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Supramolecular design of a bicomponent

topochemical reaction between two non-
identical molecules

Article in Chemical Communications · December 2012

DOI: 10.1039/c2cc37067k · Source: PubMed

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Baiju P. Krishnan Shyama Ramakrishnan

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Showcasing research from Prof. Kana M. Sureshan’s
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Laboratory, Indian Institute of Science Education and
Research, Thiruvananthapuram (IISER-TVM), Kerala,
India

Supramolecular design of a bicomponent topochemical reaction

between two non-identical molecules

The first design and execution of a topochemical reaction

between two non-identical reactants using supramolecular
chemistry approach is reported. A coassembly of two sugar-based
organogelators with complimentary reacting motifs, viz azide and
alkyne, undergoes topochemical Huisgen reaction between them.
See Kana M. Sureshan et al.,
Chem. Commun., 2013, 49, 1494.

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Supramolecular design of a bicomponent

Cite this: Chem. Commun., 2013,
topochemical reaction between two non-identical
49, 1494
molecules†‡
Published on 21 November 2012 on http://pubs.rsc.org | doi:10.1039/C2CC37067K

Received 28th September 2012,

Accepted 20th November 2012 Baiju P. Krishnan, Shyama Ramakrishnan and Kana M. Sureshan*
Downloaded by IISER Thiruvananthapuram on 10/04/2013 04:25:49.

DOI: 10.1039/c2cc37067k

www.rsc.org/chemcomm

We report the first design of a topochemical reaction between two non- monosaccharides with their structure revealed that compounds with
identical reactants using a supramolecular chemistry approach. A a diol motif capable of forming one dimensional H-bonded assem-
coassembly of two sugar-based organogelators with complementary bly are potential organogelators.8a The crystal structures of two
reacting motifs, viz. azides and alkynes, undergoes topochemical organogelators10 of this series namely a- and b-methyl-4,6-O-benzyl-
Huisgen reaction between them. idene glucopyranosides (1-a and 1-b) are known (QAKTEB and
NIQPIM; CSD search) and in both the structures, 2e3e (2-equatorial
Topochemical reactions, the reaction between preorganized reacting 3-equatorial) trans diol motifs make similar hydrogen bonded 1D
motifs in the crystal lattice, are attractive not only because they assembly with a conserved zig-zag architecture (Fig. 1A and B). The
provide a basic understanding of mechanistic and geometrical only difference between the two crystal structures is the opposite
details about a reaction, but also because they constitute a perfectly orientation of anomeric groups (highlighted as balls in Fig. 1A and B)
green chemical method as they avoid solvents, catalysts and other due to their opposite anomeric stereochemistry. We envisioned that
special reaction conditions.1 Due to the restricted mobility in the an equimolar mixture of two structurally similar gelators with
crystalline state, close proximity and proper orientation of the essential 2e3e diol and benzylidene motifs conserved but anomeri-
reacting motifs in the crystal lattice are important pre-requisites cally substituted with complementary reacting motifs (CRMs; e.g.
for a topochemical reaction to occur.2 The design of topochemical azides and alkynes) in opposite anomeric stereochemistry (a and b)
reactions constitutes a difficult task owing to the inherent difficulty might coassemble to form a 1D-assembly wherein the CRMs of
in designing a crystal structure with such fine control.3 Especially
more difficult is the design of bimolecular topochemical reaction
between non-identical partners due to the difficulty in cocrystalliza-
tion of two components with predesigned packing.1k,4 Pursuing our
interest in organogels5 and topochemical reactions,1l,6 we herein
illustrate the concept of designing a thermal topochemical reaction
between two non-identical reactants by preorganising the reactants
through their coassembly in their bicomponent gel and their
thermal reaction after arresting at their coassembled state.
It is well known that hydrogen bond based gelators adopt a
similar molecular arrangement in their organogel fibers and
crystals7 and hence the crystal structure can be used as a tool to
predict the gelation ability.8 Shinkai et al. established that 4,6-
O-benzylidene monosaccharides constitute an important class
of hydrogen bond-based organogelators.9 Shinkai’s correlation
of the gelation abilities of different methyl 4,6-O-benzylidene

School of Chemistry, Indian Institute of Science Education and Research,

Thiruvananthapuram, Kerala 695016, India. E-mail: kms@iisertvm.ac.in;
Fax: +91 4712597427; Tel: +91 4712599412 Fig. 1 (A) and (B) Chemical structure and crystal packing diagram of 1-a and 1-b
† Dedicated to Prof. Uday Maitra. respectively. The anomeric group is highlighted as a ball model and the hydrogen
‡ Electronic supplementary information (ESI) available: Experimental details, atoms are omitted for clarity. (C) Proposed coassembly and the subsequent
methods, materials and figures. See DOI: 10.1039/c2cc37067k topochemical reaction. (D) Chemical structures of organogelators 2–4.

1494 Chem. Commun., 2013, 49, 1494--1496 This journal is c The Royal Society of Chemistry 2013
 View Article Online

Communication ChemComm

adjacent molecules approach each other and they might undergo

thermal topochemical reaction if frozen in the coassembled state
(Fig. 1C). Any attractive interaction between these CRMs will be an
additional drive for this type of coassembly.
Both a- and b-methyl-4,6-O-benzylidene-galactopyranosides (2),
which also possess the 2e3e diol motif, are reported to be excellent
organogelators.9a This correlation between the 2e3e diol motif and
the gelation ability clearly indicates that the mode of assembly
leading to the gelation of both the anomers of 2 may be similar to
the mode of assembly in 1-a and 1-b, one dimensional H-bonded
zig-zag assembly. Based on the structural resemblance with 2 and
the fact that 4,6-O-benzylidene derivatives of D-galactose fulfil the
Published on 21 November 2012 on http://pubs.rsc.org | doi:10.1039/C2CC37067K

optimal structural requirement to be a gelator,9a,11 we expected

b-azide 3 and isosteric a-propargyl analog 4 to be organogelators.
Also, as azides and alkynes can have attractive interactions such as
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CH


N, CH


p, p


p, etc., we expected diols 3 and 4 to coassemble
as in Fig. 1C.
We have synthesized diols 3 and 4 (Fig. 1D) and investigated their
Fig. 2 (A) Comparison of the 1H NMR spectrum of an equimolar solution of 3 and
ability to congeal various organic solvents (ESI‡). As anticipated, 4 (blue) with an overlay of 1H NMR spectra of 3 (red) and 4 (black) in DMSO-d6.
both the diols showed organogelation properties in non-polar (B) Comparison of the 1H NMR spectrum of an equimolar solution of 3 and 4
solvents. In general, azide 3 was a better gelator in terms of the (blue) with an overlay of 1H NMR spectra of 3 (red) and 4 (black) in benzene-d6.
number of solvents that got congealed, critical gel concentration and (C–E) Toluene gel (1.5 wt%) of 3, 4 and their 1 : 1 mixture respectively. (F) NOESY
spectrum of an equimolar mixture of 3 and 4 in benzene-d6 showing cross peaks
Tgel (ESI‡). To our satisfaction, both the diols congealed benzene,
between hydroxyl groups of 3 and 4. (G) TGA spectra of xerogels of 3, 4 and their
toluene and chlorobenzene in common. Gratifyingly, different equimolar mixture.
mixtures of 3 and 4 also formed gels with benzene (also in toluene
and chlorobenzene). Interestingly, the gel formed by their equimolar
3 and 4 is supportive of the proposed coassembly based
mixture was stronger than the gels formed by the individual diols or
packing.
other mixtures as evidenced from Tgel measurements (ESI‡).12 The
A comparison of Powder X-ray Diffraction (PXRD) patterns of
involvement of hydrogen bonding in the gelation of 3, 4 and their
the xerogels gave convincing evidence for the different packing
mixture was established using IR and NMR spectroscopy (ESI‡).
in the gel of the equimolar mixture. A physical mixture of the
In DMSO-d6, a non-gelling solvent, the 1H NMR of the mixture
xerogels of 3 and 4 showed PXRD patterns identical to the
was identical to the simple overlay of individual 1H NMR spectra of
overlaid PXRD spectra of xerogels of 3 and 4 as anticipated.
3 and 4 in DMSO-d6 (Fig. 2A) as expected. Interestingly, the 1H NMR
However, the PXRD pattern of the xerogel of the equimolar
of the equimolar mixture of 3 and 4 in benzene-d6, a gelling solvent,
mixture was different from that of the physical mixture of
was different from the simple overlay of their individual spectra
xerogels of 3 and 4 or overlaid spectra of xerogels of 3 and 4
(Fig. 2B) in benzene-d6 at similar concentrations. This striking
(Fig. 3D). This confirms that the equimolar mixture undergoes
difference clearly suggests that there is interaction between the
gelation via uniform coassembly of 3 and 4 but not via
two gelators in gelling solvents. This supramolecular association,
self-assembly of 3 and/or 4.
presumably a coassembly of the two gelators, might be responsible
In order to check the topochemical Huisgen reaction in this
for the superior gelation ability of the mixture (Fig. 2C–E). Providing
coassembly, the xerogel of the equimolar mixture of 3 and 4 was
additional evidence for the coassembly, the NOESY spectrum of a
gel of the equimolar mixture of 3 and 4 in benzene-d6 showed cross
peaks between protons of 3 and protons of 4 (Fig. 2F). Thermo-
gravimetric analyses (TGA) revealed that the xerogel of the
1 : 1 mixture showed enhanced thermal stability than the xerogels
of 3 and 4 (Fig. 2G) suggesting a different packing involving
both 3 and 4.
Scanning Electron Microscopy (SEM) images of xerogels of
individual gelators and their equimolar mixture showed fiber
like morphologies (Fig. 3A–C). While diol 4 showed thicker
fibers, diol 3 showed entangled 3D networks of thinner fibers in
accordance with its better gelation ability. Interestingly, xero-
gels of the mixture showed cross-linked fibers with multiple
Fig. 3 SEM images of xerogels of (A) 3, (B) 4 and (C) equimolar mixture of 3 and
junction nodes (Fig. 3C), which might facilitate an enhanced 4. (D) P-XRD images of (i) xerogel of 3 (black) and xerogel of 4 (blue) overlaid,
3D fiber network, material stability and efficient gelation. This (ii) mixture of xerogels of 3 and 4 (red) and (iii) xerogel of an equimolar mixture
difference in morphology of xerogels of the mixture from that of of 3 and 4 (green).

This journal is c The Royal Society of Chemistry 2013 Chem. Commun., 2013, 49, 1494--1496 1495
 View Article Online

ChemComm Communication

In conclusion, for the first time, we have designed and

executed a thermal topochemical reaction in a coassembly of
two low molecular weight organogelators decorated with com-
plementary reacting motifs, viz. azides and alkynes. We have
designed two 4,6-O-benzylidene-D-galactopyranoside organo-
gelators with b-azide and a-propargyloxy motifs at the anomeric
position which coassemble in a 1 : 1 molar ratio in non-polar
solvents such that the azide and alkyne functionalities are
close enough to undergo thermal 1,3-dipolar cycloaddition. The
coassembly of the two gelators was proved by different NMR
techniques, Thermogravimetry, Powder-XRD of the xerogels and
SEM imaging of the xerogels. The xerogel of this co-assembled gel
Published on 21 November 2012 on http://pubs.rsc.org | doi:10.1039/C2CC37067K

underwent topochemical cycloaddition reaction upon mild heat-

ing, following sigmoidal kinetics as expected of a topochemical
Fig. 4 (A) Possible arrangement of 3 and 4 in the coassembled state and their
topochemical reaction in the xerogel state. SEM image of the xerogel is shown in reaction. This is not only the first report on the topochemical
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the background. (B) and (C) Kinetics of the topochemical reaction at 55 1C. reaction between two non-identical reactants in their supramole-
cular coassembly but also the first example of application of
organogelators for thermally activated topochemical reactions.
kept at a constant temperature of 55 1C and the reaction was
monitored by TLC.13 The reaction was almost complete in one week Notes and references
with the formation of both the possible products 5 and 6 in a 1 : 1 1 (a) Photochemistry in Organized and Constrained Media, ed.
ratio.14 We have followed the kinetics of this reaction by using NMR V. Ramamurthy, VCH Publishers, New York, 1991; (b) J. N.
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J. Trotter, Acc. Chem. Res., 1996, 29, 203; (c) V. Ramamurthy and
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( j) L. R. Macgillivray, G. S. Papaefstathiou, T. Friscic,
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products, cannot be ruled out. 14 Compounds 5 and 6 or their mixture are not organogelators.

1496 Chem. Commun., 2013, 49, 1494--1496 This journal is c The Royal Society of Chemistry 2013

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