Catalytic Oxidation of Alcohols: Recent Advances

ARTICLE IN PRESS Catalytic Oxidation of Alcohols: Recent Advances Maximilian N. Kopylovicha,*, Ana P.C. Ribeiroa, Elisabete C.B.A. Alegriaa,b, Nuno M.R. Martinsa, Luísa M.D.R.S. Martinsa,b, Armando J.L. Pombeiroa,* a Centro de Quı́mica Estrutural, Instituto Superior Técnico, Universidade de Lisboa, Av. Rovisco Pais, Lisbon, Portugal Chemical Engineering Department, ISEL, R. Conselheiro Emı́dio Navarro, Lisboa, Portugal *Corresponding authors: e-mail address: maximilian.kopylovich@tecnico.ulisboa.pt; pombeiro@tecnico. ulisboa.pt b Contents 1. Introduction 2. Aerobic and Peroxidative Oxidations 2.1 Metal Catalysts 2.2 Organocatalysts, Organic Radicals, and Other Additives 2.3 Prospective Substrates and Oxidation Agents 3. Acceptorless Dehydrogenative Oxidations 4. Oxidative Desymmetrizations 5. Cascade and Sequential Reactions 6. Conversion of Renewable Sources and Hydrogen Production 6.1 Transformation of Renewable Materials into Added-Value Compounds 6.2 Alcohol Oxidation for Hydrogen Storage and Production 7. Irradiation-Promoted Oxidations 7.1 Photocatalytic Oxidations 7.2 MW-Promoted Oxidations 7.3 Others 8. Catalysts Recyclization 8.1 Heterogeneous Solid Oxides, Alloys, and Related Materials 8.2 Supported Catalysts 8.3 Nano, Dispersed and Micellar Catalysts 8.4 ILs and Related Systems with Phase Division 8.5 Other Directions Acknowledgments References Advances in Organometallic Chemistry ISSN 0065-3055 http://dx.doi.org/10.1016/bs.adomc.2015.02.004 # 2015 Elsevier Inc. All rights reserved. 93 94 94 101 106 112 120 125 130 130 132 134 135 138 143 144 145 148 150 152 155 157 157 91 ARTICLE IN PRESS 92 Maximilian N. Kopylovich et al. ABBREVIATIONS [C2mim] 1-ethyl-3-methylimidazolium [C4mim] 1-butyl-3-methylimidazolium [C4py] 1-butyl-pyridine [C6mim] 1-hexyl-3-methylimidazolium [C8mim] 1-octyl-3-methylimidazolium 1-Me-AZADO 1-methyl-2-azaadamantane N-oxyl 2IBAcid 2-iodobenzoic acid ABNO 9-azabiciclo[3.3.1]nonane N-oxyl Aliquat N-methyl-N,N-dioctyloctan-1-ammonium chloride AZADO 2-azaadamantane N-oxyl Bmim 1-buthyl-3-methylimidazolium BOX bis(oxazoline) bpyO bis(oxazoline)α,α0 -bipyridonate Cp cyclopentadienyl Cp* pentamethylcyclopentadienyl DESs deep eutective solvents DIAD diisopropyl azodicarboxylate DKR dynamic kinetic resolution DPIO 4,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl Emim 1-ethyl-3-methylimidazolium FDCA 2,5-furandicarboxylic acid HFCA 5-hydroxymethyl-2-furancarboxylic acid HMB hexamethylbenzene HMF 5-hydroxymethylfurfural Hmim 3-methylimidazolium IBA iodosobenzoic acid IBX o-iodoxybenzoic acid IBXF o-iodoxybenzoic acid with a fluorous tag IL ionic liquid (room-temperature) ketoABNO 2,2,6,6-tetramethylpiperidine-1-oxyl LED light emitting diode MOF metal-organic framework MW microwave NAD nicotinamide adenine dinucleotide NBS N-bromosuccinimide NHC N-heterocyclic carbene NHPI N-hydroxyphthalimide Nor-AZADO 2-azanoradamantane N-oxyl NP nanoparticle NT nanotube NTf2 bis(trifluoromethylsulfonyl)imide OKR oxidative kinetic resolution Oxone potassium peroxomonosulfate KHSO5 PINO phthalimide-N-oxyl SPB surface plasmon band TBAB tetrabutylammonium bromide ARTICLE IN PRESS Catalytic Oxidation of Alcohols 93 TBHP tert-butyl hydroperoxide (tBuOOH) TBN tert-butyl nitrite TEMPO 2,2,6,6-tetramethylpiperidine-1-oxyl radical TOF turnover frequency TON turnover number 1. INTRODUCTION The oxidation of alcohols to carbonyl-containing compounds1,2 or their full oxidations3,4 are among the central reactions in organic chemistry5,6 and are of interest for the development of environmentally benign processes,7,8 production of new materials9,10 and energy sources.11,12 Due to their pivotal role in industrial fields and expected further applications,13 these reactions continue to attract a great attention, disclosing new catalysts,14,15 substrates, oxidants with peculiar features, and applications.1 Concerning the oxidants, stoichiometric oxidations with transitionmetal compounds or sulfoxides are still in common use, despite the formation of a large amount of undesirable products.1 The most used oxidants include small organic molecule-based reagents, e.g., Dess-Martin periodinane, Swern, Moffatt, Corey-Kim oxidants, SO3/pyridine, some of them being moisture-sensitive and expensive (e.g., N,N0 -dicyclohexylcarbodiimide, oxalyl chloride), or metal-based systems (such as Jones, Collins, Oppenauer reagents, pyridinium chlorochromate (PCC), pyridinium dichromate, barium permanganate, manganese dioxide, ruthenium tetroxide, silver carbonate).1 A recent environmental compatibility and sustainability approach leads toward aerobic oxidations with transition-metal catalysts (based on Pd, Ru, Fe, Cu, Pt, Au, Ir, Rh, etc.,) and dioxygen or hydrogen peroxide as oxidants.1–14 The use of molecular oxygen as a stoichiometric reoxidant in combination with a catalytic metal has practical advantages due to the favorable economics associated with O2 and the formation of environmentally benign by-products (water and hydrogen peroxide). Advances on the development of new methodologies, oxidation agents, catalysts and applications have been regularly surveyed,1–13 and the field continues to be one of the most extensively and actively investigated areas of current organic synthesis. Newly developed green oxidations of alcohols usually involve active and selective recyclable catalysts that ideally should ARTICLE IN PRESS 94 Maximilian N. Kopylovich et al. work with dioxygen, air, or other cheap oxidants, not leaving aside toxic or wasteful by-products. However, despite some remarkable advances, only few of the known methods are capable of offering an economic and practical oxidation toward a particular industrially important transformation. Many of the found catalytic systems suffer from high reagent cost, instability, employment of hazardous metals or oxidants, harsh reaction conditions, operational complexity, functional group incompatibility, or production of unprocessable wastes.1 Thus, there is a continuing demand for new catalytic systems that could overcome such challenges. Moreover, other perspectives for alcohol oxidation have been tested, including atom-efficient transformations (e.g., direct synthesis of esters), hydrogen transfer and production, oxidation of natural substrates, such as cellulose, cascade and sequential reactions, etc. The achievements in the alcohol oxidations until 2010 have been covered in several books, book chapters, and reviews,1–15 and thus the current work focuses on the most recent advances in the 2010–2014 period with some notable examples back to 2005. 2. AEROBIC AND PEROXIDATIVE OXIDATIONS Aerobic and peroxidative oxidations of alcohols, in particular of benzylic alcohols, are typical model reactions due to their importance and generality; inexpensive O2, H2O2 or tert-butyl hydroperoxide (TBHP) oxidants, and simple procedures are usually involved.1–15 In this section, an overview of some interesting catalytic systems, which were lately introduced into the field of alcohol oxidation, is presented. This concerns mainly homogeneous systems, since recent advances on heterogeneous catalysts are included in Section 8. Moreover, a glance at new substrates and oxidants which could successfully be used in a near future and make a difference in terms of efficiency, selectivity, economy and/or sustainability of the processes, is also presented. 2.1 Metal Catalysts Historically, Pt, Pd, Ru, Ir, and Rh complexes were among the most effective catalysts for alcohol oxidation. The series was expanded to 3d metals, e.g., V16 and Cu/TEMPO systems (TEMPO ¼ 2,2,6,6tetramethylpiperidine-1-oxyl radical) which have been reinvented and developed,17,18 and now includes representatives of most of the groups ARTICLE IN PRESS Catalytic Oxidation of Alcohols 95 and subgroups of transition and even nontransition metals.19 In contrast to those of noble metals, the newly introduced catalysts based on abundant 3d and related metals typically operate by redox mechanisms that usually involve one-electron (radical) processes.20 During the last decade, the search for new and effective catalysts for alcohol oxidation has mainly concentrated on finding cheaper and more effective metal–ligand combinations,21 achieving regio- and enantioselective reactions,22 and explaining mechanistic details of action of known catalytic systems.23 In addition, recent reports24,25 showed that there is a continuous interest concerning the structural details of the catalysts. For instance, the role of nuclearity in multinuclear copper(II) complexes26,27 was recently discussed.28 When using dioxygen, a challenge to overcome concerns the fact that it is a four-electron oxidant, while the aerobic oxidation of alcohols to carbonyl compounds involves two electrons. Apart from that, partially reduced oxygen species are usually more reactive than O2 itself. Hence, the introduction of special “oxidation buffer agents” which can balance the specific energetic requirements of the substrates with the possibilities of the oxidant is an important task, and complex, metallorganic, or organocatalysts can play the role of such agents. Generally, the effective catalytic systems contain an organic component, as a ligand in a coordination compound or an additive, but sometimes simple salts, such as Mn(II) acetate,29 can be efficient catalysts. Organic ligands in complexes can play different roles: adjust electronic and steric properties, provide the required solubility or arrangement of central metal ions or protect them from overoxidation or reduction. Thus, a systematic study of the catalytic activity of palladium complexes with commercially available pyridine-containing ligands30 found the conditions where precipitation of Pd black does not occur. Similarly, tertiary phosphine oxides (O]PR3) can be used as ligands for Pd catalysts.31 Recently, the importance of the trinuclear Pd3O2 intermediate [(LPdII)3(μ3–O)2]2+ (L ¼ 2,9-dimethylphenanthroline) in Pd-assisted catalysis was unveiled.32 This trinuclear compound is a product of oxygen activation by reduced palladium species and is an important intermediate in the aerobic oxidation of alcohols.32 The introduction of new ligands is an important task in the development of new catalytic systems. For instance, some polymers can be used as macroligands to host metal ions. The polymer ligands can not only provide the catalyst reutilization, but also stabilize the central metal ions and prevent their aggregation (e.g., precipitation of palladium black, if a Pd(II) catalytic system is applied).33 This approach is rather attractive and combines the ARTICLE IN PRESS 96 Maximilian N. Kopylovich et al. Scheme 1 Synthesis of Pd(II) complexes with poly(l-lactide) and poly(caprolactone) macroligands.33 advantages of homogeneous and heterogeneous catalysts. It involves coordination of Pd(II) by 4-pyridinemethylene-end-capped poly(l-lactide) and poly(caprolactone) (Scheme 1). The polymer-anchored catalysts are soluble under the applied catalytic conditions, but upon addition of n-pentane or methanol the polymer-anchored catalyst precipitates, and thus can be easily separated from the reaction mixture. These catalysts are effective in the oxidation of several primary and secondary alcohols with O2.33 In a related work, a design of enzyme-inspired star block-copolymers with branched topologies and protein-like tertiary or quaternary structures was performed.34 These polymers incorporate hydrophilic, superhydrophobic, and polydentate metal-binding sites and self-assemble in water, their mode of assembly being controlled by the composition of the polymer. An important feature of the star block-copolymers is that they incorporate perfluorocarbons and, due to that, their emulsions in water can attract and preconcentrate O2 in the vicinity of the active metal site. Addition of Cu(II) and TEMPO leads to an effective catalytic system for oxidation of alcohols to aldehydes in water.34 A series of tetradentate pyridyl-imine terminated Schiff-bases, bis(pyridyl-imine) terminated siloxane and other related polymers, can be used as ligands to host copper(II) ions.35 These CuBr2/polyL/TEMPO catalytic systems (polyL stands for polydimethylsiloxane derived pyridyl-imine terminated ligand) are effective for aerobic oxidations of primary and secondary alcohols under aqueous conditions. Chiral N,O-ligands, e.g., inexpensive L-proline, can also be used to prepare copper catalysts that are particularly effective for the oxidation of sterically hindered, allylic or heterocyclic alcohols such as 1-(3-pyridyl)ethanol, 1-(2-furfuryl)ethanol, ARTICLE IN PRESS Catalytic Oxidation of Alcohols 97 2-thienyl, 2-furyl and 3-pyridyl methanol.36 Related mono- and dicopper(II) aminopolyalcoholates were easily prepared by self-assembly and also studied.37 The selectivity parameters for oxidative transformations were measured and discussed, supporting free-radical mechanisms. Copper complexes with hydrazone ligands have been extensively studied during the last decade as oxidation catalysts38 and their family continues to grow. For example, an easy to synthesize and to handle trinuclear dihydrazone copper(II) complex [Cu3(L)(μ2–Cl)2(H2O)6] can be used as a reusable (up to eight runs) catalyst for the selective oxidation of a wide variety of alcohols, not being deactivated by N/S-heteroatom-containing substrates.39 Copper-containing metal-organic frameworks (MOFs) based on 5-(4pyridyl)tetrazole building blocks, easily prepared in situ by 1,3-dipolar cycloaddition between 4-cyanopyridine and azide in the presence of copper(II) chloride, were successfully applied40 as precatalysts for the low power (10 W) microwave (MW)-assisted peroxidative oxidation of secondary alcohols leading to the corresponding ketones with yields up to 86% and turnover frequencies (TOFs) up to 430 h 1 after 1 h, in the absence of any added solvent or additive. Zr(IV)-based robust MOFs with open Fe- and Cr-monocatecholato metal sites on the structure of the organic linkers were prepared by postsynthetic metal exchange,41 an approach that allows good control over the number of metal-binding sites, and can be used as a facile and efficient way to obtain MOFs that cannot be directly synthesized under solvothermal conditions. The Cr-metalated MOFs are efficient, versatile, and reusable heterogeneous catalysts for the oxidation of alcohols to ketones with TBHP or H2O2 as oxidants. Biomimetic Cu(II) and Fe(II) complexes with bis- and tris-pyridyl amino and imino thioether ligands and vacant (or potentially so) coordination positions (Fig. 1)42 are active as catalyst precursors for the solvent- and halogen-free MW-assisted oxidation of 1-phenylethanol by TBHP, in the presence of pyridazine or other N-based additives. Maximum TOF of 5220 h 1 (corresponding to 87% yield) was achieved just after 5 min of reaction time under the low power MW irradiation. The same authors reported43 the catalytic activity of related copper, iron, and vanadium systems with mixed-N,S pyridine thioether ligands. The Cu and Fe complexes proved to be useful catalysts in various MW-assisted alcohol oxidations with TBHP, at 80 °C. Thus, S-containing ligands can also be used to create effective catalyst precursors. Another green and easy to prepare iron-based catalyst, [Fe(BPA)2] (OTf )2, with the commercially available bis(picolyl)amine (BPA) ligand ARTICLE IN PRESS 98 Maximilian N. Kopylovich et al. Figure 1 Iron and copper complexes with bis- and tris-pyridyl amino and imino thioether ligands.42,43 Figure 2 Bis(picolyl)amine 8-hydroxyquinolinate (HQL). (BPA), thymine-1-acetate (THA), and (Fig. 2), chemoselectively oxidizes a variety of secondary alcohols in the presence of primary ones into the corresponding hydroxy ketones within 15 min at room temperature with 3 mol% catalyst loading and H2O2 as oxidant.44 The complex can also be generated in situ and operates similarly to the preformed one. In situ generated iron chloride complexes with thymineacetate or 6-(N-phenylbenzimidazoyl)-2-pyridinecarboxylic acid ligands (Fig. 2) have been also recently synthesized and proved to be selective and convenient catalysts for the oxidation of benzylic and allylic alcohols.45 They can be applied to sensitive compounds like perillyl alcohol, geraniol, or carveol, while diols can be oxidized in good yields without oxidative cleavage of products. Mechanistic investigations reveal that thymine-acetate possesses organocatalytic activity for the oxidation of alcohols. Aerobic alcohol oxidations with vanadium catalysts continue to widen their substrate scope and applications.16,46 The recently studied mechanism of the intramolecular oxidation of benzyl alcoholate ligands in 8-hydroxyquinolinato(L) vanadium(V) complexes of the type [LV(O) (OR)] resembles those proposed for certain metalloenzyme-catalyzed ARTICLE IN PRESS Catalytic Oxidation of Alcohols 99 Scheme 2 Key steps in base-assisted dehydrogenation for [LV(O)(OR)] complexes.20 oxidations and involves unusual ligand exchange and intermolecular deprotonation at the benzylic position (Scheme 2).20 This biomimetic pathway differs from the previously identified hydride-transfer and radical mechanisms for transition-metal-mediated alcohol oxidations. As a result, new ways to enhance the activity and selectivity of vanadium catalysts were proposed. They include the control of the outer coordination sphere and application of ligands with appropriately positioned pendant bases to serve as proton shuttles. Related V complexes show activity toward the oxidative decomposition of pinacol with CdC bond cleavage and aerobic oxidation of 4-methoxybenzylalcohol and other lignin model compounds.47 Other oxidovanadium(V) complexes with cis-2,6-bis-(methanolate)-piperidine ligands of the type depicted on Scheme 3 were applied as catalysts to convert prochiral alkenols into 2-(tetrahydrofuran-2-yl)-2-propanols, 2-(tetrahydropyran-2-yl)-2-propanols, oxepan-3-ols and epoxides, upon oxidative alkenol cyclization with TBHP as oxidant (Scheme 3).48 These catalysts are rather stable and possess improved chemoselectivity, e.g., epoxidation of geraniol occurs enantioselectively. It was ruled out the vanadium(V) tert-butyl peroxy complex formation is a key step to activate peroxides. Silver N-heterocyclic carbene (NHC) catalysts (Scheme 4) can be applied not only for the selective oxidation of alcohols to aldehydes or carboxylic acids but also for further tandem one-pot synthesis of imines ARTICLE IN PRESS 100 Maximilian N. Kopylovich et al. Scheme 3 Example of cis-2,6-bis-(methanolate)-piperidine ligands (A) and oxidation of alkenols by TBHP, catalyzed by the piperidine-derived vanadium complexes (B).48 Scheme 4 Example of a Ag(NHC) catalyst (A) and oxidation of alcohols to aldehydes or carboxylic acids and tandem one-pot synthesis of imines catalyzed by them (B).49 Figure 3 Example of a ruthenium(III) catalyst for the aerobic oxidative dehydrogenation of benzyl alcohols.51 (see Section 5).49 Rhodium porphyrin complexes have been successfully applied as catalysts for the selective oxidation of functionalized alcohols, since they tolerate a variety of functional groups, such as methoxy, C]C, and thiofuran moieties.50 The proposed catalyst is robust and does not degrade under the studied conditions (1 atm O2, 80 °C, 7 h). A porphyrin rhodium(III) methoxide complex was identified as a key intermediate in the proposed mechanism. The cyclometalated complex bearing phenylpyridine [RuCl(ppy)(tpy)][PF6] (ppy ¼ 2-phenylpyridine; tpy ¼ 2,20 :60 ,200 -terpyridine) (Fig. 3) is an example of ruthenium(III) catalysts ARTICLE IN PRESS Catalytic Oxidation of Alcohols 101 Figure 4 Ionic catalyst containing a Ru(III)-complex cation and a α-Keggin-type phosphotungstate anion.52 for the aerobic oxidative dehydrogenation of benzyl alcohols to benzaldehydes.51 The complex was also applied as catalyst for the one-pot synthesis of benzonitriles from benzyl alcohol with ammonia. Another approach to develop new catalysts is the combination of different metals and even different types of complexes in one system, e.g., an ionic compound containing a Ru(III)-complex cation and a α-Keggin-type phosphotungstate anion (Fig. 4).52 This compound is robust because the phosphotungstate [PW12O40]3 anion, although exhibiting a negligible contribution to the activation of benzyl alcohol, significantly stabilizes the structure. On the other hand, due to the high polarity and the ionic nature of the complex, ionic liquids (ILs) can be used as solvents. The catalyst was applied as an efficient catalyst for aerobic oxidations of alcohols, free of base and nitroxyl radical, and can be reused at least 5 without significant loss of activity.52 Other heteronuclear complexes are effective for the solvent-free peroxidative (with H2O2) oxidation of primary and secondary alcohols, e.g., a trinuclear complex with a dicopper(II)–monozinc(II) center.53 Finally, it is noteworthy that the alcohol oxidation reaction can be used for the direct one-pot synthesis of coordination compounds. The application of such a technique allows to generate in situ aldehydes, ketones, and other carbonylic derivatives, which are not available commercially, are unstable or cannot be prepared and isolated by conventional methods. As a result, new interesting coordination compounds can be prepared with ligands which are not attainable by usual synthetic methods.54 2.2 Organocatalysts, Organic Radicals, and Other Additives In spite of their high activity, catalytic systems that employ transition metals exhibit a number of disadvantages. For instance, substrates with a chelating ability can bind to the metal and hamper the reaction. Moreover, the presence of the metal can show some environmental impact. Therefore, new transition-metal-free systems have been searched for.7,13,15 One of such systems mimics the Anelli–Montanari protocol (see below) and employs an oxoammonium salt that carries out substrate oxidation while NO2 ARTICLE IN PRESS 102 Maximilian N. Kopylovich et al. (generated in situ from nitric acid, nitrates, nitrites, or hydroxylamine) regenerates the salt.55 It is possible to operate aerobic NOx systems under halide free conditions; however, participation of halides (bromine, hypobromous acid, nitrosyl bromide, or nitrosyl chloride) as active co-oxidants can potentially widen the scope of such reactions. In addition, an efficient transition-metal-free catalytic system mediated by N-bromosuccinimide (NBS) for the aerobic oxidation of various aromatic alcohols, under mild conditions, was recently reported.56 For instance, benzyl alcohol is oxidized to benzaldehyde with 99% conversion (94.5% selectivity) by the 2,3-dichloro-5,6-dicyano-1,4-benzoquinone–NaNO2–NBS system under 0.3 MPa of O2 for 2 h at 90 °C. The NH4NO3/TEMPO/H+ catalytic system was reported57 as efficient, under mild aerobic conditions, for the chemoselective oxidation of a comprehensive range of alcohols, including those bearing oxidizable heteroatoms (S, N, O), alkyl-, cycloalkyl-, and allyl-type substituted substrates. Very recently, tetra-n-butylammonium bromide was successfully applied58 as simple but efficient organocatalyst for the peroxidative (with TBHP) oxidation of a variety of functionalized benzylic/allylic alcohols under mild conditions. It shows excellent selectivity for secondary benzylic alcohols over aliphatic alcohols. The analog tetra-n-butylammonium iodide was employed for the alfa-oxyacylation of ketones by benzylic alcohols leading to alfa-acyloxyketones, with TBHP, affording moderate to good yields.59 1,2-Di(1-naphthyl)-1,2-ethanediamine efficiently catalyzes the oxidation of alcohols by using TBHP as oxidant. Secondary benzyl alcohols are oxidized in almost quantitative yields, and the catalyst displays a high activity toward hindered cycloaliphatic secondary alcohols.60 Quinine-derived urea has been identified as a highly efficient organocatalyst for the enantioselective oxidation of 1,2-diols using bromination reagents as the oxidants, at ambient temperature, to yield a wide range of α-hydroxy ketones in good yield (up to 94%) and excellent enantioselectivity (up to 95% ee).61 Nitroxyl radicals (Fig. 5), such as 9-azabiciclo[3.3.1]nonane N-oxyl (ABNO), 9-azabiciclo[3.3.1]non-3-one N-oxyl (ketoABNO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-2-azaadamantane N-oxyl (1-Me-AZADO) and 2-azanoradamantane N-oxyl (Nor-AZADO), and especially TEMPO, are widely used promoters for the aerobic oxidation of alcohols to the corresponding carbonyl compounds due to their high efficacy and selectivity.1–23 TEMPO is applied62 in industrial processes using aerobic or Anelli–Montanari1,63 type (neither aerobic nor peroxidative) conditions (Scheme 5): the oxoammonium salt of TEMPO carries out ARTICLE IN PRESS Catalytic Oxidation of Alcohols 103 Figure 5 Structures of the nitroxyl radicals (from left to right) 2,2,6,6tetramethylpiperidine-1-oxyl (TEMPO), 2-azaadamantane N-oxyl (AZADO), 1-methyl-2-azaadamantane N-oxyl (1-Me-AZADO), 2-azanoradamantane N-oxyl (NorAZADO), 9-azabiciclo[3.3.1]nonane N-oxyl (ABNO), and 9-azabiciclo[3.3.1]non-3-one N-oxyl (ketoABNO). Scheme 5 Anelli–Montanari's oxidation of alcohols.63 the alcohol oxidation in the presence of an excess of hypochlorite (which may lead to undesirable chlorinated by-products). In contrast, few applications of nitroxyl radicals as catalysts for the peroxidative oxidation of alcohols were reported, despite its enhancing effect on the alcohols conversion to the respective ketones.38f,40,64 Compared to TEMPO, the less hindered AZADO and ABNO radicals exhibit significantly enhanced reactivity toward a wide range of alcohols, including structurally hindered secondary alcohols that TEMPO fails to efficiently oxidize due to the steric congestion near its active center.65,66 ARTICLE IN PRESS 104 Maximilian N. Kopylovich et al. O O N O NHPI N OH O O PINO Figure 6 N-hydroxyphtalimide (NHPI) and phthalimide-N-oxyl nitroxyl (PINO) radical. Moreover, the sterically unhindered Nor-AZADO is more catalytically active than AZADO, 1-Me-AZADO, ABNO, and TEMPO in the aerobic oxidation of alcohols to their corresponding carbonyl compounds.65 Other related to TEMPO additives have been used, e.g., N-hydroxyphthalimide (NHPI) which generates the nitroxyl radical phthalimide-N-oxyl (PINO, Fig. 6).66 The PINO radical and analogs have been utilized for a range of aerobic oxidation reactions. PINO has been shown to be more reactive than TEMPO, but it is not stable and is usually formed from NHPI in situ by means of an initiator. Usually the stable nitroxyl radicals alone cannot directly catalyze the oxidation of alcohols with dioxygen or peroxide, so they rely on the assistance of various cocatalysts that play an important role in activating the oxidation agent. The most used cocatalysts are first row transition-metal complexes where Cu compounds with various N-donor ligands account for the prime ones.67 In many instances this combination serves as some kind of model to compare catalytic properties of copper compounds. For example, the performances of two asymmetric tetranuclear (with the {Cu4(μ–O)2(μ3– O)2N4O4} core) and dinuclear (with the {Cu2(μ–O)2N2O2} core) copper(II) complexes were compared in the catalytic TEMPO-mediated aerobic oxidation of benzylic alcohols.28 In spite of their similarity, the complexes perform differently: the tetranuclear copper(II) (R) complex is highly active leading to yields up to 99% and TONs up to 770, while the (S,R)-2 dinuclear complex is not so efficient under the same conditions. However, no solid explanation of the activity differences was proposed. Nevertheless, almost all of the nitroxyl radicals are quite inefficient for the oxidation of aliphatic and secondary alcohols, and require an additional base for the oxidation to proceed. One of the reasons for the poor performance toward secondary alcohols is believed to be the steric hindrance (as secondary alcohols are bulkier) and the involvement of a bimolecular reaction between the Cu-alkoxide and TEMPO on the final step.55 This can be overcome by switching from TEMPO to the less sterically hindered nitroxyl radicals ABNO68 or AZADO.69 Thus, Cu(I)/ABNO systems effectively ARTICLE IN PRESS Catalytic Oxidation of Alcohols 105 oxidize a wide range of secondary alcohols, including substrates with bulky groups close to the alcohol group, and still be effective for primary alcohols. With the AZADO catalyst system, substrates containing primary and secondary alcohols can also be oxidized in good to excellent yields.69 In contrast to the Cu/TEMPO combination, Fe(III)/TEMPO systems readily catalyze the aerobic oxidation of secondary alcohols and do not usually require any base or less sterically hindered ligands.70 The best activity was observed with weakly coordinating solvents (e.g., dichloroethane) unlike Cu/TEMPO systems, which normally are more active in acetonitrile. The performance and substrate scope (primary and secondary allylic, benzylic, or inactivated aliphatic alcohols) of Fe–TEMPO catalyst systems is improved by the presence of NaCl.70e The use of cobalt and manganese cocatalysts with TEMPO and its derivatives has been known for some time.71 The utilization of nitrate salts suggests that the oxoammonium salt oxidizes the substrates. There are also reports with heterogeneous Cu/Mn oxide cocatalysts; in these cases low loadings of TEMPO were used while the heterogeneous catalyst can be recycled. Cobalt can be used as a sole effective metallic component within a Co(NO3)2/dimethylglyoxime/TEMPO system, with only 1 mol% catalyst loadings.71 The polyoxometalate H5PV2Mo10O40 oxidizes TEMPO to form the oxoammonium salt, which then electroxidizes alcohols to their corresponding carbonyl compounds.72 Despite the fact that this system was used industrially (by DSM),62 it has received little research interest. Recently, functionalized TEMPO was immobilized with an IL on silicacoated magnetic nanoparticles (NPs).71d They catalyze the aerobic oxidation of a range of alcohols and the catalyst can be separated with an external magnet and recycled 10  without a significant loss of activity. The first example of vanadium as the sole metallic component in TEMPO-catalyzed aerobic alcohol oxidation in acetonitrile was recently reported.73 However, the catalyst did not perform well with secondary aliphatic alcohols, even with extended reaction times. The level of sophistication of stable radicals may increase in the near future as it is becoming clear that the performance of catalysts can be improved by tuning both steric and electronic effects of these radicals.74 Another successful approach is the use of heterogenized radicals. Thus, a free-radical porous coordination polymer [Cu(DPIO)2(SiF6)] [DPIO ¼ 4,7-bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl] (Fig. 7), possesses one-dimensional channels with incorporated nitroxyl catalytic sites. ARTICLE IN PRESS 106 Maximilian N. Kopylovich et al. Figure 7 4,7-Bis(4-pyridyl)-1,1,3,3-tetramethylisoindolin-2-yloxyl (DPIO). When dioxygen or air is used as oxidant, this polymer acts as an efficient, recyclable, and widely applicable catalyst for selective oxidation of various alcohols to the corresponding aldehydes or ketones.75 Fullerene has been employed successfully to anchor TEMPO moieties and used as an organocatalyst for the aerobic oxidation of primary and secondary alcohols in the presence of tert-butyl nitrite (TBN) as cocatalyst. The reaction showed a general applicability to various alcohols, and the catalyst was recovered easily and could be recycled for at least seven cycles with no loss in catalytic activity.76 In a rare example of Fe/TEMPO catalyst systems, Fe(NO3)3 or NaNO2 were not used; instead, the FeCl3
6H2O/TEMPO combination was coupled with a silica support.77 In a related study, TEMPO was covalently bound to silica and combined with FeCl3
6H2O/NaNO2.78 The TEMPO immobilization was performed via reductive amination of 4-oxo-TEMPO with amine-functionalized mesoporous silica SBA-15.79 A very efficient oxidation with a loading of 0.01 mol% of the expensive TEMPO radical, with 8 mol% FeCl3
6H2O and 10 mol% NaNO2, in toluene and O2 (1 atm) at 25 °C, was achieved. The iron salt and heterogeneous TEMPO could be recycled at least for five cycles. A continuous flow approach employing a microreactor with a Fe–TEMPO system comprised of a heterogeneous iron oxide NP catalyst stabilized on a mesoporous aluminosilicate support, was reported.80 A 42% yield of benzaldehyde is achieved in a single pass of the reactor, but the reaction conditions were rather harsh (120 °C and 35 bar of pure oxygen). 2.3 Prospective Substrates and Oxidation Agents Finding or developing new starting materials for the aerobic and peroxidative oxidation of alcohols is an important task, namely by widening the range of substrates that are of significance in fine chemical synthesis. For instance, since aldehydes are produced by the oxidation of alcohols, while aldehydes by themselves can participate in further reactions, aminoxidation of alcohols to nitriles and amide synthesis can be directly achieved using Mn catalysts for both the transformations in one-pot (Scheme 6).81 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 107 Scheme 6 Proposed reaction pathway for the ammoxidation of alcohols over MnO2.81 Scheme 7 Oxidation of indole carbinols using the Fe(NO3)3
9H2O/TEMPO catalytic system.82 Indole derivatives with carbonyl units (Scheme 7) have been found in natural products, possess versatile bioactivity and are important intermediates in many organic syntheses. The introduction of the carbonyl moieties to indoles significantly enhances their reactivity and can be achieved by oxidation of indole carbinols with Fe(NO3)3
9H2O/TEMPO/NaCl at room temperature and atmospheric pressure of dioxygen, using toluene as a solvent.82 NaCl is an important accelerator for the oxidation, reducing the reaction time from 15 to 2 h; however, the role of Cl is not quite clear. Recently, there has been a growing interest in new processes to derive chemicals and fuels from renewable carbon feedstocks, e.g., lignocellulosic biomass (see below). Hence, the study of homogeneous oxovanadium and copper catalysts toward aerobic oxidation of lignin model compounds is of great potential.83 In this transformation, the vanadium catalyst affords primarily ketone products, while the copper catalytic system leads to the products of CdC bond cleavage reactions, thus reflecting different mechanisms of oxidation. Glycerol is another substrate of importance in the biomass conversion, and it can be transformed to the added-value dihydroxyacetone with a supported palladium catalyst.84 A related approach is the added-value modification of nature-derived materials. For instance, pullulan, a polysaccharide extracted from the fermentation medium of the Aureobasidium pullulans bacteria, can be functionalized by the introduction of various groups, such as carboxylic ones, what significantly influences its hydrophilicity, etc.85 ARTICLE IN PRESS 108 Maximilian N. Kopylovich et al. Vanadium was found to be active in the direct transformation of 4-pentenols to 3-acyloxy-γ-butyrolactones, using TBHP as oxidant (Scheme 8).86 In this interesting oxidative conversion, stereocenters of the tetrahydrofuran moiety retain their configuration. Allylic alcohols can be also oxidized to stereodefined α,β-unsaturated aldehydes/ ketones with the retention of the C]C double-bond configuration, using Fe(NO3)3
9H2O/TEMPO/NaCl, under atmospheric pressure of oxygen at room temperature.87 The same catalyst was found to be effective in the conversion of homopropargylic alcohols to the corresponding homopropargylic ketones, which can be further isomerized to 1,2-allenic ketones.88 Iron catalysts are also effective in the oxidative tandem assembly of 3-(2-oxoethyl) indolin-2-ones from N-arylacrylamides and alcohols through the respective 1,2-difunctionalization of the C]C double bond in N-arylacrylamides (Scheme 9).89 Until very recently, it was very difficult to prepare carbonyl compounds from alcohols with strong electron-withdrawing groups adjacent to the RCHOH moiety. For instance, 2,2,2-trifluoroethanol is often used as a solvent in oxidation reactions since it has been considered inert to the oxidation. However, the aerobic oxidation of “inert” perfluoro-substituted alcohols to their corresponding carbonyl derivatives has been recently achieved using Pt(II) complexes with dipyrido[3,2-a:20,30-c]-phenazine ligands (Scheme 10).90 Thus, in the presence of H2SO4 and O2 oxidant, trifluoroethanol was successfully oxidized to trifluoroethyl trifluoracetate with >98% selectivity. Scheme 8 Oxidation of 4-pentenols to 3-acyloxy-γ-butyrolactones.86 Scheme 9 Synthesis of 3-(2-oxoethyl)indolin-2-ones from N-arylacrylamides and alcohols.89 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 109 Scheme 10 Aerobic oxidation of perfluoro-substituted alcohols.90 Scheme 11 Oxidation of imidazole diol to give imidazole-4,5-dicarbaldehyde.91 A bimetallic Pt/Bi/C catalyst was used for the oxidation of imidazole diol to give imidazole-4,5-dicarbaldehyde, a key step in the synthesis of pro-drugs of hepatitis C virus replicase inhibitors (Scheme 11).91 Introduction of new oxidative agents is also an important task since “classical” oxidants, such as pyridinium chromates, are mostly not ecological, produce high amounts of toxic agents and frequently are not enough selective. The oxidations with eco-friendly air, oxygen, and peroxides, in spite of being very attractive in theory and in small scale, encounter a number of difficulties when being scaled-up. In fact, oxidations in flammable organic solvents will virtually eliminate such oxidants due to risk of inflammation and other hazards related to high oxygen pressures. Apart from that, in many instances these attractive oxidants do not operate or the reaction goes through an undesired way, e.g., overoxidation or production of a number of by-products. Hence, the search for new effective oxidants is continuously growing. Rather selective and mild oxidizing agents, namely hypervalent halogen compounds or their precursors (e.g., iodic acid, o-iodoxybenzoic acid (IBX), NaBrO2, NaBrO3, etc.), have been widely introduced into the field of alcohol oxidation during last few years.92 IBX, the less hazardous 2-iodosobenzoic acid (IBA) and even the commercially available 2-iodobenzoic acid (2IBAcid) (Fig. 8) in the presence of Oxone (potassium peroxomonosulfate KHSO5), an environmentally acceptable reagent, as a co-oxidant, were shown to be effective for the oxidation of many primary and secondary alcohols in user- and eco-friendly solvent mixtures.93 In this process, the reduced form of IBX, namely IBA is used in catalytic amounts and is reoxidized with Oxone, in aqueous media, thus providing an attractive green protocol. ARTICLE IN PRESS 110 Maximilian N. Kopylovich et al. However, the application of IBX as oxidant can lead to some unpredictable results, e.g., unexpected dehomologation of primary alcohols to one-carbon shorter carboxylic acids (Scheme 12).94 In this case, the combination of IBX and molecular iodine affords a different type of active hypervalent iodine species, which was isolated and shown to be crucial for the reaction. It should be noted that usually the dehomologation involves complicated multistep procedures, whereas in the described oxidative strategy it proceeds smoothly in one step. Another useful direct oxidation of secondary alcohols was performed using performic acid within just 15 min.95 The modification of IBX with a fluorous tag (IBXF, Fig. 9) allows its easy recovery since insoluble fluorous IBA can be separated from the reaction mixture by simple filtration, and be reused without significant loss of its activity.96 Further, IBXF can be easily generated in situ from cheaper and readily available Oxone. Other modifications of the oxidizing component are being introduced. For instance, a combination of NH2OH
HCl and NaIO4 was recently proposed for the selective and mild oxidation of alcohols to the corresponding carbonyl compounds at room temperature.97 Concerning particular substrates, a wide range of β-hydroxyketones was Figure 8 o-Iodoxybenzoic acid (IBX), 2-iodosobenzoic acid (IBA), and 2-iodobenzoic acid (2IBAcid). Scheme 12 Oxidative dehomologation of primary alcohols to one-carbon shorter carboxylic acids.94 Figure 9 Modified IBX with a fluorous substituent for its easy recovery.96 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 111 selectively oxidized in quantitative yields to β-diketones with IBX being a superior oxidant for this transformation.98 The ability to perform oxidations without generating species harmful for potential intermediates of further transformations is important to perform multistep synthesis, such as the domino reactions described below (Section 5). In this respect, the use of aryl halides as readily available, stable and cheap oxidants (hydride acceptors) is a powerful option due to the production of inert, dehalogenated aryl by-products in anaerobic conditions. Commercially available Pd and Ni complexes with NHC ligands were found to be active in a temperature-controlled domino oxidation/R-ketone arylation with aryl halide.99 tert-Butylnitrite (t-BuONO) was recently introduced as a convenient and easy-removable oxidant for an environmentally benign conversion of primary and secondary benzylic alcohols to ketones and aldehydes, which can be readily isolated by simple evaporation of the reaction mixtures since t-BuONO decomposes giving only volatile side products.100 The oxidation requires neither metal-based reagents nor organic catalysts and presumably involves a nitrosyl exchange and a subsequent thermal decomposition of benzylic nitrites. t-BuONO can potentially be recovered, since several alkyl nitrites are known to be industrially produced from the corresponding alcohols and gaseous NO under an O2 atmosphere. A related approach was realized for the simple, high-yield conversion of various achiral and chiral alcohols to carbonyl compounds using TEMPO or AZADO in conjunction with BF3
OEt2 or LiBF4 as precatalysts and t-BuONO as oxidant.101 A NO+/NO
pair was used for mild anaerobic nitroxide reoxidation, which allowed the oxidation of enantiomerically pure substrates without racemization. K3[Fe(CN)6] was applied as a secondary oxidant in a chemoselective osmium(VI)-catalyzed oxidation of benzylic, allylic, and propargylic alcohols.102 An uncommon oxidation agent, diisopropyl azodicarboxylate (DIAD), can be used as an effective terminal oxidant103; in this case 1,2-diols were oxidized to hydroxyl ketones or diketones depending on the amount of DIAD used. Diaziridinone (Scheme 13) as oxidant allows the reactions to Scheme 13 Diaziridinone as oxidant for acid- or base-sensitive substrates.104 ARTICLE IN PRESS 112 Maximilian N. Kopylovich et al. be performed in neutral conditions and makes it compatible with acid- or base-sensitive substrates.104 As a result, various acyclic and cyclic secondary benzylic alcohols with alkenyl, alkynyl, thioether, silyl ether, amide, carbamate, ketal, ester, and heterocyclic moieties were effectively oxidized to ketones in 73–99% yields, while no racemization of stereocenters occurred during the oxidation. A sure way of avoiding the toxic chemicals is the use of electric current as oxidizing agent. Thus, an electrochemical process for selective oxidation of 1,2-diols to the corresponding α-hydroxyketones in water using [Me2SnCl2] catalyst, KBr, and platinum electrodes has been introduced.105 The “Br+” ions, generated at the anode, are oxidants, while OH ions, electro-generated at the cathode, play the role of a base. However, in this synthetic strategy the toxicity of organotin catalysts is a drawback. This issue was addressed in another study,106 where methylboronic acid [MeB(OH)2] was used as a safe alternative. In this case, the selective oxidation of 1,2-diols to the corresponding α-hydroxy ketones in aqueous medium most probably occurs through the formation of boronate esters. 3. ACCEPTORLESS DEHYDROGENATIVE OXIDATIONS The aerobic and peroxidative oxidations, described in the previous sections, in spite of being very attractive in many aspects, can produce a considerable amount of by-products. Other problems concern the overoxidation and risk of explosion due to the coexistence of oxygen and organic reactants or solvents. In this respect, the oxidant- and acceptor-free dehydrogenation (Scheme 14) is an alternative environmentally friendly route for the conversion of alcohols into aldehydes or ketones, since gaseous H2 can be easily separated from the reaction mixture. Moreover, this strategy is much more attractive from the atom efficiency viewpoint and can provide a promising route for H2 synthesis and storage.107 Thus, some advances in the oxidant- or acceptor-free oxidations based on the catalytic dehydrogenation of alcohols accompanied by the evolution of hydrogen gas will be described in this section. Some new attractive processes were introduced recently in terms of efficiency and substrates, and examples are described below. Scheme 14 Oxidant- and acceptor-free dehydrogenation of alcohols. ARTICLE IN PRESS Catalytic Oxidation of Alcohols 113 To date, the homogeneous catalytic systems for the dehydrogenative oxidation of alcohols are mainly based on ruthenium,107,108 iridium,109 and, in much lesser extent, other transition-metal107 complexes and metal-organic compounds. For instance, water-soluble Cp*Ir (Cp* ¼ pentamethylcyclopentadienyl) complexes with α-hydroxypyridine109 or bipyridonate109 ligands have shown a high catalytic activity in dehydrogenative oxidation of a wide variety of primary and secondary alcohols and reversible dehydrogenation–hydrogenation between 2-propanol and acetone. The Ir catalysts are reusable109 and can accelerate the selective dehydrogenation of biologically important and complex molecules, e.g., β-estradiol to give estrone (Scheme 15).109 Production of hydrogen gas can be also achieved with the Ir catalysts from 2-propanol, thus providing a prototype for a hydrogen storage system, based on the interconversion between 2-propanol and acetone (Scheme 16).109 The reversible transformation with hydrogen storage and evolution was repeated several times without loss of the catalytic activity. A ligand-promoted mechanism of dehydrogenation was proposed, Scheme 15 Selective dehydrogenation of β-estradiol to estrone.109 Scheme 16 Interconversion between 2-propanol and acetone.109 ARTICLE IN PRESS 114 Maximilian N. Kopylovich et al. acting the ligand as a proton acceptor in the activation step and as a proton donor in the dehydrogenation step, thus playing a dual role in cooperative catalysis.109 A detailed discussion of the mechanism of the pH-dependent alcohol dehydrogenation in aqueous solution catalyzed by related [C,N] or [C,C] cyclometalated Cp*Ir complexes was recently reported,110 generally supporting the mechanistic suggestions and demonstrating a significant dependence of the studied reaction system on pH. Structurally related PCP-pincer Ir complexes (Fig. 10) were synthesized by straightforward [4+2] cycloaddition and employed as catalysts in the acceptorless dehydrogenation of alcohols.109 Such complexes can be easily modified with a functional sidearm that is capable of interacting with the catalytic site, thus making them suitable candidates for catalytic studies involving ligand–metal cooperation. The H2 formation involves an intramolecular cooperation between the structurally remote functionality and the metal center.109 A theoretical study on the mechanism of the acceptorless alcohol dehydrogenation mediated by the iridium catalyst [Cp*Ir(bpyO)] (bpyO ¼ α,α0 bipyridonate) suggests that the metal–ligand cooperative work involves aromatization/dearomatization of the bpyO ligand.111 On the basis of those results, the new ruthenium catalyst [(HMB)Ru(bpyO)(H2O)] (HMB ¼ hexamethylbenzene) was designed (Fig. 11).111 Another ruthenium catalyst formed from the Ru-hydride precursor [RuH2(PPh3)3CO] and pincer PNP-ligands effectively promotes the dehydrogenative oxidation Figure 10 PCP-pincer Ir complex.109 Figure 11 [(HMB)Ru(bpyO)(H2O)] ruthenium catalyst.111 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 115 of alcohols with TOFs as high as 1.4  104 h 1 after 20 min, at moderate temperatures (<100 °C) and without an activation additive.111 The reaction is an example of an efficient hydrogen production from a renewable alcohol source. The proposed mechanism may also involve intramolecular concerted hydrogen loss from a dihydride Ru species followed by the outersphere dehydrogenative step (Scheme 17).111 Ruthenium compounds also can be used for transfer hydrogenations. Thus, a series of water-soluble Ru(II) half-sandwich complexes with 2,20 -bipyridine ligands behave as catalysts in the transfer hydrogenation of different ketones to the corresponding alcohols, using a mixture of sodium formate/formic acid as a hydrogen source.112 Interestingly, some obtained alcohols can be selectively deuterated at the benzylic carbon, if the catalytic transfer hydrogenation is performed in D2O (Scheme 18). This fact can be explained by the fast reversible deuteration of the hydride intermediates Scheme 17 Mechanism of alcohol–ketone interconversion coupled with the hydrogen evolution step.111 Scheme 18 Catalytic transfer hydrogenation and deuteration of acetophenone.112 ARTICLE IN PRESS 116 Maximilian N. Kopylovich et al. Ru(X2) (X ¼ H, D) in the presence of D2O and formic acid. The deuteration is faster than the hydrogenation process; as a result, a selective deuteration of phenylethanol at the benzylic carbon occurs. Using this principle, selective deuteration of different unsaturated organic substrates can be developed.112 The above mentioned ruthenium- or iridium-PNP type complexes catalyze a one-pot dehydrogenation/aldol condensation/hydrogenation sequence (Guerbet reaction), thus allowing to prepare β-alkylated ketones.113 A dehydrogenative CdC bond formation occurs in this and related cases114,115; the proposed catalytic sequence involves hydrogen abstraction by cooperation of an acidic side arm with the metal-hydride site, ligand exchange step with alkoxide formation, catalyst regeneration by hydride elimination and formation of the carbonyl product; metal independent aldol condensation; and finally, transfer of H2 to the aldol condensation product to give the β-alkylated ketone (Scheme 19). Substitution of noble metals in catalysts for cheap and abundant materials is of obvious importance, and many works concern the preparation and study of the latter systems. Thus, cobalt(II) alkyl complexes of aliphatic PNP pincer ligands (Fig. 12) are active precatalysts for the hydrogenation of ketones and the acceptorless dehydrogenation of alcohols under mild conditions; in this case the alcohol dehydrogenation likely proceeds through a cobalt(I)/(III) redox cycle.116,117 Scheme 19 Preparation of β-alkylated ketones via the dehydrogenation/aldol condensation/hydrogenation sequence (Guerbet reaction).113 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 117 Figure 12 Cobalt(II) complex with a PNP pincer ligand as a catalyst precursor for acceptorless dehydrogenation of alcohols.116,117 Scheme 20 Dehydrogenation of 2-pyridylmethanol using [CpFeCl(CO)2] as a precatalyst.118 2-Pyridylmethanol derivatives can be effectively (with TONs up to 67,000) dehydrogenated in an acceptorless way using an iron catalyst, e.g., derived from [CpFeCl(CO)2].118 A cocatalyst NaH is also used; its role is possibly to provide the corresponding sodium alkoxide, which then reacts with the above iron complex to give the active Fe-alkoxide complex (Scheme 20). The attached pyridine ligand then displaces one of the CO ligands, forming a Fe-N,O-metallacycle, from which an iron hydride complex and 2-pyridinecarboxyaldehyde are produced by a β-hydride elimination. The oxidative addition of 2-pyridinylmethanol coupled with the H2 reductive elimination completes the catalytic cycle (Scheme 20).118 It should be mentioned that this work is somehow related to the finding119 that in the Fe-catalyzed oxidation of alkanes, pyrazinecarboxylic acid, and analogs have a significant promoting effect. In general, the iron catalysts seem to be very promising, and interesting works on Fe-catalyzed acceptorless dehydrogenations of alcohols appear ARTICLE IN PRESS 118 Maximilian N. Kopylovich et al. every year. Thus, an operationally simple and economical process can be mentioned, employing, as a catalyst, a mixture of readily available Fe(III) acetylacetonate, 1,10-phenanthroline and K2CO3.120 In spite of generally being rather active, the above systems suffer from the drawbacks common for homogeneous catalysts, and thus a number of heterogeneous catalysts have also been introduced.121 For instance, a nonoxidative dehydrogenation of benzyl alcohol was achieved using a hydrotalcite-supported gold catalyst.122 Nanocrystalline rhenium particles concern another example of a reusable heterogeneous catalyst to convert secondary and benzylic alcohols to ketones and aldehydes, through catalytic acceptorless dehydrogenation via a novel γ-CH activation mechanism.123 Curiously, primary alcohols and aldehydes act as inhibitors of the dehydrogenation reaction. A heterogeneous rhodium-on-carbon system catalyzes the dehydrogenation of primary and secondary alcohols to the corresponding carboxylic acids and ketones in water under basic conditions.124 Upon production of the carboxylic acids, water played a role of an oxygen source. Pt nanoclusters, supported on metal oxides, were also tested, and it was found that the support, e.g., amphoteric alumina, plays an active role in the catalytic process, reacting with alcohols and yielding an alkoxide and a proton.125 β-Hydrogen elimination from the alkoxide to a low coordinated Pt0 site affords Pt-H and ketone; then protolysis of Pt-H by a neighboring proton regenerates the Pt0 site with release of H2 gas.125 Nickel is a well-recognized promoter of hydrogen-transfer reactions and hence it is expected that it can be applied in the reactions under discussion. In accord, acceptor-free dehydrogenation of secondary alcohols by Ni NPs supported on alumina has been reported.126 Low-coordinated Ni0 sites and metal/support interfaces are believed to play significant roles in the catalytic cycle.126 Nanocopper(0) on alumina [Cu(0)/Al2O3] NPs were prepared by a simple procedure from copper aluminum hydrotalcite and shown to be an efficient catalyst for the acceptor- and oxidant-free dehydrogenation of alcohols and amines.127 TiO2-supported Co NPs are also catalytically active in the acceptorless dehydrogenation of various aliphatic secondary alcohols to the corresponding ketones.128 The acceptorless dehydrogenation can be used not only for simple transformation of alcohols to the corresponding ketones or aldehydes or for ARTICLE IN PRESS Catalytic Oxidation of Alcohols 119 hydrogen storage and production, but also as an essential part of more complicated multistep procedures or tandem reactions. Moreover, apart from the usual model alcohols, other uncommon alcohols are also being introduced into the field and some of them are mentioned below. Thus, dehydrogenative [Ir]-catalyzed coupling of arylhydrazines and alcohols can be used for the selective synthesis of arylhydrazones, where no N-alkylated by-products were generated (Scheme 21).129 This route is more straightforward and potentially can compete with the traditional synthetic ways to arylhydrazones based on condensation of arylhydrazines or aryldiazonium salts with carbonyl compounds. Dehydrogenative lactonization of diols is an efficient way to various lactones (Scheme 22).130 The lactone formation is found to be catalyzed by a recoverable stable dicationic iridium complex with 6,60 -dihydroxy-2,20 bipyridine ligands, and employs a variety of benzylic and aliphatic diols in aqueous media. In comparison with the esterification of hydroxyl acids, hydroacyloxylation of olefinic acids and Baeyer–Villiger reaction of cyclic ketones, the dehydrogenative lactonization of diols proceeds without any oxidant; hence, it is more environmentally benign and atom economical. The transformation of alcohols to carboxylic acids with no oxidant or hydrogen acceptor uses water as the oxygen atom source with concomitant emission of dihydrogen gas.131 The reaction is catalyzed by a ruthenium complex at a low loading (0.2 mol%) in basic aqueous solution (Scheme 23).131 The same or related complexes are active in many other tandem reactions which involve dehydrogenative oxidation; the proposed mechanism of the catalysis involves a metal–ligand cooperation and both O2 and H2 generation at a single metal center.132,133 Scheme 21 Direct synthesis of arylhydrazones through dehydrogenative [Ir]-catalyzed coupling of arylhydrazines and alcohols.129 Scheme 22 Dehydrogenative lactonization of diols to lactones.130 ARTICLE IN PRESS 120 Maximilian N. Kopylovich et al. Scheme 23 Conversion of alcohols to carboxylic acids with water as the oxygen source, catalyzed by a Ru complex.131 4. OXIDATIVE DESYMMETRIZATIONS The oxidation of alcohols is extensively used to introduce asymmetry to starting compounds. For this, chemo-, regio- or stereospecific oxidations are employed, e.g., upon the Oppenauer reaction, where the substrate is oxidized by transfer of hydrogen atoms to a sacrificial ketone, such as acetone. And vice versa, alcohols can be a source of hydrogen and hence a reducing agent in asymmetric hydrogenations. For instance, asymmetric transfer hydrogenation of ketones (Scheme 24) is an efficient and relatively simple method to introduce asymmetry in a molecule.134 Other examples include OKR of racemic secondary alcohols (Scheme 25A), oxidative desymmetrizations of meso-diols,136 etc. The kinetic resolution is generally defined as a process where two enantiomers of a racemic mixture are transformed to products at different rates. Thus, one of the enantiomers of the racemate is selectively transformed to product, whereas the other is left behind. This method allows to reach a maximum of 50% yield of the enantiopure remaining sec-alcohol. To overcome this limitation, a modification of the method, namely dynamic kinetic resolution (DKR), was introduced. In this case, the kinetic resolution method is combined with a racemization process, where enantiomers are interconverted while one of them is consumed (e.g., by esterification, Scheme 25B). Therefore, a 100% theoretical yield of one enantiomer can be reached due to the constant equilibrium shift. In most of the proposed DKR processes, several catalytic systems, e.g., enzymes and transition-metal catalysts, work together. Both reactions (transfer hydrogenation of ketones and the reverse oxidation of secondary alcohols using ketone as a hydrogen acceptor) can be promoted by a catalyst. The process can involve a temporary oxidation of a substrate with hydrogen transfer to a transition-metal complex. A classical example of catalytic desymmetrization is a regioselective oxidation of polyoles, in particular 1,2-diols to form the corresponding ARTICLE IN PRESS Catalytic Oxidation of Alcohols 121 Scheme 24 An example of asymmetric transfer hydrogenation of ketones.134 Scheme 25 Oxidative kinetic resolution (A) and dynamic kinetic resolution (B) of racemic secondary alcohols.135 Scheme 26 Regioselective oxidation of 1,2-diols using organotin catalysts.136 α-hydroxyketones, versatile synthetic intermediates, and an important class of biologically active products.136 Organotin compounds are renowned catalysts in this transformation, which proceeds through the formation of stannylene acetal as an intermediate (Scheme 26). Other metal catalysts, e.g., palladium complexes with pyridine ligands137 also are used for the chemoselective aerobic conversion of unprotected diols into the corresponding hydroxy ketones. Pd–(NHC) complexes are catalytically active as well; propene evolution was detected in this case, indicating that the reaction most likely proceeds through a reductive elimination from a palladium-allyl-hydride intermediate.138 A wide range of vicinal diols and polyols, including 1,2-diols, triols, and tetraols, were oxidized selectively at the secondary alcohol to afford α-hydroxy ketones, using chiral palladium complexes with pyridinyl oxazoline derived ligands as catalysts and benzoquinone as the terminal oxidant.139 The ligand geometry and environment have a significant influence ARTICLE IN PRESS 122 Maximilian N. Kopylovich et al. on the activity and chemoselectivity of the catalytic system. Neocuproine can be also used as a ligand to create effective Pd catalysts for the chemoselective oxidation of unprotected vicinal polyols under mild reaction conditions.140 Oxidative lactonization of 1,5-diols to cyclic lactones and stereospecific oxidation of (S,S)-1,2,3,4-tetrahydroxybutane [(S,S)-threitol] to (S)-erythrulose (Scheme 27) were performed with these Pd-neocuproine catalysts. It is noteworthy that generally the selective oxidation of unprotected polyols and in particular vicinal diols at the secondary alcohols is a difficult task due to competitive CdC bond cleavage, overoxidation, poor chemoselectivity, etc. Concerning the mechanism,140 the facile formation of chelating protonated diolates from the vicinal diols leads to both higher rates and chemoselectivity for oxidation of the secondary alcohol moiety due to easy β-H elimination from the secondary Pd alkoxide. In contrast, for mixtures of primary and secondary alkoxides, the formation of the primary Pd alkoxide is preferable and thus the selectivity for oxidation of primary alcohols to form aldehydes is observed in this case. The readily available diamine ( )-sparteine was also applied as a chiral ligand for Pd to create an effective system for asymmetric catalysis, in particular kinetic resolution of benzylic, allylic, and cyclopropyl secondary alcohols and desymmetrization of meso-diols.141 A related oxidative desymmetrization of meso-diols with a chiral Ir(Cp*) catalyst can be achieved with good efficiency and enantiocontrol.134 In some cases, an oxidative lactonization occurs if primary meso-diols are used as staring materials (Scheme 28). Chiral N-sulfonyldiamine ligands are used to create effective chiral bifunctional amidoiridium catalysts for the asymmetric aerobic oxidation of meso- and prochiral diols to give up to >99% ee of hydroxyl ketones and 50% ee of lactones.143 These catalysts can be also applied for an efficient oxidative kinetic resolution of racemic secondary alcohols affording R enantiomers with >99% ee and with 46–50% yields.135 Scheme 27 Stereospecific synthesis of (S)-erythrulose.140 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 123 Scheme 28 Enantiospecific preparation of lactones by oxidative lactonization of primary meso-diols.142 Scheme 29 Chemoselective oxidation of a secondary alcohol moiety with Ru catalysts.144 Chemoselective oxidation of a secondary alcohol moiety can be also performed with ruthenium catalysts with phenylindenyl ligand.144 The selective oxidation of 1-phenylethanol to acetophenone from a mixture of phenylethanol isomers, without oxidizing the other isomer, can then be achieved (Scheme 29). In general, only the secondary alcohol moieties are oxidized and the catalyst can be used for the chemical separation of isomers or specific oxidation of highly functionalized molecules. The OKR of unactivated racemic alcohols with dioxygen of air as the hydrogen acceptor was effectively performed at room temperature with [(aqua)Ru(salen)] complexes as catalysts.145 ARTICLE IN PRESS 124 Maximilian N. Kopylovich et al. Scheme 30 Conversion of 2-diols to the corresponding chiral α-ketoalcohols.146 The use of cheaper, more available, and recyclable catalysts is a common aim in the catalytic studies. Under this perspective, many recent publications on the oxidative asymmetric resolution of alcohols involve 3d and other abundant metals. Thus, in situ combination of copper(II) triflate and (R,R)-Ph-BOX [BOX ¼ bis(oxazoline)] with NBS as an oxidant allowed to achieve a desymmetrization of 2-diols to afford the corresponding chiral α-ketoalcohols (Scheme 30).146 An air-stable, well-defined (NHC)–Ni0 complex is another effective 3d-metal catalyst for the mild anaerobic catalytic oxidation of secondary alcohols with such functionalities as ether, tertiary amine, and alkene, using nonanhydrous, -degassed 2,4-dichlorotoluene as both oxidant and solvent.147 The use of inexpensive, stable, and ease to handle chlorinated solvent as an oxidant makes this catalytic system attractive for multistep synthesis and scaling-up; moreover, potentially harmful and/or difficult to remove species are less likely to be formed. Additionally, primary benzylic and alkylic alcohols are unreactive with this catalytic system and can be recovered. Jacobsen’s chiral Mn(III)-salen complexes constitute another example of cheap and available catalysts for the enantioselective oxidation of racemic secondary alcohols148; in this case, sodium hypochlorite (NaClO) was used as oxidant. Related macrocyclic chiral Mn(III) salen complexes were applied for the OKR of secondary alcohols with diacetoxyiodobenzene [PhI (OAc)2] and NBS co-oxidants, in a biphasic dichloromethane-water solvent mixture149; the catalyst can be easily recycled up to 7  without losing its performance. Transition metals can be eliminated from the catalytic systems. Thus, a quinine-derived urea organocatalyst is effective in the enantioselective oxidation of a wide range of diaryl-substituted meso-1,2-diols using bromination reagents as oxidants (Scheme 31).61 The method is simple, operates at ambient temperature and utilizes available reagents to yield α-hydroxy ketones in good yields (up to 94%) and enantioselectivities (up to 95% ee). ARTICLE IN PRESS Catalytic Oxidation of Alcohols 125 Scheme 31 Enantioselective oxidation of diaryl-substituted meso-1,2-diols.61 Moreover, both chemo- and enantioselectivities can synergistically contribute to the specific oxidation of racemic diols bearing different substituents, affording a hydroxy ketone as the sole product.61 5. CASCADE AND SEQUENTIAL REACTIONS Some products of partial oxidation of alcohols, in particular aldehydes, are widely used as starting materials in many organic transformations. Hence, they potentially can react in situ with other components of the reaction mixture, giving, e.g., alkenes, imines, or α-functionalized carbonyl compounds. Selectivity issues will arise, but a proper choice of reaction conditions, catalysts, and other additives would hopefully provide good yields and selectivities. Sometimes the products of partial oxidation of alcohols can be isolated from the reaction mixture after the oxidation step and transformed further using the same catalyst.150 In this case, one deals with a sequential transformation; an example is given by the conversion of aromatic alcohols to the corresponding aldehydes using molecular oxygen and a copper– TEMPO catalytic system,38e where the formed aldehydes can be isolated and transformed further with the same copper catalyst to nitroalcohols upon the nitroladol (Henry) reaction (Scheme 32). If further transformations of the products of partial oxidation of alcohols occur in one-pot, the term tandem or cascade reactions is generally used.151 Hydrogen atoms attributed to the starting alcohols can be combined with an inorganic oxidant,152 leave the reaction environment, e.g., as H2,12 or be transferred to another substrate, forming an added-value product. In the last case, cheap and easy to operate alcohols are usually used as sacrificial reducing agents. At the same time, in many instances the hydrogen atoms can ARTICLE IN PRESS 126 Maximilian N. Kopylovich et al. Scheme 32 Sequential transformation of alcohols to aldehydes and further to nitroalcohols.38e O Catalyst R OH 1/2 R R O + H2 151 Scheme 33 Dehydrogenative coupling of alcohols to esters. interact with an intermediate.151 Formally, a net oxidation of alcohols in this case will not occur, but only a proton transfer. However, if one considers the mechanism of such transformations, an oxidation (dehydrogenation) is an essential rate-determining step. Hence, appropriate catalysts and specific conditions of the alcohol oxidation (dehydrogenation) can serve as a starting point in the search of new catalytic systems for the sequential or tandem reactions of this type. As mentioned above, hydrogen atoms, removed from the alcohol substrate, can return to form the product; however, if the final hydrogenation step could not occur, a product that is more oxidized than the starting material is obtained. The formation of esters from alcohols and of amides from alcohols and amines concern the most representative and studied reactions of this type. In these cases, aldehydes, formed on the first oxidation stage from alcohols, undergo Tishchenko- and Cannizzaro-type reactions, where esters or carboxylates and alcohols are formed upon fusion or disproportionation of aldehydes, respectively. Thus, the dehydrogenative coupling of alcohols to esters with evolution of H2 (Scheme 33) is one of possible effective variants of the tandem reactions with net oxidation of alcohols.151 In contrast to the normal esterification of an acid and alcohol, in which an equilibrium mixture is formed, the evolved hydrogen, which is valuable by itself, can shift the equilibrium to completion. Generally, the direct catalytic transformation of alcohols to esters, without the use of the corresponding acid or acid derivative, is a very attractive approach. Ru(II) hydride complexes based on electron-rich PNP and PNN ligands of the type depicted on Fig. 13 (which can undergo aromatization/ dearomatization steps) efficiently and selectively catalyze the acceptorless dehydrogenation of primary alcohols to esters and H2 with high TONs ARTICLE IN PRESS Catalytic Oxidation of Alcohols 127 Figure 13 Ru(II) catalysts for acceptorless dehydrogenation of alcohols to esters.132 under relatively mild conditions.132 This provides a convenient method for the synthesis of esters in view of its high efficiency, simplicity, and facile product isolation. Thus, the reaction gave up to 99% of ester yield after 6 h at 115 °C, if started from 1-hexanol. Concerning the mechanism, it was demonstrated that hemiacetal formation from the aldehyde and alcohol followed by its dehydrogenation is more likely to occur than a Tischenkotype reaction involving the aldehyde.132 Heterogeneous reusable catalysts, e.g., supported platinum-based ones, can be utilized for the acceptor-free dehydrogenative coupling of alcohols to esters under additive-free and solvent-free conditions at 180 °C, with isolated yields within the 53–91% range.153 The activity depends on the support material and on the loaded transition metal. Thus, the SnO2 support contains Lewis acid sites that activate carbonyl groups of adsorbed aldehyde intermediates, while the Pt/SnO2 combination possesses the best promoting activity among the studied transition metals, e.g., Ir, Re, Ru, Rh, Pd, Ag, Co, Ni, and Cu loaded on SnO2. Direct amide bond formation is a rather important transformation since the amide functionality is a widely spread unit in synthetic intermediates, pharmaceuticals, polymers, and natural products. Hence, its simple formation from cheap and convenient starting materials, such as alcohols, is of interest. Generally, the amide bond construction is similar to the ester formation, but the competing N-alkylation process significantly complicates the picture (Scheme 34). Thus, the search for effective and selective catalysts is of a clear need. Magnetic Fe3O4@EDTA–Cu(II) NPs can be such catalysts if benzyl alcohols and amine hydrochloride salts are used as substrates with TBHP as an oxidant. The corresponding amides are formed, while the catalyst can be easily recovered by magnetic forces and reused several times without loss of activity.154 To improve selectivity in the oxidation-addition cascades, the catalysts and reducing agents should be able to maintain the catalytic cycle and respond to the polarity differences between different radicals. Using this approach, synthesis of side-chain-extended tetrahydrofurans from alkenols and acceptor-substituted alkenes was achieved.155 Co-1,3-diketones were ARTICLE IN PRESS 128 Maximilian N. Kopylovich et al. Scheme 34 Competing processes in the dehydrogenative formation of amides from alcohols and amines.154 used as catalysts to activate molecular oxygen for the oxidative cyclization (Scheme 35); a sequence of polar and free-radical reactions is believed to occur in this case. Dehydrogenation of alcohols to aldehyde or ketone allows subsequent bond construction steps which would not be possible for the parent alcohols. Hence, a variety of iridium, rhodium or ruthenium phosphine, pincer and related complexes, that are efficient catalysts for the dehydrogenation of alcohols, can potentially be applied for the related hydrogen-transfer reactions, thus leading to new added-value compounds.8 The hydrogen atoms transfer to a sacrificial hydrogen acceptor, such as a carbonyl compound or an olefin which is reduced to the corresponding alcohol or alkane. Apart from the described examples, other interesting alcohol transformations can occur where different reactions are combined to give new and sometimes unpredictable result. For instance, oxidation of one substrate and reduction of another one can lead to the same product, thus favoring ARTICLE IN PRESS Catalytic Oxidation of Alcohols 129 Scheme 35 Alkenol oxidation—olefin addition cascade catalyzed by Co complexes.155 Scheme 36 Oxidation of methanol to formate salts.152 sustainability and profit, if the product is of a significant added value. The process where methanol is effectively transformed into formate salts (Scheme 36)152 can serve as an example of a reaction of this type. In this case, catalytic methanol dehydrogenation is combined with bicarbonate hydrogenation giving high TONs (>18,000), TOFs (>1300 h 1) and yields (>90%) of formate salt. The bicarbonate behaves as a very convenient hydrogen acceptor since the same product (formate) is formed from bicarbonate hydrogenation and methanol dehydrogenation, while utilization of hazardous gases and chemicals was avoided. The obtained formate salts are essential chemicals with a variety of uses. The current industrial production of formates involves absorption of hazardous, flammable, and difficult to transport carbon monoxide under high pressure in solid sodium hydroxide at 160 °C. A related direction concerns the elaboration of coupled systems, where one substrate is oxidized into an added-value product, while another one is reduced, also giving a product with added value. As an example, coupled systems for selective oxidation of aromatic alcohols to aldehydes and reduction of nitrobenzene into aniline156 can be mentioned. This oxidation– reduction coupling was realized using a cadmium-based composite material as a photocatalyst under visible-light illumination, giving, in one-pot, 45% and 26% yields of benzaldehyde and aniline, respectively. ARTICLE IN PRESS 130 Maximilian N. Kopylovich et al. 6. CONVERSION OF RENEWABLE SOURCES AND HYDROGEN PRODUCTION 6.1 Transformation of Renewable Materials into Added-Value Compounds Many renewable raw materials contain hydroxyl groups connected to carbon atom and hence can be considered as alcohols. One of the most abundant renewable raw materials is lignocellulosic biomass, which is composed of carbohydrate polymers such as cellulose or hemicellulose, and an aromatic polymer lignin. The lignocellulose is mainly used in construction materials, as firewood and for production of biofuels, such as biodiesel, ethanol, and hydrogen, which are sustainable alternatives to fossil fuels with reduced CO2 emission. However, modification and effective conversion of biomass to fine chemicals with an added value is an active field of modern research. For instance, partial oxidation of cellulose157 can lead to significant change in its properties, e.g., ability to absorb water, what is important for many applications. Generally, the conversion of components of lignocellulose into value-added chemicals is a complex process with many parallel reactions, low yields and sometimes unpredictable results. One of the current directions is the direct conversion of biomass to formic or acetic acids.158 In this process, remarkable total yields up to 80% were achieved.159 Vanadium-substituted phosphomolybdic acids (H3+nPVnMo12 nO40) were used as catalysts, while molecular oxygen was employed as an oxidant.159 The catalysts could be reusable at least 3  without significant loss of activity. Both formic and acetic acids are of wide use in chemical, pharmaceutical, and agricultural industries; formic acid is also recently evaluated as a perspective carrier to store or generate H2.160,161 Other important added-value products, such as levulinic acid,162,163 sorbital,164 ethylene glycol,165 5-hydroxymethylfurfuran,155 lactic,166 glycolic,167 and gluconic168 acids can also be prepared by selective conversion of cellulose and biomass-derived carbohydrates. Different conditions were proposed to optimize the yields and selectivities; the use of supercritical conditions or various catalysts and additives are among the most used procedures to increase effectiveness of these conversions. For instance, Keggintype heteropolyacid catalysts H5PV2Mo10O40 can effectively accelerate the conversion of mono-, olygo-, and polysaccharides to formic acid with good yields.152 Similar phosphomolybdic acid H3PMo12O40 was used as a ARTICLE IN PRESS Catalytic Oxidation of Alcohols 131 bifunctional catalyst to accelerate both the hydrolysis of cellulose and the subsequent oxidation reactions to give glycolic acid with molecular oxygen in a water medium.167 Conversion of substrates, readily available from natural sources, e.g., glycerol (a cheap by-product of biodiesel production), furans, or carbohydrates, is another direction which is under active exploration.84,169 The liquid phase oxidation of glycerol with bimetallic Au/Pd and Au/Pt catalysts supported over MgO leads to an enhanced glycerol conversion and increased selectivity toward oxidized C3 products under mild conditions and without the addition of a base.170 Another tested bimetallic catalyst allows proceeding with a selective oxidation of biomass alcohols to the corresponding aldehydes by a visible-light-driven synergistic photoelectrochemical (PEC) catalysis system with Au/CeO2–TiO2 nanotubes (NTs) as photocathodes and using mild conditions.171 The obtained conversion of benzyl alcohols was up to 98% while the selectivity toward benzaldehyde was >99%. Along with conventional catalysts, thermostable enzymes are prospective agents to convert glycerol into materials with high added values, e.g., dihydroxyacetone, a synthetic precursor and sunless tanning agent. Using glycerol as a second substrate, (R)-1,2-propanediol can be also produced from hydroxyacetone in a one-enzyme bioelectrocatalytic reactor.172 In this study, NAD-dependent Thermotoga maritime glycerol dehydrogenase (TmGlyDH) was employed as a main catalytic agent, while the NAD(H)-cofactor can be immobilized and regenerated electrochemically. It was also demonstrated that TmGlyDH can be a useful catalyst for producing optically active products, e.g., for resolving a racemic mixture of 1,2propanediol leaving the (S)-enantiomer aside. Other important and available derivatives of biomass are sugars, and their conversion and application are also of a high recent interest.163,164 For instance, novel magnetically separable carbonaceous nanohybrids were recently prepared from porous starch.173 These porous polysaccharidederived materials are highly magnetic up to 450 °C and thus can be used as recyclable catalysts, e.g., oxidations of benzyl alcohols and xylose dehydration to furfural. Gold-based materials were shown to be more active than other metal catalysts in the first step of the 5-hydroxymethylfurfural (HMF) oxidation, leading to 5-hydroxymethyl-2-furancarboxylic acid (HFCA) very quickly, even though they showed less activity for the subsequent conversion of HFCA to 2,5-furandicarboxylic acid (FDCA).174 The HMF oxidation over ARTICLE IN PRESS 132 Maximilian N. Kopylovich et al. Au and Pt catalysts in the presence of high amounts of NaOH was recently investigated with the use of isotopically labeled dioxygen and water. The source of inserted oxygen was shown to be water rather than oxygen. Unfortunately, in all the studied gold-based samples, process efficiency and catalyst stability were rather low. However, the modification of Au-based catalysts with Pt or Pd metal produced stable and recyclable catalysts.175 Bimetallic Au8Pd2 species supported over active carbon have the highest activity and stability for the production of FDCA. 6.2 Alcohol Oxidation for Hydrogen Storage and Production The reserves of fossil fuels are rapidly depleting, while the pollution caused by their intensive exploitation constitutes another challenge to overcome. Among the alternative and clean fuels, hydrogen is one of the most promising ones since it can be produced from renewable resources, has a high energy density, zero carbon emission, etc. However, other physical properties of H2 make its handling a rather difficult task. Therefore, easy-to-handle hydrogen-containing sources should be introduced, for instance, alcohols which can be used to store and transport H2. Thus, methanol contains 12.6% hydrogen, but the hydrogen production from alcohols generally involves rather costly oxidative steam reforming at high temperatures (over 200 °C) and pressures (25–50 bar) and with noble metal catalysts to obtain high purity hydrogen. Furthermore, these methods usually generate greenhouse gases, while the evolved CO itself may result in poisoning the involved catalysts. Alcohol dehydrogenation in the presence of a hydrogen acceptor (e.g., O2, H2O2, acetone) concerns another perspective, but the application of the hydrogen acceptor contradicts the atom economy and hence efficient acceptorless alcohol-dehydrogenation protocols are of clear need. Many molecular catalysts have been applied for the hydrogen production from alcohols,12,176 ruthenium and rhodium ones being particularly effective in catalytic acceptorless dehydrogenations.107 Thus, an efficient lowtemperature aqueous-phase methanol dehydrogenation process, which is facilitated by ruthenium complexes with pincer-type ligands, e.g., [RuHCl(CO)(HN(C2H4PiPr2)2)], has been described.177 Hydrogen generation by this method proceeds at 65–95 °C and ambient pressure with catalyst TOFs up to 4700 h 1 and TONs above 350,000; furthermore, no base is needed. It was demonstrated for the first time that it is possible to efficiently dehydrogenate the thermodynamically less-favorable primary ARTICLE IN PRESS Catalytic Oxidation of Alcohols 133 aliphatic alcohols below 100 °C. This eventually could make the use of methanol as a practical hydrogen carrier a feasible task. Concerning heterogeneous catalytic systems, Rh is an active metal for the ethanol steam reforming to produce H2.178,179 In the reforming the supports also play an important role regarding the activity, selectivity and stability of the catalyst, and MgAl2O4 was found to be an appropriate support for the Rh catalysts.180 This Rh/MgAl2O4 system is believed to be a bifunctional catalyst,177 whereas the activation of ethanol takes place both on the metal particle and on the support basic and acidic sites leading to the formation of intermediate compounds (Scheme 37). Ethylene can also be formed giving a lower hydrogen production [Scheme 37 (3)] due to deactivation of the acidic sites with coke formation on the support.177 Primary alcohols, like ethanol, can be obtained from biomass fermentation, but their further transformations toward other carriers with higher energy density are still under development. However, a simple but effective non-catalytic way for the production of high purity hydrogen from primary alcohols under basic conditions has been recently reported.181 At this reaction, one mole of ethanol reacts with one mole of a base (e.g., NaOH, Scheme 38) giving two moles of H2 and one mole of sodium acetate. One of the main advantages of this approach is that no catalyst is required, and no environmentally harmful gas, such as CO or CO2, is produced in the process. The authors also report that temperature is a key factor affecting the rate of gas generation: as long as the temperature is higher than 120 °C, hydrogen and carboxylate are produced. If methanol is used as a starting material, the corresponding formate salts can be produced apart from H2. Since formates, in their turn, have been evaluated as potent carriers to store and produce hydrogen,160,161 additionally giving valuable oxalates or carbonates, the overall process has a high Scheme 37 Ethanol steam reforming main reaction (1) and intermediate ethanol dehydrogenation (2) and undesirable ethanol dehydration (3). Scheme 38 Production of hydrogen from ethanol.181 ARTICLE IN PRESS 134 Maximilian N. Kopylovich et al. Scheme 39 Proposed177 mechanism for H2 generation with ethanol as substrate. potential to become a large-scale industrial process. The water tolerance of the reaction was also tested, and it was found that high water content inhibits the hydrogen production. In spite of that, the normal bioethanol can be used to generate high purity hydrogen at a relatively low temperature; although the rate of the reaction might be lowered, a moderate extension of the reaction time would overcome this obstacle. The proposed177 mechanism of this transformation (Scheme 39), as supported by GC/MS and isotope labeling studies, involves removal of the hydroxide proton of ethanol by a base, thus giving a molecule of water and ethanolate (Step 1). This anion rapidly reacts with water to give H2, acetaldehyde, and hydroxide ion (Step 2). Reaction of the hydroxide ion with the carbonyl carbon of acetaldehyde (via nucleophilic addition) forms alkoxide which is then deprotonated and gives a dianionic Cannizzaro intermediate, that subsequently reacts with another molecule of acetaldehyde to give the final product (sodium acetate) and a molecule of ethanol (Step 3). 7. IRRADIATION-PROMOTED OXIDATIONS Acceleration and promotion of chemical reactions by irradiation with radiowaves of various frequency ranges is an established field with multiple applications in laboratory practice and in industry. However, new and interesting results in this topic continue to appear. The range of radiowave frequencies is expanding and now includes virtually all the spectrum starting from gamma-rays and down to the low-energy MW irradiation. Although there is some consensus on the key steps of photochemical reactions promoted by short-waves, e.g., UV and visible-light irradiations, it is still debatable the mechanism by which low-energy irradiation, such as MWs, influence the reaction kinetics.182 In this section, we shall not discuss the ARTICLE IN PRESS Catalytic Oxidation of Alcohols 135 mechanisms of irradiation action (there are plenty of books and reviews on that matter, e.g., see Refs. 182c,d and references therein), but focus on some recent applications of irradiation in alcohol oxidation. 7.1 Photocatalytic Oxidations Photochemical reactions induced by ultraviolet and visible-light irradiations mainly involve transformations of molecules by direct absorption of light. In these cases, the energy of the UV/vis photon is significantly higher than the energy of Brownian motion and is generally high enough to directly cleave molecular bonds. Application of photocatalytic and photoelectrocatalytic methods for partial oxidations of both aliphatic and aromatic alcohols to give aldehydes and other related products, mainly using titanium oxide as catalyst, have been reported by several groups.183,184 Due to its direct and in many cases specific action toward certain chemical bonds, many irradiation-induced processes are more sustainable, consume less energy, generate less by-products, or can be conducted using greener solvents than analogous conventional reactions. On the other hand, sometimes the application of irradiation is crucial to perform a specific reaction, which cannot be conducted with a reasonable rate or would not occur at all, if the irradiation was not applied. For instance, piperonal, a compound of great importance for cosmetics, agrochemical, and pharmaceutical chemistries, is traditionally synthesized by isomerization and subsequent oxidation of safrole, an ingredient of some rather expensive and rear essential oils. Other proposed routes involve harmful reagents or environmentally unsafe heavy metals, while selective photocatalytic oxidation (Scheme 40) allows to use piperonyl alcohol, a common chemical which is approximately 500  less expensive than piperonal, as a starting material, under organic-free aqueous conditions, using UV irradiation and cheap commercial TiO2 as a photocatalyst.184 The photocatalytic oxidation of aromatic alcohols to aldehydes in water with rutile and anatase TiO2 NPs under UV light irradiation was studied.185,186 For example, photooxidation of four-substituted aromatic alcohols to the corresponding aldehydes, over rutile TiO2 NPs, showed Scheme 40 Photocatalytic synthesis of piperonal from piperonyl alcohol.184 ARTICLE IN PRESS 136 Maximilian N. Kopylovich et al. 45–74% of selectivity.185 The effect of surface and physical properties of TiO2 NPs on the selective photocatalytic activity under UV light irradiation was also investigated. Photocatalytic selective oxidation of ethanol to acetaldedyde in a fluidized bed photoreactor was studied on new structured photocatalysts based on direct supporting the VOx/TiO2 on the surface of commercial ZnS-based phosphors.187 Oxidation of benzylalcohol to benzaldehyde with yields and selectivity values higher than 40% and 80%, respectively, was reported using ferric ions as homogeneous catalysts and oxygen as an oxidant under UV-solar simulated radiation. To avoid occurrence of side reactions, in consequence of generated undesired reactive OH radicals188 due to the possible Fe(III) aquo-complexes photolysis, reactions were carried out at pH 0.5.189 Selective oxidation of benzyl alcohol to benzaldehyde and reduction of nitrobenzene into aniline, under visible-light illumination, was also reported using CdS/g-C3N4 composite as a photocatalyst.156 The conversion of the alcohol into the aldehyde was achieved by direct holes oxidation, and the reduction of nitrobenzene into aniline by direct electrons reduction. The CdS/g-C3N4 photocatalyst exhibits enhanced photocatalytic activity and excellent photostability relatively to single g-C3N4 and CdS, with an optimum percentage of CdS of ca. 10 wt.%. Under irradiation of visible light (420 nm) for 4 h under N2 purge conditions, the yields of benzaldehyde and aniline are 44.6% and 26.0%, respectively.156 Recently, the first example of a MOF as a promising visiblelight photocatalyst toward the selective oxidation of alcohols to their corresponding aldehydes was reported. The Zr-based MOFs, Zr-benzenedicarboxylate (UiO-66) and its derivative Zr-2-NH2-benzenedicarboxylate (UiO-66(NH2)) were prepared via a solvothermal method and successfully applied to photocatalytic reactions.190 A reaction mechanism, involving the production of charge carriers and photogenerated electrons and holes, was proposed. Oxygen reacts with an electron to form O2 , while the photogenerated holes can directly oxidize the organic reactive substrates to carbonium ions. The formed superoxide radicals further react with the carbocations, which leads to the final products.187 A recent development in this field concerns the construction of a photoreactor with membrane separation, namely pervaporation, in order to prevent overoxidation of the aldehyde. A significant rate acceleration was observed in the copper-catalyzed aerobic photooxidation of sugars using a Cu–TEMPO catalytic system (Scheme 41).191 It was suggested that the transformation of the ARTICLE IN PRESS Catalytic Oxidation of Alcohols 137 Scheme 41 Synthesis of tetrahydroazapanes through light-activated Cu/TEMPOcatalyzed aerobic alcohol oxidation.191 Scheme 42 Plasmon-mediated oxidation of sec-phenethyl (R ¼ CH3) and benzyl (R ¼ H) alcohols in the presence of supported AuNP.192 Cu(II)–TEMPO intermediate is induced photochemically. Curiously, this reaction is highly selective for primary alcohols, and hence tetrahydroxyazepanes can be easily synthesized via specific oxidation of a benzyl glucoside, followed by reductive amination and nucleophilic ring expansion (Scheme 41). Recent technological advances and price drop in laser and light emitting diode (LED) production allow extensive studies on how an irradiation of a certain wavelength influences a specific chemical reaction. Thus, a comparative study of laser, LED, and MW irradiations in both sec-phenethyl and benzyl alcohol oxidations to acetophenone and benzaldehyde, respectively, was performed.192 Excitation with monochromatic 530 nm LEDs (Scheme 42) gave yields as good or better than the corresponding laser and MW techniques, with a maximum conversion of 95% after 20 min. Hence, LEDs can provide a new economical and power-efficient alternative ARTICLE IN PRESS 138 Maximilian N. Kopylovich et al. Scheme 43 Possible pathways in the plasmon excitation of supported metallic nanoparticles.192 light source for plasmon-mediated reactions, where Au metal nanoparticles (AuNPs), with a strong absorption of the surface plasmon band (SPB) within the visible region, favor ejection of electrons and induce a variety of photoinitiated electron transfer pathways (Scheme 43). A highly efficient and selective catalytic oxidation of biomass alcohols to the corresponding aldehydes was shown to be driven by a synergistic action of visible-light and electrochemical catalysis employing Au/CeO2–TiO2 NTs as photocathodes, under mild conditions.171 At the bias potential of 0.8 V and under visible-light irradiation for 8 h, the conversion of benzyl alcohol was 98% with the selectivity toward the benzaldehyde formation being >99%. 7.2 MW-Promoted Oxidations The use of MW irradiation with a lower energy than UV–vis light has attracted much attention in synthetic organic chemistry due to the improvements on efficiency, rates of reactions, and energy consumption, as well as on selectivities, what has positioned this technology as a useful alternative energy source in organic synthesis, with an environmentally friendly nature. The MW-enhanced chemistry is based on the efficiency of the interaction of molecules with electromagnetic waves generated by a “microwave dielectric effect”. This process mainly depends on the ability of a specific ARTICLE IN PRESS Catalytic Oxidation of Alcohols 139 mixture (substrates, catalyst, and solvents) to absorb MW energy and convert it into heat. Polar molecules have good potential to absorb MWs and convert them to heat energy, thus accelerating the reactions compared with the conventional heating. The ability of a specific material (e.g., a substrate or a solvent) to convert electromagnetic energy into heat is known as loss tangent, tan δ. A reaction medium with a high tan δ is required for efficient absorption and consequently rapid heating. Despite alcohols and most of organic compounds have a considerably lower dielectric constant compared to water, they heat much rapidly in a MW field on account of their high tan δ. Furthermore, polar components (such as ILs) can be added to increase the absorbance of a reaction medium. However, the nature of the MW effect is still debatable and in some cases it has been proved that it concerns a heating effect.182 External infrared (IR) temperature controllers mainly used in MW-assisted homogeneous reactions do not accurately monitor the sample temperature and usually tend to understate its value.182 There are many examples of the successful application of MW-assisted chemistry to organic synthesis; these include the use of benign reaction media, solvent-free conditions, and application of solid supported and reusable catalysts. Over the past few years, it was demonstrated that many transition-metal-catalyzed bond transformations can be significantly enhanced by employing MW heating under sealed-vessel conditions, in most cases without requiring an inert atmosphere. Recently, a MW-promoted procedure for one-pot, two-step conversion of aryl alcohols to aryl fluorides via aryl nonafluorobutylsulfonates (ArONf ) was reported (Scheme 44).193 Moderate to good yields were achieved by this MW-assisted palladium-catalyzed fluorination sequence. The in situ conversion of aryl alcohols to aryl nonaflates using CsF as base, followed by the MW heated (180 °C) fluorination, catalyzed by [Pd2(dba)3] (dba ¼ dibenzylideneacetone) and t-BuBrettPhos [2-(di-tert-butylphosphino)20 ,40 ,60 -triisopropyl-3,6-dimethoxy-1,10 -biphenyl], allowed full conversion after 30–60 min reaction.191 Scheme 44 MW-assisted fluorination of aryl triflates.191 ARTICLE IN PRESS 140 Maximilian N. Kopylovich et al. Rhodium- and ruthenium-catalyzed hydrogen-transfer type oxidation of secondary alcohols (e.g., 5-tetradecanol, cyclododecanol, cyclooctanol, 4-t-butylcyclohexanol, or 1-p-tolyl-1-hexanol) lead, in moderately to excellent yields, to the corresponding ketones by employing MW heating at 140 °C for 15 min, using 2 equiv. of methyl acrylate as hydrogen acceptor and 5 mol% of [RhCl(CO)(PPh3)2] as catalyst, in a water/N,N-DMF solvent mixture (Scheme 45).194 No conversion was observed in the absence of MW irradiation. Primary alcohols, such as n-heptanol, n-tridecanol, or 1,7-heptanediol, were oxidized using a 2.5 mol% of [RuCl2(PPh3)2] and 2 equiv. of methyl vinyl ketone under solvent-free conditions and MW heating at 120 °C for 15 min (Scheme 46).194 The accelerating effect of MW irradiation in the synthesis of ketones from secondary alcohols with TBHP as oxidant was also reported.26,38,40,64 In fact, in the presence of the dicopper(II) [Cu2(Hedea)2(N3)2]
(0.25H2O) (Hedea ¼ N-ethyldiethanolamine) complex with the {Cu2(μ–O)2} diethanolaminate core, the oxidation of 1-phenylethanol is dramatically accelerated when the reaction mixture is subject to MW irradiation, achieving a very high yield of acetophenone (91%) after 15 min of reaction, in contrast with the 51% acetophenone formed after 30 min when using conventional heating.26 Several recent examples use copper(II) or Cu(II)/TEMPO catalytic systems,38 namely alkoxy-1,3,5-triazapentadien(e/ato) copper(II) complexes (yields up to 97% and TONs up to 485 after 60 min, and TOFs of up to 1170 h 1 after 10 min reaction)195 or bis- and tris-pyridyl amino and imino thioether Cu and Fe complexes, with a maximum yield of acetophenone of 99% after 30 min at 80 °C. The maximum TOF of 5220 h 1 (corresponding to 87% yield) was achieved just after 5 min of reaction time under the low MW power of 10 W.42,43 Scheme 45 MW-assisted oxidation of secondary alcohols with a Rh(I) catalyst and methyl acrylate.194 Scheme 46 MW-assisted oxidation of primary alcohols with a Ru(II) catalyst and methyl vinyl ketone.194 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 141 In a related study, the same group reported a series of mixed-ligand dinuclear manganese(II) Schiff base complexes as catalysts for the MW-assisted oxidation of alcohols (Scheme 47).64 Acetophenone yield of 81% is obtained using a maximum of 0.4% molar ratio of [Mn(H2L)– (py)2]2(NO3)2
2CH3OH (H2L ¼ hydrazone Schiff base) catalyst relative to the substrate (1-phenylethanol) in the presence of TEMPO and in aqueous basic solution, under mild conditions.64b A recent publication has described the efficient ruthenium-catalyzed C-3 reductive alkylation of 4-hydroxycoumarin by dehydrogenative oxidation of benzylic alcohols.196 The optimization of reaction parameters, such as type of catalyst, type of solvent, activation method, reaction time, temperature, and base, was performed (Scheme 48). Under optimized conditions, using [RuCl2(PPh3)3] as catalyst in tert-amyl alcohol under MW irradiation at 140 °C for 2 h, afforded a satisfying selectivity/conversion. MW irradiation also permitted a shorter reaction time for the selective solvent-free oxidation of primary, secondary, allylic, and benzylic alcohols with pyridinum sulfonate chlorochromate and pyridinum sulfonate fluorochromate as oxidizing agents, compared with the use of solvent. For example, cholest-5-en-3-ol acetate and cholest-5-en-3-ol benzoate were chemoselectively oxidized at position 7 in solvent-free conditions with 82% and 87% conversion, respectively.197 A CrO3-catalyzed oxidation of homopropargyl alcohols with TBHP under MW irradiation was found to be an efficient and rapid alternative Scheme 47 MW-assisted solvent-free oxidation of 1-phenylethanol to acetophenone with a Mn catalyst.64 Scheme 48 MW-assisted selective C-3 alkylation of 4-hydroxycoumarin with benzyl alcohol.196 ARTICLE IN PRESS 142 Maximilian N. Kopylovich et al. for the preparation of 1,2-allenic ketones. The optimal conditions were achieved when a solution of 1-phenylbut-3-yn-1-ol in CH2Cl2 was irradiated with MW at 40 °C for 0.2 h in the presence of 5 mol% of CrO3 as catalyst and 3 equiv. of TBHP as oxidant.198 Tetrabutylammonium decatungstate(VI) was reported to possess a catalytic activity in the oxidation of selected alcohols with hydrogen peroxide as oxidant, in 1,2-dichloroethane/water or acetonitrile/water solvent systems. A pronounced accelerating effect on the reaction rate was observed when a MW conditions were used.199 A highly active (NHC)-Pd catalytic system can be applied for anaerobic oxidation of secondary alcohols at very mild temperatures. This procedure allows assessing one-pot domino procedures for the synthesis of R-arylated ketones from secondary aryl alcohols with very good yields.200 Recently, a successful use of MW heating for the synthesis of [Pd(acac)Cl(NHC)] and [PdCl2(3-Cl-pyridine)(NHC)] complexes was reported.201 This protocol affords the desired compounds in yields comparable to those obtained using conventional heating, but drastically reduces the reaction times. A similar protocol was thereafter applied to the synthesis of a series of [Ni(Cp)Cl(NHC)] complexes (Scheme 49).202 These complexes where applied in the catalytic anaerobic oxidation of alcohols using 2,4chlorotoluene as solvent and oxidant.202 In recent years, the use of a room-temperature IL in MW-assisted synthesis, as a (co-) solvent and/or (co-)catalyst, is becoming an increasingly exploited area since the ionic nature of ILs allows them to absorb MW energy very efficiently. Besides that, ILs exhibit several inherent benefits, such as low vapor pressures, high thermal and chemical stability, and nonflammability. The role played by the combination of an IL and MW was explored in the activation of H2O2 in [hmim]Br (hmim ¼ 3-methylimidazolium) used as catalyst and solvent, under MW irradiation, for the metal-free chemoselective oxidation of various alcohols into the corresponding carbonyl compounds.203 In addition, a new metal-free methodology for the synthesis of anthraquinone has been reported.203 Scheme 49 MW-assisted synthesis of [Ni(Cp)Cl(NHC)] complexes, catalysts for alcohol oxidation.202 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 143 A Co(II) complex supported on SBA-15 has been employed as a highly active and reusable catalyst in the selective oxidation of various alcohols, in which the improved rates of reactions (from hours to minutes), yields and even selectivities in some cases, illustrate the usefulness of MW protocols as alternative methodologies in organic synthesis.204 A MW-assisted solvent-free peroxidative oxidation of benzyl alcohol to benzaldehyde catalyzed by magnetic Ni-doped MgFe2O4 NPs was also successfully performed.205 The catalyst is reusable and eco-friendly, is applied in a small amount and the reaction time is short.205 7.3 Others Application of ultrasounds is among the new alternative techniques which can accelerate heterogeneous reactions by increasing the surface (dispersity) of the reagents, while in homogeneous systems ultrasounds assist the even heat distribution in a reactor.206 Thus, the application of ultrasounds or of MW irradiation may substantially shorten the reaction time in oxidations of alcohols with PCC, from hours to minutes.207 It is believed that the ultrasound produces erosion on the PCC surface and therefore accelerates its interaction with the organic substrates. The involvement of ultrasonic irradiation in a drastic reduction on the amount of PCC used was also indicated.207 An ongoing approach on the development of new environmentally friendly protocols involves the combination of MW and ultrasonic irradiation in ILs. In this context, MW and ultrasound activation methods have been used in the oxidation of five- to eight-membered cyclanols in the presence of H2O2/H2WO4 and several hydrophobic ILs as cocatalysts.208 Quantitative oxidation of cyclohexanol, after only 30 min at 90 °C, was achieved in the presence of [Aliquat][NTf2] (Aliquat ¼ N-methyl-N,Ndioctyloctan-1-ammonium; Ntf2 ¼ bis(trifluoromethylsulfonyl)imide), prepared from Aliquat 336 (Scheme 50) which is well known as a phase transfer Scheme 50 Aliquat 336 (A) and [Aliquat][NTf2] (B). ARTICLE IN PRESS 144 Maximilian N. Kopylovich et al. Scheme 51 Ultrasound- and microwave-assisted one-pot oxidation of cyclohexanol to ε-caprolactone in [bmim][BF4] ionic liquid.209 agent. The aliphatic cation of Aliquat is more efficient than the cyclic cations of other tested ILs.208 Another example of application of MW and ultrasound methods is the one-pot, tandem oxidation of cyclic alcohols to their respective lactones using KHSO5 (potassium peroxy-monosulfate) as oxidant and an IL as a solvent (Scheme 51).209 Ultrasound and MW irradiation reduced the reaction time for the cyclohexanol oxidation by Oxone®, catalyzed by a TEMPO nitroxyl radical, in the presence of tetrabutylammonium bromide (TBAB) in [bmim][BF4] (bmim ¼ 1-butyl-3-methylimidazolium), from 8, using normal heating, to 5 and 0.5 h, respectively, with similar yields of ca. 80%. A new class of ILs with peroxymonosulphate anions was also synthesized and employed in the model oxidation.209 As illustrated above, the activation of substrates, intermediates, and other components of reaction mixtures by using irradiation of different kinds can efficiently influence the reaction kinetics in the oxidation of alcohols. Further application of such techniques should widen the spectrum of used substrates and obtained products in this field. 8. CATALYSTS RECYCLIZATION Many homogeneous catalytic systems, in spite of being very active and interesting from a fundamental point of view, frequently cannot lead to a practical solution of technological problems due to their high cost, instability, and difficulty of isolation and recyclization. Hence, efforts have been devoted to obtain recyclable catalytic systems, which, even if being less active than state-of-art homogeneous representatives in a single use, can be reused many times. Several methods can be used to achieve recyclable catalytic systems, such as the following ones: (i) utilization of heterogeneous solid catalytic materials; (ii) formation of dispersed nano-, sol-gel, and micellar systems; (iii) phase division, where a homogeneous catalyst and substrate are usually well soluble in one solvent, while the product is soluble in another solvent, ARTICLE IN PRESS Catalytic Oxidation of Alcohols 145 which in its turn is unmixable with the first one. Other methods that generally lead to such approaches have also been developed, e.g., immobilization of intrinsically homogeneous catalysts on a solid support without significant loss of their activity. In this case (supported catalysis), advantages of homogeneous catalysts (e.g., enantioselectivity) can be combined with easy recovery and reutilization. Another idea concerns the use of physical forces, e.g., magnetic fields, to remove a catalyst from the reaction mixture. Some advances in the development of these and related ideas on catalysts recyclization and improvement of the overall activity of catalytic systems for alcohol oxidation are discussed below. 8.1 Heterogeneous Solid Oxides, Alloys, and Related Materials Many classical heterogeneous catalysts, e.g., oxides and related compounds with incorporated ruthenium, gold, palladium, or platinum were found to be effective for the aerobic oxidation of alcohols.2 Silver210 and cobalt211 were also included in this catalytic family and have raised expectations regarding the availability of the catalysts. Benzyl alcohol is a typical model substrate to test a chosen heterogeneous catalyst and the reaction conditions. Apart form the formation of benzaldehyde, disproportionation, and dehydration can occur to give toluene or dibenzyl ether (Scheme 52).212,213 Therefore, selective, active, and recyclable heterogeneous catalysts are highly sought after. In this respect, AuPd alloys supported on activated carbon, as well as the monometallic Pd and Au were tested for alcohol oxidation in the presence of O2.212 The AuPd alloy possesses a higher catalytic activity than the monometallic Pd (TOF of 38 or 54 h 1 for Pd or AuPd alloy, respectively) maintaining a high Scheme 52 Reaction scheme for benzyl alcohol oxidation.212 ARTICLE IN PRESS 146 Maximilian N. Kopylovich et al. selectivity (>94%) toward benzaldehyde. An unexpected result was that, under the same conditions, Au was inactive. The better activity was explained by the smaller interatomic distances in AuPd which leads to a better contact between the reagents and the catalyst. A direction in the development of heterogeneous catalysts for alcohol oxidation concerns the synthesis of composite materials, such as Fe(III) substituted Keggin-type clusters dispersed in amorphous silica matrix (PWFe/SiO2).214 This composite was tested as a catalyst in the oxidation of alcohols into aldehydes using H2O2 as a “green” oxidant. Under mild reaction conditions, the catalyst showed a high activity and selectivity, with good yields for all the tested substrates. The good catalyst activity was attributed to the large surface area owing to its micro and mesoporous structures and strong acidity of the polyoxometalate component. The stability and reusability of the catalyst was also quite good; moreover, the catalyst preparation is simple and direct from cheap starting materials. An example of a flow chemistry process is the Oppenauer oxidation of secondary benzylic alcohols using partially hydrated zirconia and various carbonyl compounds as oxidants (Scheme 53).215 The authors applied this procedure to electron-rich and electron-deficient substrates, with improvement in temperature (as low as 40 °C) and an easy reaction workup. The reuse of the catalyst was performed several times, without loss in catalytic efficiency. Cerium(IV) oxide-based heterogeneous catalysts are of interest in oxidation owing to the unique redox properties of cerium, i.e., if the crystallite size of ceria decreases, an increase in the oxygen vacancy defect concentration occurs leading to attractive catalytic properties.216 On the other hand, due to peculiar catalytic activity of gold, its combination with other materials is also appealing, especially when the combination promotes the Scheme 53 Example of the flow Oppenauer oxidation of secondary benzylic alcohols.215 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 147 development of eco-friendly processes. Thus, the composite Au/CeO2 material was prepared by two distinct methods: homogeneous deposition–precipitation (ACH) and direct anionic exchange (ACD).216 The efficiency of the two processes of catalyst fabrication was evaluated using the aerobic oxidation of benzyl alcohol under solvent- and base-free reaction conditions. The ACH catalyst exhibited a higher alcohol conversion (64.5%) than the ACD catalyst, (53.8%), with comparable selectivities. This difference, according to the authors, is due to presence of smaller sized gold NPs and the higher number of oxygen vacancy sites on the ceria surface: the ACH sample shows surface oxygen vacancies, whereas no oxygen vacancies were found in the ACD sample. The conversion of benzyl alcohol increased with reaction time, while the selectivity on benzaldehyde diminished with an overproduction of benzyl benzoate. The recyclability of both catalysts decreased after repeated use, which might be due to their structural changes. The related Au/CeO2–Al2O3 catalyst was reported to be effective for the heterogeneous oxidation of 1-tetradecanol, used as a model to test the catalytic potential of the material toward other fatty alcohols.217 All the reactions were performed at 80–120 °C with molecular oxygen at atmospheric pressure and no added base. The highest conversion was 38%, while the reaction selectivity was up to 70% for tetradecanoic acid and up to 80% for tetradecanal. Hydrotalcites have also attracted much attention as useful precursors for the development of new environmentally friendly catalysts. Thus, Pt/Au alloy NPs, supported over hydrotalcites and with soluble starch as a green reducing and stabilizing agent, are catalysts for the selective aerobic oxidation of glycerol and 1,2-propanediol.218 The reaction conditions were mild; the oxidation was being performed in aqueous solution with no base added and using molecular oxygen. The authors tested the individual metals as catalysts and concluded that the bimetallic catalyst exhibits some synergetic properties. The high activity and selectivity of these bimetallic catalysts suggest that Pt atoms gain more electrons than Au atoms in PtxAuy–starch/ hydrotalcites as a result of the alterations of geometry and due to two types of electron transfers: (i) from the starch ligand to both Au and Pt atoms and (ii) from Au to Pt atoms.218 A series of cobalt-doped vanadium phosphorus oxide catalysts was prepared using a classical organic method, followed by calcination, and tested for the oxidation of benzyl alcohol with TBHP as an oxidant.219 The catalytic activity increases with the temperature growth up to 68% at 90 °C, while selectivity toward benzaldehyde varies in different solvents and reaches ARTICLE IN PRESS 148 Maximilian N. Kopylovich et al. Scheme 54 Oxidation of benzyl alcohol with cobalt-doped vanadium phosphorus oxide catalysts.219 100% following the trend: acetonitrile > chloroform > toluene > dioxane. The effect of the competitive adsorption between the solvents and benzyl alcohol was discussed. The proposed mechanism (Scheme 54) involves two active phases, where a suitable V5+/V4+ balance is required; the presence of Co increases the average oxidation number of vanadium creating higher amount of V5+ species, which are essential for the reversible V4+/V5+ redox mechanism. A porphyrin-containing cellulose derivative, namely hematin-appended 6-aminocellulose, performed well as a catalyst for the oxidation of guaiacol and synapyl alcohol.220 The catalytic material is insoluble in most alcohols and can be considered as a heterogeneous biomimetic catalyst. The high oxidation activity and stability of the catalyst might be due to the cellulose backbone that inhibits the self-aggregation of the hematin moieties. To probe the potential of the cellulose backbone as a chiral catalyst, oxidation of sinapyl alcohol was performed, but the material showed no chiral behavior. 8.2 Supported Catalysts Supported catalysts can be considered as heterogeneous ones which, however, exploit some features of homogeneous catalytic systems, e.g., selectivity and activity. Thus, vanadium-substituted phosphotungstic acid immobilized on amine-functionalized MCM-41 exhibited high activity and selectivity in the oxidation of aromatic alcohols to the corresponding aldehydes with H2O2, even after five cycles.221 It should be mentioned that not only catalytic systems as a whole, but also their components, in particular the most expensive and unstable ones, can be immobilized and reused. For instance, fullerene has been employed as a molecular support for TEMPO ARTICLE IN PRESS Catalytic Oxidation of Alcohols 149 and this combination was further applied for the oxidation of primary and secondary alcohols using the Anelli protocol.76 In another study,222 MCM-41 was used as a support, to anchor undecatungstophosphate. The thus prepared bifunctional catalyst was tested for oxidation, as well as esterification, of benzyl alcohols. Kinetic studies revealed that the reactions follow first order kinetic patterns, and the low values of activation energy for esterification and oxidation are indicative that the reaction rate is governed by a chemical step. A curious feature of the supported tungstophosphate is the drastic change in selectivity of the reaction with time and increasing temperature. Thus, 100% selectivity toward benzaldehyde was achieved for 2 h but, after 24 h, the selectivity shifted toward benzoic acid. When the reaction temperature increases, the conversion of the alcohol also grows, as well as the selectivity toward benzoic acid. This overoxidation of benzaldehyde to benzoic acid might be due to the high acidity of the catalyst or an effect of the support. The effect of a support can provide a key to achieve a high catalytic efficiency: for instance, TONs up to 63,000, with selectivities of 99% and conversions between 71% and 99% were reached when the oxidation of activated, nonactivated, and heterocyclic alcohols were studied with 1 atm of molecular oxygen and a zirconia supported ruthenium catalyst.223 Such high TONs were attributed to the properties of ZrO2 surface, in particular hydroxyl groups and coordinatively unsaturated Lewis acidic-basic Zr4+ and O2 pairs. Developments in support materials, which can facilitate electron transfer, increase long-term stability of the catalysts, etc., are highly desirable. With Pd dispersed over mesoporous SiO2, the oxidation of crotyl and cinnamyl alcohols (Scheme 55) can be achieved with high efficiency.224,225 Scheme 55 Cinnamyl alcohol oxidation.224 ARTICLE IN PRESS 150 Maximilian N. Kopylovich et al. The generation of atomically dispersed Pd2+ surface species at low palladium loadings promotes the high activity in the oxidation of allylic alcohols. The hierarchically ordered nanoporous Pd/SBA-15 was shown to be a key factor to obtain aldehydes upon oxidation of sterically hindered allylic alcohols, such as phytol and farnesol. The results show how important is the capability of support materials to stabilize the metal oxide and to provide a specific pore size thus promoting the mass-transfer. A robust matrix material can also be employed as a support to accommodate active metals, metal oxides, polymers, etc. Thus, three-dimensional graphene-based frameworks were used to support copper phthalocyanines and show improved thermal and chemical stability combined with a competitive overall cost and availability.226 This catalyst was successfully tested for the selective aerobic oxidation of alcohols to the corresponding carbonyl compounds. The high catalytic activity of the material was related to π–π interactions between benzene moieties of reactants and graphene that favor the interaction of the reagents and catalytic sites. On the other hand, the presence of basic sites on the support also helps to improve the selectivity. The use of more active and stable reducible oxides, like TiO2, has some advantages over the nonreducible ones, such as SiO2 and Al2O3. Thus, mesoporous titanium dioxide (anatase) was applied as a support for the deposition of gold NPs and the thus prepared material was used for the vapor phase oxidation of benzyl alcohol.227 The activity of the catalyst decreased in terms of TOFs against metal loading, what is probably due to agglomeration of gold NPs and hence lower number of available active metal sites. 8.3 Nano, Dispersed and Micellar Catalysts One of the ways to enhance the activity of heterogeneous catalytic systems consists in providing a high surface/volume ratio, i.e., its dispersion down to nano-scale. Thus, nanoporous stainless steel (NPSS) electrode materials with copper and a film of palladium were fabricated and it was found that their porosity and the presence of Cu improve the long-term stability of the Pd film on the surface.228 The presence of Cu has a significant effect on the catalytic activity, the reaction kinetics, and poisoning tolerance of the NPSS/Cu/Pd electrode. The electrode was tested for the electrooxidation of glycerol, with comparable results of those of palladium-carbon catalysts, possibly due to the large electrochemically active surface area. Palladium clusters can be encapsulated in a microporous silica shell; the thus prepared heterogeneous catalyst was tested for the solvent-free aerobic ARTICLE IN PRESS Catalytic Oxidation of Alcohols 151 oxidation of various hydrocarbons and alcohols and showed a high activity, with TOF values of up to 54,740 h 1.229 The activity and selectivity depend on the sizes of pores and substrates. The recyclability was also tested and reached 20 without any loss of activity for benzyl alcohol oxidation; the corresponding overall TONs were estimated to be ca. 280,000. Palladium NPs supported on carbon NTs functionalized with various organosilane modifiers have been tested for the selective aerobic oxidation of benzyl alcohol and quasi-TOFs based on the active surface area as high as 288,755 h 1 were obtained.230 The high selectivity toward benzaldehyde was ascribed to the low surface hydrogen concentration leading to diminished formation of toluene, and to the low surface basicity that hampers the disproportionation of benzaldehyde to form benzoic acid. In addition, the basic catalyst support facilitates small and highly dispersed Pd NPs with narrow size distribution, what favors the high activity of this catalyst even after five consecutive runs. Another related catalytic system, composed of conjugated microporous polymers, with encapsulated palladium NPs with 1.6–3.5 nm in size, showed a high catalytic activity for the benzyl alcohol oxidation to benzaldehyde with conversions and yields up to 74%.231 Gold-containing poly(urea-formaldehyde) microparticles were prepared by in situ polymerization using a series of stabilization agents and tested for the selective oxidation of glycerol.232 The glycerol conversion and the glyceric acid selectivity have opposite behaviors when the size of gold particle decreases. If the surface of stabilizer is hydrophobic, then the selectivity to C3 products in the resulting catalysts is enhanced. If stabilizers with hydrophilic surfaces are applied, the formation of C–C bond cleavage products is preferable. Besides oxidants, solvent, and other components of the reaction mixture, the environmental compatibility of the catalyst support also deserves to be addressed. Novel biocompatible thiol-functionalized fructose-derived nanoporous carbon support produced by hydrothermal carbonization can significantly diminish the environmental impact.233 This porous carbon material with supported gold NPs was highly active in selective aerobic oxidation of several alcohols to the corresponding aldehydes and ketones. The catalyst was easily recovered and reused 6  without leaching of metals or loss of activity. The proposed mechanism for the catalytic oxidation of alcohols involves the base promoted deprotonation of alcohol to form alkoxide on the Au surfaces. Then gold catalyzes the β-hydrogen elimination to produce the corresponding aldehyde, along with the formation of O2 and H2O.233 ARTICLE IN PRESS 152 Maximilian N. Kopylovich et al. Another remarkable direction concerning the promotion of the catalyst active area and its recyclability consists in the use of micelle catalytic systems. They can combine the advantages of homogeneous and heterogeneous catalyzes, because, on one hand, the catalyst and substrate are soluble in one solvent, while the product possesses a high solubility in another one. The transfer of the product to another phase shifts the equilibrium, while the catalyst is responsible for the kinetics. Usually micellar structures are used, where catalysts are confined within the small droplets of one solvent, separated from another one and commonly stabilized by surfactants. Thus, enzyme-inspired star block-copolymers with limited branching were tested in catalytic systems for the oxidation of alcohols in water, in particular for a Cu/TEMPO-catalyzed alcohol oxidation reaction34 90% conversion of benzyl alcohol to benzaldehyde was obtained after 44 h of reaction. The fact that the polymers are able to preconcentrate molecular oxygen is of particular significance for further developments. 8.4 ILs and Related Systems with Phase Division The use of ILs234 and supercritical fluids235 in catalytic oxidation has been regarded as a new possibility for catalyst recycling and enhancing the product yield and selectivity. For instance, recycling of expensive TEMPO is not a trivial task due to the homogeneous character of most of the TEMPO catalytic systems. Moreover, in some cases there are also drawbacks due to overoxidation that lower conversion and selectivity. The replacement of organic solvents by ILs can provide an effective strategy to avoid such problems.236 Thus, a vanadium-based catalyst, TEMPO and sulfonic acid cocatalysts were grafted on the [C4py][BF4] IL and used for oxidation of alcohols with H2O2 as oxidant, exhibiting good activity and recyclability.237 In another example, TEMPO was incorporated into supported [C6mim] [BF4] IL (Fig. 14A) and exhibited high activity for alcohol oxidation using bis(acetoxy)iodobenzene (BAIB) as the terminal oxidant.236 The catalyst can Figure 14 Strategies for immobilizing TEMPO on ILs. [C4mim][BF4] supported TEMPO (A) and IL@SBA-15-TEMPO (B).79 ARTICLE IN PRESS Catalytic Oxidation of Alcohols 153 be recycled together with the IL without loss of the efficiency for several cycles. TEMPO can be also grafted on SBA-15 solid support be combined with 1-methyl-3-butylimidazolium ([C4mim][Br]) IL thus forming the IL@SBA-15-TEMPO catalytic system (Fig. 14B).79 This system possesses improved selectivity and good recyclability for the oxidation of alcohols to aldehydes and ketones with TBN as an oxidant in AcOH. Deep eutective solvents (DESs) are a novel class of ILs that are generally obtained by the interfusion of quaternary ammonium salts and hydrogen bond donors (e.g., amides, amines, alcohols, and carboxylic acids). Their ionic nature and relatively high polarity provide good solubility for many ionic species, such as metal salts. They also have other advantages over common ILs, such as the simple and easy preparation as pure phases from cheap and easily available components or high chemical stability toward atmospheric moisture and temperature. These novel DES were used to incorporate Fe(NO3)3
9H2O in TEMPO; the DES–TEMPO/Fe(NO3)3 system showed good performances in the selective oxidation of various alcohols to the corresponding aldehydes and ketones, using molecular oxygen as an oxidant and under mild and solvent-free conditions.238 As expected, the DES was easily recovered and recycled up to 5  without significant loss of catalytic activity. Glycols constitute other media which have been widely used in organic transformations as environmentally benign solvents and soluble supports for liquid phase synthesis. ILs and polyethylene glycols can also be combined in the solvent–free aerobic oxidation of alcohols to give an excellent catalytic effect and easy catalyst recovery.239 The bifunctionalized combined IL-glycol PEG1000 catalytic system ([Imim-PEG1000-TEMPO][CuCl2]) shows catalytic properties similar to those of its nonsupported counterpart in terms of yields as high as 95% with 100% conversion and selectivity toward ketone. Moreover, ([Imim-PEG1000-TEMPO][CuCl2]) could be recycled and reused without significant loss of catalytic activity after five runs. The combination of several of the above described approaches can be also used. Thus, precious metal catalysts can be supported on nanomaterials and combined with ILs.240 The supported gold NPs on graphene oxide (GO) with an ionic liquid framework (Au@GO-IL) has been shown to be a highly active, and leaching tests, such as hot filtration test and AAS analysis, indicate that the catalytic reaction is mainly heterogeneous in nature. The reusability of this catalyst was tested for 5  without a significant decrease in its catalytic activity.240 Also using gold NPs, but performing oxidation under MW ARTICLE IN PRESS 154 Maximilian N. Kopylovich et al. irradiation, 2-hydroxybenzyl alcohol was converted successfully to 2-hydroxybenzaldehyde in the presence of 3-chloroperoxybenzoic acid and hydrogen peroxide in methanol.241 Pd NPs are commonly used in oxidations of alcohols and can be incorporated into an IL.242 The morphology of the particles can be suited to different applications, e.g., flower-like particles, due to their concave tetrahedral subunits, exhibited a high electrocatalytic activity toward ethanol and methanol oxidation compared with that of the commercial Pd black catalyst.242 A Pd complex containing triphenylphosphine and a Schiff base catalyst was used243 for the study of the solvent effect in carbonylation of primary and secondary alcohols to aldehydes and ketones, in the presence of NaOCl as an oxidant. By kinetic study of different proportions between the imidazolium-based IL ([C2mim][PF6]), it was shown that the acceleration of the reaction depends on the mixing proportion and that the best ratio was 1:1. Another interesting aspect of using ILs instead of molecular solvents is the possibility of bypassing steps. Thus, in a propane oxidation using rhodium (palladium)–copper–chloride catalytic systems immobilized in ILs, propane is oxidized to acetone, bypassing the isopropanol formation step.244 Methane was also studied and is oxidized under more severe conditions than propane, giving methyl trifluoroacetate as the main product. Several hydrophobic methylimidazolium-based ILs were studied in the oxidation of cyclohexanol to cyclohexanone with H2O2 and WO3 as a catalyst.245 In the biphasic cyclohexanol-ILs system, 1-octyl-3methylimidazolium chloride ([C8mim]Cl) IL was found to effectively promote cyclohexanol oxidation to 100% conversion of cyclohexanol and 100% selectivity to cyclohexanone, what is accounted for by the biphasic character of the system. The oxidation of cyclohexanol occurs in aqueous phase containing H2O2 and the catalyst, while the produced cyclohexanone is transferred to the organic phase, minimizing its further oxidation. Higher concentrations of [C8mim]Cl favor the oxidation possibly by stabilizing reaction intermediates in the catalytic process.245 A combination of a tungsten species (in particular, tungstic acid H2WO4) and an IL allowed to obtain very good results in the oxidation of five- to eight-membered cyclanols, using aqueous H2O2 MW or ultrasound activation with the ammonium-based IL Aliquat 336.208 The oxidation reaction was studied in several ILs and no effect of the aromaticity of the IL cation was observed. However, the anion of the IL seems to be important and the yields increased with its size. This can be a key factor for the choice of ILs. ARTICLE IN PRESS Catalytic Oxidation of Alcohols 155 The use of lanthanides with ILs for oxidation reactions is still scarce, but a hydrogen peroxide–urea adduct and catalytic (CF3SO3)3La in an IL was recently used for the oxidation of a secondary alcohol to ketone.246 A number of 1,2-diols, α-hydroxyketones and other aromatic, and aliphatic secondary alcohols have been successfully oxidized to the corresponding ketones with yields from 74% up to 92% and reaction times between 0.5 and 3 h. Imidazolium-based ILs were used as nonconventional media in alcohol dehydrogenase (ADH)-catalyzed reactions in enzymatic catalysis.247 When containing up to 50% of the IL, the overall conversion could be improved in some cases, while the stereoselectivity of the enzyme remained unaltered.247 Besides enzymatic catalysis, the development and use of green and efficient methods to transform lignocellulosic biomass (along with cellulose and hemicellulose) into fuels and high value-added chemicals is another appealing area. Thus, one-pot sequential oxidation and aldol condensation reactions of veratryl alcohol in the basic ionic liquid (BIL) 1-butyl-3-methylimidazolium 5-nitrobenzimidazolide, which acted as the solvent and basic additive, was studied.248 The effects of different factors, such as the type of catalyst, reaction time, reaction temperature, and the amount of BIL, on the oxidation reaction were investigated. It was shown that the catalytic performance of individual Ru@ZIF-8 (zeolitic imidazolate framework8) or CuO was very poor for the oxidation of veratryl alcohol to veratryl aldehyde. Interestingly, Ru@ZIF-8 + CuO was very efficient for the oxidation reaction and a high yield of veratryl aldehyde could be obtained, indicating the synergistic effect of the two catalysts in the BIL. The veratryl aldehyde generated by the oxidation of veratryl alcohol could react directly with acetone to form, in high yield, 3,4-dimethoxybenzylideneacetone by aldol condensation reaction catalyzed by the BIL. 8.5 Other Directions Supercritical CO2 is the most used supercritical fluid due to its favorable characteristics. Its low toxicity, relatively low critical temperature and stability allow most compounds to be extracted with little damage. In addition, the solubility of many extracted compounds in CO2 varies with pressure, allowing selective extractions. These possibilities are particularly useful in the reactions involving gaseous reagents such as oxidation with O2. Concerning the selective aerobic oxidation of alcohols, CO2 can dry wet material thus allowing to achieve a high selectivity to aldehyde by suppressing the ARTICLE IN PRESS 156 Maximilian N. Kopylovich et al. formation of carboxylic acid via the favored hydration of aldehyde.249 In another work250 it was reported that the conversion of benzyl alcohol to benzaldehyde in CO2 increased 50% for a pressure increase of 7% because the substrate and products are distributed differently in the organic and supercritical phases, thus affecting both the reaction rate and selectivity. A promising perspective for catalyst recyclization concerns the application of magnetically recoverable catalysts that can be readily collected by magnetic attraction without the use of traditional isolation methods, such as filtration, extraction or centrifugation. For instance, magnetic CoFe2O4 NPs can efficiently catalyze the oxidation of alcohols to the corresponding carbonyl products and then be easily recovered with assistance of a magnetic field and reused several times.251 If the catalytically active particles are nonmagnetic, they can be immobilized on the surface of a magnetic carrier, e.g., Fe2O3 or Fe3O4, be applied for the oxidation of various alcohols, and then be recovered by application of a magnetic field. Thus, superparamagnetic Fe3O4@EDTA–Cu(II) NPs were readily prepared and identified as an effective catalyst for the tandem transformation of benzyl alcohols and amine hydrochloride salts into the corresponding amides with TBHP as an oxidant.154 After completion of the reaction, the catalyst can be removed from the reaction vessel by assistance of an external magnet and reused at least 6  without significant loss of its activity. Bifunctional bimetallic alloys in which both catalytic and magnetic functions are simultaneously provided can be applied. For example, Co-Pd bimetallic alloy NP catalysts were prepared and employed for the aerobic oxidation of a variety of alcohols in water.252 The catalysts then were magnetically recovered and reused for further oxidation. Leaching of Co and Pd was in orders of only 10 3 mol% and 10 6 mol%, respectively. The proposed explanation for the low leaching involves an electrolytic “protection” of the more expensive Pd component: Eox of Pd(0)/Pd(II) (0.95 V) is higher than Eox of Co(0)/Co(II) (0.28 V). Hence, Co(0) is oxidized by Pd(II) to give Co(II), while Pd(0) aggregates with initial NPs and remains within the catalyst. To conclude, heterogeneous catalysis with a wide range of possible supports is rather appealing for industrial applications. The easy recovery and recycling ability are two of the most desirable features. Although some concerns still remain regarding their limited catalytic activity and deactivation,253 many of the recently introduced protocols for the alcohol oxidation involve supported transitional metal catalysts.174 The pursuit of oxidation systems without such limitations and which are readily accessible, stable, inexpensive, environmentally acceptable, and can promote selective oxidation under mild reaction conditions is still under way. ARTICLE IN PRESS Catalytic Oxidation of Alcohols 157 ACKNOWLEDGMENTS This work has been partially supported by the Foundation for Science and Technology (FCT), Portugal (FCT Doctoral Program CATSUS, UID/QUI/00100/ 2013 and “Investigador 2013” [IF/01270/2013/CP1163/CT0007] programs). N. M. R. M., A. P. C. R., and M. N. K. express gratitude to FCT for doctorate, post-doc fellowships and working contract, respectively. REFERENCES 1. Tojo G, Fernandez M. Oxidation of Alcohols to Aldehydes and Ketones. New York, USA: Springer; 2006. 2. 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