provide us the information
about their origin—(π → π*) or (n → π*). The typical radiative lifetimes of singlet
1
(n → π*) states are 10−6 − 10−3 s and of 1(π → π*) states are 10−9 − 10−6 s,
whereas of typical triplet 3(n → π*) states are 10−4 − 10−2 and 3(π → π*) states
are 1–102 s.
6.15.2 Determination of the Yield of Products
The ratio of the products formed in a photochemical reaction in the absence and in
the presence of a quencher can be determined from the ratio of φ/φq of the Stern–
Volmer equation related to triplet state quenching:
u=uq ¼ 1 þ K q 3 s½Q
where φ and φq are the quantum yields of product formation without and with the
quencher, respectively; Kq is the rate constant for quenching; [Q] is the concen-
tration of the quencher and 3τ is the triplet lifetime in the absence of the quencher.
The Stern–Volmer equation has a linear form and the quantity Kq3τ is measured as
the slope of the plot of φ/φq against [Q] for different quencher concentrations.
The value of Kq in the range of 109–1010 mol−1 dm3 s−1 is found in most cases and
hence a value of 3τ may be obtained.
6.15.3 Determination of the Lifetime of Intermediates
The kinetics of a photochemical reaction is determined by the laser flash photolysis

technique developed by Norrish and Porter in 1949 [27]. They won 1967 Nobel
Prize in Chemistry for this invention. The basis of this flash photolysis is to irradiate
the sample solution with a very short, intense pulse of light in the nanosecond
timescale using Q-switching and then to monitor the changes in the sample system
with time by some spectroscopic methods (absorption or emission spectroscopic
study). The monitoring device is fast enough to observe the transient states before it
decay. The mechanism of a photochemical reaction relating to a number of free
radical species and their electronic excited states can be explored. The development
of mode-locked titanium sapphire laser, sub-picosecond (ps) pulses can be pro-
duced and is usually pumped by continuous wave (CW) argon or Nd-YAG (neo-
dymium–yttrium–aluminum garnet) lasers. The pulses coming from this laser up to
the order of a few femtoseconds (fs, 10−15 s) will be useful to detect the singlet state
free radicals of usual lifetimes 5–100 fs.
Whereas Q-switched Nd-YAG laser pulses are useful to monitor the triplet state
diradicals of usual life times of 10–800 ns (10−9 s), the pump-pulse or probe pulse
technique is useful to study the photochemical change that occurs in the picosec-
onds time scale. It includes cis–trans-isomerisation, internal conversion [S1 (v = 4)
to S1 (v = 0)], energy transfer and electron transfer processes, etc. This method also
enables to determine the order of a photochemical reaction.
6.15.4 Low-Temperature Matrix Studies
Many photochemical reactions are carried out at low temperatures as low as 4 K to

slow down the reaction rate for the study of the lifetimes of the reactive interme-
diates. The most useful matrix materials are solid argon, solid neon and solid
nitrogen. The initial photoproduct is trapped within a rigid matrix that inhibits the
decay of the reactive species in diffusion process. For example, δ-hydroxy-α, β, γ,
δ-unsaturated valerolactone 1 on photochemical decomposition gives cyclobutadi-
ene 2 and carbon dioxide. The intermediate and the products of this reaction are
characterized in low-temperature matrix isolation process.
O hν O hν'
O + CO2
8K 8K
1 O 2
1. Wardle B (2009) Principles and applications of photochemistry. Wiley, New

York.
2. Wayne CE, Wayne RP (2005) Photochemistry. Oxford University Press,
Oxford.
3. Turro NJ, Ramamurthy V, Scaiano JC (2010) Modern molecular photochem-
istry of organic molecules. University Science Books, Sausalito.
4. Balzani V, Credi A, Silvi S, Vanturi M (2006) Artificial nanomachines based on
interlocked molecular species: recent advances. Chem Soc Rev 35:1135.
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Chapter 7
Photochemistry of Alkenes, Dienes,
and Polyenes
7.1 Introduction
Alkenes, dienes, and polyenes on absorption of light are activated to their electronic
high-energy singlet and triplet states. These electronic excited states before return to
ground states undergo various chemical reactions. These reactions have a con-
spicuous role in the areas of material engineering, nanotechnology, and
supramolecular chemistry. These reactions may occur on direct irradiation or in the
presence of a sensitizer. These reactions are broadly classified into four types, cis–
trans-isomerizations, electrocyclic reactions, cycloaddition reactions, and rear-
rangement reactions. These reactions may occur in a concerted process or in a
non-concerted stepwise process.
7.2 Cis–Trans-Isomerizations
7.2.1 Cis–Trans-Isomerizations of Alkenes
The cis–trans-isomerization is the classic photochemical reaction of olefins. The

photochemical reaction of an alkene or diene depends on the extinction coefficient,
ε, of that compound at any wavelength of light used for the reaction. A trans-isomer
is thermodynamically more stable than its cis-isomer and hence has higher
extinction coefficient in longer wavelength of light and low extinction coefficient in
shorter wavelength of light. Thus, in a photochemical reaction of an alkene at
higher wave length, trans-isomer is isomerized to its cis-form and maintains a
photochemical equilibrium when both cis- and trans-isomers are in equilibrium and
no further conversion occurs. Similarly, when the reaction is carried out at lower
wavelength, cis-isomer is converted into its trans-isomer to attain a photochemical
equilibrium. For a particular wavelength of a monochromatic light, the composition

216 7 Photochemistry of Alkenes, Dienes, and Polyenes
of the photochemical equilibrium or photostationary state for a cis–trans-iso-

merization is given by

½trans ec /c!t
¼
½cis et /t!c
The cis–trans-isomerization of alkenes is believed to take place via an excited

state in which the two sp3 carbons are twisted by 90° with respect to each other. This
twisted geometry is referred to the perpendicular (p) state. This p-state geometry is
believed to be the minimum energy geometry for both the singlet (s-) and triplet
(t) excited states. A molecule in the p-state may undergo bond rotation in either
direction to return to either cis- or trans-ground states (Fig. 7.1). The return to
ground state from singlet excited state requires repairing of electrons by a nonra-
diative process, whereas return to ground state from the triplet excited state requires
an intersystem crossing. Unconjugated alkenes absorb light in far-UV region (175–
200 nm), while conjugated alkenes absorb in UV region (220–330 nm).
For example, irradiation of trans-stilbene 1 at 310–320 nm gives a photosta-
tionary state having more than 90 % of cis-isomer [1].
H Ph 315 nm H H
C C C C
Ph 1 H Ph Ph
〈10% 〉 90%
Detailed study on the composition of the photostationary state indicated that it is

controlled by the wavelength of irradiation. Direct irradiation leads to isomerization
via singlet state intermediate and irradiation in the presence of a sensitizer involves
triplet state intermediate. Direct irradiation involves HOMO–LUMO interaction of
the ethylenic double bond of stilbene. The study on the composition of photosta-
tionary state of cis- and trans-stilbenes using various photosensitizers revealed that
the use of sensitizers (e.g., benzil, fluorenone, and 1-naphthyl phenyl ketone)
having triplet excitation energies of 52–58 kcal/mol selectively excites E-stilbene
resulting higher Z:E ratios, while use of sensitizers (e.g., benzophenone and ace-
tophenone) having triplet excitation energies above 65 kcal/mol brings the Z:E
ratios slightly higher than 1 [2].
Z- and E-2-Butenes in liquid neopentane undergo cis–trans-isomerization and it
is competitive with their photochemical [2+2]-cycloadditions [3] . When the alkene
solution is sufficiently diluted with an inert solvent, the isomerization becomes the
dominant reaction. The triplet energy of sensitizer PhH is higher (*84 kcal/mol)
than that for 2-butene (*80 kcal/mol) and hence triplet energy transfer is
H R hυ H R hυ H H
R H R H R R
Fig. 7.1 Mechanism of photochemical cis–trans-isomerization of alkenes

7.2 Cis–Trans-Isomerizations 217
favorable, while in case of ketone triplet having energy less than 70 kcal/mol, the
reaction proceeds through a 1,4-diradical intermediate.
CH3
hυ
H3C CH3
H3C PhH or Ph2CO
Among cycloalkenes, cyclohexenes, cycloheptenes, and cyclooctenes and their

1-methyl derivatives undergo cis–trans-isomerization on irradiation. Trans-isomers
of cyclohexenes and cycloheptenes are very unstable because of high ring strain and
hence are isolated by trapping in hydroxylic solvents like methanol [4]. For
example, the photo-induced isomerization of cis-1-methylcyclohexene 2 to trans-
isomer is trapped in methanol as methanol adduct [4].
CH3
CH3 CH3 CH OH/H+ CH3 OCH3
hν 3 +
Irradiation of cis- and trans-cyclooctenes yields approximately equal amounts of

both isomers in a photostationary state because of their almost equal extinction
coefficients. Both these isomers can be isolated [5].
hν
Cycloheptenes and cyclooctenes on irradiation, in addition to cis–trans-iso-

merization, undergo ring contraction and carbene insertion reactions [6, 7].
CH
H
hν + Ref. 6
CH
hν
+ + Ref. 7
Norbornene 3 on irradiation results very unstable trans-isomer, but in methanol

gives stable products by hydrogen abstraction and other radical coupling processes [8].
hν CH2OH CH2OH +
+
2
3 H-CH2-OH
The photolysis of cyclopentene and norbornene 3 in inert solvent pentane results

in several rearranged products from hydrogen abstraction and radical coupling [9].
185 nm
+ + +
pentane
27% 28% 14% 5%
185 nm
pentane + + +
10% 10% 3% 1%
7.2.2 Cis–Trans-Isomerization of Dienes
Photosensitized cis–trans-isomerization of 1,3-pentadiene 4 has been studied in

detail. It is observed that sensitizers whose triplet excitation energies are above
60 kcal/mol give photostationary mixtures containing about 55 % of the trans-
isomer, while sensitizers having triplet excitation energies below 60 kcal/mol give a
variety of photostationary mixtures containing 65–80 % trans-isomer, and are less
efficient as sensitizers. Possibly the sensitizers having Et greater than 60 kcal/mol
transfer its triplet energy to either cis- or trans-1,3-pentadiene triplet excited states
in a diffusion-controlled process, so that the composition of photostationary mix-
tures depends on the decay processes of the pentadiene triplets, while sensitizers of
Et less than 60 kcal/mol transfer their energy inefficiently to 1,3-pentadiene triplets,
resulting the photostationary mixtures of high content of trans-isomer because the
system prefers to produce the cis-diene triplet. Benzophenone-induced photo
excitation of 1,3-pentadiene 4 results in both cis–trans-isomerization and dimer-
ization [10].
hν + + +
sens
4
While direct irradiation of E,E-2,4-hexadiene 5 gives only E,Z-2,4-hexadiene

from singlet excited state, triplet-sensitized reaction gives both E,Z- and Z,Z-
2,4-hexadienes. The singlet state reaction proceeds with just one terminal double
bond rotation involving allylic methylene or cyclopropane methylene diradical with
just one double bond rotation, whereas the triplet excited state reaction proceeds
with “double” double bond rotation [11].
7.2 Cis–Trans-Isomerizations 219
hν
E,Z
5 sens +
hν Z,Z
E,Z
S1 or C-Me one bond rotation
T1 two bond rotation
7.3 Photochemical Electrocyclic and Addition Reactions
Photochemical-concerted electrocyclic reactions of alkenes and dienes are sym-

metry allowed processes. For alkene system, the HOMO (SOMO) is the excited
alkene π* orbital and the LUMO is the π* of ground-state alkene. The interactions
of HOMO and LUMO produce two new σ bonds in the product, cyclobutane.
HOMO
LUMO
Similarly, photochemical reaction of 1,3-butadiene into cyclobutene and its

reverse process are symmetry allowed processes in disrotatory mode of cyclization
and opening of ring.
HOMO
ψ3
Experimental results on the photochemical reaction of cyclobutene to butadiene

ring opening indicated that the reaction is not straightforward concerted. The ring
opening is accompanied by fragmentation products, acetylene and ethylene and
rearrangement to methylene cyclopropane [12]. Similarly, trans- and cis-
3,4-dimethylcyclobutenes 6 on direct irradiation give mixtures of three isomeric
2,4-hexadienes in different ratios [13]. The photochemical cyclization of butadiene
also gives a mixture of products [14].
hν
+ + +
185 nm
6, cis hν, 185 nm

+ + values in brackets are
pentane for cis isomer
trans, cis: 2.8 (3.5) 2.6 (6.0) 1.0 (1.0)

6, trans
hν + + +
+
These fragmentation and rearrangement products from the photolysis of buta-

diene and cyclobutene can be interpreted by considering the different excited states
and their return to the ground states through conical intersections (CIs). In addition
to singlet and triplet excited states, a Rydberg excited state is also observed from
irradiation of alkenes and dienes. A Rydberg state is an excited state in which one
electron in a nonbonding orbital is far from immediate neighborhood bond being
excited. The Rydberg states are designated by symbol “R” and occur at energies
similar to and sometimes lower than those required for π → π* transitions.
The Rydberg state of alkene is designated as π, R(3s), to specify that one
electron from the π bond remains in the π orbital and the other has been shifted to
the Rydberg orbital, which resembles the 3s orbital of a helium atom. The Rydberg
state of an alkene is considered to have a partially ionic structure such that a net
positive charge remains on a carbon atom. The Rydberg state of ethylene is shown
below.
H H
H H
a Rydberg state of ethylene
The CI is analogous to transition state in thermal reactions. In the photolysis of

butadiene, the CI is considered as a tetraradicaloid, which on different modes of
electron repairing produces cyclobutene, bicyclo [1.1.0]-butane, and a carbene. The
carbene intermediate on rearrangement and fragmentation gives cyclopropyl-
methylene, acetylene, and ethylene.
7.3 Photochemical Electrocyclic and Addition Reactions 221
+
4
3 (1,4/2,3) (1,3/2,4)
pairing pairing
2
1
H
a
a +
(1,3)-pairing
followed by1,3-
hydrogen shift
The transformations of 3,4-dimethylcyclobutenes 6 are believed to occur

through a twisted CI in which all the π electrons are unpaired similar to that formed
from excited singlet butadiene. Passage through this CI results in a mixture of
stereoisomers [15].
Cyclic dienes such as cyclooctadiene 7, where s-cis-conformation is dominant,
undergo electrocyclization to give the dominant products. Similarly, acyclic con-
jugated diene such as 2,3-dimethyl-1,3-butadiene 8 undergoes electrocyclization on
direct irradiation from the s-cis-diene conformation preferentially [16].
H H3C
H3C H3C
hν hν
257 nm 257 nm H C H3C
7 dis H 8 CH3 3
1,3-Pentadiene 4 on direct irradiation undergoes cis–trans-isomerization as well

as cyclization to 1,3-dimethylcyclopropene and orbital symmetry allowed electro-
cyclization to 3-methylcyclobutene [17].
5 H3C CH3
3 hν
4 + +
1 2
4 H3C
The cyclopropene arises from (2,4)-pairing followed by hydrogen migration.

CH3
CH3
H2C CH3
H3C
H
2,4-Hexadienes 9 on direct irradiation undergo both cis–trans-isomerization and

stereospecific electrocyclic reaction [18].
1 CH3 CH3
CH3 CH3
3
2 hν hν
dis dis
4 CH3
6 CH3 CH3
5
9 9 CH3
(trans-cis) (trans-trans)
Cyclic heteroannular diene 10 on irradiation also produces cyclobutene by either

electrocyclization or by isomerization in one ring to E-isomer, followed by [2+2]-
cycloaddition reaction [19]. The isomerization of one ring into E-isomer was ver-
ified by trapping in MeOH.
hν
dis
H H
10
hυ con
Un-conjugated
system
Electron-withdrawing or electron-donor substituent in 2 and/or 3 position of acyclic

dienes directs the reactions to proceed through both concerted and non-concerted
processes. For example, 2-cyano-1,3-butadiene 11 gives 12 and 13 [20].
NC NC NC
hν
+
Ref. 20
11 CN 12 13
(by conceretd (by nonconceretd
process) 1:3.2 process)
minor major
The major bicyclic compound, 1-cyano-bicyclo [1.1.0]-butane 13 is formed

through a diradical intermediate.
NC
NC NC
2,3-Di-tert-butyl-1,3-butadiene 14 gives only 15 in a non-concerted process due

to steric factor [21].
t
Bu
t hν
Bu Ref. 21
t
Bu 15 Bu
t
14
Photosensitized reaction of 1,3-butadiene gives a mixture of cis- and trans-

1,2-divinyl cyclobutanes and 4-vinyl cyclohexene [22].
3
hν 2
+ + 4
sens 1
Ref. 22
5
6
The ratio of the products, divinyl cyclobutanes to cyclohexene depends on the

triplet energy (Et) of the sensitizer used. With sensitizer Et > 60 kcal/mol, both s-
trans and s-cis are excited having a preference to s-trans. With sensitizer Et
− * 55 kcal/mol, the s-cis is preferentially excited. Excited s-trans gives only
cyclobutanes, while the excited s-cis conformer gives both cyclobutane and cyclo-
hexene. For instance, sensitizer benzophenone of Et 68.5 kcal/mol gives divinylcy-
clobutanes and cyclohexene in a ratio of 92:8, whereas sensitizer benzil of Et
53.7 kcal/mol, gives divinylcyclobutanes and cyclohexene in a ratio of 55:45 [22].
1-Aryl cyclobutene 16 on direct irradiation undergoes cycloreversion through
zwitterionic S1 excited state to give arylacetylene. When the reaction is carried out
in MeOH (hydroxylic solvent) only addition product is found [23].
CH3C N
hν ArC CH + CH2=CH2
Ar 16 Ar
CH3OH
Ar
OCH3
Cyclohexenes and cycloheptenes containing an acyloxy group at the allylic or

homoallylic positions give regiospecific addition products in polar solvents due to
formation of intermediate oxonium ions of different stabilities. For example,
methyl-3-cyclohexene-1-carboxylate 17 in acetic acid on benzene-sensitized irradi-
ation gives methyl trans-4-acetoxy cyclohexane carboxylate 18 as major product
along with small amount of methyl trans-3-acetoxy cyclohexane carboxylate 19 [24].
O Me O
C Me O
CO2Me H Me O
CO2Me
hν O
+ H O
AcOH, PhH OAc
H less stable
17 more stable
TS
CO2Me CO2Me
OAc
18 OAc 19
97 : 3
Such regioselectivity is not observed in cis-20 and trans-4-cyclohexene-1,2-

dicarboxylates because the participation of oxygen functions in the TS is not sig-
nificant in the stability of the TS.
CO2Me AcO CO2Me AcO CO2Me

+
CO2Me CO2Me CO2Me
20 25% 71 : 29
The photochemistry of hexatriene system is interesting. Unsubstituted

1,3,5-hexatriene 21 undergoes cis–trans-isomerization, whereas substituted hexa-
trienes undergo cyclization by both concerted and non-concerted processes [25].
A triene 22 having conformation that is s-cis at both C(2)–C(3) and C(4)–C(5) as
well as Z at C(3) and C(4) is favorable to electrocyclization [26]. An extended
conformation of 1,3,5-hexatriene is not favorable for electrocyclic process. For
example, trienes 23–25 on irradiation give products in non-concerted processes [27].
hν
21
Z-form E-form
3 4
2 5 hν
1 6 6e process Ref. 26
H3C 22 CH3 con H3C CH3
(in conceretd process)
3 4 H3C
2 5
H3C CH3 hν
H3C CH3 +
1 6
H3C
23 1,6-pairing 1,4-pairing
3 4
2 6
H3C hν
5 +
1 H3C
24 H3C
1,5: 6,4: 2,3-
1,4-pairing
pairing
3 4
5 CH3 CH3
2
H3C 6 hν
CH3 Ref. 27
1 25 CH3 H3C
The products of the trienes (23–25) are formed in non-concerted processes from
a tetraradicaloid CI via a diradical intermediate [28]. The formation of products
from a triene depends on its ground-state conformation.
3 4 3 4 4 4 6
4,6-pairing 3
5 1,6-pairing 1,5-pairing 5 6
2 5 5
2
2
1 6 1 1 1
6
CI
1,4-pairing
5
3 4
6
2
1
7.4 Photochemical [2+2]-Cycloaddition and Dimerization

Reactions
Open-chain alkenes undergo dimerizations in stereospecific ½p2s þ p2s paths in low

yields on irradiation. For example, photodimerization of Z-2-butene 26 gives two
products in which cis-geometry of methyl groups is retained. Similarly, the
cycloadducts from irradiation of E-2-butene 27 retains the trans-geometry of
methyl groups [29]. Irradiation of Z-2-butene also gives cycloadduct with E-stil-
bene in a similar manner [29].
hν
+
26
hν
+
27
Ph
Ph hν
+
Ph large excess
Ph
Cyclohexene 27a undergoes photo-induced dimerization in the presence of xylene

sensitizer to give a mixture of stereoisomers in non-stereospecific manner [30].
H H H H H H
hν
+ +
xylene
27a H H H H H H
2% 2.6% 1%
1-Phenylcyclohexene 28 also undergoes dimerization on irradiation directly or in

the presence of sensitizer to give tail-to-tail products. This indicates that both singlet
and triplet excited states diradical intermediates are involved in the formation of the
products [31].
Ph Ph Ph Ph
Ph Ph
Ph
hν +
or
hν, sens H H H H
28
Stereocontrolled photoaddition of allene to cyclopent-1-ene-1-carboxaldehyde 29

has been utilized in the introduction of exocyclic methylene group in steviol 30 [32].
OH
CHO CHO CH2
Et2O, hν CH2
+
pyrex
29
CO2H 30
Intramolecular [π2+π2]-cycloadditions are also observed in cyclic 1,4-dienes 31

[33] and 32 [34] and indene 33 [35].
hν
Ref. 33
31
H H
hν
Ref. 34
32 H H
hν
Ref. 35
sens
66%
33
Conjugated dienes such as cyclopentadiene undergoes photodimerization

through triplet sensitization in the presence of benzophenone to give both [4+2] and
[2+2]-cycloaddition products in equal proportion [36].
Ph2CO (sens)
2 + +
330 nm
[4+2] [4+2] [2+2]
7.5 Photochemical Rearrangements
Several alkenes and dienes undergo rearrangement reactions on irradiation. For

example, methylene cyclopropane 34 in acetonitrile on irradiation gives 35 as major
product along with minor products 36 and 37 (about 3 %) via the formation of
2,2-diphenylcyclobutylidene intermediate [37].
7.5 Photochemical Rearrangements 227
Ph Ph Ph Ph O Ph
Ph hν Ph
Ph Ph
+ +
34 Ph MeCN 35 36 37
29%
5,5-Diphenyl-cyclohexa-1,3-diene 38 on direct irradiation undergoes electro-

cyclic ring opening to give 39 as major product, and on photosensitization gives 40
as major rearrangement product [38]. The rearrangement proceeds by
1,2-sigmatropic shift of one of the phenyl rings in the triplet excited state followed
by the formation of a diradical intermediate and consequent formation of a cyclo-
propane ring. This rearrangement is known as the di-π-methane rearrangement.
3
2 4 hν
5
1 Ph 6e electrocyclis process
6 Ph dis motion of ring Ph Ph
opening 39
38
1,1-diphenyl hexatriene
major product
3 Ph H
2
H Ph
4 3
2 4
5 hν
1 Ph 5 +
6 Ph sens 1
6 Ph Ph
trans 40 cis
91:9 minor
major
products.
Ph Ph
7.5.1 The di-π-Methane Rearrangements
Direct or sensitized irradiation of 1,4-dienes and other related molecules in which

two π systems are separated by an sp3 carbon atom gives vinyl or π-substituted
cyclopropanes as major products. These reactions are known as the di-π-methane
rearrangements or Zimmerman rearrangements because Howard Zimmerman
group of the University of Wisconsin, USA had made major works on these
reactions [39]. The rearrangements are believed to undergo apparent 1,2-vinyl
migrations, followed by formation of a new σ bond between the lateral carbon
atoms, giving rise to vinyl cyclopropanes. The π system may be a vinyl, aromatic
ring, acetylenic, or allenyl moiety. For examples, 1,4-dienes 41 and 43 give 42 and
44, respectively [39, 40].
R
3
2 hν R
4 3 4
1 5
5
Ph 3 Ph Ph
2 hν 4
4 Ph Ref. 39
3 5
1 5 Ph 42
41
Phenyl substituted vinyl group has higher preference of cyclization than methyl
substituted vinyl group because of higher stabilization of radical character.
H3C 3 CH3 H3C 3 CH
3
4 2 hν Ph
5 4 2 Ref. 40
Ph 5 1 CH3
Ph
PhH3C
44 H3C 1 CH3
43
7.5.1.1 Mechanism and Stereochemistry
Di-π-methane rearrangement may occur from either a singlet or a triplet excited

state [41]. The reaction involving singlet excited state occurs in a concerted process
and this mechanism is followed in acyclic dienes and cyclic systems in which
concerted process is sterically feasible by free rotation of the π systems, whereas the
reaction from the triplet excited state occurs in various bicyclic systems where both
the π systems are in rigid structural environments and prohibited for free rotation.
The singlet reaction is analogous to sigmatropic shifts of π system. The reaction
proceeds by a 1,2-sigmatropic shift of one π system followed by reorientation of π
electrons to form a new σ bond between the lateral carbon atoms in a concerted
process. As the new σ bond is formed between C(3) and C(5) using back lobe of the
C(2)–C(3) σ bond by disrotatory motion, anti to the migrating π system results in
the inversion of configuration at C(3). The process is photochemically allowed
because the orbital array corresponds to Mobius topology of 6e process (two σ
bonds and one π bond) with one phase change between the C(1) and C(2) positions
as depicted in Fig. 7.2. The uncyclized π system retains its E- or Z-configuration
present in the starting material. In Fig. 7.2, the double bond between C(1) and C(2)
retains its E- or Z-geometry in the product.
Fig. 7.2 The orbital array of 4 4

di-π-methane rearrangement 5
2 5
through singlet excited state 2 3 3
1
1
The following examples confirmed the predicted stereochemical course of the

rearrangement [42, 43].
Et CH3
Me Et Me Et
hν Me Me
+ Ref. 42
Ph Ph
Ph CH3 Me Ph Me
PhH3C Ph
Z
Z 1 6 H
2
1
3 4 hν 2
Ref. 43
5 3
5
Ph
Ph 4 38%
1-phenyl-3-methyl- cis-endo
3-(cis-1-propenyl)- 1-methyl-5-phenyl-6-
1 endo-cis-6-propenyl
cyclohex-1-ene
2
bicyclo [3.1.0]-hexane
5
Ph 3
4
The triplet reaction proceeds through the formation of a cyclopropyl diradical

intermediate. The less stabilized diradical center utilizes its odd electron density to
open the cyclopropane ring and forms a more stabilized 1,3-diradical intermediate.
The new 1,3-diradical intermediate gives the cyclopropane derivative as major
product of the reaction. Thus, in a 1,4-diene system, a terminus substituted with aryl
groups will cyclize in preference to an unsubstituted or alkyl-substituted terminus
because of the greater stability of the diradical intermediate by delocalization with
aryl groups. The following example of 43 demonstrated the mechanism of the
triplet reaction [40].
H3C 3 CH3 H3C CH3 H3C CH3 H3C CH3

hν Ph + Ph
4 2 Ph
1 Ph Ph
Ph 5 CH3 Ph CH3
PhH3C Ph CH3 H3C
PhH3C H3C CH3 H3C CH3
43 less stable Z-configuration
diradical centre
7.5.1.2 Regioselectivity
The rearrangement proceeds rapidly in 1,4-dienes when phenyl or other groups are
present as substituents in the vinylic parts. In allylic aromatic analogs, aromatic ring
migrates in preference to a vinyl group. When two different substituents are present
in two positions of the vinylic systems, the regioselectivity of the reaction depends
on the relative stability of the diradical species. For example, compound 44 gives
compound 45 through a diradical intermediate, where phenyl group stabilizes the
diradical much more compared to carbomethoxy group [44].
Ph Ph Ph
MeO2C MeO2C MeO2C
5 5
4 4
1 hν 1
3 3
2 2
44
Ref. 44
MeO2C Ph
45 (100%)
In dihydrobenzobarrelene 46, field effect of the substituent controls regioselec-

tivity of the rearrangement. The exclusive formation of product 47 from syn-
7-hydroxy-7,8-dihydrobenzo barrelene 46 can be interpreted by greater stabilization
of the diradical 48 due to field effect of hydroxyl group over the competitive
diradical 49 [45].
HO 7 H HO H
2 8 1 6 hν
4 5 Me2CO
3
46 48 47 OH
favourable diradical
HO H
disfavoured 49
7.5.1.3 Substituent Effect on Central Sp3-Carbon
When the central sp3-carbon of 1,4-diene is unsubstituted, the di-π-methane rear-

rangement is less favorable. The reaction of 1,1,5,5-tetraphenyl-2,4-dideuterio-1,4-
pentadiene 50 illustrates the fact [46].
3 H D Ph Ph
D D
hν H D H
D Ph +
Ph Ph
Ph 5 Ph Ph 1 Ph D
Ph Ph H
Ph
50
In this case, the stable diradical is formed by hydrogen atom migration from C(3)
because it produces a more stable allylic radical. The formation of products can be
explained by the following mechanistic path.
H H
H H 3 D
H
D D D D D 4 2
hν D 1,3-pairing H Ph
D
Ph Ph Ph 5 Ph Ph 1 Ph Ph
Ph Ph Ph Ph Ph Ph
Ph
Ph
50 more stable
1,4:3,5-pairing
Ph 5 Ph
D H
4 3
Ph 1 D
2
Ph H
Similarly, 1,5-diphenyl-1,4-pentadiene 51 on irradiation undergoes intramolec-

ular [2+2]-cycloaddition in preference to di-π-methane rearrangement [47].
hν
+ Ph
Ph Ph
51 Ph Ph Ph
Delocalized aryl substituents on the central sp3-carbon accelerates the reaction

rate by stabilization of 1,3-diradical. For example, the diene 52 gives 53 in high
yield [48].
Ph Ph
Ph Ph
hν
Ph2CO (sens) Ph
Ph Ph CO2Me Ph
MeO2C MeO2C
CO2Me
52 53 95%
The reaction of 52 in direct irradiation gives a different product 54 [48]. Possibly

in the singlet state, better zwitterionic nature of carbomethoxy-substituted vinyl
group makes it to take part in cyclization in preference to diphenyl-substituted vinyl
group. In this case, the formation of cyclopropane ring is controlled by the dipolar
nature of the diradical.
Ph Ph Ph Ph Ph
hν Ph
52 CO2Me δ+ CO2Me
δ-
Ph O
Ph CO2Me
Ph CO2Me C O2 Me
Ph Ph
OMe Ph 54
95%
Some other typical examples of the di-π-methane rearrangements [49–61] are:

hν
1.
Me2CO Ref 49
50%
hν
2. Ref. 50
Hg vapour
80%
hν
3. Ref. 51
Hg vapour
H
hν
4. Ref.52
Ph
H
Ph Ph Ph
H Ph
hν
5. + H Ref. 53
Ph H
Ph minor
major
Ph Ph Ph
hν Ph
Ref. 54
6. Ph
Ph Ph
major
Ph Ph Ph Ph
H
hν
7. Ref. 55
Ph
Ph
Ph Ph H
80%
hν Ref. 56
8.
Me2CO
56%
Ph
Ph Ph hν Ph
9. Ref. 53
Ph
Ph Ph
Ph
hν
10. Ref. 57
PhCOMe
95%
hν Ref. 58
11.
hν
12. Ref. 59
Ph
Ph
hν Ref. 60
13.
Me2CO
50%
hν
14.
Ref. 61
94%
7.5.2 The aza-di-π-Methane Rearrangements
Photochemical rearrangement reactions of 1-aza-1,4-dienes and 2-aza-1,4-dienes

from their triplet states to form corresponding cyclopropylimines are known as the
aza-di-π-methane rearrangements [62].
1
3
2 5
N 4 hν
4
N
1 5 sens 3 2
cyclopropylimine
The rearrangement is analogous to the di-π-methane rearrangement and is

considered as 1,2-shift of imino group from C(3) to C(4), followed by a σ bond
formation between C(3) and C(5). For example, the photorearrangement of
β,γ-unsaturated imine 55 gives cyclopropyl imine 56, which on hydrolysis gives
cyclopropane aldehyde 57 [62].
*
OHC PhCH2NH3
hν
Ph Ph Ph N PhCOMe Ph N
Ph Ph N Ph
Ph Ph
Ph less stable Ph
55 radical center
OHC
H3O
Ph
Ph N Ph
Ph Ph Ph N Ph
57 40% 56 Ph
This reaction is useful for synthesis of cyclopropane carboxylic acids.

Similarly, the β,γ-unsaturated oxime acetate 58 gives 59 in high yield [63].
NH2OAc/H hν
CHO
Me2CO H Ref. 63
N
OAc sens
58 N OAc
59 76%
The presence of an electron-withdrawing group in the oxime ester improves the

yield of the reaction. For example, oxime 60 gives 61 in 90 % yield [64].
hν Ph
N Me2CO (sens) Ph
Ph O-COPh N
Ph OCOPh
61
60
90%
7.5.3 The tri-π-Methane Rearrangements
The light-induced rearrangement reactions of trivinyl methanes to give divinyl

cyclopropane and vinyl cyclopentene derivatives are known as the tri-π-methane
rearrangements. The reaction involves both singlet and triplet excited states. For
sterically congested molecular systems, the triplet excited states are involved and
proceed through the formation of cyclopropyl dicarbinyl diradical intermediates,
which rearrange to more stable vinyl-allyl-carbinyl diradicals by cyclopropane ring
opening and give the products. The ratio of cyclopentene to cyclopropane deriva-
tives in a reaction depends on the involvement of the excited state. The following
examples of tris-diphenylvinyl methanes (62, 65 and 68) are illustrative [65–67]:
Ph Ph
Ph Ph
Ph
hν Ph
Ph + Ph Ref. 65
direct Ph
Ph Ph Ph Ph Ph
Ph
Ph
62 Ph Ph 64
63
(63 Z,E : 64, 49,19 : 32)
Ph Ph
Ph Ph
Ph Ph Ph
Ph
hν Ph
Ph + Ph
direct Ph Ref. 64
Ph Ph Ph Ph Ph
Ph
Ph
Ph Ph Ref. 66
65 67
66
(66 Z,E : 67, 31,17 : 52)

p-NC-Ph Ph-p-CN
Ph p-NC-Ph Ph-p-CN Ph
p-NC-Ph Ph-p-CN Ph
Ph Ph
hν Ph
Ph + Ph
+ Ph
Ph Ph-CN-
Ph Ph Ph H Ph
Ph Ph H Ph Php -CN-p
Ph
Ph Ph
68 69 70 71
hν, 50 min
68 69 + 70 + 71 Ref. 67
34 : 27 : 39 %
hν, 20 min
53 : 17 : 0 %
In direct irradiation, the products are formed through a single intermediate,

whereas for sensitized reactions, the products are formed through two-stage inter-
mediates following the di-π-methane path. In two-stage process, in the second
stage, the transoid intermediate is dominant due to its greater stability and provides
the major product of the reaction. The formation of the products can be rationalized
by the following mechanism:
Ph
Ph
Ph Ph
p-NC-Ph Ph-p-CN Ph Ph
Ph-p-CN Ph Ph-CN-p
Ph
Ph-CN-p
Ph-CN-p Ph 71
hν Ph Ph
cyclopentene derivative
direct S1 'cisoid' diradical
Ph Ph Ph Ph
Ph Ph
68
hν sens Ph p-NC-Ph Ph-p-CN
T1 Ph
Ph-CN-p Ph
p-NC-Ph Ph-p-CN Ph-CN-p Ph
H Ph
Ph Ph
Ph 69 and 70
Ph
'transoid' diradical
more stable cis and trans- cyclopropane derivatives
(2nd stage)
Ph Ph Ph Ph
cyclopropane diradical
(1st stage)
7.6 Problems
7.6.1. Suggest a mechanistic rationalization for each of the following reactions and
mention the major product when more than one product is formed
(a) hν
+
Me2CO (sens)
H
hν Ph
(b) H (c) H hν
Ph Ph Ph H Ph
Ph
(d) hν (e) (f) hν
hν
(g)
HOH2C CH2OH CH2OH CH2OH
hν (>210 nm)
+ +
hexane
7.6 Problems 237
7.6.2. Predict the structure(s) of the principal product(s) formed in the following
reactions:
Ph hν CO2Me
(a) •
H (b) hν
Ph Ph +
Ph Ph MeOH
MeO2C
(c) hν (d) hν
PhCOMe
hν (254 nm) hν
(e) (f)
Ph Ph
MeCN N
CO2Et
O
(g) hν , Et2O
O
MeO
7.7 Further Reading
1. Coyle JD (1986) Introduction to organic photochemistry. Wiley, New York

2. Kagan J (1993) Organic photochemistry, principles and applications. Academic
Press, New York
3. Coxon JM, Halton B (2011) Organic photochemistry, 2nd edn. Cambridge
University, London
4. Gilbert A, Baggot J (1991) Essentials of molecular photochemistry. Blackwell
Scientific Publications, Oxford
5. Mayo PD (ed) (1980) Rearrangements in ground and excited states, vol 3.
Academic, New York
6. Mattay J, Griesbeck AG (1994) Photochemical key steps in organic synthesis.
Wiley- VCH
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Chapter 8
Photochemistry of Carbonyl Compounds
8.1 Introduction
Carbonyl compounds undergo various photochemical reactions in both gas and

liquid phases. The reactive excited states of saturated ketones are the n → π* states,
whereas that of conjugated ketones are π → π* states. Both these n → π* and
π → π* transitions of carbonyl compounds may occur by singlet or triplet excited
states. Both singlet and triplet excited states of a carbonyl compound react in
different rates to give same type of products in different ratios. For saturated
ketones, the activation energies for singlet and triplet excited states are about 80–85
and 75–80 kcal/mol, respectively, and hence require UV light of wavelengths of
about 270–280 nm (near-UV region), whereas for conjugated ketones, the activa-
tion energy for singlet and triplet excited states is below 80 kcal/mol, in the range
45–78 kcal/mol, and require light in the far-UV region (310–330 nm). The excited
carbonyl groups have radical characters at both carbon and oxygen and the dipole
moments of the excited states are reduced compared to that of ground states due to
transfer of electron density from an orbital localized on oxygen to the orbital of
carbon atom [1]. The excited states of saturated and conjugated ketones can be
represented as hybrid structures of diradical and dipolar forms.
R R R
h
C O C O C O
R' R' R'
O h
O O
The high reactivity of the excited states of carbonyl compounds is due to

half-filled orbital of oxygen.
C O C O
electronic structure of a carbonyl

group in its n, * excited state

242 8 Photochemistry of Carbonyl Compounds
Important photo-induced reactions of carbonyl compounds are the reduction of

carbonyl compounds by hydrogen abstraction, fragmentation, cycloaddition to
alkenes and rearrangement.
8.2 Hydrogen Abstraction and Fragmentation Reactions
One of the common reactions of photo-excited carbonyl compounds is hydrogen

abstraction from solvent or some other hydrogen donor substrate. The hydrogen
donor substrate may be other compound or the same compound. The hydrogen
abstraction generates free radical intermediates. These free radicals may react among
themselves or with ground-state molecules to give the products. Many aromatic
ketones on irradiation undergo abstraction of hydrogen atom from alcohols to give
pinacol type diols in a photoreduction process. For example, benzophenone is
reduced to pinacol-like diol 1 on photoirradiation in 2-propanol [2]. The quantum
yield of the reaction is 2.0. It indicates that two molecules of benzophenone are
reduced by photoexcitation of its one molecule. The reaction is believed to take place
through triplet state of the carbonyl compound in the following steps:
h *
Ph2C=O Ph2C=O
triplet
* + (CH ) CHOH Ph2COH + (CH3)2COH
Ph2C=O 3 2
(CH3)2COH + Ph2C=O Ph2COH + (CH3)2C=O
2Ph2COH Ph2C CPh2

OH OH
1
The photoreduction efficiency of ortho-alkyl benzophenone derivatives is greatly

reduced by intramolecular enolization process, known as photoenolization reaction.
For example, ortho-ethyl benzophenone 2 on photoirradiation in deuterated
hydroxylic solvents gives deuterated ethyl benzophenone 3 by photoenolization
without reduction [3].
CH2CH3 CH3 CHCH3

CH ROD CHCH3
h
H
O OH O-D
O*
2 Ph Ph Ph Ph
CHDCH3
O
3 Ph
8.2 Hydrogen Abstraction and Fragmentation Reactions 243
For some aromatic ketones, the reactive dienols undergo electrocyclization to

cyclobutenols [4].
CH3 CH3
CH3
O h OH dis OH
R
CH2R C H 4e process of
electrocyclic H
R ring closure
The reactive enols 4 may be trapped as Diels–Alder adducts 5, for example, with
dimethyl acetylenedicarboxylate [4]:
R
R HO R
CO2Me CO2Me -H O CO2Me
2
OH
+
CO2Me CO2Me
5
4 CO2Me H
Another important photochemical reaction of both aliphatic and aromatic ketones

is the fragmentation reaction. Unconjugated ketones on photoexcitation undergo
α-cleavage followed by decarbonylation and subsequent reactions of alkyl radicals
to give product(s). All these processes are collectively known as Norrish type-1
cleavage reactions [5]. These reactions take place both in gaseous and liquid phases.
O
h
R C R' R C O R' R + CO + R' R R'
The energies of α-cleavage and subsequent coupling, disproportionation and

hydrogen abstraction processes depend on the structure of the ketones and stability
of the radical fragments that are ejected. For examples, dibenzylketone 6 and t-butyl
ketone 7 undergo photolytic α-cleavage readily in solution to give a mixture of
products [6, 7]. In unsymmetrical ketones, α-cleavage preferably takes place at the
site of alkyl group that can form relatively more stable free radical.
O
h Ref. 6
PhCH2 C CHPh2 PhCH2CH2Ph + PhCH2CHPh2 + Ph2CHCHPh2
6
25% 50% 25%
O O
h
PhCH2 C CHPh2 PhCH2 C + CHPh2
more stable radical
O
PhCH2 C PhCH2 + CO
3 PhCH2 + 3 CHPh2 PhCH2CH2Ph + PhCH2CHPh2 + Ph2CHCHPh2

O O
h (CH3)3CH + (CH3)2C=CH2 + CH3CH=O Ref. 7
(H3C)3C C CH3 H3C C + C(CH3)3
7
The formation of the products can be explained as follows:
(CH3)3C + H CH2 C(CH3)2 (CH3)3CH + (CH3)2C=CH2
O
H3C C + H CH2 C(CH3)2 (CH3)2C=CH2 + CH3CH=O
Cyclic ketones 8–10 also undergo similar α-cleavage, decarbonylation and

hydrogen abstraction reactions to give products [6, 8].
Ph Ph Ph
Ph
h -CO
O O Ref. 6
Ph Ph Ph
8 Ph
O
Ph O
Ph Ph
h Ph -CO Ph Ph Ph Ph
Ph Ph Ph Ph Ph Ph
H Ph Ph
9
Ref. 8
Ph
CHPh2
Ph
O O Ph Ph
Ph Ph h Ph Ph -CO Ph Ph Ph Ph
Ph Ph Ph Ph Ph Ph
a
10
a Ref. 8
Ph
Ph
Some cyclic ketones undergo α-cleavage followed by intramolecular hydrogen

abstraction to form unsaturated aldehydes. Usually abstraction of hydrogen takes
place from δ-carbon. In addition to aldehyde, ketene is also formed involving
another path in some cyclic ketones. The following examples [9, 10] are illustrative:
O O O
CH3 CH3 CH3
h H
H Ref. 9
O O O
h CH2 H
H Ref. 10
O
CH2CH2CH2CH3
CH2 O C C
H H
Cyclic ketones 11 and 13 on photoirradiation in hydroxylic solvents give acetals

12 and 14, respectively, by α-cleavage followed by formation of carbene and
subsequent reaction with solvent [11].
O h O
O ROH O Ref. 11
ROH OR
11 carbene H
12
O
O O O OMe
h MeOH
MeOH H
13 14
68%
O
11%
Aliphatic and aromatic ketones having propyl or longer alkyl group as a car-
bonyl substituent on photoirradiation undergo intramolecular hydrogen abstraction
preferably from γ-carbon to give diradicals, which on β-cleavage or ring closure
give ketones and cyclobutanol derivatives. The β-cleavage results in the formation
of an alkene and a new carbonyl compound.
All these reaction processes are collectively known as Norrish type-II cleavage
reactions [12]. The hydrogen abstraction occurs through a chair-like conformation
to generate a diradical intermediate. Usually the β-cleavage is the dominant reaction
for both aryl and allyl ketones, but high yields of cyclobutanols may be found when
favorable gauche conformation of the diradical intermediate is feasible [13]. For
example, aromatic ketones 15 and 16 give major products from the β-cleavage [13].
OH R1
O H O* H
h 1
OH ring closure R
R CHR1 R CHR R CHR 1
R, R1= alkyl, aryl -cleavage

OH
R C CH3 + H2C CHR1
R CH2
O
R' H OH
H R'
R
R' HO
H
CH H
R' C R H
O Gauche
H O
H
R
R
H
H R' OH
HO + H2C CHR'
H R CH2
R H
Anti
Ph Ph OH
O h OH OH Ph Ph
Ph
+ +
Ph Ph T1 state Ph Ph Ph CH2 Ph
15 major Ph
minor
Ph CH3
OH
H HC O
O OH Ph H2C
h Ph Ref. 13
Ph Ph
H H
16
Whereas aromatic ketones 17 and 19 with an α-substituent give cyclobutanol 18

and cyclopropyl ketone 20 as major products, respectively [4, 13].
O OH
h O
Ph
Ph + + CH2=CH2
Ph
17
18
89% 11%
O OH O O
h - MsOH
Ph CH2Cl2 Ph Ph Ph
20
19 OMs OMs
In aliphatic and allylic ketones, the reaction proceeds through both singlet and
triplet excited states, but in aryl ketones, intersystem crossing is very fast and triplet
state is more effective. For examples, aromatic ketones give major β-cleavage
products from their triplet excited states. Aliphatic 2-hexanone 21 gives acetone,
propylene and 1,2-dimethylcyclobutanol by both β-cleavage and ring closure pro-
cesses [14]. Unsaturated aliphatic isomesityl oxide 22 gives major product
cyclobutane 23 and minor products by α-cleavage [14].
j p y p y g
OH CH3
O H OH H3C
h
H3C CH CH3 + H2C CH-CH3 +
H3C CH2
21
OH
H h 57%
O 23
O O O O
22 + +
11% 6%
Diastereomeric aliphatic ketones 24 undergo Norrish type-II reactions to give

stereospecific cis-and trans-alkenes 25 and 26 from their singlet excited states by
exclusive β-cleavage process [15].
O
H3C CH2CO2CH3 H CH2CO2CH3
h
CH3-C-CH2-CH-CH-CH2-CO2CH3 +
H CH3 H3C CH3
H3C CH3 25 26
24 Z E
erythro 98.9% 1.1%

threo 1.1% 98.9%
High yields of cyclobutanols are obtained in the reactions of cyclic ketones 27

and 28 [16, 17].
O CH3 HO CH2
H3C H3C
CH2 hν
Ref. 16
27
OH
hν Ref. 17
O
28 76%
In ketone 29, γ-hydrogen is not available and hence δ-hydrogen abstraction takes
place to give cyclopentanol 30 as major product.
CH3
CH2-CH3 CH-CH3
h OH Ref. 18
PH
CH2-C-Ph CH2-C-Ph
30
29 O OH
α-Diketone 31 and β,γ-unsaturated ketones undergo γ-hydrogen abstraction to

give only cyclization products 32 and 33 because in each case, one radical site of
the diradical intermediate is allylic type such that its π system is orthogonal to the C
(2)–C(3) bond and prevents β-cleavage process [19].
CH2CH3 CH2CH3 OH OH
H CH2CH3
O CH3 OH CH3 Ph
h OH CH3 Ph Ref. 19
+
Ph Ph 3
O
Ph 2 O
O O major minor
O
31 32 33
Acyclic α,β-unsaturated ketone 34 on Norrish type II reactions gives both

normal product 35 and a by-product 36 due to resonance hybrid of the diradical
intermediate [20].
O OH OH
CH2-CHR2 h CH2-CR2 CH2-CR2 Ref. 20
H3C H3C H3C
CH2 CH2 CH2
34
O R
HO HO R
R
R
H3C R R H3C
H3C 36
H2C 35
Alicyclic α,β-unsaturated ketone 37 on irradiation yields major product 38 from

Norrish type II reactions and minor products 39 and 40 from other paths [21].
H CH3 CH3 CH3

O CH OH HC OH HC
CH2 h CH2 CH2
O
H3C
37 39
-H
CH3 O Ref. 21
HO
40
38
In ketones 41 and 42, hydrogen abstraction takes place from β- or ε-carbon [22, 23].
OH HO
O
C N(CH3)2
C h CH2-CH-N(CH3)2
CH2-CH2-N(CH3)2
Ref. 22
41 OH OH
h
O + Ref. 23
42 H H
3 : 1
Norrish Type II reactions of 2-benzylcyclohexanones 43–45 are interesting. In

2-benzylcyclohexanone 43, the exclusive β-cleavage gives the product, whereas
2-benzoyl-2-methylcyclohexanone 44 gives the major product from cyclobutanol
ring opening. 2-(2-Methylbenzoyl)-cyclohexanone and 2-(2-methylbenzoyl)-
2-methylcyclohexanone 45 give hexahydroanthracenone derivative 48 as the only
product. The direction of hydrogen abstraction and stability of the intermediate
diradicals control the reaction course [24–26].
O O
O O O OH
-cleavage Ref. 24
h
43 82% 46
O
O O O OH
CH3 CH3 O CH3
h
CH3 Ref. 25
44 H-O O
47
79%
O O CH3 O OH CH2 OH O
R R R R
h
OH
45 O H2C H2C
R= H, CH3 O
R
OH 48
R= H, 57%
R= CH3, 93% Ref. 25, 26
α,β-Unsaturated ketone 49 in the presence of catalytic amount of acid or base

undergoes hydrogen abstraction from γ-carbon followed by isomerization to give
β,γ-unsaturated ketone 50 [27].
OH CH3
O OH
CH(CH3)2 h C(CH3)2 CH3
H3C H3C H3C
trace base
49
O CH3 Ref. 27
CH3
H3C
50
β,γ-Unsaturated cyclic ketones 51 and 52 on irradiation undergo α-cleavage

followed by isomerization of the diradical intermediates to yield the major products
53 and 54, respectively [28, 29].
O O CH3 O CH3 O
CH3
CH3 CH3 CH3 Ref. 28
h
CH3
51 53 H3C
C(CH3)2
CH3 CH3 H3C
h
Ref. 29
O O
H3C CH3 52 H3C CH3 O 54
Irradiation of cyclopentenone in hydrocarbon solvents results in intermolecular

hydrogen abstraction, followed by recombination of the resulting radicals to give
the addition products [30].
O O O
O-H O
+ + + Ref. 30
h
H
Cyclohexenone 55 and cycloheptenones undergo photo-induced acid-catalyzed

addition reactions with hydroxylic solvents via cis–trans-isomerization [31].
O O O O
h H+ MeOH Ref. 31
OMe
MeOH
55
8.3 Cycloaddition and Rearrangement Reactions of Unsaturated Carbonyl Compounds 251
8.3 Cycloaddition and Rearrangement Reactions

of Unsaturated Carbonyl Compounds
Cyclopentenone 56 having a propyl-like substituent undergoes photosensitized

intramolecular hydrogen abstraction to give rearrangement product 57 and
cycloadduct 58 [30].
O O O O
h
Ph
Ph2CO -H transfer
H
56
Ph Ph Ph
60% 57
cyclization
major
Ph
O Ref. 30
58
8%
α,β-Unsaturated cyclic ketone 59 undergoes intermolecular cycloaddition reac-

tions with alkenes and dimerization on irradiation. The reactions take place in a
π–π* triplet excited state via the formation of 1,4-diradical intermediates. The
stability and relative efficiency of the diradicals determine the regioselectivity of the
addition products [32, 33].
O O
O O O
O
h +
+
2
more stable
59 less stable because radical at -
carbon and lone pair on oxygen have
repulsive interactions
O
Ref. 32
20%
O O O O
h
+ Ref. 33
59
O
The dimerization occurs via the formation of a 1,4-diradical intermediate fol-

lowed by addition with a ground-state molecule.
O O O
O O O O
h
3
+
(n, *)
O
O O
O
O O
The stereochemistry of ring junction was studied in the cycloaddition of

cyclohexenone with isobutene. The trans-ring closure is the preferred cyclization
process because of minimum steric repulsion [34].
O O O O
O
h
+ H
H H H H H
less stable less stable due to
stable gauche due to steric anti conformation
conformation repulsion
O reactants
O
H H
H H
major minor
4,4-Dialkylcyclohexenone 60 undergoes photo-induced lumiketone rearrange-

ment to form cyclopropane derivative 61 [35].
O O R'
1 1 R
2 4
6 2 h 6
5 4 3 5 3
R R' 60 61
The rearrangement is stereospecific with inversion of configuration at C(4). It is

a photochemically allowed [π2a + σ2a ] cycloaddition process. The mechanism is very
similar to the di-π-methane rearrangement. It involves 1,2-shift of C(5)–C(4) σ
bond to C(3) followed by formation of a new σ bond between C(2) and C(4) using
back lobe of C(4)-p-orbital.
6
6
1 1
O 5 O
5 3
R 2
4 3 2
R' 4
R' R
An alternative mechanism via triplet excited state was also proposed by

Zimmerman with chiral diradical intermediate.
O O O O O R'
R
h
3
(n, *; , *)
R R' R R' R R' R R'
For example, cyclohexanone 62 gives stereoisomeric products 63 and 64 with

inversion of configuration at C(4) [36].
R
O O
O
h Pr(n)
+ Ref. 36
(S)
(R) Pr(n) Pr(n)
62 63 64
This lumiketone rearrangement also occurs in steroid 65 and 4-alkyl-4-aryl

cyclohexenone 66 to produce 67 and 68, respectively [37, 38].
H OAc H OAc
D H
D H
h
Ref. 37
O
65 67
O
4 h
1
O 3 Ref. 38
2 66 68
O
4,4-Diphenylcyclohexenone 69 on irradiation gives products 70 and 71 via

di-π-methane rearrangement from n → π* transition of carbonyl group [39].
O O O
H H
h
+
(n *) Ph Ph
H H Ref. 39
Ph Ph Ph Ph
69 major 70 minor 71
In 4,4-diarylcyclohexenone 72, the aryl group having electron-withdrawing

substituent migrates in preference to phenyl group to give major product 73 [40].
g p p y g p g j p
O O
h H
CN
H Ref. 40
Ph 73
major product
72 CN
These compounds undergo photo-induced rearrangement from a triplet excited

state via a TS similar to that of di-π-methane rearrangement. The reaction is a
stereospecific concerted process. In the TS, the migration of one aryl group from C
(4) to C(3) and bridging between C(2) and C(4) occur in a concerted mechanism to
yield kinetically controlled endo-product as major product [39].
p
O
5 6
Ar H
4 O Ar O
Ar 1 Ar
3 2 H
Ar
major product
X
Mobius system of 4e
Alternative mechanism involving diradical intermediate was also proposed [39].
O O O O
h H
Ar Ar
H H
Ar Ar Ar Ar Ar Ar
Cyclohexenone 74 with alkyl and vinyl substituents in C(4) gives major product
75 by migration of vinyl substituents. Similarly with alkyl and aryl substituents in C
(4) position, aryl substituent will migrate in preference to alkyl substituent to give
the major product of the reaction. In both cases, the migrating vinyl or aryl group
stabilizes the radical character of the TS.
TS.
O O O
H H
254 nm +
Ref. 39
H H
74 major 75 minor
β,γ-Unsaturated ketones and aldehydes undergo photo-induced rearrangement

reactions to give cyclopropyl-ketones and aldehydes, respectively. These reactions
are known as the oxa-di-π-methane (ODPM) rearrangements [41]. These reactions
occur in both acyclic and cyclic systems. The following examples are illustrative
[41–44]:
O
h
1. Ref. 41
PhCOMe Ph
Ph O
93%
2. h
Ref. 42
t-BuOH, pyrex
O
O
>50%
3. CHO h Ref. 43
PhCOMe Ph CH=O
Ph
90%
H CHO
CHO
h
4. Ref.43
PhCOMe, 5 min
Ph
Ph
90%
O
h Ref. 44
5.
Me2CO
O
H
70%
These reactions proceed through the π → π* excited singlet and triplet states of
the carbonyl compounds. The efficiency of the reactions depends on aryl substi-
tution at the γ-carbon and disubstitution at the α-carbon. Possibly, the substituents
at these carbons stabilize the intermediate triplet diradical. The efficiency of the
intersystem crossing of singlet S1 state to triplet T1 state also determines the out-
come of the reaction [45].
In singlet state, the reaction is believed to take place by 1,2-sigmatropic shift of
acyl group followed by sigma bonding between C(4) and C(2).
O R
O
2 R'
R 1 3
4
R'
The reaction of triplet excited state of the carbonyl compound is believed to

proceed through a σ bond formation between C(1) and C(3) followed by cleavage
of the cyclopropane ring in the resulting diradical to form a relatively more stable
diradical and subsequent cyclization of the diradical.
O O R
O O
h 3 R'
R' 1 R'
2 4 R
R 1 3
4
3
( , *) R
2 R'
less stable more stable
Some β,γ-unsaturated ketones undergo both 1,2- and 1,3-acyl migrations in

photoexcitations. For example, 1,2-dimethylcyclopent-2-enyl methyl ketone 76
gives 77 and 78.
O
O O
h
+ Ref. 45
sens
H
77 78
76
1,2-acyl migration 1,3-acyl migration
The product from 1,3-acyl migration was formed from a triplet excited state
through the formation of new σ bond between C(1) and C(4), followed by cleavage
of the cyclobutane ring of the diradical.
O O
O 1
3 1 h 3 cleavage of C(1) - C(2) bond
2
4 4 2
sens
3 78
( , *)
In β,γ-unsaturated ketones, where both di-π-methane and oxa-di-π-methane

possibilities are available, only one of these paths is favored due to steric interaction
and stabilization of 1,3-diradical. Thus, enone 79 gives 80 solely by
oxa-di-π-methane path [46].
h
O O O
Me2CO or Ref. 46
PhCOMe
80
79
70%
3
4 2
O ODPM O O O
1
more stable
(benzylic stability
of the diradical
1 2 1
2 O O O
5
3
DPM 3
4
less stable diradical
The dienone 81, where both DPM and ODPM processes may be feasible, the
DPM process is preferred due to involvement of weaker ethylenic π system and
charge transfer stabilization of its 1,3-diradical [47].
O hν COPh O
DPM T1
81
O
O
more stable
diradical
O
H
+
O
minor product
major product
34%
52%
Spirocyclic β,γ,δ,ε-unsaturated ketone 82 undergoes ODPM rearrangement to

give two photoproducts 83 and 84 from its triplet excited state on irradiation at
λ ≥ 340 nm. The major product comes from larger spin orbit coupling interaction
and better orbital overlaps due to less atomic motion, whereas 82 on irradiation at
254 nm undergoes electrocyclic ring opening of cyclohexadiene ring to give tri-
enone 85 and on irradiation at 300 nm gives aromatic aldehyde 86 by α-cleavage
and β-H abstraction [48].
6 R' R R' R
R' 7 R
5
h O
8 3 4 O
O1 T1
2 83
340 nm
major product O
82
R' R O R nm
254
O 30
0n 85
m
84 82 R'
minor product
R = R' = H
OHC 86
The presence of a carbomethoxy or hydroxymethyl group at C(2) position of the

1,3-diene system of the spiroketone 87 gives only one photoproduct 88 with
inversion of configuration at the chiral C(6) [49].
O R
O R 11
1 2 3 2 1 10
8 7 h , MeCN
9 6 3 4
7 9
10 11 5 4 5 8
6
88
R= CO2Me 87 11-carbomethoxy-trans -tricyclo
2-(carbomethoxy) spiro- [5.4.07.11] undeca-9-en-2-one
[5.5]-undeca-1,3-diene-7-one
In bis-β,γ-unsaturated ketones, the products are obtained from two successive

oxa-di-π-methane rearrangements. For example, 89 gives 90 [50].
.
O
h
O O Ref. 50
PhH, pyrex
O 89 90 40%
Some other typical examples [51–55] of ODPM rearrangement are:

O
O
h
1. Ref. 51
Me2C
O
(sens)
h O
O
2. Me2CO Ref. 52
60%
CHO
h
3. CHO
H
m-MeO-PhCOMe Ref. 53
15 min H
52%
O
O
h
4. Ref. 54
PhCOMe
5. h
O
Ref. 55
PhCOMe
O H
85 90%
Cross-conjugated cyclohexadienones on photoirradiation undergo rearrange-

ments to give cyclopentenones fused with a cyclopropyl ring, called lumiketones.
These rearrangements are known as Lumiketone rearrangements. Detailed study on
the mechanism of the reactions indicated that the reaction proceeds from a triplet
excited state via the formation of a ground-state zwitterion intermediate. The
zwitterion undergoes 1,4-charge transfer followed by migration of a sigma bond
C(3)–C(4) from C(3) to cation site C(6) to give the product. For example,
4,4-diphenylcyclohexadienone 91 gives lumiketone 92 as major product on pho-
tolysis [56].
p y
O O O O O
1 1
6 2 h 2 6
3 5 Ph
5 3 n * T1 S0 4
4 Ph Ph
Ph Ph Ph Ph Ph Ph Ph 92
91 singlet triplet n-state
An alternative mechanism was also proposed.

O O O
1,4-shift Ph Ph
Ph Ph
Ph Ph
The Woodward–Hoffmann rules predict an inversion of configuration at C(4) for

such [1, 4]-sigmatropic shift.
Ph Ph
- -
O Ph O Ph
4e Mobius system (one node) 4e Huckel system (0 node)

1,4-shift with inversion is allowed 1,4-shift with retention is forbidden
The rearrangement of α-bromoketone 93 in the presence of a base gives 94, with

inversion of configuration at C(4) [57].
O O O
O
Br
t
KO Bu Ph
H H H H
Ph Ph Ph
Br Br
Br Br 94
93
Direct irradiation of 4,5,5-triphenylcyclohex-2-en-1-one 95 gives four products

96–99, whereas irradiation of 4-methyl-5,5-diphenylcyclohex-2-en-1-one 100 gives
only one product 101 on photorearrangement [58].
O O O O O
1
6 2 h
Ph Ph + Ph + Ph + Ph
3
Ph 5 4 Ph Ph Ph Ph Ph Ph
96 97 98 Ph
95 Ph 60 % 16 % 99 Ph
16% 4%
O 1 O
4
h 3 2
Ph Ph
Ph
Ph
101 Me
Me
100 ± 93-97 %
Both these cyclohexenones react with their triplet excited states through dirad-
ical intermediates. In both cases, the cyclobutanone products were formed by the
homolytic cleavage of C(4)–C(5), followed by attack of diphenyl methyl radical
center on C-2. In the former cyclohexenone 95, bicyclic ketone and rearranged
cyclohexenones were formed by a phenyl migration mechanism.
. .
O O . O
O O
h Ph Ph
Ph . Ph Ph
1 sc Ph Ph Ph Ph
Ph Ph . Ph exo
95 Ph . isomer
. O
O
Ph
Ph
Ph Ph
Ph . Ph
H
. . .
O O O O
O
h
Ph Ph . Ph Ph Ph
. . Ph
Ph Ph Ph Ph Ph
less stable
Ph Ph Ph Ph
.
O O
. Ph
Ph Ph Ph
Ph Ph
more stable
. .
O O O
O
h
Ph Ph . Ph
1 sc .
Ph Ph Ph Ph
Ph
Me Me Me Me
triplet diradical
8.4 Isomerization of Unsaturated Carbonyl Compounds
6,6-Dimethyl-2,4-cyclohexadienones 102 undergo photoisomerization to 103 by

cleavage of the 1,6-bond from their n,π* singlet excited states [59].
g g
O Me
1 Me O
2 6 HOEt O OEt
Me 354 nm
3 Me Me Me
EtOH
R 4
5
H+ R
R 102 R R 103
R= H, Me R
8.4 Isomerization of Unsaturated Carbonyl Compounds 261
α-Tropolone methyl ethers 104 on photoisomerization and hydrolysis give

cyclopentenone derivatives 105 [60].
O
O O O
OMe 4
MeO 1 MeO 5 3
2 7 h h h
R
3 6 H2O 1 2
4 5 R
R CO2Me
R
104 i
105
R = H, Pr
The isomerization is believed to take place by formation of a new σ bond

between C(2) and C(5), followed by formation of a zwitterion and its rearrange-
ment. The resultant isomer undergoes nucleophilic addition of H2O and cleavage of
cyclobutene ring to give 105.
O O OMe OMe
MeO OMe O O
R + R
h h _
104 R R
OMe OMe O
+ O O MeO
h
R _ R
H2O
R
H O OH O
MeO O
R R
R 105
CO2Me CO2Me
8.5 Cycloaddition Reactions of Carbonyl Compounds

with Alkenes
The photochemical [2 + 2]-cycloaddition reactions of carbonyl compounds with

alkenes are known as the Paterno–Buchi reactions This reaction was first reported
by the group of E. Paterno and G. Buchi on the reaction of benzaldehyde with 2-
methyl-2-butene to give 3,4,4-trimethyl-2-phenyloxetane 106 [61]. Benzophenone

reacts with isobutene and E/Z-2-butene to give isomeric oxetanes 107–110 in a
regioselective manner [62, 63].
2 3
h Ph
PhCH=O + Ref 61
1O 4
106
h
Ph2C=O + + Ref 62
Ph O Ph O
107 Ph 9 : 1 Ph 108
Ph Ph
h
Ph2C=O + Ph O + Ph O Ref 63
109 110
major 6 : 1 minor
The study of the mechanism of the reactions indicated that for aromatic carbonyl
compounds, the reaction occurs through a triplet excited state of the carbonyl
compound, whereas for aliphatic carbonyl compounds through both singlet and
triplet excited states of the carbonyl compound. The reaction is stereospecific for
aliphatic carbonyl compounds and gives syn adduct. For cyclic alkenes, kinetically
controlled endo-isomer is the major product. The regioselectivity of this cycload-
dition reaction depends on the stability and steric interactions of the intermediate
diradical. In the reaction of benzophenone with isobutene, the major product is
derived from the stable diradical.
h . . . .
Ph2C=O + [Ph2C=O]* Ph2C=O Ph2C=O
S1 T1
. .
. + .
Ph2C O Ph2C O
more stable less stable
spin inversion spin inversion
O Ph O
Ph
Ph 107 Ph 108
. major product minor product
The lifetime of S1 is too short and so oxetane formation is much faster than C–C
bond rotation. The formation of oxetane can be explained from FMO approach. The
frontier orbital interactions between half occupied n orbital of carbonyl oxygen
atom (LUMO) with the π orbital of electron rich alkene (HOMO) take place to form
a C,C-diradical.
LUMO O . O
.
HOMO
8.5 Cycloaddition Reactions of Carbonyl Compounds with Alkenes 263
The reaction of benzophenone with dihydrofuran gives regioselective and almost

single product 111 [64].
The overall yield of the reaction is 98 % as a >98:2 regioisomeric mixture. The
major isomer is 88:12 endo/exo-mixture. It can be explained from consideration of
the stability of plausible triplet diradicals and the ISC of the conformers in the
formation of the product. The endo-conformer shows favorable spin–orbit inter-
actions for formation of a sigma bond in the product, whereas exo-conformer faces
steric restriction for this orbital interaction.
H H Ph
O
h
+ PhCH=O + O
O Ph O
111 O H H
> 98 : 2
H H
O O O
H + H
Ph Ph Ph
O O O
111 H H exo
endo
88 : 12
Ph Ph
h O . .
+ Ph2C=O . . Ph + no product
O PhH O O
Ph O
less stable
more stable because radical
O center in the furan moiety
Ph is stabilized by secondary
O Ph orbital interaction with
111 oxygen p-orbital
only product
O Ph O H
H Ph
O H H
O
3 3
A B
ISC ISC
H
H Cyclization O H
O Ph 1 1
A B
Cyclization Ph
H O H
O H
H H
O H O Ph
Ph H
O O H
H
exo
endo
The reaction of furan with benzaldehyde gives unusual exo-product 112 in

high yield by reversal of regioselectivity and stereoselectivity [65]. The triplet
diradical 113 is more stable than 114 by 16.5 kcal/mol due to allylic stability. The
exo-conformer of triplet diradical 113 undergoes favorable ISC to give singlet

diradical of enlarged lifetime due to secondary orbital interactions and gives pro-
duct 112 of high diastereoselectivity, exo/endo, 98:2.
Ph H Ph H Ph
. . H
hν H H
+ PhCH=O
O O O Ref 65
O O O O
113 H H
preferred diradical 112 exo endo
for allylIic stability dr, 212 : 1
+ 94%
O
no product
. .
Ph
O H
114
less stable diradical
The exo-product 112 results from the diradical 113 via intersystem crossing
(ISC) from triplet to singlet conformer (1C) and ring closure as follows:
Ph H
H O Ph O
O O
3
Ph H 3
C ISC D ISC
H H Ph H
1 1 endo product
C D
O O
O O
H H
exo
The reaction of dihydrofuran with β-naphthaldehyde gives high exo-selective

product. Possibly the singlet excited state of the carbonyl compound was respon-
sible for such exo-selectivity [66].
H H
CHO O O H
+ 2-naphthyl
O H +
O O 2-naphthyl
H H
dr, 98 : 2
57%
Irradiation of enol ether of dihydropyran 115 in the presence of benzaldehyde

gives major diastereoselective exo-product 116 due to steric and stereoelectronic
factors [67] and kinetically controlled endo-product 117 as minor product.
H H
O O
1. PhCHO, h O
EtO EtO EtO H + EtO Ph
.
O
EtO O 2. HCl EtO O H . Ph EtO O
H Ph EtO H H
..
115 preferred diradical H2O
.. exo endo
IS
O O
C
H H OEt
H3O+
EtO O + EtO O H
H Ph . O
HO HO EtO
116 H Ph 117 H H O . H
Ph
exo endo
98 : 8 (57%)
Homobenzvalene 118 on irradiation in the presence of ethyl phenylglyoxalate

gives regio- and stereoselective endo-product 119 due to absence of steric interaction.
O
+ h
O Ref 68
Ph CO2Me . O
. Ph
118 Ph CO2Me CO2Me
70 % 119
preferred diradical because
of allylic like stability by
adjacent cyclopropane ring
High cis-diastereoselectivity was observed in the photoaddition reaction of chiral

allyl alcohol 120 with propionaldehyde 121 [69]. Possibly the hydrogen bonding
between singlet and triplet excited states of propionaldehyde and the substrate in the
exciplex favored the formation of the threo-isomer 122, whereas 1,3-allylic strain
prevents the formation of the erythro-isomer 123. The cis-diastereoselectivity can
be explained by consideration of its optimal conformation where steric interactions
are minimized. The effect of the hydrogen bonding in the stereoselectivity of the
product was rationalized on the fact that when the reaction was carried out in
MeOH, such diastereoselectivity was dropped. The preferred conformation for cis-
threo-diastereoselectivity is controlled by hydrogen bonding-induced gauche steric
interactions with chiral methyl and strong spin–orbit coupling in the triplet diradical
intermediate for facile ISC to singlet state [69].
OH HO
O H OH
+ hν O O
Et H H + HH
PhH Et Et
121 120
H H
122 123
threo, cis erythro, cis
.> 95 : 5
Et Et
O H
Et O H Et O
O H O H
H O
OH H OH H
H
Me ISC 1,3
H A
H H
preferred cis threo exciplex threo, cis less preferred cis erythro
exciplex
Cis–trans-selectivity of threo-isomer is controlled by spin–orbit coupling

interaction of the diradical.
OH OH
O O H
cis-threo isomer trans-threo isomer
Similar to furan, photocycloaddition of 1,3-cyclohexadiene 124 to propanal

gives exclusively exo-oxetane 125 from the reaction of singlet excited propanal to a
ground-state 1,3-cyclohexadiene [70].
H H
h O O
+ EtCHO Et + H
124 H H H Et
125 70% 30%
exo endo
In contrast to furan, the exo-diastereoselectivity of the product oxetane in the

photocycloaddition of spiro [4.2]-heptadiene 126 to benzaldehyde was reduced
substantially to an exo/endo-ratio of 3.5:1 by the spiro-cyclopropane ring. The
opposite exclusive endo-diastereoselectivity was found in the reaction with methyl
ester of phenyl pyruvate 127. Possibly endo-orientation of the large carbomethoxy
group reduces the population of the exo-conformer and only the endo-conformer
undergoes the ISC process and forms the carbon–carbon bond in the latter case [71].
H Ph
Ph H
O O
PhCHO
+
h PhH
126 exo endo
3.5 : 1
Ph MeO2C
CO2Me Ph
O O
PhCOCO2Me 127
+
h PhH
126 endo exo
19 : <1
The reaction of non-symmetrical allene with aliphatic aldehyde gives little re-
gioselectivity. For example, 3-methyl-1,2-butadiene (dimethyl allene) 128 with
propanal 121 gives 129 and 130 in the ratio of 2:1 [72].
H H
h
+ Et CHO Et O + Et O
128
121
129 130
2 : 1
69 %
Possibly both steric and electronic factors play the key roles in the stability of
intermediate diradical.
+ Et CHO C . . C
. major product
O CH Et . O
H
Et
minor product
The reaction of 1-ethoxy-allene 131 with 3-methylbutanal 132 gives regiose-

lective product 133 as a 2:1 mixture of diastereoisomers [72].
O h H
H H OEt
+ C CH
. OEt
OEt H . O
H H O CH iPr
131 132 133 80 % (Glc)
preferred diradical
40% (isolated)
Some other typical examples [71, 73–78] of the Paterno–Büchi reaction are:
MeO
Ph
H H
OMe Ph
PhCOOMe
1. O + O Ref. 71
O h PhH O H O H
endo 95 : 5 exo
H H
t-Bu OTMS O t-Bu O t-Bu
h
2. + PhCHO + Ref. 73
t-Bu t-Bu OTMS
Ph OTMS Ph t-Bu
92 : 8
Ph
PhCHO
3. N O Ref. 74
N h
O O
exo 98 %
Ph Ph
H
Ph2CO Ph Ph
4. Ph Ph
+ H Ref. 75
O h , 18 h O O
OH O Ph O H
OH
HO
exo 73 : 7
Ph2CO O
5. Ref. 76
PhH,h Ph
exo Ph
81 %
6. H h
Ref .77
PhH
O O
83 % Me
Me
h OH
7. Ref. 78
N Me2CO N
Me Me 92%
8.5.1 Limitations
The Paterno–Büchi reaction of carbonyl compounds to alkenes fails when the

energy difference between the triplet and ground states of carbonyl compound is
greater than that between the corresponding states of the alkene. In such case, the
excited triplet state of carbonyl compound transfers its excess energy to the alkene
and returns to its ground state. The generated triplet excited state of the alkene
undergoes dimerization. For example, the irradiation of acetophenone
(ET = 74 kcal/mol) in presence of norbornene 134 gives mainly norbornene dimers
135 and 136 because the energy difference between the triplet and ground states of
acetophenone is greater than that of the corresponding states of norbornene.
Acetophenone serves here as photosensitizer, whereas the reaction of norbornene
with benzophenone (ET = 69 kcal/mol) gives cycloaddition product 137 [79].
PhCOMe
+
h
134 135 136
h
Ph2CO
O
Ph
137
Ph
Photo-induced addition reaction of aromatic carbonyl compounds with alkynes

gives unstable oxetanes. For example, the reaction of benzaldehyde with 2-butyne
138 gives α,β-unsaturated carbonyl compound 139 by cleavage of intermediate
oxetane [80].
Me
h . Me Me O
PhCH=O + .
PhHC O PhHC C C Me
H O Me
Me
138 Ph 139
Benzaldehyde reacts with 1-hexyne 140 to give 141–143 without cycloaddition

product [81].
h O H O O OH OHOH
PhCHO + C4H9C CH Ph C C C C Ph + Ph C C Ph + Ph C C Ph
1.5 h
140 H2 C H 142 H H H
141 4 9 143
48 % 13 % 12 .%
+ other products
Possibly this reaction of benzaldehyde takes place by radical pair formation and
hydrogen abstraction mechanism to give major product 141.
O OH
h
2 PhCHO Ph C. + PhCH .
OH
OH OH
. .
2 PhCH
PhHC CHPh
OH O OH O
PhCH . + Ph C. PhCH CPh
O O O
. PhCHO
Ph C. + HC C C4H9 Ph C C C C4H9 Ph C C C C4H9
H H H
O
O O . O H O
Ph C. . H H H
Ph C C C C Ph Ph C C C C Ph
H
C4H9 141 H C4H9
The photosensitized reaction of benzaldehyde with p-benzoquinone 144 in

supercritical CO2 does not give oxetane. It gives 2-benzoyl-1,4-hydroquinone 145
in a radical–radical coupling, followed by enolization through triplet excited state of
the quinone [82]. This method is effective as environmentally benign method for
synthesis of 2-acyl-1,4-hydroquinone without using benzene or other hazardous
solvent.
O O OH
h (254 nm) PhCHO
+ PhCHO
Ph2CO (sens) PhCO
SC-CO2
O 144 O O
OH OH
O
O
145 OH H Ph
Ph O
(44%)
8.6 Problems 271
8.6 Problems
8:6:1. Predict the structure including stereochemistry of the expected product(s)

for the following reactions. For the reactions, where more than one product
is formed, indicate the major and minor products with justification.
(a) (b) h
PhH, h
H + PhCHO
Hg lamp
H 14 h
O
O
(c) Me h
O (d)
PhH, h Ph
+
Me Ph
O OTBDMS
O
(e) O h h
(f)
Ph PhH
Me Ph
O
(g) ( h) O
h h 313 nm
O H
(i) (j) H CO2n-Bu h

O h +
O O
(sens)
Ph
O O
(k) h (l) Ph h
O Ph
Me pentane
Me H
O Ph
(m) (n)
O h t- h
Bu
Me
8:6:2. Provide a mechanistic rationalization for each of the following reactions:
CH3 OHC
H3C CH2 H H
(a) h (b) h
CH2 O O
C O MeOH,
O H r. t.
O H
O
O
(c) h (d) h
sens O
O
Me
Me O O OMe
(e) (f) h
h OH
+ Me2CO Me MeOH Me
N N
Me Me Me
Me
(g) O O (h) O
254 nm
h (Hg-lamp) Me2C = O
PhH O
Michler's ketone
O O
O O O O
(i) (j) h
h
(k) CHO CHO (l)

h
h
+
m-MeO-PhCOMe O O
(sens) 2h
O
(m) h (n) O h (>210 nm) HO H OH

O CHO hexane + +
n-hexane
O O
(o) h (313 nm) ( p)
O h
O PhH
8.7 Further Reading
1. Zimmerman HE (1982) Topic in photochemistry, Top Curr Chem 100:45

2. Zimmerman HE, Armesto D (1996) Synthetic aspects of the di-π-methane
rearrangement, Chem Rev 96:3065
3. Hixon SS, Mariano PS, Zimmerman HE (1973) The di-π-methane and oxa
di-π-methane rearrangements, Chem Rev 73:531
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Chapter 9
Photochemistry of Aromatic Compounds
9.1 Introduction
Aromatic compounds on absorption of light undergo photoexcitation to produce

excited singlet and triplet states. Each of these excited states may undergo different
chemical reactions in proceeding back to the ground state. The triplet excited state of
relatively long lifetime, frequently undergoes photochemical reactions such as iso-
merization, cycloaddition, and di-π-methane rearrangement. These reactions are
useful for synthesis of various types of strained compounds for industrial applications.
9.2 Photoisomerization Reactions of Aromatic

Compounds
Liquid benzene on photoirradiation gives a very small amount of benzvalene (tri-

cyclo[3.1.0.0.2.6]-hex-3-ene) through formation of a diradical as an intermediate [1].
1 1 1
2 254 nm 2 3
6 1,3-bonding 6 2
31,3-bonding 6 5 4
5 5 3
4 benzvalene
4 0.05 %
The presence of the diradical intermediate was supported by the fact that irradiation
of benzene in acidic hydroxylic solvents gives addition products with solvent [2].
OMe
MeOH, H MeOH
+
h
OMe

278 9 Photochemistry of Aromatic Compounds
1,3,5-Tri-t-butylbenzene undergoes photoisomerization to give various products

of valence isomers [3]. Possibly bulky tert-butyl group in the aromatic ring induces
steric interaction to facilitate this isomerization. It is difficult to predict the exact
mechanism of formation of all these photoproducts.
t
Bu t
Bu
t
Bu t
But t Bu
Bu But
1,4-bonding
t
But Bu t
Bu t
Bu
7.3 % t
Bu 7.1 %
< 0.7%
But 20.6 %
1,3-bonding
t
But Bu
64.8 %
prismane
Similarly 1,2,4-tri-t-butylbenzene gives a Dewar benzene.

t
Bu
t
Bu t
h Bu
But t
Bu
t
Bu
9,10-Dimethoxy-octamethylanthracene on photoirradiation isomerizes to

9,10-Dewar isomer [4].
OMe
OMe
h
toluene-d8
- 78oC
OMe OMe Ref. 4
9.3 Photocycloaddition Reactions of Aromatic

Compounds with Unsaturated Compounds
Photoirradiation of benzene and its derivatives with alkenes give ortho-, meta-,
para-cycloaddition products. In most cases, either meta- or ortho-adducts are
obtained as major products [5]. Bryce-Smith and Gilbert suggested a prefulvene
type diradical intermediate mechanism (Path A) for the meta-adduct [6], whereas
Morrison and Srinivasan groups [7, 8] proposed the exciplex mechanism (Path B)
for the formation of these adducts (Scheme 9.1). The exciplex intermediates
undergo photo-induced electron transfer processes between donor (D) and acceptor
(A) to produce radical ion pairs as intermediates, stabilized by coulombic interac-
tions to give adducts [9].
9.3 Photocycloaddition Reactions of Aromatic Compounds … 279
1 *
1
2 3 1 *
hν hν
+
6 4 Path A Path B
5
prefulvene 1,3-pairing exciplex
1,3-addition
4 3
6
5 2 3 1
5
6 1 1
4 +
6 4
2 2
5 3
meta-adduct meta (m)- + ortho (o)-
6
5 1
2
3 4
para (p)-
hν D A hν
A A* [AD]* D* D
[AD]* = { [A*D] [AD*] A D A-D

exciplex
Scheme 9.1 Mechanism for formation of photochemical adducts from the reaction of aromatic
compounds with alkenes
The relative efficiencies of ortho- and meta-cycloadditions of ethenes to arenes

depend on the stability of the exiplex and the polarity of the solvent. In general,
higher stability of the exciplex favors meta-addition and ground-state ion pair
formation between electron donor–electron acceptor favors ortho-addition. In meta-
additions of benzene, endo-selectivity is preferred because of greater stability of
endo-sandwich exciplex, whereas in ortho-additions, electron donor alkenes give
endo-products and electron acceptor alkenes give exo-products preferentially. In
substituted benzenes, ortho-additions are preferred with polar arene substituents
than for alkyl substituents. The lifetime of exciplex is longer in solvent of medium
polarity such as in diethyl ether and dimethoxyethane, but decreases in polar sol-
vents due to formation of ion pair.
h D A h
A A* [AD]* D* D
[AD]* = { [A*D] [AD*] A D A-D

exciplex
The charge transfer process between donor and acceptor molecules influences
the mode of the addition reaction. This charge transfer process is closely related to
the free energy change ΔG of the radical ion pair formation and can be calculated
from the Rehm–Weller equation [9] using the oxidation potential of the donor (D),
the reduction potential of the acceptor (A), and the excitation energy of the excited
species and coulombic interaction energy by the radical ion pair at the encounter
distance for their interaction.
MG ¼ EOx
1=2 ðDÞ E1=2 ðAÞ MEexcit þ MECoul
Red
The value of ΔECoul depends on the dielectric constant of the solvent used.
The photoreaction of benzene with alkenes depends on the electron donor and
electron acceptor ability of the alkene. Both poor electron donor and electron
acceptor olefins react with benzene to give preferentially meta-cycloadducts, where
ΔG values are greater than 1.4–1.6 eV. For example, propene, isobutene, cyclo-
butene, cyclopentene, and cyclohexene give meta-cycloadducts as major products
[10]. All other olefins having strong electron donor and electron acceptor abilities
mainly give ortho-cycloadducts. In ortho-cycloadduct, strong electron donor
alkenes such as 2,3-dihydrofuran and 3,4-dihydropyran give endo-isomers as major
products, whereas strong electron acceptor alkenes such as acrylonitrile give exo-
isomers as major products. Alkenes such as 1,3-dioxoles and 1,3-dioxol-2-one
(vinylene carbonate) give meta-adducts as major or sole products with benzene,
where ΔG values are very low. Only exception is that the tetramethyl 1,3-dioxole
gives ortho-adduct only with benzene because of destabilization of the intermediate
exiplex by the steric crowding and formation of ion pair complex.
2,2-Dimethyl-1,3-dioxole gives ortho-adduct as major adduct with ortho:meta ratio
of 3.5:1. One interesting feature of the meta-adducts of benzene is that cycloalkenes
give predominantly the endo-isomers due to greater stability of endo-sandwich
exciplexes by an electronic interaction, whereas 1,3-dioxoles give predominantly
exo-isomers because of repulsive effects of the oxygen atoms of electron-rich cis-
enediol ethers with the partly negative charged arene in the endo-configuration of
the zwitterionic species of exciplex. Vinylene carbonate, where oxygen atoms in the
five-membered ring are electron deficient due to carbonyl group, enjoys an elec-
tronic attraction with the negatively charged allylic moiety of the arene to stabilize
the endo-configuration and gives preferentially endo-isomer [11]. As for examples,
1,1-dimethoxyethene, 2,4-dihydrofuran, 3,4-dihydro-2H-pyran, tetramethyl-1,3-
dioxole give only ortho-cycloadducts [12]. The ΔG values of these olefins are
less than 1.4 eV.
Electron donor-substituted benzenes such as anisole and toluene on reaction
with neutral and weak electron acceptor alkenes usually give meta-cycloadducts
as major products along with minor ortho-cycloadducts [13]. If the alkene is a
strong electron acceptor such as acrylonitrile or acrylate, maleic anhydride, and
maleimide, the ortho-cycloadducts are the major products [13]. In such cases,
ΔG values are negative. Photocycloaddition of 1,3-dioxoles and vinylene car-
bonate to anisole gives meta-adduct preferentially on similar grounds of selec-
tivities as described for benzene with electron donor substituent at C-1 carbon
atom of arene [14]. The solvent is an important factor in the photochemical
reactions of anisole with electron-withdrawing alkenes. In aprotic solvent, it
gives cycloadduct as major product, whereas in protic solvent gives substituted
product as major product. For example, photoirradiation of anisole with acry-
lonitrile in acetonitrile solvent gives 1,2-ortho-cycloadduct as major product
(73 %). When this reaction is carried out in methanol, para-substituted product
was found as major product (49 %) along with a small amount of ortho-sub-
stituted product (10 %) [15]. Such contrasting results in methanol can be
explained from the formation of ion pair complex in protic solvent and its
stabilization as cation radical, which favors the substitution in para- and ortho-
positions [16]. The relative ratio of the cycloadduct is markedly affected by the
polarity of the solvent. It is evident in the reactions of acrylonitrile with anisole
in presence of cyclohexane and acetonitrile as solvent. In cyclohexane, the ratio
of 1,2- and 3,4-cycloadducts is 2:1, whereas in acetonitrile it is 20:1. Possibly,
polar solvent stabilizes the ground-state ion pair or excited state complex for
1,2-ortho-orientation of arene and alkene [17].
Electron acceptor-substituted benzenes such as benzonitrile and
α,α,α-trifluorotoluene generally give ortho-cycloadducts with olefins [18]. Some
exceptions are also found. For example, photo addition of 1,3-dioxol-2-one to
benzonitrile gives only meta-adduct of endo-configuration having electron acceptor
substituent at C-2 and C-4 carbon atoms. Possibly the electron acceptor ability of
benzonitrile is higher at ortho- and para-positions [19]. The following examples are
illustrative:
4 3
H
H 5
6 H
H 1
– + 2
h meta only (endo major)
1. + Ref. 10
+ H
H H 4
3 H
5
– 6 1
+ 2
meta-exo (minor)
O
O
O H
O
H H
O O
O hν H
2. + O +
O Ref.11
vinylene carbonate
endo exo
5 : 1
H O
O H
O h H H
O O
3. + O + + H
O H
O
H H
exo Ref. 12
endo
major
94 : 4 meta : ortho = 3.5 : 1
Me O Me h
4. + Me
O Me Me O Me Ref. 12
Me
O
Me
ortho (only)
H CN H
CN h Hg-lamp CN
5. + + Ref. 12
20 oC
H H
exo (73%) endo (17%)
H
O O
O O
OMe H
OMe
O OMe
h
6. + meta-exo exo 37 % Ref 15
O +
H O
anisole 1,3-dioxole O
O O
OMe H
meta-endo OMe
endo 18 %
O
O
O H
O
OCH3 H H
O O O
254 nm H
7. + O +
O MeOH Ref. 14
OCH3 OCH3
endo exo
65 : 35
OCH3
CN H3CO H CN
h H H
+ Ref. 14
cyclohexane
H H
20 : 13
OCH3 OCH3
CN CN
h
8. +
Ref. 15
CH3CN
H
73%
OCH3 OCH3
h CN
+ Ref. 16
MeOH
CN
49% 10%
OCH3 OCH3 OCH3 OCH3 OCH3

H+ transfer
e- transfer – from MeOH -
+ + + Ref. 17
CN CN CN -H+
H
ion pair CN
CN
CN CN
h
9. + Ref. 18
hexane
H
ortho (only)
CN CN
CO2Me h CO2Me
10. + Ref. 13
CH3CN
exo H
O
O
O CN H
4 5 6 O
CN H H
O 3 O O
254 nm
O 2 7 H Ref. 14
11. + 8 +
O MeOH NC
1
endo exo
45 : 55
Benzene and other aromatic hydrocarbons undergo photoaddition reactions with

furan, dienes, and allenes to give para-like adducts as major products by allowed
(π4s + π4s) cycloaddition process. For examples, benzene reacts with furan in molar
ratio 1:1 gives para-like adduct as major product [19] and with allene gives para-
and meta-adducts in a ratio of 2:1 [20].
3' O
2' O 1'
O 4' O
h
+ 5' 1 + +
2 6
3 4
5 meta
para
50 % 30 % 11 %
h CH2
+ +
CH2
Ref. 20
allene
meta-(1,3) para-(1,4)
1 : 2
Naphthalene on direct photoirradiation with 1,3-diene gives [4 + 4]-cycloaddi-

tion product, which undergoes [3,3]-sigmatropic shift to give stable product [21].
2 1
3
H
h 1 3,3
2
+
3
less stable due to strained

trans double bond
1-Cyanonaphthalene undergoes photoaddition with furan to give [4 +4]-

cycloadduct [22].
CN O
NC
h
+ O Ref. 22
The presence of an excess amount of furan in the reaction with benzene resulted
in ortho-adduct as minor product by [2 + 2]-cycloaddition process [19].
O h O
+ + other products mentioned
above
10 %
[2 + 2]-Photoadducts of aromatic hydrocarbons, benzene and naphthalene, and

N-methylindole with acetylenic compounds often undergo electrocyclic ring
opening to give stable products. The following examples are illustrative:
CO2Me CO2Me
CO2Me
h h
+ Ref 23
CO2Me CO2Me
CO2Me
Ph Ph Ph
Ph
Ph
h h
+
Ref 24
Ph
, > 180 oC
Ph
Ph
CO2Me CO2Me CO2Me
h
+ h CO2Me
sens.
N dis
N N
Me CO2Me CO2Me
Ref. 25
Me Me
benzazepine (major)
Intramolecular photoaddition of alkene part with aromatic ring takes place in

non-conjugated aryl olefins when two π systems in a molecule are in close prox-
imity and are separated by four sigma bonds. For example, cis-6-phenyl-2-hexene 1
in solution undergoes intramolecular 1,3-cycloaddition to give two meta-adducts 2
and 3 by the formation of exciplex [26].
6 7 2 7
6 Me
1 hν,Hg-lamp 10
1
+ H
10 cyclopentane H
3 11 Me
1 11 3h H H
2
3
major minor
Similarly, 3-methoxy-4-butenyloxyacetylbenzene 4 gives ortho-cycloadduct 5 [27].
O Me O Me
h
Ref 27
OMe
O OMe
O
4 80 %
5
Indene derivative 6 undergoes intramolecular [2 + 2]-cycloaddition and rear-

rangement reactions through triplet excited state [28].
Ph
Ph Me
hν
Me
thioxanthone
sens Me Me
6 Me Me
T1
Me Me
H Ph
Me Me Ref.28
Ph + Ph
+
Me Me
H Me Me Me
34% 33% 26%
Photo-induced intramolecular [2 + 2]-cycloaddition reaction also occurs in 7

bearing a non-conjugated aromatic ring and acetylenic π system [26].
h
Ph(CH2)3C C-CH3
254 nm
7
CH3
Aromatic compounds of aromatic rings in close proximity undergo
photo-induced [2 + 2]-cycloadditions, e.g., 8 gives 9.
h
Ref 29
8 9
Regioselective [2 + 2]-photoaddition of 4-methyl-1-cyanonaphthalene 10 with

TME gives [2 + 2]-cycloadduct 11 selectively at 1,2-position [30].
CH3
CN H3C
CH3
H3C CH3 NC
h
+ CH3
H3C CH3 PhH
10 CH3 TME 11 CH
3
Both regio- and stereo-selectivities of [2 + 2]-photocycloaddition of ethene 12 to

1-cyano-4-hydroxymethylnaphthalene 13 is observed to get 14 by the hydrogen
bonding between the addends [31].
CH2OH
CH2OH
HO
+ h CH3 CH2OH
Me Me HH
NC Me
CN
14
13 12 major product due to H-bonding
Phenanthrene 15 undergoes [2 + 2]-photoaddition reaction with maleic anhy-

dride at 9,10-positions to give 16 [32].
O
H H O
+ h
O O
O H H O
15 16
9.3.1 Photo-Diels–Alder Cycloaddition Reactions

of Aromatic Compounds
Indole 17 undergoes photo-induced radical cation Diels–Alder reaction with

cyclohexa-1,3-diene 18 in the presence of triphenylpyrylium tetrafluoroborate to
give diastereoselective product 19 [33].
1. Cat. A (5 mol%), CH2Cl2

AcCl, h , 13 h
+
N 2. NaHCO3
17 H N
18 19
Ac
Ph 70%, endo / exo = 3 : 1
BF4– AcCl
product
Ph O Ph N N
Cat. A H H
Naphthalene undergoes [4 + 2]-cycloaddition reaction with reactive dienophile,

N-methylthiazolinedione (MTAD) 20 to give cycloadduct 21 via both singlet and
triplet excited MTAD and also by direct irradiation of ground-state charge transfer
(CT) complex at 458 nm [34]. The cycloadduct 21 on irradiation with triplet sen-
sitizer xanthone at 300 nm gives di-π-methane rearrangement product 22, which
may be utilized as a potential source of diazasemibullvalene 23.
O
O
N h sens N
N N O N
N N N N N
N
22 23
20 O 21
O O
1-Acetyl naphthalene 24 on photoirradiation undergoes [4 + 2]-cycloaddition

reaction with chiral electron acceptor alkene, (S)-(2-methoxymethyl-1-
pyrrolidinyl)-acrylonitrile and its (R)-enantiomer 25 to form chiral (+)- and
(−)-cycloadducts 26 as sole products, which on hydrolysis afford 1,4-diketones 27

in *97 % enantiomeric excess. The absolute configuration of 26 at C-1, C-4 and
C-9 carbons has not yet determined, but the methoxymethylpyrrolidinyl moiety had
probably syn-geometry to the benzenoid ring [35].
O
OMe O
N CuSO4 / Na2HPO4 / H2O CO
NC N h CN rt
9
COMe Me
MeO cyclohexane 1 [H2O]
25 4 27
24 -aminoacrylonitrile 26 76 %, de> 97
derivative (62%) %
1,1-Diphenylethene derivative 28 (as electron donor) undergoes photo-[4 + 2]-

cycloaddition reaction with 1,4-dicyanobenzene (as electron acceptor) in the
presence of phenanthrene sensitizer to give isoquinoline derivative 29 in a PET
process. The reaction takes place in a highly polar exciplex and/or a contact radical
ion pair generated in a PET process, followed by air oxidation [36].
Ph Ph
Ph H R
CN R R
h ( >334 nm) N air N
Ph
R + N H
Ph Phenanthrene (sens) C [O]
28 PhH, rt
CN 29
NC
R = H, Me, Et, n-C5H11 exciplex CN (23%) CN
9.4 Photo-Induced Hydrogen Abstraction and Addition

Reactions of Aromatic Compounds
Photoexcited aromatic hydrocarbons undergo hydrogen abstraction and 1,4-addi-

tion reactions with cycloalkanes and hydroxylic compounds. These reactions are
believed to take place through an excited singlet state of arene to form an exciplex
or ion pair as an intermediate, which on back electron transfer dissociates into
triplet diradical and undergoes proton abstraction from solvent or amines, followed
by addition of an alkyl or aryl unit to give the products [37]. The following
examples of this reaction are illustrative:
9.4 Photo-Induced Hydrogen Abstraction and Addition Reactions … 289
H
1.
H H H H
H
N H H H
N
,h H-transfer H N
2. + H H
N
H H H H
H
* H NHR
RNH2
3.
h
S1 ( *) T1 H H H H
6
OAc
6 H
4 5 H OAc
4 5
AcOH OAc H
4. 3
+ + +
1
h 3 1
2 2
H OAc 6-endo 6-exo
CH2 Me OMe
h
5. + MeOH
1-methoxy-naphthalene (sens)
In entry 4, the reaction proceeds through a prefulvene diradical intermediate.
9.5 Photocyclization Reactions of Aromatic Compounds
cis-3-Styrylpyridine 30 undergoes photocyclization to give azaphenanthrene 31 [38].

9 10
8
1
N N O2 N
h 7 2
-H2O
6 5HH4 3 31
30
Irradiation of diphenylethynyl cycloalkene 32 in isopropanol gives cyclic product,

diphenylbenzo cycloalkene 33 up to 21 % dependent on the alkene ring size [39].
Ph
Ph
CH2 n hν CH2 n
iPrOH
Ph
32 33
Ph
n = 2-4
9.6 Photorearrangement Reactions of Aromatic

Compounds
Irradiation of 3-methoxyphenol 34 in presence of AlBr3 gives lumiketone type

product, 4-methoxybicyclo[3.1.0]-hex-3-en-2-one 35. This reaction provides a
useful method for synthesis of bicyclo[3.1.0]-hex-3-en-2-ones [40].
O
OH 2
AlBr3 (2 eq.) 6 1
3
h , Cu2Cl2
5 4 OMe
OMe 4.5 h 35
34
38 %
The reaction is believed to proceed through a para-protonated Lewis complex.
The presence of excess AlBr3 prevents the cyclo-reversion of the product.
OH OAlBr3 AlBr3 O
O
AlBr3 (2 eq.) AlBr3
h , Cu2Cl2
OMe OMe
4.5 h H H OMe OMe
The light-induced rearrangement of phenyl esters 36 into hydroxy aryl ketones

37 and 38 is called the photo- Fries rearrangement [41]. Because of low yields, this
procedure has little commercial importance. The reaction is usually carried out in an
aprotic solvent and involves a radical mechanism by the homolytic cleavage of the
O-acyl bond from its singlet excited state [41].
O
O OH OH
O R O O O
C
h O + O
+ R
R H 37 38
36 R O R O
The rearrangement is found to be of intramolecular nature. The radical pair

remains in the solvent cage and their recombination in the cage affords the acyl
migration products, while hydrogen abstraction by the phenoxy radical from the
solvent leads to the formation of phenol as by-product. When the reaction of phenyl
acetate was carried out with deuterated phenol in methanol, the major products were
ortho- and para-hydroxy acetophenones and phenol. Only trace amounts of
crossover products were obtained [42]. The presence of methoxy substituent at
meta- and para-positions increases the yield of ortho-Fries product [43]. For
example, 39 gives 40.
9.6 Photorearrangement Reactions of Aromatic Compounds 291
OCOCH3 OH OH OH OH OH OH
COCH3 COCH3
h
+ + + + +
CH3OH
d5 23 % d5 d5
35.7 % COCH3 COCH3
0.7 %
34.7 % 1.1 %
Me
O O OH O
i-
PrOH Me
MeO OMe h MeO OMe
39 40
63.6%
This reaction occurs in nature when a plastic bottle made of polycarbonate

(polyethyleneterephthalate) exposed to sunlight (wavelength 310 nm) leads to
leaching of phthalate from the plastic [44].
The reaction rate of aromatic esters in photo-Fries rearrangements is greatly
influenced by the steric and electronic factors of the substituents. For example, the
reaction rates of 4-t-butylphenyl-4-substituted benzoates 41 are influenced by the
substituent in 4-position. Usually EWG accelerates the rate of the reaction and ERG
retards the rate [45].
O
C X OH O Reaction rate
O X I0
C X
h mole.cm-2sec-1 x 108
CMe3 46.6 ± 2.2
41 CMe3 CN 135.6 ± 6.1
CMe3
It indicates that both the radicals remain associated as pair at all stages of the
reaction because of attractive forces between them.
3,5-Dimethoxybenzyl derivatives 42 (X = OAc, O(PO)(OEt)2) undergo pho-
torearrangement in alcohol solution to give 1,3-dimethoxy-5-
methylenecyclohexa-1,3-dienes 43 in low yields (*16 %) [46].
CH2X CH2 X
5 6
X
h 4 H
MeO OMe MeO OMe MeO 3 2 1 OMe
42 43
Dibenzocyclopentanemethylene derivative 44 undergoes photosensitized-oxa-di-

π-methane rearrangement to give 45 in 96 % yield [47].
H
O CHO
h
m-MeO-PhCOMe (sens)
44 45
Aromatic β,γ-unsaturated oximes, oxime esters, acyl hydrazones and semicar-
bazones 46 undergo photochemical rearrangements into their cyclopropane
derivatives 47. These rearrangements are known as the aza-di-π-methane (ADPM)
rearrangements [48].
h
Ph
Ph N R sens Ph
Ph 46 47
N R
R = OH, OAc, NHCONH2
For example, oxime 48 gives 49.

OH
N
N OH PhCOMe Ref 48
48 49
63 %
9.7 Photooxidation Reactions of Aromatic Compounds
Aromatic hydrocarbons 50 and 51 react with oxygen under the influence of light to
give cyclic peroxides. Oxygen in its singlet state serves as a dienophile.
O2, h AcOH
OMe Ref. 49
OMe [2+2]
50 O CO2Me CO2Me
OMe MeO O
9,10-dimethoxyphenanthrene
O2, h O
CS2 O Ref. 50
51 [4+2]
9.8 Photodimerization Reactions of Aromatic Compounds
Anthracene and 9-substituted anthracene undergo photodimerization via a singlet

excimer, which collapses to dimer in a symmetry allowed [π4s + π4s]-cycloaddition
process. Substituted anthracenes with different substituents at C-9 position give
dimers having head-to-head geometry of the substituents [51, 52].
9.8 Photodimerization Reactions of Aromatic Compounds 293
h
2 Ref. 51
OMe CN
h
+ MeO Ref 51, 52
NC
The kinetic study indicated that the reaction takes place in the following steps:
A+h 1 *
A
1
A +A
1 * (A A)*
(eximer)
1 *
A A+h
f
(fluorescence)
1
(A A)* 2A +h e
(emission)
1
(A A)* A A
A = anthracene
9-Substituted anthracene 52 gives dimer 53 of head-to-tail regiochemistry [51].

R R
h
2
52
53 R
R = Me, Et, OMe, CN, Cl, Br
Similarly, 2-methoxynaphthalene 54 undergoes photodimerization to give two

isomeric products 55 and 56 [53, 54].
R
R
h R +
R
54 R
55 56
R = OMe
Methyl naphthalene 2-carboxylate and 2-cyanonaphthalene 57 undergo pho-

todimerization followed by [2 + 2]-cycloaddition to give cage compounds 58 [55].
R
R
h
57 R
58
R = CO2Me, CN
meta-Methoxystyrene 59 undergoes photodimerization in the presence of

acceptor sensitizer to give tetralin derivatives 60 and 61 via the cyclization of
dimeric cation radical [56].
OCH3 OCH3
h
A.– +
m-dicyanobenzene (sens) +
59
A = Acceptor sensitizer
OCH3 OCH3 OCH3
OCH3 OCH3
+
+
H3CO 61 60
9.9 Photosubstitution Reactions of Aromatic Compounds
Photo-induced aromatic substitution reactions occur through an electron transfer

process, which creates an aromatic radical anion or aromatic radical cation as
intermediate. This intermediate couples with the electrophile or nucleophile radical
to give the product. This mechanism is called SRN1 (where the abbreviations stand
for substitution, radical, nucleophilic, and first order). Photoirradiation of aromatic
compounds in the presence of nucleophiles gives nucleophilic-substituted products
different from those of thermal reaction. For example, 3,4-dimethoxynitrobenzene
on UV irradiation in presence of hydroxide ion gives 3-hydroxy-substituted pro-
duct, while on heating gives 4-hydroxy-substituted product [57].
O OMe OMe
OMe OMe O
h
OH– OH–
NO2 NO2 NO2
Usually electron-withdrawing group such as nitro group makes the ortho- and
meta-positions positive relative to the para-position in the excited state, whereas the
ortho- and para-positions are positive relative to the meta-position in the ground
state. With electron-releasing substituent, the reverse is observed, i.e., the ortho-
and meta-positions are negatively charged relative to para-position in the excited
state. For example, 4-methoxynitrobenzene gives 4-methoxyphenol [57].
OMe OMe
h
OH–
NO2 O
9.9 Photosubstitution Reactions of Aromatic Compounds 295
Photoirradiation of phenylacetate dianions 62 with aryl bromides and iodides in

liquid ammonia gives isomeric arylated phenyl acetic acids 63 and 64 [58].
Ar
ArX, h
+
liq. NH3
HC CO2M CH2CO2H CH Ar
M 62 63 HO2C 64
M = Li+, Na+ or K+
Irradiation of azulene 65 in presence of aryl iodide gives 1-arylazulene 66 [59].

Ar
h
ArI
65 66
Photochemical nucleophilic-substitution reactions of cyanobenzene with allenes

take place in a radical coupling process at the less heavily substituted radical site by
donor–acceptor property. For example, 1,2,4,5-tetracyanobenzene 67 reacts with
1,1-dimethylallene 68 in the presence of diphenyl to give 69 as major product [60].
MeO MeO
NC CN h NC NC
+ +
NC CN MeCN: MeOH (3:1)
NC CN NC CN
67 68 Ph–Ph 45 min 69
TCB major product
5%
37 %
Possibly diphenyl acts as a co-donor to drive the reaction in the forward direction.
The major product of the reaction is formed in a stepwise process as follows:
h
TCB TCB*
TCB* + DP TCB + DP DP = diphenyl

DMA = dimethyl allene
DP + DMA DP + DMA TCB = tetracyano benzene
H
+ OMe
+
MeOH H OMe
OMe
MeO
OMeNC CN NC
+
CN - CN NC -
NC CN NC CN NC CN
Similarly, 1,4-dicyanobenzene 70 on reaction with tetramethylallene 71 gives

substituted product 72 [60].
OMe
CN
h
+
MeCN: MeOH (3:1)
70 CN 71
Ph–Ph 45 min CN 72
42 %
Aromatic amine 73 undergoes photo-induced alkylation with pentafluoro
iodobenzene to give 74 and 75 through a radical coupling mechanism in a
photo-induced electron transfer process [61].
NMe2 NMe2 NMe2 NMe2 NMe2 NMe2

H C6F5
h - HI
+ C6F5I + C6F5 + I- C6F5 + +
PET
73 74 75
H C6F5 C6F5
9.10 Problems
9:10:1. Predict the structure (s) including stereochemistry of the product (s) of the
following reactions with plausible mechanism of formation:
OH
Me
h h 0 oC
(a) (b)
+
O Me
(c) h (d) h
N PhH S Ph
O
OMe CN
(e) OMe CN h MeOH (f) Me Me h
+ +
hexane
Me Me
(g) h (h) h
+
t
H CN BuOH 254 nm
9.10 Problems 297
(i) (j)
CN h 313nm CN Me
+ h
+
OMe Michler's ketone
(sens)
NC
(k) O Ph h DCA (sens) (l)
Ph
O2, MeCN Conc. soln.
Ph Ph
N O h
DCA = 9,10-dicyanoanthracene H
Dil. soln.
MeO OMe h
(m) 1. h
+ +
2. H
9:10:2. Suggest the plausible mechanism for each of the following reactions:
O
N2 OH
(a)
h
O CH2Cl2
O
Ph
H
(b) Ph Me hν (334 nm) Me
Ph
Me Me MeCN
1,4di-CN-C6H4 / Phenanthrene HMe
Me 71%
1. Turro NJ (1978) Modern molecular photochemistry. Benjamin-Cummings,

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87:811
4. D’Auria M, Racioppi R (2013) Oxetane synthesis through the Paterno–Buchi
reaction. Molecules 18:1138
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Chapter 10
Photofragmentation Reactions
10.1 Introduction
The light-induced reactions of organic compounds for introduction of functional

groups at unactivated carbons through free radical attack are known as the
photofragmentation reactions. The fragmented free radicals are generated by the
cleavage of heteroatomic bonds such as C–H, C–N, C–O, C–S, O–N, etc. The most
common reactions of this class are the Barton reaction, the hypohalite reaction and
the Hofmann-Loffler-Freytag reaction. A variety of reagents and conditions have
been employed in these reactions to improve the yields of the reactions. Each of
these procedures follows a generalized mechanism (Scheme 10.1). The first step of
each procedure is the homolytic cleavage of an heteroatomic bond, X–Y, where X
is a heteroatom, O or N, and Y is H, halogen or NO, to form either an oxygen or a
nitrogen radical. The generated heteroatom radical undergoes hydrogen abstraction
normally from the d-carbon in a quasi-chair- like six-atom transition state to gen-
erate a new carbon radical, which in absence of any competing external radical,
adds the radical Y to form a C–Y bond. The presence of a good competing radical
source, such as iodine, diverts the reaction to produce an iodohydrin, which can
cyclize to form a tetrahydrofuran derivative in most cases. The most common
means of generating the requisite oxygen radical (1: X=O) is by thermolysis or
photolysis of a nitrite ester (Barton reaction), a hypochlorite, or a hypoiodite.
Sometimes, lead tetraacetate-iodine or mercuric oxide-iodine variation of the
hypoiodite reaction is carried out to improve the yield of the product. The requisite
nitrogen radicals (1: X=N) are generated by heating or photoirradiation of appro-
priate N-haloamines with sulfuric acid or other acid to afford pyrrolidines or
piperidines (Hofmann-Loffler-Freytag reaction). Sometimes, very weakly basic or
neutral medium is employed to increase the stability of nitrogen radicals. Extensive

302 10 Photofragmentation Reactions
Y R hν or Δ H R
X X R X
δ
α
-Y
1 H-abstraction
-HI ring closure
from δ-C
H Y H R H I
X R addition X addition of external X R
1 of Y radical scavenger I2
X = O; Y = NO : The Barton reaction

= O; = Cl : Pyrolysis of hypochlorite
= O; = I : Pyrolysis of hypoiodite
= O; = H : Pb(OAc)4-I2/HgO-I2 variation of hypoiodite reaction
= H; = Cl : The Hoffmann-Loffler-Freytag reaction
Scheme 10.1 Generalized pathway for photofragmentation reaction
study on these reactions indicated that these free radical functionalizations work
best with rigid substrates that hold the reacting atoms in fixed geometries. Many of
these intramolecular free radical based functionalizations have been reviewed [1, 2].
10.2 The Barton Reaction
The photolysis reactions of nitrite esters into d-nitrosoalcohols are known as the
Barton reactions. The reaction involves the cleavage of O–N bond of nitrite ester,
followed by an intramolecular abstraction of d-H by the alkoxy radical and for-
mation of carbon-nitrogen monoxide bond by radical coupling. The abstraction of
hydrogen is very fast and takes place through a six-membered transition state. In
many cases, nitroso alcohols are either tautomerize into oximes or dimerize.
Labeling studies using nitrogen-15 revealed that the nitrosyl radical is transferred
intermolecularly but not in a cage process [3]. The oxime derivatives are further
hydrolyzed or oxidized.
R H H O OH
R H R R N
O N O hν O O H N O O H R N
+ N O O H
oxime
O O
R N N R
H O O H
10.2 The Barton Reaction 303
This reaction has been extensively utilized in the functionalizations of steroids

and other bioactive organic compounds. For example, 11-hydroxycorticosterone
acetate 2 was converted into aldosterone-21-acetate 3 [4].
O O
O N O H OAc
OAc OAc O
HO O
NOCl hν + NO
C5H5N PhMe
O O 75 minO
2
H
O N O O O
OAc HO N O O OAc
CH OAc
HO HO
1. Ac2O/Py
2. 5 % NaNO2
O O
O 3
N O N O
+ +
RHC N OH + N O RHC N RHC N
OH
OH
H
R C N N O RCH O + N N O
O
H
RHC O R
RCH O + R OH
OH
In steroid 4, d-hydrogen abstraction takes place from C(19)-methyl instead of C

(18)-methyl to give oxime 5 [5].
18 C8H17 C8H17
O N O HO
HO N
19
hν
H H
AcO AcO
H 4 H
5
66%
Other important applications of Barton reaction are in the synthesis of alkaloids

and terpenoids. For example, the photolysis of nitrite 6 was used in a crucial step in
the synthesis of alkaloid perhydrohistrionicotoxin 7 [6]. The oxime 8, formed in
about 20 % yield, gives the spirolactam ring of the alkaloid 7 on Beckmann
rearrangement.
hν
OH
C5H5N OH
O N O NOH N
6 8 H
7
The Barton reaction (BR) is also used in key steps in the synthesis of tetracyclic
triterpenoid, azadiradione 9 [7] and 3(a-hydroxyethyl)-quinuclidine 10 for syn-
thesis of cinchona alkaloids [8].
O O
OEt OEt
O P O P
OEt OEt
OH HO3S-O-NO
O O
Py ONO
O 0 oC, 30 min O
O O O
OEt OEt
O P O P
OEt OEt
hν OH OH
O
O O
DCM, 50 oC
N O N OH
BR O O
28% O OAc
9
O-N=O OH
H H
H
H
hν
N N
HO=N 10
50%
10.3 The Hypohalite Reactions
The photolysis of a tertiary hypohalite (readily prepared from the corresponding

alcohol) generates alkoxy radical, which abstracts hydrogen from d-carbon to
produce an alkyl radical. The resulting alkyl radical abstracts halogen atom from a
second molecule of the hypohalite to give d-haloalcohol, which is cyclized in
presence of a base to form a tetrahydrofuran derivative [9]. These reactions are very
similar to the Barton reaction and proceed through a six-membered cyclic transition
10.3 The Hypohalite Reactions 305
state. For example, the hypochlorite 11 of 2-methyl-2-hexanol gives tetrahydro-

furan derivative 12.
Cl hν ROCl
11 O
H O HO Cl HO
OH Cl - Cl
O
O 12
This method has been used in the synthesis of steroids for introduction of
functionality at angular methyl groups. For example, steroid 13 gives 14 [10].
HO O
18
1. Cl2O
19
2. hν
3. -OH
13 14
In the hypochlorite reactions, intramolecular hydrogen abstraction competes

with b-cleavage and other reactions. For example, tertiary hypochlorite 15 on
photolysis gives ketones and alkyl chlorides from b-cleavage and radical coupling
reactions [10].
O O O
O hν
Cl Cl + + + + + MeCl
Et Cl
95%
15
Thus, the hypochlorite reactions can produce several products by multiple

functionalizations. Moreover, the stabilities of the hypochlorites are much less
compared to hypoiodites. Hence, the photolysis of hypoiodites is more convenient
method for generation of oxyradicals. The generated oxyradicals on hydrogen
abstraction from d-C give the corresponding alcohols. The hypoiodites are unstable
at reaction temperatures and are prepared in situ by irradiation of the solutions of
the corresponding alcohols in presence of N-iodosuccininimide (NIS), acyl
hypoiodites or diacetoxyiodobenzene (DIB), lead tetraacetate, mercuric acetate or
mercuric oxide and iodine. The resulting alcohols form either five-membered ring
oxides or acetals [11]. For example, the steroid acetate 16 gives cyclic ether 17 in
75 % yield [12].
R
R R
hν H
Pb(OAc)4, I2, CCl4 AcO
AcO AcO Br
Br Br O
OH O I
16
R= R R R
I
I
O
AcO AcO AcO (75%)
Br Br Br 17
OH OH
The hypohalite method of lead tetraacetate and iodine has been applied in the
preparation of a key intermediate 18 for the synthesis of labdane diterpene, manool
18a [13].
O O
OH O OH
Pb(OAc)4, I2
hν
18 (60%) H 18a
Sesquiterpene, cedrol 19 and its derivatives are functionalized at remote carbons

to produce 20 and 21 using DIB, diphenylselenium acetate and iodine or HgO and
bromine. The use of HgO/Br2 and selenium acetate improves the yield of the
reaction [14, 15].
DIB/Ph2Se(OAc)2
OH I2, hν O -H O
or 20 21
HgO, Br2 , hν DIB: 56% -
19 20
Ph2Se(OAc)2: 67% 31%
HgO: 60% 25%
O
+X O
X
X = I, Br
21
10.3 The Hypohalite Reactions 307
The hypoiodite method was also applied in a key step in the synthesis of
antibiotic talaromycin 22 from 22a [16].
HO
O O
HgO, I2, hν
O O
CCl4 O
O O HO
Ph O O OH
22a
Ph 22
major product (55%)
10.4 The Hofmann-Löffler-Freytag Reaction
The thermal or photolytic decompositions of N-halogenated amines in acid (H2SO4

or CF3CO2H) solutions followed by basification of the reaction mixture to produce
pyrrolidines or in some cases piperidine derivatives are known as the Hofmann-
Löffler-Freytag(HLF) reactions. For example, N-chlorobutylamine 23 gives
N-butylpyrrolidine 24 [17].
Cl 1) 85 % H2SO4, hν
N N
23 2) -OH 24
The reaction proceeds by homolytic cleavage of protonated halogenated amine

to a nitrogen cation-radical, which undergoes an intramolecular abstraction of
hydrogen atom from the d-carbon via a six-membered cyclic transition state to form
an alkyl radical. The resulting alkyl radical abstracts a halogen atom from another
molecule of halogenated amine to form a protonated d-halo amine 23a. The pro-
tonated d-halo amine on basification undergoes intramolecular cyclization to form
pyrrolidine derivative by elimination of hydrohalic acid [18].
Cl Cl hν H
H .
N N . N
Cl
H H
.
H Cl Cl
H R2NHCl OH ..
N N
N H+
H 23a H
H
HCl
N
When an N-halogenated amine contains two d carbons, the abstraction of hydrogen

atom preferably takes place from secondary and tertiary d-carbons. For example,
N-chloro N-butyl N-pentyl amine 25 gives only 1-n-butyl-2-methylpyrrolidine 26.
1-Amylpyrrolidine 27 is not detected.
1) H2SO4,hν
N N
N -
2) OH
25 Cl 27
26 (not detected)
This reaction was utilized by Hofmann for synthesis of ö-coneceine 28 from

N-bromoconiine 29 [19].
1) H2SO4, 140 oC
N N
2) NaOH
Br
29 28
Loffler reported the synthesis of alkaloid nicotine 30 using this reaction [20].
1) H2SO4, Δ
N N
Br Me 2) NaOH N Me
N 30
This reaction has been used extensively for introduction of functionality in

different classes of nitrogen heterocycles. For example, this reaction has been used
in the synthesis of alkaloid, N-methylgranatinine 31 from N-chloro-N-
methylcyclooctylamine 32 [21] and in a key step in the synthesis of steroidal
alkaloid, dihydroconessine 33 [22].
H2SO4, hν, Cl1

or NMe
H2O2, dark
N Cl
24 %
Me
32 31
Me NCl H
MeHN H
N-chlorosuccinimide 1) H2SO4, hν
2) -OH
HO HO
Me H Me
H N H N H
Cl
HO HO
33 90%
10.4 The Hofmann-Löffler-Freytag Reaction 309
The HLF reactions have been applied in the synthesis of different classes of
organic compounds using usual acidic medium. The following examples [23–28]
are illustrative:
H 1) H+, hν
1. H
CO2Et Ref. 23
Cl N 2) NaOH N CO2H
H H
85 %
L-proline
1
n-Bu 1) H2SO4,hν
2. 2 Ref. 24
N Cl 2) NaOH N
4 3
5
n-Bu
25 %
3-butyl-5-methyloctahydroindolizine
2 8
1) CF3CO2H, hν 1
3. 4
Cl 3 5 Ref. 25
2) NaOH O 7 N
Me 6
Me
26 %
6-methyl-6-azabicyclo[3.2.1]-octan-4-one
12
11 13
1 159 14
1) CF3CO2H, hν 2 10 Kobusine type alkaloids
16 N 8 Ref. 26
4. N Cl 3 7
2) NaOH 5
4 6
39 %
(±) 6,15,16-iminopodocarpane-8,11,13-triene
5. H2SO4-HOAc, hν 290 C
o
Cl N . HCl Ref. 27
N 10 min
1h N
Cl 85 % H 34 %
1-methyladamantano-[1,2b]-pyrrolidine
Cl N
N
1) CF3CO2H, hν H Ref. 28
6. H
2) NaOH H H
H H HO
HO H 85 %
H
Conessine derivative
Kimura and Ban modified the condition of HLF reaction in weakly basic
medium to improve the yield of the reaction [29]. The authors suggested that
weakly basic medium increased the stability of the intermediate alkyl radical. They
reported the synthesis of dihydrodeoxyepiallocernuine 34 using this reaction con-

dition [29].
H H
Et3N, hν
N N NH . N N N
Cl
H H
30 %
34
Baldwin and Doll modified the HLF reaction condition in neutral medium to
prepare a key tricyclic intermediate 35 for the synthesis of alkaloid gelsemicine 36
and found better yield of the product [30].
O O
O O
O OMe
hν
N Cl N Ac
NO N
Ac H
35 (43 %) 36 OMe
2-aza-7-oxatricyclo[4.3.2.04,8]-undecane
Suarez et al. modified the HLF reaction condition in neutral medium for the
substrates, N-nitroamides, N-cyanamides, N-phosphoramidates and N-carbamates
of steroids 37 for synthesis of five membered nitrogen ring compounds 38 using
iodine and DIB or LTA as oxidizing agent, and a tungsten lamp for irradiation with
visible light [31]. All these substrates react with hypervalent (trivalent) iodine
reagents to generate nitrogen-radicals via homolytic cleavage of iodoamide inter-
mediate. The nitrogen radicals undergo hydrogen atom abstraction from d-carbons,
followed by addition of iodine radical to d-C and ring formation via elimination of
HI (Scheme 10.2). N-Phosphoramidate substrate gives an excellent yield and other
substrates give fair to good yields. The following examples are illustrative [31–33].
H I
R N EWG Pb(OAc)4 or DIB R N EWG R N EWG δH-atom
I2, hν abstraction
H
H
R R I
N EWG +I N EWG base R N EWG
-HI
EWG = NO2, CN, P(O)(OR)2, CBz, Boc
Scheme 10.2 The generalized pathway of HLF reactions in neutral medium

10.4 The Hofmann-Löffler-Freytag Reaction 311
Pb(OAc)4 or DIB
I2, hν N X
37
AcO H AcO H 38
NH-X
X = NO2 63%
= P(O)(OEt)2 99%
= CN 64%
Using this modified reaction condition, Suarez et al. synthesized oxoindolizidine

39 in 82 % yield [34].
PhI(OAc)2, I2
N N
hν or Δ
O H
O 39
1-aza-bicyclo-[4.3.0]nonan-2-one
Yokoyama et al. applied the HLF reaction in neutral medium to sulfonamides 40

and found excellent yield of 41 [35].
O O
NH DIB
SO2CF3 I2, hν / Δ
40 N
41 SO2CF3
72%
10.5 Problems
10:5:1. Predict the product(s) with structure and stereochemistry of the following
reactions:
OAc
1) ONCl, Py
(a) (b) H hν
2) hν, DCM, 0 oC ONO
n-Bu OH H H PhMe
OAc AcO
(c) (d) Br
H hν
t-
Bu N hν
PhMe
H R
O
AcO
H
ONO H2N O
(e)
O Hg(OAc)2
I2, hν
10:5:2. Suggest the mechanism of the following reactions:
(a) NCS, ether, Et3N

NH N N N
then hν, (Hg lamp), 0oC, 3.5 h, N2 atm
(100%)
(b) OH
HgO O
O O
I2, hν
Cl3CH2C-O Cl3CH2CO 50%
o
(c) 1. DIB, I2, 60 - 70 C, 2 h, W-hν (CH2Cl)2
NH
Z 2. PhSH, K2CO3, MeCN N
H
Z= S NO2
O2
H2N t
HN
(d) 1) BuOCl
NMe 2) ether, rt NMe. 2HCl
hν
1. Kopecky J (1992) Organic photochemistry. Wiley VCH, New York.

2. Majitich G, Wheless K (1995) Remote intramolecular free radical functional-
izations: An update. Tetrahedron 51:7095–7129.
References
1. Heusler K, Kalvoda J (1964) Angew Chem 3:525

2. Majetich G, Wheless K (1995) Tetrahedron 51:7095
3. Barton DHR, Hesse RH, Pechet MM, Smith LC (1979) J Chem Soc Perkin Trans 1:1159
4. Barton DHR, Beaton JM (1960) J Am Chem Soc 82:2641
5. Barton DHR, Budhiraja RP, Mc Ghie JF (1969) J Chem Soc C 336
6. Corey EJ, Arnett JF, Widiger GN (1975) J Am Chem Soc 97:430
7. Corey EJ, Hahl RW (1989) Tetrahedron Lett 30:3023
8. Stotter PL, Hill KA, Friedman MD (1987) Heterocycles 25:259
9. Walling C, Padwa A (1963) J Am Chem Soc 85:1597; Akhter A, Barton DHR (1964) J Am
Chem Soc 86:1528
10. Norman ROC, Coxon JM (1993) Principles of organic synthesis, 3rd edn. ELBS with
Chapman & Hall, Oxford, p 555
References 313
11. Akhtar A, Barton DHR (1964) J Am Chem Soc 86:1528; Heusler K, Kalvoda J (1964) Angew
Chem Int Ed 3:525; Mihailovic ML, Gojkovic S, Konstantinovic S (1973) Tetrahedron
29:3675
12. Hadd HE (1978) Steroids 31:453
13. Ceccherelli P, Curini M, Marcotuillio MC, Mylari BL, Wenkert E (1986) J Org Chem
51:1505
14. Brun P, Pally M, Waegell B (1970) Tetrahedron Lett 331; Brun P, Waegell B (1976)
Tetrahedron 32: 1137
15. Dorta RL, Francisco CG, Freire R, Suarez E (1988) Tetrahedron Lett 29:5429
16. Kay IT, Bartholomew D (1984) Tetrahedron Lett 25:2035
17. Wolff ME (1963) Chem Rev 63:55; Wawzonck S, Culbertson TP (1959) J Am Chem Soc
81:3367
18. Corey EJ, Hertler WR (1960) J Am Chem Soc 82:1657
19. Hofmann AW (1885) Ber 18:109; Lellmann E (1890) Ber 23:2141
20. Loffler K, Freytag C (1909) Ber 42:3427; Loffler K, Kober S (1909) Ber 42:3431; Loffler K
(1910) Ber 43:2035
21. Wawzonek S, Thelan PJ (1950) J Am Chem Soc 72:2118
22. Van de Woude G, van Hove L (1973) Bull Soc Chim Belg 82: 49; ibid (1975) Bull Soc Chim
Belg 84:911; van de Woude G, Biesemans M, van Hoe L (1980) Bull Soc Chim Belg 89:993
23. Titouani SL, Lavergne JP, Viallefont P, Jacquier R (1980) Tetrahedron 36:2961
24. Sonnet PE, Oliver JE (1975) J Heterocycl Chem 12:289
25. Esposito G, Furstoss R, Waegell B (1971) Tetrahedron Lett 12:899
26. Shibanuma Y, Okamoto T (1985) Chem Pharm Bull 33:3187
27. Narayanan Vl, Setescak L (1971) J Org Chem 3: 4127
28. Hora J, Sorm F (1968) Collect Czech Chem Commun 33:2059; Van De Woude G, Van
Hore L (1973) Bull Soc Chim Belg 82:49
29. Kimura M, Ban Y (1976) Synthesis 201; Ban Y, Kimura M, Oishi T (1976) Chem Pharm Bull
24:1490
30. Baldwin SW, Doll RJ (1979) Tetrahedron Lett 3275
31. Betancor C, Concepcion JI, Hernandez R, Salazar JA, Suarez E (1983) J Org Chem 48:4430
32. Carrau R, Hernandez R, Suarez E, Betancor C (1987) J Chem Soc Perkin Trans 1:937
33. De Armas P, Francisco CG, Hernandez R, Salazar JA, Suarez E (1988) J Chem Soc Perkin
Trans 1:3255
34. Dorta RL, Francisco CG, Suarez E (1989) Chem Commun 1168
35. Togo H, Hoshina Y, Muraki T, Nakayama H, Yokoyama M (1998) J Org Chem 63:5193
Chapter 11
Photochemistry in Nature and Applied
Photochemistry
11.1 Introduction
Several photochemical processes take place in nature for the evolution of life and
their existence on earth. Such evolutionary processes of life have been brought
about by photochemical reactions of supramolecular devices. Among these pho-
tochemical reactions, photosynthesis of plants, vision of animals, and depletion of
stratospheric ozone layer are significant. Based on these photo-induced reactions,
several devices have been developed to meet the needs of our daily life. For
example, zeolites have been used in the field of sunscreens and purification of
drinking water, while porphyrin, metalloporphyrin, and fullerene have been used in
the building block of triads for the harvesting of solar energy as electrical energy.
Artificial photosynthesis systems have been developed for the most attracting
fuel-producing reaction in photolytic cleavage of water.
11.2 Depletion of Stratospheric Ozone Layer

from Photochemical Degradation
The stratospheric region of the atmosphere is located above 15–35 km from the
surface of the earth. It contains a deep layer of ozone that acts as a filter of harmful
UV radiation of sunlight to reach the earth’s surface and thus protects us from
hazardous effect of UV radiation. The massive loss of ozone in the stratosphere
occurs daily by atmospheric pollutants, UV-induced photolysis of ozone in the
presence of man-made chlorofluorocarbons (CFCs), hydrochlorofluorocarbons
(HCFCs), halons (brominated hydrocarbons), CCl4, and methylchloroform
(CH3CCl3). These halocarbons generate halogen radicals which have active roles for
photolysis of ozone. These are also derived from gaseous chlorine and hydrochloric

316 11 Photochemistry in Nature and Applied Photochemistry
acid, which are ejected from volcanoes. The massive loss of ozone has created
several holes in ozone layer, known as ozone holes.
hν . .
CFCl3 CFCl2 + Cl
hν . .
CH3Br CH3 + Br
. .
X +O3 O2 + XO
. . .
(X = Cl, Br)
hν . .
CCl4 CCl3 + Cl
hν .
Cl2 2Cl
. .
XO +O3 X + 2O2
It is a chain reaction and thus one chlorine radical can break down more than
100,000 molecules of ozone. Bromine radical is more destructive than chlorine
radical because of its extended chain cycle. These CFCs are widely used as coolants
in refrigerators, cold cleaning solvents, aerosol spray cans, and foaming products.
CCl4 and halons are widely used in fire extinguishers and air conditionings. The
lifetime of CFCs and halons are more than a century. The emission of CFC-113a
(1,1,1-trichloro-2,2,2-trifluoroethane) in the atmosphere has jumped to 45 % in
2010–2012 [1]. The ozone layer protects the UV-B (280–320 nm) radiation from
sunlight to reach the earth’s surface. Due to these ozone holes, the UV radiation
from sun reaches the earth’s surface and causes malignant melanoma, corneal
damage, cataract, DNA mutation, and enormous growth of harmful cyanobacteria.
To reduce the levels of different halocarbons in the atmosphere, an International
Treaty, known as Montreal Protocol was signed in 1987 by different countries for
the use of less hazardous chemicals as alternative to halocarbons.
11.3 Photochemical Smog in Polluted Zones

of Troposphere
The photochemical smog (polluted smoke) in the industrial areas is the product of
photochemical reactions of primary air pollutants such as nitrogen oxides (NO2,NO)
and hydrocarbons in the presence of bright sunlight. These pollutants are generated
from emissions of vehicles and industrial plants. These primary pollutants undergo
complex reactions to give secondary pollutants such as ozone, aldehydes, peroxy-
acyl nitrate (PAN), peroxybenzoyl nitrate (PBzN), and particulate matter which are
the major constituents of smog. The following are the major reactions:
11.3 Photochemical Smog in Polluted Zones of Troposphere 317
hν
NO2 NO + O
O + O2 O3
O2 + hν 2O
.
O + hν O. + e
O3 + hν O + O2
. .
O + H2O 2 OH
. .
OH + RCH3 RCH2 + H2O
.hydrocarbon .
RCH2 + O2 RCH2O2
. .
RCH2O2 + NO RCH2O + NO2
. .
RCH2O + O2 RCHO + HO2
. .
HO2 + NO OH + NO2
. .
RCHO + OH RC=O + H2O
. O
RC=O + O2 R C .
O O
O O
R C . + NO2 R C
O O O O NO2
PAN, R = CH3
PBZN, R = C6H5
The formation of photochemical smog in the industrial belt reduces the visibility
and causes health hazards such as headache, eye irritation, cough, bronchial, other
respiratory problems, and pulmonary edema (accumulation of fluids in lungs).
These pollutants also create irritations in lungs.
11.4 Photochemistry of Vision: Geometrical Isomerisation

of Retinal
Our vision involves a photochemical process in the visible light (400–800 nm). The
retina of the eye is lined with millions of photoreceptor cells, called rods and cones.
The rod cells are sensitive to dim light, whereas the cone cells are sensitive to bright
light and color vision. Hence, rod cells are black and white receptors while cone cells
are color receptors. In color vision, there are three types of cone cells corresponding
to red (*622 nm), green (*535 nm), and blue (*455 nm) light receptors. In both
rod and cone cells, small guest molecules, 11-cis-retinal 1 is held within the internal
cavity of the much larger protein host molecule (opsin) as a result of noncovalent
1 7 9 11
2 12
6
3 8 10 13 -NH2
5
14 lysin moiety
4 15 of opsin
11-cis-retinol
1 H O N-Enz
11
hν [H2O]
N-Enz
vis light
2
opsin
trans isomer cis isomer rhodopsin / conopsin
Fig. 11.1 Photochemical reaction in the vision process
bonding and thus the light-absorbing part of rod cells is called rhodopsin and of cone
is conopsin. After absorbing a photon, the 11-cis-retinal undergoes photoisomer-
ization into its geometrical 11-trans-retinal 2 having all trans-double bonds
(Fig. 11.1). This change in molecular geometry of the retinal does not fit well in the
protein opsin, and so a series of geometrical changes occur in the protein and
the attached plasma membrane resulting to set a reverse potential difference across
the nerve cell membrane. This potential difference is passed along the adjoining
nerve cell as an electrical impulse. The nerve cell then carries the impulse to the
brain, where the visual information is interpreted. Thus, eye functions as a transducer
as does CCD (charge coupled device) camera by the photochemical reactions
between 11-cis-retinal and opsin. The time taken for this whole process is of the
order of a few picoseconds. After the process, the trans-isomer is hydrolyzed and
converted to cis-isomer and attached well to opsin for the recyclic process.
11.5 Phototherapy of Neonatal Jaundice
Neonatal jaundice occurs in a newborn baby due to deposition of yellow pigment

bilirubin in brain cells and skin because of abnormal liver function. It occurs due to
rapid breakdown of hemoglobin to bilirubin in red blood cells compared to
breakdown of bilirubin in liver. Untreated baby suffers from the damage of central
nervous system. For treatment of this disease, the affected baby is subjected to
phototherapy with visible bluish-green light.
In bilirubin molecule 3, two isomerizable double bonds at C(4) and C(15)
normally exist as cis,cis-isomer. On exposure to visible blue-green light, isomer-
ization of one or both double bonds takes place to produce trans,cis-(4E,15Z)
(major) 4 and trans,trans-(4E,15E) (minor) bilirubins (Fig. 11.2) [2]. These com-
pounds can form hydrogen bonds with water molecules and become highly soluble
in water and are excreted in urine, relieving the baby from this toxic effect. The
isomerization reaction is very fast and occurs in femtosecond range.
11.6 Photosynthesis of Plants and Bacteria 319
2 3
5
4 6 7
8 4
H
N O HN OH
O HN HN O
H 9
O 400 - 525 nm H O
10 O H O HN
hν
HO H NH
11
O O O
12 19
HN
NH H 15
13 18
14 16
O
17 H
15
4
3
cis,cis-isomer of bilirubin trans,cis-isomer of bilirubin
Fig. 11.2 Cis–trans-isomerisation of bilirubin
11.6 Photosynthesis of Plants and Bacteria
Photosynthesis is a photochemical process by which green plants, sea weeds, algae

and certain bacteria adsorb solar energy and utilize it to convert the atmospheric
carbon dioxide to carbohydrates in the presence of water. The overall reaction for
all cases except for photosynthetic bacteria may be written as:
nCO2 þ nH2 O þ hv ! ðCH2 OÞn þ nO2 ; DG ¼ 500 kJ mol1 ðCO2 Þ
Photosynthetic process occurs on a large scale, fixing more than 2 × 1011 tons of
carbon from atmospheric CO2 as carbohydrates per annum.
Photosynthetic process occurs in two stages, namely light reactions and dark
reactions.
Light reactions: Solar light energy is converted into short-term chemical
energy, producing oxygen from water as a by-product. Certain reducing agents,
such as NADPH is formed by hydrogen atoms of water with simultaneous phos-
phorylation of ADP to ATP. NADPH and ATP are considered as chemical energy
produced in the light reaction of photosynthesis [3].
2H2 O þ 2NADP þ þ 2ADP þ 2Pi þ hv ! O2 þ 2NADPH þ 2H þ þ 2ATP
Dark reactions: The short-term chemical energy from light reactions is utilized
for reduction of carbon dioxide into glucose in the absence of light [4].
6CO2 þ 18ATP þ 12NADPH þ 12H þ ! C6 H12 O6 þ 18ADP þ 18Pi þ 12NADP þ þ 6H2 O
Dark reactions do not imply that these reactions take place in the dark. It implies
that these reactions occur without light but take place simultaneously with light
reactions in daytime.
Fig. 11.3 Structures of Me

chlorophyll a and chlorophyll
b
H N
Me R
C20H39O N Mg N
O H N
H
Phytol chain
MeO2C Me
O
Chlorophyll a: R = Me
Chlorophyll b: R = CHO
We will concern about the light-induced photochemical reactions of the supra-

molecules, chlorophylls. The light reaction takes place in the molecular device
located mainly in the leaves of the plants. The leaves of plants are green because
they contain the light-absorbing pigments, called chlorophylls, which absorb the
blue and red regions of visible white light, leaving the intermediate green light to be
reflected to our eyes. A chlorophyll molecule (Fig. 11.3) consists of two major
parts, one rigid, planar and conjugated porphyrin ring co-ordinated with Mg2+ ion,
and a long hydrophobic phytyl chain, which keeps the chlorophyll molecule
embedded in the photosynthetic membrane of leaves. Accessory pigments called
carotenoids (such as β-carotene; Fig. 11.4) and blue or red phycobilins (such as
phycoerythrobilin; Fig. 11.4) are also found in plants. These accessory pigments are
essential for photoprotective mechanism employed by the plants to dissipate excess
photon energy absorbed by chlorophyll as heat, thus preventing the formation of
highly reactive oxygen species and protecting the biological system of leaves from
photochemical damage. Light reactions of photosynthetic process occur in the
following sequence:
Light harvesting Several hundred chlorophyll pigment molecules act together as
the photosynthetic unit, which is made up of two basic sections, light-harvesting
antenna and the reaction center. The light-harvesting antenna allows the absorption
of light of a broad range of wavelengths and its rapid transfer to the reaction center.
The energy transfer occurs by means of the Coulombic long-range mechanism.
Excitation of the reaction center is over within a few femtoseconds. The harvested
light is transferred as electrons through an electron transport chain of Z-shape,
called the Z-scheme (Fig. 11.5).
Reaction centers Light-dependent reactions take place at two reaction sites,
photosystem-I (PS-I) and photosystem-II (PS-II) and these are connected in
Z-scheme. Both PS-I and PS-II are associated with chlorophylls a and b in different
proportions. PS-I is characterized by its absorption maxima at longer wavelength
(700 nm) and is designated as P700, P meaning pigment and 700 being the
wavelength in nanometre at which the dimer absorbs most strongly. Similarly, PS-II
is designated as P680 (maximum absorption at 680 nm). The chemical reactions
occur within a few picoseconds after absorption of light.
β-carotene
COOH
COOH
O N N N N O
H H H
phycoerythrobilin
Fig. 11.4 Structures of β-carotene and phycoerythrobilin
Fig. 11.5 Photochemical electron transport chain in a Z-scheme during light-dependent reactions
of photosynthesis. EA and ED refer to the electron acceptor and electron donor of the two
photosystems. Adapted with permission from (Wardle B, 2009 Principles and Applications of
Photochemistry, Wiley, p. 226). Copyright (2009) John Wiley & Sons
P700 chlorophyll unit absorbs light and is excited to P700*. P700* loses an
electron to the electron acceptor EA2 (P-430). EA2 transfers its electron to NADP+
through a number of electron carriers including ferredoxin. After transferring its
electron, P700* becomes P700+ (in the oxidized state) and it requires one electron
to reach its ground state.
hν
P700 P700* P700+
EA2 EA2-
Then the PS-II comes into operation. P680 absorbs light and is excited to P680*.
The excited P680* loses an electron to an unidentified electron acceptor EA1. EA1
transfers its electron to ED2 (plastocyanin). ED2 transfers its electron directly to
P700+ and P700+ returns to its ground-state P700 and continues the process of light
absorption. After losing an electron, P680* becomes P680+, which is strongly
oxidizing and its redox potential is very close to that of O2/2H2O system (E0,
+0.816 V). So, water molecule in the presence of Mn(II)-protein absorbs light and
splits into O2 and H+ with the liberation of one electron. The liberated electron is
received by P680+ and returns to its ground-state P680 for continuation of photo-
synthetic process.
hν
P680 P680* P680+
EA1 EA1-
H2 O þ hv ! 2H þ þ 1=2O2 þ 2e
NADP+ after receiving two electrons, one from PS-I and another from PS-II, is
converted to electron-rich NADPH in the presence of flavoprotein {Fd(II)} called
ferredoxin-NADP oxidoreductase.
NADP+ + H+ NADPH
2 Fd(II) 2 Fd(III)
Thus, NADPH is formed along with O2, H+ and ATP in the light phase of
photosynthesis. The products, NADPH and ATP are utilized in the dark phase of
photosynthesis for fixation of CO2 as glucose.
nCO2 þ nH2 O þ hv ! ðCH2 OÞn þ nO2
For the assimilation of one molecule of CO2 by green plants, two molecules of
NADPH are needed. To utilize H2O, both PS-I and PS-II are to be activated four
times each to produce the four electrons required to reduce 2 NADP+. Therefore, a
total of eight quanta of light will be required for conversion of one mole of CO2 into
carbohydrate.
2NADP þ þ 2H2 O þ hv ! 2NADPH þ 2H þ þ O2
Archaebacteria use a simpler method using a pigment, archaea rhodopsin similar

to vision process. The pigment changes its configuration in response to sunlight
acting as a proton pump producing a proton gradient for conversion of light energy
into chemical energy. This process does not involve CO2 fixation and does not
release O2 [5].
11.6.1 Artificial Photosynthesis
The multi-step electron transfer process in natural photosynthesis has been utilized
in the construction of various triads using porphyrin, metalloporphyrin, fullerene,
and imide as basic components for harvesting solar energy as electrical energy and
for photoreduction of water to get clean fuel hydrogen [6, 7]. Recently, tetrads,
pentads and hexads have been constructed using porphyrin, fullerene, and a
chromophoric unit as basic components for fast energy transfer process.
11.7 Photo-Induced DNA-Damage and Its Repair
Pyrimidine base, thymine 5 present in DNA, on exposure to UV light undergoes

dimerization to give 6 and 7 by [2+2]-cycloaddition reaction [8].
O O O O
H
N O
HN hν HN NH HN
2 +
UV light NH
O N O N N O O N
H H H H
5 6 7 O
Due to this dimerization, this dimeric thymine alters the structure of DNA and
consequently inhibits replication of DNA. Such mutation of DNA results in cell
death in some instances. Repairing of this mutation may be done by exposing the
DNA at longer wavelength (>300 nm) of light [9].
11.8 Conservation of Solar Energy as Electrical Energy:

Photovoltaic Solar Cells
The conversion of solar energy into direct current electricity can be achieved by the
use of photovoltaic solar cells. Photovoltaic solar cells are electrical cells based on
semiconductors that produce electricity from sunlight and deliver electricity to an
external load. It is now the third important renewable energy source after hydro and
wind power resources. More than 100 countries use solar cells for the harvest of solar
energy. According to the estimate of International Energy Agency, about 177 GW
(giga-watt) of electricity has been produced in 2014 from solar photovoltaic

installations and Germany is the world’s largest producer of electricity from solar
resource, contributing about 7 % of its annual domestic electricity consumption [10].
Commercial solar cells were initially developed using silicon-based p–n junctions as
semiconductors. Silicon-based p–n junctions were made by n-type doping with
arsenic or phosphorous atom (five valence electrons) in a silicon crystal and p-type
doping with gallium or indium (three valence electrons) in a silicon crystal. The
n-type doping provides an extra loosely bound electron that is more easily excited
into the conduction band (CB) than in case of pure silicon and results in electrical
conductivity of CB electrons; whereas p-type doping creates a hole in the silicon
crystal from which thermally excited electrons from the valence band (VB) can
move, leaving behind mobile holes. When these n-doped and p-doped silicons are
joined together into a single semiconductor crystal, the electrons in CB and VB have
an average potential energy, known as Fermi level. Excess electrons move from the
n-type side to the p-type side, resulting in a buildup of negative charge along the
p-type side and a buildup of positive charge along the n-type side at the interface.
When light is absorbed by the p–n junction, it acts as a photovoltaic cell resulting in
the promotion of electrons from the VB to CB, forming an electron hole pair. When
these p- and n-type silicon sites are connected to an external load, the electron–hole
pair tends to separate, resulting in the production of current from the flow of elec-
trons from the n-region to p-region. Thus, the n-type and p-type silicon sites become
the negative and positive pole, respectively, of the solar cell (Fig. 11.6).
Fig. 11.6 The working mechanism of a silicon p–n junction solar cell. Adapted with permission
from (Wardle B, 2009 Principles and Applications of Photochemistry, Wiley, p. 217). Copyright
11.8 Conservation of Solar Energy as Electrical Energy … 325
The original silicon-based p–n junction solar cells have been replaced by recent
organic solar cells to reduce the cost of production.
The dye-sensitized solar cells (DSSCs) have received more attention from the
industry because of high efficiency, low cost, environment friendliness, low inci-
dent light angle dependence, flexibility, etc. In these dye-sensitized solar cells,
nanometer-sized TiO2 (or Nb2O5) particles are allowed to absorb large amounts of
colored organic dye based on Ru(II) having a broad absorption range of visible
light. For example, ruthenium-polypyridine dye, Ru(4,4′-dicarboxy-2,2′-bipyr-
idine)2 (NCS)2 8 is used. The nanoparticles of TiO2 are deposited on a glass support
covered with a transparent layer of Sn-doped indium oxide (ITO) having electron
conduction property. To regenerate the sensitizing dye from its oxidized form, a
liquid electrolyte solution of iodide/triiodide mixture is used as a mediator in
between the electrodes. Usually ITO and Pt are used as working and
counter-electrode, respectively. As in a conventional alkaline battery, ITO acts as
anode and Pt as cathode in the redox shuttle electrolyte, I3/I. The schematic
structure of a nanocrystalline DSSC is shown in Fig. 11.7. Photoexcitation of the
dye results the flow of electrons into CB of working electrode TiO2 and from CB to
external circuit via working electrode ITO to counter-electrode platinum, which
reduces triiodide to iodide. The generated iodide reduces Ru(III) to Ru(II) for
continuation of light absorption process. The following reactions take place at the
electrodes:
Fig. 11.7 Schematic diagram of a dye-sensitized solar cell where semiconductor TiO2
nanoparticles are coated with Ru(II)-based dye. Adapted with permission from (Wardle B, 2009
Principles and Applications of Photochemistry, Wiley, p. 202). Copyright (2009) John Wiley &
Sons
At ITO electrode:
RuðIIÞ þ hv ! RuðIIÞ
RuðIIÞ ! RuðIIIÞ þ e ðinjected to CB of TiO2 Þ
At Pt electrode:
I
3 þ 2e ! 3I

3I þ RuðIIIÞ ! I
3 þ RuðIIÞ
COOH
HOOC
N
N
NCS
Ru
NCS
N
N
HOOC
8
COOH
The overall efficiency of the DSSC depends on the following factors:

a. The HOMO-LUMO gap of the photosensitizer (dye): the smaller the size of this
gap, the larger will be the photocurrent, due to the ability of the dye to absorb
light of longer wavelength regions.
b. The LUMO-HOMO energy levels of the photosensitizer: the LUMO energy
must be higher than that of the CB of TiO2 to allow efficient electron injection
into CB. Similarly, the HOMO energy level of the photosensitizer must be lower
than that of the redox couple of the mediator so that efficient electron transfer
can occur from mediator to photosensitizer.
c. The maximum voltage of the DSSC is given by the energy gap between Fermi
level of the semiconductor electrode (ITO) and redox potential of the mediator.
It is about 0.7 V (Voc) under solar illumination conditions.
The DSSC differs substantially from silicon p–n junction solar cells by the fact
that no holes are formed in the VB of the semiconductor.
The modern type of dye-sensitized solar cell known as the Gratzel cell was
invented by O’ Regan and Gratzel [11]. Its overall quantum efficiency for green
light is about 90 % and power conversion efficiency is about 11 % [12].
The major disadvantages of the dye-sensitized solar cells are the stability
problem of liquid electrolyte, high cost of ruthenium dye and platinum catalyst and
11.8 Conservation of Solar Energy as Electrical Energy … 327
volatile solvent of liquid electrolyte. At low temperatures, the electrolyte can freeze.
The solvents used for the preparation of electrolyte are hazardous to human health
and environment. Replacing the liquid electrolyte by a solidified melted salt has
shown some promise but suffers from higher degradation during continued oper-
ations and is not flexible.
An alternative inexpensive organic polymer-based photovoltaic solar cell has
been invented. In this device, p-type and n-type semiconductors are sequentially
stacked on top of each other. In such devices, absorption of a photon by a
π-conjugated polymer results in the formation of an excited state, where coulom-
bically bound electron–hole pair (exciton) is created. This exciton diffuses to a
region of interface of n-type semiconductor where exciton dissociation takes place
and transport of charge to the respective electrodes occurs. For example, the
photo-induced electron transfer from a donor layer (p-type) to acceptor layer
(n-type) takes place in a polymer/fullerene-based organic bilayer solar cell,
MDMO-PPV: PCBM, with power conversion efficiency of 2.5 % (Fig. 11.8) [13].
PCBM [(6,6)-phenyl-C61-butyric acid methyl ester] has been widely used as
electron acceptor in polymer/fullerene solar cells due to its greater solubility than
C60. The polymer MDMO-PPV [poly-{2-methoxy-5-(3,7-dimethyloctyloxy)}-
para-phenylenevinylene] has been used as electron donor polymer for better
absorption of solar light. This heterojunction solar cell has 80 wt% of PCBM,
which is supposed to be the main light absorber. The change of solvent from
toluene to chlorobenzene increases the efficiency by a factor of 3 [13].
Fig. 11.8 Photo-induced electron transfer from excited MDMO-doped PPV to PCBM
Fig. 11.9 Schematic device structure for polymer/fullerene bulk heterojunction solar cells.
Adapted with permission from (Gunes et al. 2007 Chem Rev 107:1324). Copyright (2007)
American Chemical Society
The general structure of the bilayer solar cells is similar to the light-emitting
diodes (LEDs). The devices are fabricated in sandwich geometry (Fig. 11.9). The
active layer is sandwiched between two contacts: an indium-tin-oxide electrode
(ITO) (cathode) coated with a hole transport layer/glass or plastic foil. The blend
polymer/PCBM solution was doctor-bladed on top of the hole conductor PEDOT:
PSS [poly(3,4-ethylenedioxythiophene) doped with polystyrene sulfonic acid]. It
improves the surface quality of ITO electrode. On the top of this polymer, about
100-nm aluminum layer is placed as an electrode (anode).
The efficiency of bilayer solar cells is limited by its exciton (charge) generation
in a 10–20 nm scale around the donor–acceptor interface. It leads to the loss of
absorbed photons further away from the interface and results in low quantum
efficiencies [14].
11.9 Photo-Induced Supramolecular Devices
Photo-induced supramolecular devices may be designed in the field of information

processing and construction of chemical-based computers in future. The operations
of these devices are monitored by their luminescence. These devices are mainly of
three types:
1. Devices based on photo-induced energy transfer. Molecular wires can be
designed to send an electrical signal by connecting the receiver components
over a long distance. For example, carbon nanotubes can be soldered in an
electron microscope to get connectivity over a long distance. Photosensitive
dendrimers may be used as antenna systems for harvesting solar energy.
11.9 Photo-Induced Supramolecular Devices 329
2. Devices based on photochemical or photo-physical process. Molecular-level

two or more inputs may be designed to get a single output by performing basic
logic operations in information processing. For example, the molecular-level
two-level AND logic gate can be designed using two covalently linked receptors
that are able to quench the luminescence of a fluorophore. When one of the
receptors acts as a host to a suitable group, it becomes inactive to quench the
luminescence of the fluorophore, but the luminescence may be quenched by the
other free receptor. The output signal as luminescence of the fluorophore can
only be observed when both the receptors are bound to suitable chemical
groups.
3. Devices based on photo-induced nuclear movements. Molecular machines
may be designed based on mechanical movements in supramolecular structures
on absorption of light. Such movements occur when the charge-transfer inter-
actions between electron donor and electron acceptor groups are weakened; for
example, a six-component bistable [2]-rotaxane that has been designed to work
as a light-driven molecular machine based on photo-induced electron transfer
reactions (Fig. 11.10) [15]. The rotaxane is composed of an electron-rich
macrocycle, [bis-p-phenylene [34]-crown-10, BPP34C10] (M) and a
dumbbell-shaped component. The dumbbell-shaped component contains a [Ru
(bpy)3]2+-based light-harvesting unit (P) with a rigid spacer (S) (p-terphenyl
Fig. 11.10 Molecular structures of the components for a light-driven molecular scale machine.
Adapted with permission from (Bolzani et al. 2006 Aust J Chem 59:193). Copyright (2006)
CSIRO Publishing
type ring) which separates the P unit from the mechanical switching moiety and
serves as a stopper. The mechanical switching moiety is composed of a 4,4′-
bipyridinium component (A1) as a strong primary π electron-accepting unit, a
3,3′-dimethyl-4,4′-bipyridinium component (A2) as a weak secondary π
electron-accepting unit and a tetraarylmethane group as the second stopper (T).
The A1 and A2 components act as stations for the macrocyclic unit M.
M encircles the A1 station in the starting state and can be moved automatically
to A2 station on absorption of light by P unit. The intramolecular mechanism for
this ring movement can be explained as follows. The light absorption of the
photoactive unit P promotes it into the excited state. The excited state transfers
an electron to the station A1 and deactivates the station. Due to deactivation, the
ring moves from station A1 to station A2. The reduced station A1− then transfers
an electron back to the oxidized P+ unit and becomes activated as electron
acceptor. Due to this electronic reset of A1 station, the ring moves back to this
station [15]. These molecules can be driven at a frequency of about 1 kHz and
are stable for about 103 Hz in solution at an ambient temperature. The ring
motion generates a mechanical power of 3 × 10−17 W/molecule [16].
Several artificial molecular machines have been designed to work as functional
elements in molecular electronics, NEMS (nano-electro-mechanical systems),
nanophotonics as nanoscale optical integrated circuits for electronic display and
light-harvesting process and in nanomedicine for drug delivery process [17]. In our
body several protein molecules act as bio-motors and machines for performing
various functions of the cells such as powering of skeletal muscles, synthesis of
ATP, and templating of DNA/RNA.
1. Blackenship RE (2002) Molecular mechanisms of photosynthesis, 1st edn.

Blackwell Science, Oxford
2. Pessarakli M (2005) Handbook of photosynthesis, 2nd edn. CRC Press, Boca
Raton
3. Hara K, Arakawa H (2003) Dye-sensitized solar cells. In: Luque A, Hegedus S
(eds) Handbook of photovoltaic science and engineering. Wiley, pp 663
4. Foyer CH (1984) Photosynthesis. Wiley, New York
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Appendix
Chapter-2
2.6.1.
CF3
(a) + (c) Me Me
CF3 (b) +
Me Me
95 : 5 Me Me
D
H
(d) OSiR3 CHO (e) (f) (g)
H H H
(h) D (i) H (j) H H (k) OMe
D
(via dis motion of 4πe)
D
D H H H H O
2.6.2. (a) Carbanion formation followed by cyclization and proton abstraction.

(b) Electrocyclic ring opening of cyclobutene, then nucleophilic addition of diazo
acetic ester, electrocyclic ring closure of oxy anion (8 p e) gives the major product
and keto-enol tautomerism and cyclization gives the minor product(major: minor,
86:14, 84 %).
(c) Electrocyclic ring opening and ring closure of cyclopropyl cation by
dis-motions.
(d) Electrocyclic ring closure by dis-motion, followed by H abstraction and addi-
tion of methoxyl radical.

Lecture Notes in Chemistry 93, DOI 10.1007/978-3-319-45934-9
334 Appendix
Me Me
Me Me
N N H N OMe
NH product
O Me O
H
Me
(e) Conrotatory ring opening and closure in the same direction

(f) Inward disrotatory ring opening of cyclopropane ring with loss of fluoride ion in a
concerted process and addition of formate ion to the resultant cyclohexenyl cation.
(g) Aziridine ring opening by con-motion and cyclization of the resultant dipolar
ion by con-motion.
(h) One electrocyclic ring opening of cyclobutene to cyclohexadiene by allowed
dis-motion due to the formation of Ag+-p complex and then opening of
cyclohexadiene.
(i) Electrocyclic ring opening of cyclobutene, formation of cyclobutene with
exo-methylene group, opening of cyclobutene and finally cyclization of 6pe.
(j) Electrocyclic ring opening of six-membered ring to an unstable product, which
on double bond isomerization gives the product.
product
2.6.3. (a) Both A and B undergo ring opening by allowed con-motion, whereas C
undergoes ring opening by forbidden dis-motion.
(b) 2-Vinylcyclopropanols in the presence of organolithium generate cyclopropyl
anions, which on conrotatory ring opening produce anions of long lifetime. These
anions undergo ring closure by dis-motion as 6e process and abstract proton from
solvent to give the products.
(c) Cyclobutene ring undergoes inward conrotatory ring opening to produce a conju-
gated triene. The triene undergoes cyclization by dis-motion to give the product.
(d) The cyclopropene ring opening by inward dis-motion in D accelerates the
removal of Cl atom as leaving group by E2-like process by participation of
p-orbitals to produce cyclohexenyl cation much faster. The resultant cyclohexenyl
ion forms acetate with solvent. Such inward ring opening in E does not favor the
removal of Cl atom because generated p-orbital is syn to Cl atom.
Chapter-3
3.7.1. (a) Electrocyclic ring opening of cyclobutene followed by D–A
cycloaddition.
(b) D–A cyclization followed by retro D–A cleavage.
(c) D–A cyclization and then loss of CO2 and MeOH from the adduct.
(d) Electrocyclic ring opening of oxirane to produce a carbonyl ylide, which
undergoes 1,3-dipolar cycloaddition.
(e) D–A cycloaddition followed by retro D–A cleavage.
(f) D–A cycloaddition of 1,3-diphenyl-2-azallyl anion.
Appendix 335
(g) [2+2]-cycloaddition.
(h) 1,3-Dipolar cycloaddition gives cis-isomer as major product through E-endo-TS.
(i) Electrocyclization of cycloheptatriene to a norcaradiene and then its D–A
cycloaddition.
(j) Electrocyclization of benzocyclobutene, then D–A cyclization and loss of H2.
(k) Loss of MeOH gives a ketene, which undergoes [2+2]-cycloaddition and retro-
ene reaction.
(l) D–A cycloaddition and then retro D–A reaction and electrocyclic ring opening
of trans-3,4-diacetoxycyclobutene.
(m) Electrocyclization of 6e process by dis-motion and then D–A cycloaddition.
(n) [6+4]-Cycloaddition reaction using exo-methylene group of the fulvene as 4p
electron system.
3.7.2.
O H O
(a) (b) (c) H
+ CO2Me (d)
Me CN
EtO Me tBu CO2Me CO2Me
H CO2Me MeO COMe
(4+4+2)-cycloaddition
[2 x (4+2) D-A] SPh
O O
(e) O (f)
O (retro D-A to ketene and then
HO electrocyclization of 6e process)
80% H
O Ph Me Me
Ph Me Ph
N N
(g) N [3+2] Ph - CO2
O O O
Ph
MeO2C Ph O
(95%) CO2Me MeO2C CO2Me MeO2C
CO2Me
MeO2C O (k) O
(h) (i) N O H (j)
MeO (to minimize
H O steric repulsion)
H CO2Me
exo (major) N
CO2Me
[8+2]-Cycloaddition (D-A cycloaddition of allyl
cation and diene)
H O Cl
(l) (m) (n) CO2Me
O CN via O
O
Cl N N
H O H
Me Me (90%) Me
(84%) OMe
Me Me Me
(o)
H H
intramolecular D-A reaction of valence isomers Me and H Me
Me 2
and rearrangement if cis -bishomobenzene
O Me
(p) (q)
[2+2]-cycloaddition (retro D-A and then electrocyclic opening of cyclobutene ring)
+
(27%) OEt
SMe
336 Appendix
Et O
(r) (s)
O [4+2]-cycloaddition
(Electrocyclic ring opening of cyclobutene to ketene
and the intramolecular [2+2]-cycloaddition)
BuO O Me
H
CO2Et Me CO2Me
N
(t) (u) (v) (w) MeO2C
N
CO2Et
(major)
N
H
Me
3.7.3.
(a) O -CO2 D-A CN

CN
O OMe -MeOH CN
H [A] [B] CN
I
(b)
I
CO2H
O
[C] HO O [D] O
O NH2 NH2
(c) H Δ CO2Me -H2O
pdt
CO2Me H
CO2Me
[E] HO H
CO2Me
(d) CO2Me DDQ CO2Me
-H2O
[F] [G] [H]
r D-A
(e) products
[I]
Appendix 337
3.7.4.
O
Δ hν
(a) + O pdt
[4+2]
O O
(b) O Δ
+
N Ph (-CO2)
O N
[3+2]
Ph
(c) 1. Δ
Me3Si N N N + pdt
2. H3O+
Ph
CO2Me
CO2Me
(d) NMe2 NMe2 retro D-A
N N Δ N N pdt
+
N N N D-A N N N -N2
CO2Me
CO2Me
(e) H O
Me Δ Me Δ
+ O pdt
H O
(f) H H
+
Me CO2Me
Δ, D-A 650 oC
(g) + O
r D-A
Chapter-4
4.10.1. (a) [1,5]-CH3 shift and then [1,5]-H shift, orbital symmetry allowed
process.
(b) [3,3]-Sigmatropic shift (Cope rearrangement), allowed.
(c) [1,5]-R shift, then [1,5]-H shift, allowed.
(d) The Claisen rearrangement and then two times of Cope rearrangements, allowed.
(e) [1,3]-Alkyl shift with inversion of configuration at chiral carbon, allowed.
4.10.2. (a) Electrocyclic ring opening of cyclobutene and then [1,5]-H shift.
(b) The oxy anionic Cope rearrangement.
(c) The Cope rearrangement.
(d) [2,3]-Sigmatropic shift.
(e) The Cope rearrangement.
(f) The Cope rearrangement.
(g) Electrocyclization of 10 p electrons and then [1,5]-sigmatropic H shift.
338 Appendix
(h) The Cope rearrangement of chair form to cis-1,2-di-trans-1′-propenylcyclo-

butane, then Cope rearrangement of boat form to cis-1,2-dimethyl.
(i) [1,5]-Sigmatropic alkyl shift, then [1,5]-sigmatropic H shift.
(j) The Cope rearrangement, then Claisen rearrangement.
(k) The Claisen rearrangement.
(l) The Cope rearrangement.
(m) [1,5]-H shift, followed by electrocyclization of 6pe system.
4.10.3.
CHO O
(a) (Claisen rearrangement (b) Ph (2,3-Sigmatropic
of allyl vinyl ether) S rearrangement)
O
Me
(c) OMe (d) HO (e) CHO

H
OH Me
(Cope rearrangement) H Me
Me H
dr, E,Z = 97:3
D
OH OH D
HO O Ph
(f) (g) (h) O
+
(Claisen)
D
(β, γ-unsaturated acid)
D
major [5,5]- minor [3,3]-
H Ph Me
Me S Me
(i) (j) (k) N (l) Cl
Ph
O
Me
H O
HO (68%, E/Z = 90±3:10) Cl
Me O
H CH3
(m) (n) Ph (o) Ar (p) (q)
Ph NH
OH
H3C n-Pr Me
CH3
(81%, ee 70% (S)) O (76%)
H
O H Ph
(r) (s) (t) Me
H N
(68%) OH O
Me
4.10.4. (a) [1,5]-H shift and then retro D–A reaction.

(b) Electrocyclic ring opening of 6 p e system, followed by [1,7]-H shift and
electrocyclization of 6 p e system.
(c) [1,5]-H shift and then [4+2]-cycloaddition.
(d) Electrocyclic ring opening of 8 p e system, then electrocyclic ring closure of 6 p e
system to produce trans-bicyclic compound, which on [1,5]-H shift gives the product.
(e) Aldol condensation to produce an allyl vinyl ether, which on Claisen and Cope
rearrangements gives the product.
(f) Electrocyclization of 6 p e system (dis) and then [1,5]-H shift.
(g) [1,5]-Alkyl shift, then [1,5]-H shift.
Appendix 339
(h) Electrocyclic ring opening of 6 p e system, then [1,7]-H shift and electrocy-
clization of 6 p e system.
(i) Two times [1,5]-H shifts, then Claisen rearrangement.
(j) The Cope rearrangement using cyclobutene, then electrocyclic ring opening of
cyclobutene.
(k) [4+2]-Cycloaddition followed by two successive [1,5]-H shift.
Chapter-5
5.7.1 (a) Major and minor products are derived from favored TS and disfavoured TS.
H CO2Me
H
MeO2C O AlCl3 O H
H
Me Me AlCl3
syn anti
preferred TS disfavored TS
(b) Ene reaction of O2.

(c) Intramolecular ene of H delivery, then retro-ene of D-delivery with cyclization.
(d) Intramolecular ene reaction via enol form of ketone.
5.7.2
OH OH OH
H
a. b. c.
+
N N
kinetically
75% only product TS TS
controlled
90% 73 : 27
80% 8 : 92 (thermodynamically
controlled)
CF3 OH
CO2Me H CH3
d. CO2Me Cl e. f. g.
H OH
Cl (ene) OOH
H
53%
35% (92% trans) preferred path C
H2
Bu CO2Me
h. i. j. k. Me3Si CO2Me
80% + +
Bu O O Me
(retro-ene) CO2Me
20% >95 : <5
Me3Si H
l. m. n. R' o.
+ SiMe2
O
O
340 Appendix
Chapter-7
7.6.1 (a) The reaction proceeds preferentially through a triplet excited state in vio-
lation of free rotor hypothesis. The major product is derived from the bond formation
between C(4) and C(6), followed by cleavage of C(5)–C(6) bond to give a stable
diradical, whereas minor product is derived by cleavage of C(4) and C(5) bond.
3
4
2
1
7
5 6
C4 -- C5 C5 -- C6
less stable more stable
Minor product Major product
(b) Reaction takes place by cis–trans-isomerization about the terminal C(1)–C(2)

bond followed by [4+2]-supra-antara cycloaddition.
Ph [4+2]
Product
Ph Ph
(c) Photochemical allowed [1,3]-H shift.

(d) Rearrangement occurs through cis–trans-isomerization, followed by rapid
intramolecular photo D–A reaction.
D-A
Product, tricyclo[5.1.0.04,8]-oct-2-ene
(e) Electrocyclic ring closure to highly strained 1,2-cyclobutadiene, which on

opening of ring gives skeletal rearrangement product.
(f) The photorearrangement occurs through cis–trans-isomerization and

intramolecular photo D–A reaction (supra-antara path).
D-A
(g) Both the bicyclic compounds (bicyclo[3.2.0]-hept-2-enes) were formed as major

products in a concerted process by orbital symmetry allowed 1,3-alkyl shift in both
Appendix 341
singlet and triplet excited states. The minor product (m-hydroxymethyllimonene)

was formed by cleavage of cyclopropane ring followed by c-H abstraction.
HOH2C HOH2C
7.6.2.
OH
Ph Ph
(a) OMe + Ph (reaction proceeds through the singlet excited state to form exciplex
Ph
Ph with solvent, then hydrogen abstraction and addition of methoxy radical)
Ph H H
5 : 1
Ph
Ph
(b) ([2+2]-cycloaddition)
MeO2C CO2Me
H
H + (reaction takes place through singlet excited state)
(c)
H
H
endo 72 : 28 exo
(d) ([2+2]-cycloaddition through triplet excited state)
(cis and trans)

Ph
Ph (di-π-methane rearrangement is the favoured path
(e) + + + through singlet excited state to produce trans-1,2-
Ph Ph Ph Ph Ph
diphenylcyclopropane as major product)
1-phenylindane
H
H O
(f) N CO2Et (g)
O
50%
OMe
Chapter-8
8.6.1.
H H
(a) (b) O O
+
(Paterno-Buchi (PB) reaction) H Ph
O H H Ph H H
80 : 20
Ph Ph Me
(c) H Ph
Me Me Me (d)
+ OH O
O O
OTBDMS +
O O Ph
OTBDMS H
58 (exo/endo, 82:18) 42 (exo/endo, 95:5) 60% 40%
(Norrish type-II-cleavage, p-orbital of cyclobutyl
radical is almost perpendicular for maximum
ovelapping to give cyclic product)
342 Appendix
OH
(e)
Ph (Only product)
O Ph O O (Major product was formed in a concerted

(f) H process in lumiketone mechanism and minor
Ph + product was obtained via phenyl migration
Ph and H transfer)
H O O
Me
Me Me
major Ph
minor H Ph
Me Me H
O
(g)
Oxa di-π-methane (ODPM) rearrangement
H O
(h) CH2CH2CHO (i) O

(α-cleavage followed by γ-H transfer) Oxa di-π-methane (ODPM)
rearrangement
n-
CO2 Bu
H Me
H
(j) (k) (l) Ph Ph
O O
(PB-reaction)
O H O Me + O
40% Ph
(exo/endo, >98:2) H
HO
Ph
(m) (n)
t-
Bu
Me
CHO
8.6.2. (a) Alpha cleavage and addition to allylic radical to cycloalkene radical.
(b) Alpha cleavage at the bridge side, then c-H transfer from the bridge c-C and
formation of p bond.
(c) Di-p-methane rearrangement through triplet excited state of the reactant.
(d) Lumiketone-like rearrangement, which involves the shift of C(4)-C(5) bond to
C(3) followed by formation of bond between C(2) and C(4).
(e) The reaction proceeds via regioselective less stable oxetane. The oxetane on
ring opening gives the product.
H Me
Me
O Product
N
Me
(f) The reaction involves an a-cleavage of excited singlet state of cyclobutanone to

an acyl alkyl diradical, which undergoes electronic rearrangement to afford an
oxacarbene. The oxacarbene intermediate was trapped in a ring expanded acetal.
Appendix 343
O + H
hν O O O O MeOH O
- -
Product
Me Me Me Me
Me + Me
Me Me Me
Me Me Me
(g) 1,2-Acyl shift or oxa-di-p-methane rearrangement of triplet state of reactant

gives the only product, dihydrobarbaralone.
(h) 1,3-Acyl migration followed by cyclopropane ring formation.
(i) Norrish type-II cleavage due to d-H abstraction by benzoyl carbonyl and
b-cleavage of the triplet diradical.
(j) Alpha cleavage followed by generation of homoallyl carbinyl diradical and
cyclization.
O O O
Cyclization
(k) ODPM rearrangement

(l) Normal ODPM rearrangement involving the first double bond (C(1)-C(2)) of the
diene system gives major product via triplet excited state whereas vinylogous ring
closure using extended double bond gives minor product. The major product is
obtained from larger spin–orbit coupling interaction due to less atomic motion.
5
6 6
5 4 6
1 5 1
4 3 3
3 + 4 2
2
2
O O
O
major minor
(m) Alpha cleavage followed by H abstraction from the b carbon.

(n) c-H Abstraction by photoexcited singlet state of alkanone gives Norrish type-II
cleavage product.
OH
H OH H O
O hν OH + +
cyclization +H
OH
(o) In direct irradiation, the reaction proceeds through singlet excited state to give
cyclobutane derivative by concerted 1,3-acyl shift, whereas in sensitized irradia-
tion, the reaction proceeds through triplet excited state to give cyclopropane
344 Appendix
derivative by ODPM rearrangement. In direct irradiation, ISC is prevented by steric

interaction of the a-methyl groups with carbonyl oxygen nonbonding electrons.
(p) Alpha cleavage to produce diradical, which induces cleavage of the cyclo-
propane ring by conjugative interaction to generate another diradical. The latter
diradical undergoes cyclization.
O* O O
Cyclization
(n π∗ excitations)
Chapter-9
9.10.1.
(a)
+
meta-endo (90%) para-(10%)

OH OH O O
Me Me OH
Me
(b) hν Me
OH O Me
H2O 2
O Me O Me
Me OH
CH3
H3C O
O O HO
HO O
Me Me O
Product Me
-H2O Me
O
(c) (d) Ph [1,3]-S
O Ph S Product
N S S Ph S
50% Ph
OMe H
(e) (f) N (g) H NC (h)
OMe H H
CN
H
H + H
Me CN
92% 8%
80%
H CN H O
(i) H (j) (k) Ph O Ph (l)
OMe NH NH
CN H Ph O O Ph HN
H Me O
O
NC in conc. soln. in dil.soln.
OMe OH O
+ O
(m) OMe H OH -H2O O
9.10.2. (a) The reaction takes place in a Wolff rearrangement via ketene. The ketene
on cyclization gives a diradical, which attacks neighboring Ph gr to give product.
Appendix 345
O H
OH
CH
O Product
OPh OPh
Ph O
(b) Intramolecular [4+2]-tandem cyclization through a chair-like TS of a radical

cation in a PET process, where alkene serves as electron donor and DCNB as
electron acceptor.
Me Me Me H
Me
Ph Ph Ph
Ph Ph
Me Me Me
cis disfavoured Ph Me trans favoured
Ph Me H
Me H Ph
DCNB
Product
Me H
H H Me
Me Me
Chapter-10
10.5.1
OH O
N N
a. via b. HO HO
via
n- OH n-
Bu O NO
N Bu AcO AcO
HO
20%
OAc
HO H
N-R
c. N d. t-
Bu t- N
H via Bu R
O O
H 92%
AcO Br
H
O
HN
e.
O
80%
346 Appendix
10.5.2 (a)
NCl N N N N N
H 79%
(b) Intramolecular amidation at C-19 methyl via phosphonamidyl radical.
19
-H
product
I N P(O)(OEt)2 N P(O)(OEt)2 cyclisation
(c) HLF reaction, where intramolecular amidation on aromatic ring via sulfon-
amidyl radical gives the product.
O O O
N I hv N -H
product
Z Z
N
Z = SO2CF3 H
Z
(d) HLF reaction, where N-chloroamine undergoes cleavage of Cl bond and then
e-H abstraction and cyclization.
Index
A Bilirubin, 314, 315

Abnormal Claisen rearrangement, 131 BINOL catalyst
1,3-acyl shift, 339 in Diels–Alder reaction, 55, 57, 60, 61, 65,
Addition reactions, 95, 96, 219, 250, 286 66, 68, 97
Alder’s endo-rule, 52 in ene reaction, 161–169
Alkenes, 38, 41, 78, 84, 88, 171, 210, 215, 216, BOX catalyst
219, 220, 225, 226, 242, 251, 262, 269, in Diels–Alder reaction, 71
276, 278, 279 2-butene photoreactions, 262
Allyl phenyl ethers, 129, 130
Allyl vinyl ethers, 129, 134, 135 C
Amine oxides, 140, 148 Carbene, 95, 217, 220, 245
Ammonium ylides, 140, 141, 143 Carbonyl compounds photochemistry, 241
AND-logic gate, 325 Carbonyl ylides as 1,3-dipoles, 89
Anionic oxy-Cope rearrangement, 124 Carroll–Claisen rearrangement, 129, 132
Antarafacial shift, 109 Cheletropic reactions, 8, 95, 96
Aromatic compounds Chlorophylls a and b, 316
photoadditions, 43, 226, 260, 266, 279, Chugaev reaction, 173
282–286 Cis–trans-isomerization, 215, 216, 218, 221,
photorearrangement, 259, 288 224, 250
Photosubstitution, 292 Claisen rearrangement
Artificial photosynthesis, 311, 319 intramolecular, 130
Asymmetric Claisen rearrangement, 134 of allyl aryl ethers, 129
Aza-Claisen rearrangement, 139 of allyl ester, 131
Aza-Cope rearrangement, 127 of allylic b-keto ester, 132
Aza-di-p-methane rearrangement, 233, 290 of aryl propargyl ether, 137
Aza Diels–Alder reaction, 72 of allyl vinyl ethers, 129
Aza-Wittig rearrangement, 146 of enol ester of allylic alcohol, 133
Azomethine ylides as 1,3-dipoles, 87 of Gosteli-type allyl vinyl ether, 134
Azulene fluorescence, 193 of N-allyl indole, 139
of N-allyl-N-aryl amine, 138
B of N-allyl-N-vinyl amines, 138, 139
Bacterial photosynthesis, 205, 315 of orthoester of allyl alcohol, 134
Barton reaction, 297–300 of propargyl ester, 137
ß-carotene, 316 stereochemistry, 14, 32, 74, 101, 107, 108,
Beer–Lambert law, 183 153, 163
Bellus–Claisen rearrangement, 129, 135 substituent effect, 52, 230
Benzidine rearrangement, 149 Conia ene reaction, 162
Benzvalene, 275 Conrotatory motion, 14, 17, 19–21, 95

Lecture Notes in Chemistry 93, DOI 10.1007/978-3-319-45934-9
348 Index
Cope elimination, 173 Dipolarophiles, in 1,3-dipolar cycloaddition

Cope rearrangement reactions, 78, 80, 81, 89
chair versus boat transition structure, 119 Dipole-dipole energy transfer, 199
of barbaralane, 123 1,3-dipoles, resonating structures, 78, 82
of bicyclic compounds, 120 DNA-damage by UV, 319
of bullvalene, 123 Dyads, 209
of semibullvalene, 123 Dye-sensitized photovoltaic cells, 321, 322
Correlation diagram
of electrocyclic reactions, 15 E
of Diels-Alder reaction, 48 Electrocyclic reactions, 6, 10, 13, 14, 19, 24,
Cycloaddition reactions 25, 28, 34, 95, 97, 215, 219, 221
[2+2]-cycloadditions, 40, 41, 43, 216, 284 of 1,3-dienes, 289
[4 + 2]-cycloadditions, 285 of 1,3,5-trienes, 17, 23
[4+4]-cycloadditions, 91 of 2,4,6,8-tetraenes, 25
[6+4]-cycloadditions, 91, 92 of charged species, 278
[6+6]-cycloadditions, 91 of cyclooctadienyl anion, 31
[8+2]-cycloadditions, 91 of cyclopropyl cations, 329
[12+2]-cycloadditions, 91, 92 of heptatrienyl anion, 110
[14+2]-cycloadditions, 91 of pentadienyl anions, 30, 31
Cyclohexene photodimerization, 225 of pentadienyl cations, 66
Cycloreversions, 8, 37 orbital correlation diagram, 10, 15, 48
stereospecificity, 14, 80
D Enantioselective, 69, 70, 134, 168
Degenerate Cope rearrangement, 123 Ene reactions, 161–163, 165, 170
Dewar benzene, 23, 276 Ermolev’s rule for electronic transition, 194
Dexter theory of energy transfer, 201, 202 Eschenmoser–Claisen rearrangement, 132
Dibenzyl ketone, photocleavage, 243 Excimer, 290
Diels–Alder reactions Exiplex, 277, 279, 286
asymmetric, 68, 72
catalysis, 59, 60, 68–72, 83 F
dienes, 45, 55, 56, 58, 60, 65, 66, 77, 96, 97 Flash photolysis, 212
enantioselective, 68 Fluorescence, 188, 189, 191–194, 196, 197,
1,5-, in Cope rearrangement, 161 200, 201, 203, 206, 211
1,3-, in Diels–Alder reactions, 285 Fluorescence switching, 205, 207
intramolecular, 73 Forster resonance energy transfer, 199
regioselectivity of, 45 Franck–Condon principle, 184
stereoselectivity of, 68 Franck–Condon transition, 184
Dienophiles, in Diels–Alder reactions, 60–62, Fullerene–porphyrin-based dyad, 209
65
Diimide, as reducing agent Dimerization of G
aromatic compounds, 161, 171 Gosteli–Claisen rearrangement, 134
Dimethylaminobenzonitrile, TICT process, 190 Gratzel solar cells, 322
Di-p-methane rearrangement Grotthuss–Draper law, 183
of aromatic acetylene, 227 Group transfer reactions, 6, 9, 161, 171, 172
of dihydrobenzobarrelene, 230
orbital array, 228 H
regioselectivity, 337, 338 Halons, 311, 312
stereochemistry, 228 Hetero-Diels–Alder reaction, 56, 65
Substituent Effect, 230 Hofmann-Loffler-Freytag reaction, 297
1,3-dipolar cycloaddition reactions, 43, 78, 86, HOMO, 10, 14, 19, 21, 38, 45, 47–49, 55,
89 81–83, 92, 95, 113, 149, 206, 219
Index 349
Huckel topology, in Diels–Alder reaction, 14, of b,c-unsaturated bicyclic ketone, 234,

49 254, 258
Hypohalite reactions, 300 Oxy-Cope rearrangement, 125
I P
Intermolecular, 41, 85, 161, 165, 196, 250 Paterno–Buchi reaction
Intramolecular, 40, 41, 43, 75, 89, 91, 130, limitations, 269
164, 167, 169, 173, 189, 191, 226, 231, regio- and stereoselectivity, 264, 268
242, 244, 245, 251, 283, 288, 298, 301, with allenes, 62, 116, 170, 282, 293
303, 326 with chiral allyl alcohol, 265
Ireland–Claisen rearrangement, 133 with dihydrofuran and furans, 44, 51, 62,
67, 77, 89, 263, 265
J with enol ether, 265
Jablonski diagram, 188, 189 with homobenzvalene, 266
Johnson–Claisen rearrangement, 134 with spiro-[4,2]-heptadiene, 267
Phosphorescence, 188–191, 194, 197, 203, 211
K Photochemical [2+2]-cycloaddition, 38, 41, 43,
Kasha’s rule, 193 216, 225
Photochemical [4+2]-cycloaddition, 48
L Photochemical smog, 312, 313
Lewis acids, as catalysts Photocyclization, 287
in 1,3 –dipolar reactions, 8, 43, 78, 89 Photodynamic tumor therapy, 204
in Diels-Alder reaction, 44, 59, 68 Photo-fries rearrangement, 288, 289
in photochemical cyclization, 219 Photo-induced electron transfer process
Lifetimes of excited electronic states, 190, 211 of alkynes, 270
Limitations, 269 of allenes, 161
Lumiketone rearrangement, 252, 253, 259 of chiral allyl alcohol, 266
LUMO, 10, 38, 39, 45, 47, 49, 55, 56, 68, of furan, 282
81–83, 85, 92, 95, 113, 149, 187, 206, 216, of homobenzvalene, 265
219, 322 of isoxazole, 87, 88
of quinone, 61
M Photosensitization, 196, 203, 227
Marcus theory of electron transfer, 207, 208 Photosynthesis, 205, 311, 315, 317–319
Meisenheimer Rearrangement, 148 Photovoltaic cells, 319
Mobius topology, 14, 50, 228 Planck’s law, 182
Molecular machines, 325, 326 P-n junction, 320, 322
Porphyrin, generalized structure, 204, 205,
N 209, 311, 316, 319
Neonatal jaundice, phototherapy, 314 P-type doping, 320
Nitrile oxides as 1,3-dipoles, 78
Nitrile ylides as 1,3-dipoles, 80, 87 Q
Nitrones in cycloaddition reactions, 80, 88, 90 Quantum yield, 191, 193, 194, 197, 201, 211,
Nonradiative decay, 189, 216 242
Norbornene Quenching, 188, 194–197, 203, 206, 211
in photoaddition, 217, 269
in photodimerization, 225, 290, 291 R
Norcaradienes, in walk rearrangements, 119 Radiative processes of excited states, 189
Norrish types I and II cleavages, 243, 248, 339 Regioselectivity, 45, 47, 81, 86, 163, 229, 251,
N-type doping, 320 263, 264, 268
Retro-Diels–Alder reaction, 77
O Retro-ene reactions, 170
Orbital coefficients, 47, 81, 82 Rhodopsin, 314
Oxa-di-p-methane rearrangement Rod cells, 313
of b,c-unsaturated aldehydes, 254 Rydberg excited state, 220
350 Index
S [5,5]-sigmatropic rearrangement, 149

Selenoxides, elimination reaction, 173 [9,9]-sigmatropic rearrangement, 150
[1,5]-shifts, 116 Singlet oxygen in photodynamic therapy, 204
Sigmatropic rearrangements Stern–Volmer equation, 196, 211
Claisen rearrangement Stratospheric ozone layer, photochemical
intramolecular, 130 degradation, 311
of allyl aryl ethers, 129 Substituent effect on reaction rate
of allyl ester, 133 of barbaralane, 123
of allylic b-keto ester, 132 of bicyclic 1,5-diene, 120, 121
of allyl vinyl ethers, 108, 129 of bullvalene, 123
of allyl vinyl sulfides, 137 of chiral 1,5-hexadiene, 119
of aryl propargyl ether, 137 of divinyl cyclopropane, 234
of enol ester of allylic alcohol, 133 of divinyl oxirane, 122
of Gosteli-type allyl vinyl ether, 129, of homotropilidene, 122
134 of semibullvalene, 123
of N-allyl indole, 139 Sulfoxides, elimination reaction, 173
of N-allyl-N-aryl amine, 138 Sulfur ylides, 141
of N-allyl-N-vinyl amines, 138, 139
of orthoester of allyl alcohol, 134 T
of propargyl ester, 137 TADDOLs, in 1,3-dipolar reactions, 72
of propargyl vinyl sulfides, 137 t-butyl ketone photocleavage, 243
of thioallyl tropone, 138 1,3,5,-Tri-t-butylbenzene, photoisomerization,
classification of, 107 276
Cope rearrangement, 108, 119, 120 Tri-p-methane rearrangement, 234
chair versus boat transition structure, Triplet–triplet annihilation, 203
110–112, 119, 121, 138 Triplet–triplet energy transfer process, 202, 203
orbital symmetry selection rules, 107 Tropone, in cycloaddition reaction, 92, 93
[1,3]-shifts, 110, 112, 113, 117
[1,5]-shifts, 109–116 V
[1,7]-shifts, 109, 110, 112, 116, 118, 119 Vavilov’s rule, 193
[3,3]-shifts, 110, 112 Vision, photochemistry, 313
[3,3]-rearrangements, 108, 110–112, 119
suprafacial versus antarafacial, 107, 109 W
transition structures, 148 Wigner spin conservation rule, 196
[2,3]-sigmatropic rearrangements Woodward-Hoffmann rules in pericyclic
classification, 107 reactions, 4, 10, 107
of amine oxides, 141
of allyl ammonium ylides, 141 X
of allyl selenoxides, 146 Xanthate ester pyrolysis, 173
of allyl sulfonium ylides, 144
of amine oxides, 148 Y
of benzyl ammonium ylides, 140, 143 Ylides, 87, 88, 90, 140, 141, 144
Sommelet–Hauser rearrangement, 143
Wittig rearrangement, 146 Z
[3,5]-sigmatropic rearrangement, 148 Z-scheme, 316
[4,5]-sigmatropic rearrangement, 149

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Lecture Notes in Chemistry 93

The series Lecture Notes in Chemistry (LNC) reports new developments in

• provide an accessible introduction to the ﬁeld to postgraduate students and

More information about this series at http://www.springer.com/series/632

ISSN 0342-4901 ISSN 2192-6603 (electronic)

© Springer International Publishing Switzerland 2017

Printed on acid-free paper

This Springer imprint is published by Springer Nature

The part of pericyclic and photochemical reactions is the cornerstone of organic

Agartala, Tripura, India Biswanath Dinda

Part I Pericyclic Reactions

3.4 Cycloaddition Reactions of More Than Six Electrons

4.5.5 [2,3]-Sigmatropic Rearrangements of Allyl

Part II Photochemical Reactions

6.8 The HOMO and LUMO Concept of Electronic Transitions . . . 187

7.4 Photochemical [2+2]-Cycloaddition and Dimerization

10.5 Problems . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 311

Figure 1.1 Formation of bonding and antibonding orbitals . . . . . . . . . .. 5

Figure 3.4 Orbital interactions of HOMO of diene and LUMO

Figure 4.6 Suprafacial orbital interactions in chair- and boat-like

Figure 6.13 Electron movement in a triplet–triplet annihilation

donor of the two photosystems. Adapted with permission

Table 1.1 Symmetry properties of the orbital wn of a linear

Scheme 3.1 Regioselectivity of Diels–Alder reaction . . . . . . . . . . . . . . .. 46

Radical reaction involves the homolytic cleavage of a covalent bond by

© Springer International Publishing Switzerland 2017 3

Pericyclic reactions involve the continuous flow of electrons in cyclic transition

1.2 Molecular Orbitals and Their Symmetry Properties

w þ ¼UA þ UB ðbonding MOÞ

Presented + and - are mathematical signs

Fig. 1.1 Formation of bonding and antibonding orbitals

Fig. 1.2 Molecular orbitals formation in allyl systems

1.3 Classiﬁcation of Pericyclic Reactions

Table 1.1 Symmetry Wave function Nodes m-symmetry C2-symmetry

Cycloaddition reactions are characterized by the addition of two π-systems by

1, 3-Dipolar cycloadditions are another family member of cycloaddition reac-

Cheletropic reactions are a special group of cycloadditions or cycloreversions in

Cheletropic addition Cheletropic extrusion

Sigmatropic rearrangements are characterized by the movement of a σ bond to a

1.4 Concertedness of Pericyclic Reactions

High-level MO calculations suggest that these reactions proceed in concerted

1.5 Orbital Symmetry Property of Pericyclic Reactions

All pericyclic reactions conserve a deﬁnite orbital symmetry property throughout

Fig. 1.5 Huckel TS for

(a) Huckel TS of (4n)e (b) Huckel TS of (4n+2)e

1.6 Further Reading

An electrocyclic reaction is deﬁned as the thermal or photochemical conversion of

© Springer International Publishing Switzerland 2017 13

2.2 Orbital Symmetry Basis for Stereospeciﬁcity

Woodward and Hoffmann [5] suggested a complete description of the mechanisms

ψ2 bonding (low energy T.S) (one node)

ψ2 antibonding (high energy T.S)

Fig. 2.1 a Thermal electrocyclization of 4nπe conjugated system; b photochemical electrocy-

2.3 The Orbital Correlation Diagrams of Reactants

Fig. 2.2 a Thermal (a)

Table 2.1 Woodward– Acylic conjugated Reaction Motion

Mirror (m) C2-axis

Cyclobutene butadiene Cyclobutene butadiene

Fig. 2.3 a C2-axis of symmetry is maintained in thermal conversion of cyclobutene to butadiene;