Catalytic Asymmetric Oxygenations with the Environmentally Benign

Review pubs.acs.org/CR

Catalytic Asymmetric Oxygenations with the Environmentally Benign Oxidants H2O2 and O2 Konstantin P. Bryliakov* Novosibirsk State University, Pirogova 2, Novosibirsk 630090, Russian Federation Boreskov Institute of Catalysis, Pr. Lavrentieva 5, Novosibirsk 630090, Russian Federation ABSTRACT: The discovery of simple and efficient catalyst systems for the asymmetric oxofunctionalization of hydrocarbons is a challenging task of catalytic chemistry. In this paper, we give an overview of catalyst systems capable of conducting asymmetric oxygenative transformations of organic molecules and, in line with the major trend to sustainability, relying on green oxidants H2O2 and O2 as the ultimate oxygen source. The full historical period of asymmetric oxidation catalysis (1970 to the present day) is covered; both transition-metal-based and organocatalytic systems are considered. The focus of this review is the catalytic properties of the existing catalyst systems, in particular stereoselectivity, activity, efficiency, and synthetic outlook. At the same time, mechanistic peculiarities of stereoselective oxygen transfer are given attention.

CONTENTS 1. Introduction 2. Asymmetric Epoxidation Reactions 2.1. Manganese Systems 2.1.1. Epoxidations with O2 2.1.2. Epoxidations with H2O2: Manganese Salen Catalyst Systems 2.1.3. Epoxidations with H2O2: Other Manganese-Based Catalyst Systems 2.2. Iron Systems 2.3. Ruthenium Systems 2.4. Titanium Systems 2.5. Systems Based on Other Metals 2.6. Metal-Free Systems 2.6.1. Catalysts Based on Chiral Ketone and Iminium Salts 2.6.2. Polypeptide Catalysts 2.6.3. Phase-Transfer Catalysts 2.6.4. Amine Catalysts 3. Asymmetric Sulfoxidation Reactions 3.1. Vanadium Systems 3.2. Iron Systems 3.3. Titanium Systems 3.4. Systems Based on Other Metals 3.5. Metal-Free Systems 4. Asymmetric Baeyer−Villiger Oxidations 4.1. Metal-Based Systems 4.2. Metal-Free Systems 5. Transition-Metal-Catalyzed Asymmetric Olefin cis-Dihydroxylation Reactions 6. Asymmetric C−H Oxidations 6.1. Organocatalyzed α-Hydroxylation of Carbonyl Compounds © 2017 American Chemical Society

6.2. Direct Asymmetric Oxidation of C−H Groups 7. Conclusions and Outlook Author Information Corresponding Author ORCID Notes Biography Acknowledgments Abbreviations References

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1. INTRODUCTION The demand for optically pure compounds, particularly complex chiral organic molecules, is continuously growing: for example, 6 out of the 15 top worldwide-sold drugs (with net sales of $5−10 billion/year each) are chiral synthetic compounds.1 Other areas of application of chiral compounds are agrochemicals, cosmetics, fragrances, and products of fine chemical synthesis, more generally speaking. Among the approaches to accessing enantiomerically pure chiral organic molecules, i.e., (1) resolution of racemic mixtures, (2) acquisition of naturally occurring products from the “chiral pool”, (3) enzymic and biocatalytic asymmetric transformations, and (4) asymmetric synthesis, the last is the most versatile and powerful strategy, particularly when an asymmetric transformation is assisted with a chiral catalyst, capable of creating hundreds or thousands of molecules of the chiral product per catalyst molecule. The discovery of efficient catalyst systems for the chemo- and stereoselective asymmetric

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Received: March 23, 2017 Published: August 17, 2017

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DOI: 10.1021/acs.chemrev.7b00167 Chem. Rev. 2017, 117, 11406−11459

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Figure 1. Mukaiyama’s catalyst system for the aerobic asymmetric epoxidation of olefins and proposed active species of epoxidation.

the proven capability of some chiral catalysts to promote two or more different asymmetric processes, the author expects that creation of an integral picture of asymmetric oxygenation catalysis can be useful, and hopes that this review will be of service in this respect. Following the classical definition of Henri Kagan,11 the scope of the review is restricted to the truly “asymmetric catalytic transformations”, i.e., those occurring with control of the enantioselective reaction by a chiral catalyst; diastereoselective reactions on a chiral substrate involving a chiral catalyst (“double asymmetric induction”) will not be considered. The same applies to enantio- and diastereospecific catalytic reactions, occurring without creation of new centers of chirality. The synthetic power of the catalyst systems will be given moderate attention, mostly limited to catalyzed processes affording valuable chiral chemical products (approved and prospective drugs or bioactive compounds and their precursors). Some attention will be given to the biomimetic (often also referred to as bioinspired) transition-metal-based catalyst systems, mimicking the catalytic behavior of naturally occurring metalloenzymes (mainly mono- and dioxygenases).30 In the past decade, such systems have been increasingly popular in the asymmetric epoxidations, cis-dihydroxylations, and C−H oxidations with H2O2, also appearing as fruitful objects for mechanistic studies at the atomistic level. Also, some remarkable (sometimes unexpected) fundamental mechanistic peculiarities of the asymmetric oxidations (e.g., isoinversion behavior, chiral environment amplification, different rate-determining and enantioselectivity-determining steps of the overall reaction, etc.) will be mentioned in due course. The author believes that such information also can be of value for deeper understanding of the nature of catalytic action, and for the rational design of more efficient and sustainable catalyst systems.

transformations of organic compounds is thus a challenging task of modern synthetic chemistry. Catalytic asymmetric oxidations constitute an important class of stereoselective reactions, encompassing a plethora of transformations that create new asymmetric centers, occurring with the aid of chiral catalysts. However, this review is restricted to asymmetric oxygenations, i.e., the processes involving creation of new C−O bonds, with the oxygen atom stemming from the external oxidant. The choice of the oxidant is a keystone for securing “green” character of the overall process, in terms of active oxygen content, atom economy, E-factor, etc.2−9 This review encompasses asymmetric oxygenations utilizing the environmentally benign oxidants H2O2 (and its derivatives such as urea hydroperoxide) and O2, having the highest active oxygen content (47% for H2O2 and 50% for O2) and complying with the current as well as long-term environmental constraints. The usage of hydrogen peroxide appears to be more favorable since (1) it is easy to handle (in most cases directly used as a commercial 30% aqueous solution) and (2) it yields water as the only byproduct, which conclusion is corroborated by the predominance of the H2O2-based “green” catalyst systems. The shortages of molecular dioxygen as an oxidant are (1) usually the need for a stoichiometric organic coreductant, affording high-molecularweight organic byproducts, and (2) the potential risk of explosion when operating in organic solvents.10 Moreover, the most widespread mode of dioxygen activationthe reaction with a coreductant, typically aliphatic aldehyde, to form peroxycarboxylic acid as the true single oxygen atom donor is a free-radical-driven process which may affect the chemo- and stereoselectivity of the target asymmetric transformation. Hereinafter, a summary of the catalytic properties of transition-metal-mediated and organocatalyzed asymmetric epoxidations, sulfoxidations, cis-dihydroxylations, Baeyer−Villiger oxidations, and C−H oxidations, with a focus on stereoselectivity, activity, efficiency, and chemoselectivity, is given (oxidations involving naturally occurring enzymes are not considered). To our surprise, though a number of monographs and edited collections partly related to the topic,2−17 and focused reviews on various particular catalytic asymmetric oxidation reactions with green oxidants, have been contributed in the past years,18−30 there have been no comprehensive review papers covering the whole range of asymmetric oxygenations with environmentally benign oxidants. Given

2. ASYMMETRIC EPOXIDATION REACTIONS 2.1. Manganese Systems

2.1.1. Epoxidations with O2. Mn-based catalyst systems for the asymmetric epoxidation of olefins have been one of the most developed catalyst systems, relying on green oxidants. As early as in 1992, Mukaiyama and co-workers proposed chiral manganese(III) salen complexes of the type 1 (Figure 1) as 11407


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Table 1. Asymmetric Epoxidation of Conjugated Olefins with O2 in the Presence of Manganese Salen Complexes

a

PA = pivalic aldehyde, N-Me-Imd = N-methylimidazole, N-Bu-Imd = N-n-butylimidazole, N-Hex-Imd = N-n-hexylimidazole, and N-Oct-Imd = N-noctylimidazole. bIn the presence and in the absence of N-Me-Imd, epoxides with different absolute configurations were formed. cIn parentheses, the ratio of cis- and trans-epoxides is given. dThe ee values for the major and minor epoxides are given. eIn parentheses, the ratio of cis-epoxide and (2,2dichloro-3-methylcyclopropyl)benzene is given.

Figure 2. Structures of β-ketoiminato, salen, and related manganese(III) complexes.

species. Moderate to good enantioselectivities (43−77% ee) along with modest efficiencies (3−7 turnovers, TNs) have been documented for the oxidation of several cyclic olefins (Table 1). The crucial factor enhancing the epoxide yields and enantioselectivity was the addition of N-methylimidazole (ca. 0.5 equiv vs olefin), apparently serving as an external ligand. Moreover, the sense of asymmetric induction was reversed in the presence of N-methylimidazole.32−35 With catalyst 1a, the

catalysts for the epoxidation of conjugated olefins with molecular oxygen (1 atm of O2).31 The system operated in monooxygenase fashion:30 one oxygen atom of O2 was incorporated into the epoxide, the other atom transferring to the externally added (in 3-fold excess) coreductanttypically pivaladehyde (Figure 1). The proposed mechanism invoked the dioxygen activation through the oxidation of the aldehyde to the corresponding peroxycarboxylic acid, which further interacted with the manganese complex to form the active 11408


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complex 3 (Figure 2) and 4-tert-butylpyridine (additive).48 Subsequently, Pietikäinen studied the epoxidation of 1,2dihydronaphthalene and (E)-β-methylstyrene with H2O2 in the presence of a 2.5 mol % loading of complexes 1a and 1b (Figure 3).49 N-Methylimidazole appeared to be a more

optical configuration of dihydronaphthalene epoxide was (1R,2R) when using either O2/aldehyde or peracetic acid as the oxidant and (1S,2S) when using either O2/aldehyde/ imidazole or peracetic acid/imidazole as the oxidant.35 Different oxidation mechanisms were proposed for epoxidations with and without additives, invoking the oxygen atom transfer by the manganese(III) acylperoxo complex in the absence of additives and by the manganese(V) oxo complex in the presence of the organic base (Figure 1).32,35,36 Screening various heterocyclic nitrogen bases revealed N-alkylimidazoles as the best additives, the epoxide yield increasing with increasing length of the alkyl substituent (CH3 < n-C2H5 < n-C8H17; cf. entries 3−5 of Table 1).32 Structurally similar β-ketoiminatomanganese(III) complexes of the type 2 (Figure 2) showed complementary catalytic activities, demonstrating the capability of operating with good enantioselectivities without additives (Table 1).37−39 Variation of the R substituent in 2 effectively modulated the optical outcome (cf. entries 7 and 8 of Table 1). Mukaiyama’s oxidation protocol has been quite versatile, with good opportunities for modulation of catalytic activities by varying the ligand structures and/or use of N-alkylimidazole additives. It was shown that 2-alkyl-2-oxocylcopentanecarboxylates can be used as coreductants instead of aliphatic aldehydes.40 Drawbacks of these catalyst systems are the high catalyst loadings (12 mol %), the excess of sacrificial reductant, eventually leading to contamination of the reaction mixture with a 3-fold excess of carboxylic acid, and the rather narrow scope of “good” substrates, mostly limited to cyclic conjugated olefins. The epoxidation of (Z)-olefins was reported to yield significant amounts of trans-epoxides as byproducts (entries 9 and 11 of Table 1). Lee and co-workers reported that olefins could be epoxidized with O2 in the presence of a 10 mol % loading of complex 3 (Jacobsen’s catalyst) without a sacrificial reductant; the authors proposed that the actual oxidant was dichlorocarbonyl oxide. The latter was presumably generated in situ from added chloroform (organic solvent) and 6 M aqueous sodium hydroxide, the resulting dichlorocarbene being oxidized with dioxygen.41 The presence of imidazole or N-alkylimidazoles was crucial for the epoxide formation; however, hem-dichlorocyclopropanes were in most cases the major reaction products (e.g., entry 10 of Table 1). Variations of the reaction media were reported. Pozzi and coworkers suggested fluorous biphasic conditions (perfluorohydrocarbons:CH2Cl2 = 1:1) for the aerobic epoxidations in the presence of manganese complexes of the type 4 (Figure 2).42 Such conditions allowed reduction of the catalyst loading to 1.5 mol %; however, only the epoxidation of indene was highly enantioselective (Table 1, entries 12 and 13). Mn complex 5, intercalated into a ZnII−AlIII layered double hydroxide host, was reported to catalyze the epoxidation of (R)-limonene43 with up to 55% diastereomeric excess (de) and (−)-α-pinene44 with up to 98% de to O2/pivalic aldehyde in the presence and in the absence of N-methylimidazole. Several electrocatalytic45,46 and photocatalytic47 Mn-catalyzed asymmetric epoxidation protocols have been reported, exhibiting moderate enantioselectivities. 2.1.2. Epoxidations with H2O2: Manganese Salen Catalyst Systems. In 1993, Meunier and co-workers described the first manganese-catalyzed asymmetric epoxidation of olefins with H2O2: 4-chlorostyrene was oxidized with 39% ee (yield not reported) in the presence of a 3.6 mol % loading of

Figure 3. Pietikäinen’s catalyst system for the asymmetric epoxidation of olefins with H2O2.

effective additive, compared with imidazole; epoxide yields of up to 63% and enantioselectivities up to 60% ee were reported (Table 2). The oxidation protocol of Pietikäinen required less H2O2 (2.3 equiv vs olefin) than that of Meunier (4.1 equiv), apparently due to the portionwise addition of hydrogen peroxide, reducing the unproductive manganese-promoted H2O2 degradation. Later, Pietikäinen considered a broader range of Mn complexes, 1a and 1c−1f, substrates, and catalytic additives (Figure 3).50 Interestingly, simple ammonium acetate (20 mol %) appeared to be an effective additive, ensuring high epoxide yields and ee’s (Table 2), sometimes surpassing those obtained with heterocyclic nitrogen bases. Pietikäinen also suggested the oxidation protocol based on the use of solid H2O2 adducts (such as urea hydroperoxide and triphenylphosphine oxide−H2O2) in combination with maleic anhydride, assuming that the actual oxidant was in situ formed peroxycarboxylic acid.51 N-Methylmorpholine N-oxide was used as the additive. Katsuki and co-workers epoxidized a series of 2,2dimethylchromene derivatives in the presence of a 2 mol % loading of complex 6 (Figure 2) and 10 equiv of H2O2.52 The authors screened several solvents; moderate to good yields (55−79%) were documented in the best solventacetonitrile (Table 2). In the above-mentioned papers, external donors (sometimes considered as “axial ligands”) were extensively used, which improved the epoxidation yield and enantioselectivity, presumably by facilitating the O−O bond heterolysis of the initially formed manganese hydroperoxo species over the O−O 11409


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Table 2. Asymmetric Epoxidation of Conjugated Olefins with H2O2 in the Presence of Manganese Salen Complexes

a c

UHP = urea hydroperoxide, and MA = maleic anhydride. bN-Me-Imd = N-methylimidazole, and NMO = N-methylmorpholine N-oxide. Conversion reported.

homolysis pathway.53−55 Alternatively, Berkessel and coworkers proposed the pentacoordinate manganese salalen complexes of the type 7, with the imidazole “arm”.56,57 The resulting catalysts (10 mol % loading) operated in two-phase media (CH2Cl2/aqueous H2O2), demonstrating moderate enantioselectivities in the epoxidation of 1,2-dihydronaphthalene (up to 64% ee). Katsuki and Shitama contributed the second-generation manganese salen complexes of the type 8 (Figure 2): besides the imidazole arm, additional elements of axial chirality were introduced, yielding catalysts (2.5 mol % loading) exhibiting excellent enantioselectivities (97−99% ee) for the epoxidation of chromene derivatives in the CH2Cl2/ aqueous H2O2 media.58,59 The above results have been the inspiration for a series of further studies directed at the modifications of ligand structures, selection of additives, or variation of epoxidation conditions. Kureshy and co-workers epoxidized several conjugated olefins with urea hydroperoxide in the presence of a 1 mol % loading of homochiral dimeric complexes 9a60 and 9b61 (Figure 2); excellent conversions (99−100%) were reported using an only 1.2-fold excess of oxidant. However, high enantioselectivity (100% ee) was only reported for the epoxidation of 2,2dimethylchromene derivatives bearing electron-withdrawing groups (Table 2). The use of ammonium acetate as the additive ensured higher reaction rates, compared with N-donor bases. Kinetic measurements revealed first-order dependence of the reaction rate on the catalyst concentration.62 Later, several macrocyclic manganese complexes (bearing two Mn centers) were prepared and tested as catalysts in the epoxidations with urea hydroperoxide, using pyridine N-oxide as the additive.63,64 The authors demonstrated that organic carbonates (such as dimethyl carbonate and propylene carbonate) could be used as reaction media for the manganese salen-catalyzed epoxidations with urea hydroperoxide; the resulting protocol, however, required as much as 5 mol % catalyst loadings.65 Tomaselli and Sfazzetto conducted the epoxidations with H2O2 (8 equiv vs

olefin) in aqueous media, in the presence of diethyltetradecylamine N-oxide: the latter was believed to act both as a surfactant and as a cocatalyst.66 The nature of catalytically active species of manganese salencatalyzed enantioselective epoxidations with H2O2 remains speculative. Generally, it is believed that reactive MnVO species (similar to those shown in Figure 1) are formed, particularly in the presence of donor additives such as N-MeImd. However, other possibilities have been invoked, such as formation of active peroxycarboxylic species in the presence of carboxylate salt cocatalysts50,67 or in the epoxidations with urea hydroperoxide/maleic anhydride.51 The reported number of good substrates for manganese salen-catalyzed epoxidations with H2O2 has been rather small, mostly limited to several types of unfunctionalized conjugated olefins, particularly 2,2-dimethylchromene derivatives, for which the scope is much more narrow than that reported for manganese salen-catalyzed oxidations with other oxidants (such as NaOCl, ArIO, and m-CPBA), the oxidations with H2O2 usually exhibiting inferior enantioselectivities.10,11,13 Regarding substrates of other types, Brun and co-workers reported the epoxidation of allylic alcohols, such as geraniol and nerol, with moderate enantioselectivities (50−55% ee).68,69 2.1.3. Epoxidations with H2O2: Other ManganeseBased Catalyst Systems. Manganese complexes with poly-Ndonor ligands constitute another important class of catalysts for the asymmetric epoxidation of olefins with H2O2. Pioneering works were contributed by Bolm and co-workers, who proposed C3-symmetric chiral triazacyclononane-derived chiral ligands of the type 10 (Figure 4). The active Mn complex was generated in situ starting from Mn(OAc)2 (3 mol %) and 10a or 10b (4.5 mol %).70 In methanol, styrene was epoxidized with 43% ee (Table 3). (Z)-β-methylstyrene afforded predominantly trans-epoxide (with 55% ee), which may reflect a stepwise process, with formation of a relatively long-lived acyclic intermediate, capable of rotating around the Cα−Cβ single 11410


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A breakthrough was achieved by Sun and co-workers, who proposed an original ligand framework of the aminopyridine family (structures of the type 15).74 The authors reported enantioselectivities from 18% ee (for nonconjugated vinylcyclohexane) to 89% ee (for conjugated olefins). The oxidation protocol required the use of a 1 mol % loading of the Mn complex, 6 equiv of H2O2, and 5 equiv of acetic acid as the additive (Figure 5). Later, the authors designed L-proline-

Figure 4. Triazacyclononane-derived chiral ligands.

bond.54 Later, L-proline-derived triazacyclononane 11 was prepared and used as a ligand for the preparation of a dinuclear Mn catalyst.71 Generally low enantioselectivities were reported: 15−26% ee. Some other examples of chiral manganese triazacyclononane-derived catalysts of the types 1272 and 13 and 14,73 affording epoxides with low enantioselectivities (up to 23% ee), have been reported.

Figure 5. Catalyst system of Sun.

derived C1-symmetric manganese complexes 16a and 16b (Figure 6).75 Such modification of the ligand framework, in

Table 3. Asymmetric Epoxidation of Olefins with H2O2 in the Presence of Chiral Mn Complexes with N-Donor Ligands

a

EHA = 2-ethylhexanoic acid, DMBA = 2,2-dimethylbutyric acid, and ACA = 1-adamantanecarboxylic acid. bConversion reported. cee of the predominantly formed trans-epoxide. 11411


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Figure 6. Chiral manganese aminopyridine complexes. OTf− = −OSO2CF3.

combination with the reduction of the catalyst loading to 0.2 mol %, and the use of a low temperature (−20 °C) improved the enantioselectivity (to 94% ee for substituted chalcones) and oxidant efficiencyso that a 2-fold excess of H2O2 was enough to achieve high epoxide yields (60−99%). The C2-symmetric complex 17 also exhibited high enantioselectivities for the epoxidation of substituted chalcones (up to 96% ee), yet with higher catalyst loadings (0.5 mol %).76 Interestingly, complex 18 (Figure 6) exhibited catalytic activity in the presence of sulfuric acid (3.0 mol % to substrate), demonstrating high enantioselectivity at −20 °C (Table 3).77 Regarding the role of H2SO4, it was proposed to facilitate the formation of a highvalent Mn−oxo species via heterolytic O−O bond cleavage of a presumed Mn(III)−OOH precursor.77 Bryliakov and co-workers epoxidized olefins with H2O2 in the presence of bioinspired complexes 19a and 20a: at low catalyst loadings (0.1 mol %) and at low temperature (−30 °C), H2O2 decomposition was virtually suppressed, such that 1.3 equiv of oxidant was sufficient for achieving high olefin conversions (up to 100%).78 Remarkably, the authors observed that the replacement of AcOH with isobutyric acid improved the epoxidation enantioselectivity. Subsequently, several carboxylic acids were tested as additives in combination with catalyst 20a,

revealing 2-ethylhexanoic acid (EHA) (Figure 7) as the most effective one, ensuring the highest enantioselection in the series.79 A similar conclusion was subsequently reported by Sun and co-workers, who studied the epoxidations on complexes 16c and 17 and the (S,S)-bipyrrolidine-derived analogue of 17 using EHA as the additive.80 Apart from the structure of the carboxylic acid, the effects of the variation of the steric and electronic characteristics of the Mn complexes on the catalytic performance were tested using complexes 20 and 21.81 The introduction of more sterically demanding substituents on the ligand (complexes 20a, 20b, and 21) did not lead to obvious improvement of the optical yields (and the chemical yields dropped). Pinene-derived complexes of the type 22, proposed by Costas and co-workers, displayed moderate epoxidation enantioselectivities.82 On the contrary, the introduction of electron-donating groups into the ligand structures (complexes 20a and 20c− 20h) substantially enhanced the enantioselection, which approached 98−99% ee in some cases (Table 3).81,83 The enantioselectivity increased monotonously with an increase of the electron-donor properties of the substituents, with catalyst 20e displaying the best results. Within this paradigm, Sun and co-workers synthesized complex 23 (Figure 6), which 11412


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The nature of the catalytically active species, responsible for the enantioselective oxygen transfer, and the epoxidation mechanism have been most extensively studied for the Mnbased catalysts with bipyrrolidine-derived aminopyridine ligands of the type 20 (Figure 6). On the basis of combined spectroscopic, enantioselectivity/stereoselectivity, Hammett, kinetic isotope effect, and isotopic (18O) labeling data, the following epoxidation mechanism has been proposed (Figure 9),79,81,93 assuming the key role of the elusive oxomanganese-

Figure 7. Structures of some carboxylic acids used as additives in manganese- and iron-catalyzed asymmetric epoxidations with H2O2.

demonstrated enantioselectivities up to 98% ee, with 2,2dimethylbutyric acid (DMBA) as the additive.84 Using catalyst 20c, several Δ5-unsaturated steroids were epoxidized with good yield and α-epoxide selectivity (Figure 8).83 Gao and co-workers reported the epoxidations in the presence of manganese species, formed in situ from manganese(II) trifluoromethanesulfonate and N-donor ligands of the type 24.85,86 While electron-deficient olefins, e.g., α,β-unsaturated ketones, were the preferred substrates for the manganese aminopyridine complexes discussed above, Gao’s catalysts (with Mn loadings of 0.2−0.5 mol %) catalyzed the epoxidation of electron-rich conjugated olefins with H2O2 (4 equiv) with high enantioselectivities (up to 99% ee); 1-adamantanecarboxylic acid was the additive of choice. At the same time, the epoxidation of α,β-unsaturated ketones (such as chalcone) proceeded with poor yield and enantioselectivity (Table 3).85 Manganese-based catalysts for asymmetric epoxidations with H2O2 with ligands of other types have been scarce in the literature, in fact limited to manganese porphyrins, exhibiting moderate stereoselectivities.87−90 Very recently, the influence of the ligand structure and the nature of the additives on the catalytic performance of manganese aminopyridine and related complexes has been systematically surveyed.29 There were several reports addressing practical aspects of the development of epoxidation catalysts. Abdi and co-workers reported that catalyst 19b could be recycled and reintroduced into the asymmetric epoxidation.91 Sun and co-workers used graphene oxide (bearing carboxylic moieties) instead of carboxylic acid additives, which was claimed to simplify the workup.92

Figure 9. Proposed mechanism for the “carboxylic acid-assisted” asymmetric epoxidation of olefins with H2O2 in the presence of manganese aminopyridine complexes. Reprinted from ref 93. Copyright 2016 American Chemical Society.

(V) active species. Partial erosion of stereochemistry in the course of (Z)-stilbene epoxidation, in combination with Hammett−Brown data, hint at the formation of an acyclic, presumably cationic, intermediate (Figure 9). The role of the carboxylic acid apparently consists in (1) the promotion of the O−O bond heterolysis of the initially formed manganese(III) hydroperoxo species and (2) stabilization of the high-valent oxomanganese species, with concomitant enhancement of their enantioselectivity. Manganese aminopyridine-based and related bioinspired catalysts have demonstrated very good efficiencies (sometimes exceeding 1000 turnovers), rich possibilities for modulating the

Figure 8. Epoxidation of Δ5-unsaturated steroid. 11413


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29 was rather similar; in some cases, 29 exhibited higher enantioselectivity compared with its Mn counterpart 17.76 Bryliakov, Talsi, and co-workers examined the effect of different carboxylic acids on the epoxidations in the presence of complex 30a;79 as in the case of the analogous Mn complex 20a, the enantioselectivity increased upon the use of bulky carboxylic acids, 2-ethylhexanoic acid demonstrating the best results. Costas and co-workers suggested an alternative strategy for manipulating the catalytic properties of iron aminopyridine complexes consists in the introduction of electron-donating groups. In the resulting series of complexes, the enatioselectivity increased with increasing electron-donating properties of the substituents, complex 30b being the best catalyst.101 In addition, the authors screened 14 different carboxylic acids (including several optically pure chiral compounds) and selected 2-ethylhexanoic acid and (S)-ibuprofen (Figure 7) as the most effective additives, ensuring the best enantioselection. Later, the same group systematically tested 16 N-protected amino acids as the additives; proper matching of the chiralties of the additives and of catalyst 30b was shown to improve the enantioselectivity. Moreover, the use of amino acids was claimed to broaden the scope of epoxidation at the expense of α-alkyl-substituted styrenes.102 Finally, catalyst 31 was developed, capable of conducting the epoxidation of cyclic aliphatic enones and dienones with up to 99% ee in the presence of 2-ethylhexanoic acid.103 The principles of ligand design for iron-catalyzed asymmetric epoxidations have been summarized in a recent review paper.104 Bioinspired iron aminopyridine complexes have been very fruitful objects for the investigation of the mechanism of enantioselective epoxidation with H2O2. To date, it has been commonly accepted79,101,104 that the epoxidation is conducted by active, formally oxoiron(V), complexes of the type [((S,S)pdp)FeVO(OX)]2+ (where OX = OH or OAc); there have been examples of spectroscopic detection of oxoperferryl complexes exhibiting direct reactivity toward olefinic substrates.105−108 However, there may be some peculiarities associated with the donor properties of the ligand substituents. In particular, while unsubstituted catalyst 30a in the presence of H2O2 gave rise to an epoxidation-active, EPR-visible, low-spin oxoiron(V) complex with large g-factor anisotropy (g1 = 2.66, g2 = 2.42, g3 = 1.71),79 structurally similar catalyst 30c, bearing electron-donating groups, afforded a high-valent, low-spin iron complex with small g-factor anisotropy (g1 = 2.071, g2 = 2.008, g3 = 1.960).109 The reason for this difference is the electronic structure of the latter intermediate, which is better represented as [((S,S)-pdp*)•+FeVIO(OAc)]2+; the unpaired spin density predominantly resides at the organic ligand core, rather than on the iron center (Figure 12), which leads to the much smaller gfactor anisotropy.109 Crucially, the unsubstituted catalyst 30a also displays an intermediate with small g-factor anisotropy, provided that branched carboxylic acids (such as EHA and EBA, Figure 7) are taken as the additive; the electronic structure of the resulting species (g1 = 2.069, g2 = 2.007, g3 = 1.962) is best represented as [((S,S)-pdp)FeIVO(•OC(O))R]2+.110 The catalyst systems displaying the intermediates with small g-factor anisotropy are always more enantioselective than the systems featuring the intermediates with large g-factor anisotropy: their higher enantioselectivities are apparently caused by the additional stabilization via delocalization of the unpaired electron over the aminopyridine ligand or the carboxylic moiety (Figure 12).109,110

activity and stereoselectivity (by tuning the ligand structure and the structure of the additives), and generally high enantioselectivities (up to 99% ee under optimized conditions) for the epoxidation of substrates such as α,β-unsaturated ketones and substituted 2,2-dimethylchromenes. At the same time, nonconjugated olefins, electron-rich conjugated olefins, or trisubstituted olefins are in most cases epoxidized with low yields and enantioselectivities. 2.2. Iron Systems

Complexes of iron, the most abundant transition metal on earth, have attracted the considerable attention of chemists working in asymmetric epoxidation with H2O2. Apparently, one of the first examples of Fe-mediated enantioselective olefin epoxidation with H2O2 was reported by Jacobsen and coworkers, who studied the epoxidation of (E)-β-methylstyrene in the presence of different metal salts and chiral peptides; the combinatorial approach revealed a series of iron/peptide combinations, affording trans-β-methylstyrene epoxide with low optical purity (up to 15−20% ee).94 Beller and co-workers prepared the active catalysts in situ from FeCl3, dipicolynic acid (H2dipic), and an organic base (of the types 25 and 26) serving as the source of chirality (Figure 10).95−97 Typical loadings

Figure 10. Catalyst system of Beller.

were 5 mol % FeCl3, 10 mol % H2dipic, 12 mol % organic base, and 2 equiv (vs substrate) of H2O2; the reaction was conducted in tert-amyl alcohol.95 After a number of chiral organic bases were screened, structure 25 appeared to be the best chiral auxiliary (Table 4). The stereoselectivities were in most cases moderate; the catalyst system exhibited a preference for the epoxidation of bulky conjugated trans-olefins, with the enantioselectivity approaching 97% ee in one case (Table 4).96 In a subsequent mechanistic study, the authors hypothesized the formation of a high-valent oxoiron intermediate.97 Kwong and co-workers prepared dinuclear complex [Fe2O(26)Cl4] starting from FeCl2 and ligand 26 (Figure 10).98 In the presence of a 2.0 mol % concentration of the catalyst, 1.5 equiv of H2O2, and 10 equiv of acetic acid, several conjugated olefins were epoxidized with moderate to good yields (50− 100%) and moderate enantioselectivities (17−43% ee, Table 4). Sun and co-workers synthesized iron(II) complexes 2799 and 28100 (Figure 11), which (at 2 mol % loadings) showed moderate to good enantioselectivities for the epoxidation of α,β-unsaturated ketones, such as chalcone derivatives, in the presence of AcOH. At −20 °C, enantioselectivities of up to 98% ee were reported.100 The catalytic performance of complex 11414


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Table 4. Asymmetric Epoxidation of Olefins with H2O2 in the Presence of Chiral Fe Complexes

a

EHA = 2-ethylhexanoic acid, IBU = (S)-ibuprofen, and NPha-ILE = Npha-protected isoleucine (see Figure 7 for the structure). bConversion reported. cSubstrate used in high excess; the yield is based on the oxidant.

co-workers tested chiral complexes of the type 34 (Figure 13) in the epoxidation of para-substituted styrenes with molecular oxygen (in the presence of isobutyraldehyde as the coreductant).113 Using a 0.3 mol % concentration of catalyst 34, good conversions (50−92%) and moderate enantioselectivities (16−30%) were achieved; the addition of pyridine Noxide was reported to improve both the conversion and enantioselectivity, without alteration of the absolute configuration. Che and co-workers synthesized chiral ruthenium porphyrin 35 (Figure 13), which surprisingly was reported to catalyze the epoxidation of conjugated olefins with molecular oxygen (8 atm) without any sacrificial reductant, performing 1−21 turnovers.114 Enantioselectivities up to 73% ee were achieved (Table 5). Much later, Katsuki and co-workers synthesized the “secondgeneration” ruthenium salen complexes 36a−36c (Figure 13).115 Using a 5 mol % concentration of the Ru catalyst, both (Z)- and (E)-β-methylstyrenes were epoxidized with 1 atm of molecular oxygen (without coreductant but under visible light irradiation) with high enantioselectivities (up to 92% ee) within 36−48 h. Isotopic labeling data provided evidence that the active species can exchange its active oxygen with added water; the latter also served as the proton transfer mediator for this reaction. Apparently, the role of light irradiation was to facilitate the NO dissociation; later, complexes 36d and 36e, containing a coordinated water molecule, were prepared, which allowed the epoxidation of conjugated olefins to be conducted in the dark.116 The system demonstrated slightly higher enantioselectivities for the

Simonneaux and co-workers studied the asymmetric epoxidations in the presence of chiral iron porphyrin complexes of the type 32 (Figure 11).111,90 The catalysts operated in aqueous methanol in the presence of an imidazole additive, affording epoxides of substituted styrenes with 8−70% ee. The highest enantioselectivity of 78% ee was reported for the epoxidation of 1,2-dihydronaphthalene.111 It is worth mentioning that, in 1989, a synthetic analogue of bleomycin 33 was reported, which catalyzed the epoxidation of (Z)-β-methylstyrene with H2O2 with moderate enantioselectivity (45% ee), performing six turnovers.112 Moreover, the same catalyst afforded cis-β-methylstyrene epoxide with 51% ee when molecular oxygen was used as the oxidant (in the presence of coreductant 2-mercaptoethanol), yet benzaldehyde was the major oxidation product. Overall, iron aminopyridine-based catalyst systems for the asymmetric epoxidation of olefins with H2O2 exhibit somewhat lower efficiencies than their manganese counterparts, and comparable enantioselectivities. The reported scope of good substrates of iron catalysts is apparently somewhat broader, including, besides α,β-unsaturated ketones and 2,2-dimethylchromene derivatives, some (E)-olefins, α-alkyl-substituted styrenes, cinnamic acid esters, cyclic aliphatic enones, and dienones. Common drawback of some Mn- and Fe-based systems is pronounced H2O2 decomposition, leading to oxidant overconsumption; to suppress that, H2O2 is typically added rather slowly, preferably under syringe-pump conditions. 2.3. Ruthenium Systems

Complexes of ruthenium, a close analogue of iron, have been studied in green epoxidations since 1997, when Kureshy and 11415


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Figure 11. Chiral ligands and iron complexes used in iron-catalyzed enantioselective epoxidations. OTf− = −OSO2CF3.

epoxidations with 1 atm of O2 pressure at 0 °C, compared with the aerobic epoxidations at 25 °C. In 1999, Mezzetti and co-workers reported the first enantioselective olefin epoxidation with H2O2 in the presence of ruthenium complexes. The authors synthesized chiral ruthenium complexes of the type 37 (Figure 13) with phosphonoimino P,N,N,P-donor ligands and used them as catalysts (1 mol %) for the epoxidation of several conjugated olefins.117,118 In spite of the addition of a high excess of H2O2 (7.1 equiv), generally moderate conversions (6−39%, with a few exceptions) and moderate enantioselectivities (4−41% ee) were reported.117

In 2004, Beller and co-workers screened a series of ruthenium complexes of the types 38 and 39, prepared in situ by mixing [{Ru(p-cymene)Cl2}2], 2,6-pyridinedicarboxylic acid, and the corresponding chiral ligands.119 Using a 5 mol % concentration of the catalyst and 3 equiv of H2O2, moderate to good epoxide yields, along with enantioselectivities up to 84% ee, for the epoxidation of conjugated olefins were achieved. The addition of acetic acid was reported to improve the yield and ee. Subsequently, the authors screened a series of chiral auxiliaries (29 structures),120 examined the effect of solvents (tert-amyl alcohol showed the highest yields and ee’s), and evaluated the reaction mechanism.121 Hammett analysis witnessed electrophilic active species (ρ+ = −2.45) and and electron-deficient 11416


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Figure 12. Formation of the active species with small g-factor anisotropy. Reprinted from ref 110. Copyright 2016 American Chemical Society.

transition state. A number of the structurally related ruthenium pyridinebisimidazoline complexes of the type 40 were screened in the epoxidations with H2O2, demonstrating good yields (63% to >99%) and somewhat lower enantioselectivities (1− 71% ee) compared with their pyridinebisoxazoline counterparts of the types 38 and 39.122−124 To date, ruthenium-based catalyst systems for the enantioselective epoxidations with O2 or H2O2 have remained rather underdeveloped. The substrate scope is limited to conjugated aromatic olefins, and the activities and enantioselectivities are lower compared to those demonstrated by Mnand Fe-based catalyst systems. High catalyst loadings (up to 5 mol %) are typical for ruthenium-based oxidation catalysts, which eventually results in overconsumption of H2O2. On the other hand, the ability of some Ru systems to conduct catalytic asymmetric epoxidations with O2 without a sacrificial reductant is remarkable, yet at the moment can hardly be of synthetic value. 2.4. Titanium Systems

Complexes of titanium, the second most abundant transition metal on earth, have been well represented as catalysts of enantioselective olefin epoxidations with H2O2. The milestone work by Katsuki and co-workers, who synthesized the pseudoheterochiral bis(μ-oxo)titanium(IV) complex 41 (Figure 14), with the salalen ligand bearing additional elements of axial chirality, was published in 2005;125 very high epoxide yields and enantioselectivities (up to >99% ee) were reported for the epoxidation of several conjugated substrates, particularly indene and 1,2-dihydronaphthalene (Table 6). Dichloromethane and ethyl acetate were identified as the best solvents for this system. Although the standard epoxidation procedure required a 1 mol % concentration of the titanium catalyst, much higher efficiencies were achievable (4600 turnovers in one case).125 Besides conjugated olefins, catalyst 41 efficiently catalyzed the epoxidation of aliphatic olefins (Table 6); in this case, 2 mol % catalyst loadings, along with a 1.5-fold excess of H2O2, were used.126

Figure 13. Chiral ruthenium complexes used in enantioselective epoxidations with H2O2 and O2.

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Table 5. Asymmetric Epoxidation of Olefins with O2 and H2O2 in the Presence of Chiral Ru Complexes

a

Py-N-O = pyridine N-oxide. bTurnover number.

were successfully applied to the epoxidation of cis-alkenylsilanes with >99% ee (10 examples).131 The same catalyst 41 also catalyzed the epoxidation of a series of (Z)-enol esters to afford the epoxides in high yield with high enantioselectivity ranging from 86 to >99% ee.132 It is worth noting that catalyst 43a/ Ti(OiPr)4 (5 mol %) was found to efficiently conduct the monoepoxidation of conjugated dienes, affording the corresponding chiral allylic cis-epoxides (19 examples) with 72−98% ee.133 The pioneering studies by Katsuki and co-workers inspired the interest of other research groups. Berkessel and co-workers developed a common synthetic procedure for the preparation of salan ligands (including nonsymmetrically substituted) of the type 45 (Figure 15).134 The resulting catalysts, prepared in situ from 10 mol % Ti(OiPr)4 and 10 mol % 45, showed good performance in the epoxidation of electron-rich substrates such as indene and 1,2-dihydronaphthalene, while the yields of epoxides of nonconjugated olefins were low (6−9%). Sun and co-workers developed salan and salalen ligands of the types 46 and 47 (Figure 15). The resulting in situ prepared catalysts conducted the epoxidation of conjugated olefins with good to high optical yields (44−82% ee and 99% ee in one case) with 3 equiv of 50% H2O2 (Table 6).135 Subsequently, the same group reported the synthesis of four biaryl-bridged salalen ligands and intramolecular dinuclear titanium complexes of the type 48 obtained therefrom; the latter, used at 1 mol % loading, exhibited improved enantioselecytivity for the epoxidation of terminal aromatic olefins, compared to its nonbridged prototype.136 Aiming at developing efficient catalysts for the epoxidation of nonconjugated dienes, Berkessel and co-workers synthesized a series of chiral salalen ligands, 49 (Figure 15), stemming from cis-1,2-diaminocyclohexane.137 The latter, in combination with Ti(OiPr)4, demonstrated good to high enantioselectivities for the epoxidation of terminal aliphatic olefins (up to 95% ee for 1-octene), while “traditional” substrates (1,2-dihydronaphthalene, (Z)- and (E)-β-methylstyrene) were epoxidized with moderate optical yields (31−85% ee). Subsequently, a library of nonsymmetrically substituted salalen ligands bearing one element of axial chirality (of the type 50) were presented.138 Improved 1-octene epoxidation enantioselectivity was reported (96% ee), with the reaction conducted in neat olefin.138 A similarly high ee was achieved for the epoxidation of 1-decene,

Figure 14. Katsuki’s titanium salalen catalyst system for the enantioselective olefin epoxidations with H2O2.

A serious drawback of the above catalyst is the sophisticated synthesis of the second-generation salalen complex. In 2006, the same group presented a series of more easily available chiral salan ligands, 42 (Figure 15); the latter, in combination with Ti(OiPr)4, were used to generate the catalytically active species in situ.127 Titanium salan catalysts required 5 mol % loadings and a 1.2-fold excess of H2O2, and in most cases showed somewhat lower enantioselectivities as compared with the salalen complexes 41 (Table 6). The authors screened 14 salan ligands of the type 43 (Figure 15), and found that the introduction of ortho-substituents onto the Ar rings improved both the epoxide yield and enantioselectivity.128 Another improvement was the addition of phosphate buffer (pH 7.4− 8.0), which allowed the reduction of the catalyst loading to 1 mol % without loss of conversion and suppressed the formation of byproducts.129 The same groups proposed the C1-symmetric L-prolinederived salan ligands of the type 44; the latter served as efficient chiral auxiliaries for the titanium-catalyzed epoxidation of styrenes.130 With ligand 44d (10 mol %) and Ti(OiPr)4 (10 mol %), several substituted styrenes were epoxidized with 96− 98% ee, values which are among the highest reported for these challenging substrates. In subsequent publications, the authors enriched the substrate scope of their systems. In particular, titanium salalen complex 41 and in situ prepared combination 43a/Ti(OiPr)4 11418


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Table 6. Asymmetric Epoxidation of Olefins with H2O2 in the Presence of Chiral Ti Complexes

substantially affected the rate of epoxidation of para-substituted styrenes, the epoxidation enantioselectivity remained independent of the nature of the substituent.143,144 The reason for this apparent violation of the widely invoked “reactivity−selectivity rule” is the stepwise epoxidation mechanism, with separated rate-determining and enantioselectivity-determining steps (Figure 17). The enantioselective oxygen transfer is performed in an intramolecular and most likely concerted fashion (as stems from the retention of the stereochemistry of (Z)-stilbene epoxidation).141 The obtained kinetic profiles of epoxidations witnessed that titanium salalen catalysts are both more active and more efficient as compared to their direct titanium salan analogues.144 An important feature of titanium salan and titanium salalen catalysts is the presence of at least one N−H moiety, which is essential for efficient catalysis. Density functional theory (DFT) calculations of the presumed monoperoxotitanium salalen active species (Figure 18) estimated that the distance between one of the peroxo oxygens to the amine H is ca. 2.50 Å, which may be considered as evidence for weak, essentially electrostatic, hydrogen bonding.141 The slightly higher negative

using ligand 51/Ti(OiPr)4 (0.5 mol % each) and 0.5 mol % pentafluorobenzoic acid as the additive, serving to shorten the reaction time.139 The nature of the catalytically active species was discussed. Katsuki and co-workers140 and Berkessel and co-workers137 successfully isolated and X-ray characterized binuclear μ-oxo-μperoxotitanium salan and salalen complexes, respectively (Figure 16), supposed to be the precursors (reservoirs) of the actual active species. Berkessel and co-workers detected mononuclear titanium salalen complexes, formed in situ from the salalen ligand and Ti(OiPr)4, by high-resolution mass spectrometry, and hypothesized that the active species was also a mononuclear species.142 Bryliakov and co-workers examined a series of trans-1,2-cyclohexanediamine-derived titanium salan and salalen complexes of the types 52143 and 53,144 and found them, under optimized conditions, to exhibit enantioselectivities approaching those of the second-generation titanium salalen catalysts. The electrophilic nature of the active oxidants in the epoxidations conducted by titanium salan143 and titanium salalen complexes was established.144 Remarkably, while the presence of electron-donating or -withdrawing groups 11419


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Figure 15. Salalen and salan ligands and the corresponding titanium complexes.

2.5. Systems Based on Other Metals

Mulliken charge at the N−H hydrogen-bonded oxygen (−0.40 vs −0.43 to −0.44) suggests that nucleophilic attack of the substrate should preferentially occur at the other oxygen of the coordinated peroxo group.141 Overall, titanium salan- and salalen-based catalyst systems have demonstrated high enantioselectivities and good substrate scope, ranging from electron-rich conjugated olefins to nonconjugated terminal aliphatic olefins. Their other advantages are negligible H2O2 decomposition (which thus can be added in one portion), high epoxide selectivities, and rich opportunities for fine-tuning the ligand structure and hence the catalytic properties. Regarding their shortcomings, one could invoke the relatively high catalyst loadings (up to 10 mol % in some cases) and inactivity toward electron-deficient olefins (e.g., α,β-unsaturated ketones).

Among other transition-metal-based epoxidation catalysts with H2O2, platinum complexes demonstrated interesting results in the epoxidation of terminal nonconjugated olefins. In 1987, Strukul and co-workers synthesized chiral platinum complexes [(54)Pt(CH3)(CH2Cl2)][BF4] with ligands of the type 54 (Figure 19).145 The latter catalyzed the epoxidation of propene and 1-octene with low to good yields and moderate enatioselectivities (31−41% ee), performing up to 108 catalytic turnovers within 72 h. Later, the epoxidation of other electrondeficient substratesα,β-unsaturated ketonesin the presence of platinum complexes with ligands 54a−54d was reported.146 The epoxide yields and enantioselectivities were low to moderate (0−72% ee); the latter decreased with time, approaching in most cases nearly 0 values. This loss of enantiomeric excess was ascribed to some consecutive 11420


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Figure 18. DFT-optimized structures of the proposed active species of the titanium salalen-catalyzed oxidations: [(42d)Ti(η2-O2)(EtOAc)] and [(42d)Ti(η2-O2)(H2O)]. Reprinted with permission from ref 141. Copyright 2016 John Wiley and Sons.

Figure 16. Molecular structures of μ-oxo-μ-peroxotitanium salan ([(43a)Ti(μ-O)(μ-O2)Ti(43a)])140 and salalen ([(49b)Ti(μ-O)(μO2)Ti(49b)])137 complexes. Key: C, gray; N, blue; O, red; Ti, light gray. Hydrogens and solvent molecules were omitted for clarity. Reprinted with permission from ref 141. Copyright 2016 John Wiley and Sons.

At −5 °C, moderate stereoselectivities were reported for the epoxidation of (Z)-β-methylstyrene (36% ee, Table 7) and αpinene (41% de), using chiral amines 56a and 56b, respectively, as the chirality sources.150 Formation of significant amounts of the major side products1,2-diolswas documented. Herrmann, Kühn, and co-workers tested a library of chiral pyrazole-derived ligands (of the type 57) and diols of the type 58 (Figure 19) as chiral auxiliaries in the MTO-catalyzed olefin epoxidations with H2O2.151 (Z)-β-methylstyrene was epoxidized with 0−27% ee, and with 41% ee in one case, yet with very low conversion (5%). Burke and co-workers tested six different chiral nonracemic 2-substituted pyridine ligands in the MTO-catalyzed epoxidations of conjugated and nonconjugated olefins with 2 equiv of UHP and reported low enantioselectivities: 2−12% ee.152 Beller and co-workers reported good yields (up to >99%) but low enantioselectivities (not exceeding 19% ee) for the epoxidations of several conjugated aromatic olefins with H2O2 in the presence of in situ generated methyltrioxorhenium complexes with pyridinebis(oxazoline) ligands of the type 59.153 Complexes of metals other than Mn, Fe, Ru, Ti, and Pt have been rather rarely involved in catalyzed asymmetric epoxidations with H2O2 and O2. Some examples are listed below. Park and co-workers developed a heterogenized copper(II) complex (immobilized on mesoporous silica) with chiral proline-derived ligand 60, which catalyzed the asymmetric epoxidation of several α,β-unsaturated ketones with H2O2 and UHP under solvent-free conditions with good conversions (up

reactions, probably a further oxidation, leading to products undetectable by GC analysis.146 Much later, the authors reported the enantioselective epoxidation of terminal olefins in the presence of 2 mol % pentafluorophenylplatinum complexes of the type 55 (Figure 19).147 Using 1.0 equiv of H2O2, moderate to very high epoxide yields (up to 99%) were reported, along with good enantioselectivities, approaching 98% ee in one case (Table 7). The ee could be improved by reducing the temperature to −10 °C; the reactions were slow, requiring 20−48 h. The selectivity toward the terminal double bond was remarkable, typically 100%; conjugated styrene was not epoxidized at all under the same conditions. The above Pt catalyst systems represent a rare example of nucleophilic transition-metal-based oxidants. Typically, the catalyst operated in dichloroethane;145−147 however, the reaction could also be realized in water−surfactant media (in some cases with enhancement of the yield or enantioselectivity).148,149 To explain the observed nucleophilicity, the authors invoked the Pt−OOH oxygentransferring active species.149 Corma and co-workers developed the enantioselective olefin epoxidation with H2O2 in the presence of rhenium complexes; the catalysts were generated in situ from methyltrioxorhenium (MTO; 2.5 mol %) and a series of chiral amines (2.5 mol %).

Figure 17. Proposed mechanism of titanium salan- and titanium salalen-catalyzed olefin epoxidations. Reprinted with permission from ref 144. Copyright 2016 Elsevier. 11421


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99%) and good to excellent enantioselectivities (83−99%) were reported for the epoxidation of several substituted chalcones in THF at +35 °C. This catalyst system was subsequently applied to the epoxidation of 2-arylidene-1,3-diketones with good to excellent enantioselectivities (82−99% ee).156 Belokon and co-workers found that octahedral cobalt complexes with chiral O,N,N-donor ligands can conduct the epoxidation of chalcones with 30% aqueous H2O2 (5 equiv) in two-phase media in the presence of inorganic bases.157 Complex 62 showed the highest enantioselectivity in the series (up to 55% ee). It is worth noting that cobalt salen complexes were tested as catalysts of aerobic olefin epoxidations in the presence of aliphatic aldehydes as sacrificial reductants. While 63a (Figure 19) exhibited very low enantioselectivity (2−4% ee),158 complex 63b was more stereoselective in the presence of pyridine N-oxide, affording trans-3-nonene epoxide with 55% ee.159 Kureshy and co-workers reported good efficiencies (>300 turnovers) but moderate enantioselectivities (14−41% ee) for the epoxidation of several olefins with O2/isobutyraldehyde in the presence of chiral nickel(II) Schiff base complexes 64.160 There was a report on the preparation of a silica-supported cobalt casein complex which catalyzed the epoxidation of cinnamyl alcohol to optically pure 3-phenylglycidol with up to 92% optical yield by molecular oxygen (1 atm) without any sacrificial reductant.161 Katsuki and co-workers proposed an approach to the epoxidation of allylic alcohols with H2O2 and UHP, using catalysts (4 mol %) generated in situ from Nb(OiPr)5 and chiral salan ligands of the types 42c (Figure 14) and 65 (Figure 19).162 Moderate to good epoxide yields (40−82%) and enantioselectivities (36−95% ee) were reported. Wang and Yamamoto successfully applied a tungsten bis(hydroxamic acid) complex, 66 (2−5.5 mol %), to the epoxidation of allylic alcohols with H2O2, reporting in several cases enantioselectivities in the range of 80−95% ee and high yields.163 2.6. Metal-Free Systems

Metal-free, or organocatalytic (i.e., based on catalysts composed of carbon, hydrogen, nitrogen, sulfur, and phosphorus), epoxidations constitute another representative class of asymmetric epoxidations with H2O2 and O2. There are several major types of organocatalysts, namely, (1) those based on chiral ketones and iminium salts, (2) polypeptide catalysts, (3) phasetransfer catalysts, and (4) amine catalysts. Catalysts of the first type are able to catalyze the epoxidation of unfunctionalized olefins, while the others can only conduct the epoxidation of α,β-unsaturated ketones and aldehydes. This section will provide a brief overview of the above catalyst systems. More detailed considerations can be found in refs 19, 28, and 164−170. 2.6.1. Catalysts Based on Chiral Ketone and Iminium Salts. In 1999, Shu and Shi found that, in the presence of K2CO3, hydrogen peroxide can react with acetonitrile (the reaction solvent) to form peroxyimidic acid, which converted the organocatalystfructose-derived ketone 67 (Figure 20) to the active dioxirane, capable of transferring the oxygen atom to olefins.171 For five di- and trisubstituted (E)-olefins, high enantioselectivities of 89−95% ee were reported, along with good yields (55−90% within 7−18 h), using a 30 mol % loading of the chiral ketone and 3 equiv of H2O2 (Table 8).171 In a subsequent work, the authors reported a broader substrate scope, with enantioselectivities up to 99% ee, and showed the possibility to reduce the catalyst loading to 10 mol %.172 A

Figure 19. Chiral ligands and catalysts used in asymmetric epoxidations in the presence of complexes of other metals.

to 92%) and enantioselectivities (up to 84% ee) in the presence of triethylamine additive (Table 7). The solid catalyst was successfully recycled several times.154 Feng and co-workers reported the epoxidation of α,βunsaturated ketones with 3 equiv of H2O2 in the presence of 5−10 mol % scandium catalyst, which was prepared in situ from Sc(OTf)3 and chiral ligands of the type 61.155 High yields (70− 11422


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Table 7. Asymmetric Epoxidation of Olefins in the Presence of Complexes of Various Metals

a

Yield based on the oxidant; the substrate was in excess. bAt very low conversions, corresponding to 1.2 catalytic turnovers. cAt low conversions, corresponding to 16 catalytic turnovers. dConversion reported. eOnly terminal epoxide was formed. fPoly(oxyethylene) alcohol (C12H25− C18H37)(OCH2CH2)5. gCorresponding to the reported 9% H2O2 conversion and 82% epoxide selectivity; olefin was taken in excess (olefin:H2O2 = 4:1). hIBA = isobutyraldehyde.

fosfomycin, to afford the epoxide with 100% conversion and 68% ee.173 There have been a few modifications of the ketone-catalyzed epoxidations with H2O2. The structurally related oxazolidinonecontaining ketones of the type 68 (Figure 21) were also shown to be capable of catalyzing the epoxidation of (Z)-olefins with good to high enantioselectivities (Table 8).174 Romney and Miller reported a peptide-embedded trifluoromethyl ketone catalyst, 69 (Figure 21), which, taken at 10 mol %, catalyzed the epoxidation of conjugated olefins with 8 equiv of H2O2 with moderate to good optical yields.175 The reaction was proposed to proceed via transiently generated chiral dioxirane. There was a report on the use of iminium salt organocatalysts of the type 70 (Figure 21) for the asymmetric epoxidation of unfunctionalized olefins with H2O2.176 Moderate enantioselectivities (up to 46% ee) were reported for the epoxidation of 1phenylcyclohexene. The authors used carbonates and hydrocarbonates as cocatalytic additives, capable of yielding percarbonates (HCO4−) upon the interaction with H2O2; the percarbonates converted the iminium cation into the oxaziridinium one, the latter being able to transfer the oxygen atom to the olefinic double bond. More recently, the authors reported the use of diphenyl diselenide for the same purpose:

Figure 20. Catalyst systems of Shu and Shi.

plausible catalytic cycle was proposed (Figure 20). Catalyst 67 was applied to the asymmetric epoxidation of cis-1-propenylphosphonic acid, a precursor of a broad-spectrum antibiotic, 11423


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Table 8. Asymmetric Epoxidation of Olefins with H2O2 in the Presence of Chiral Ketones and Iminium Salts

a

EDTA= ethylenediaminetetraacetic acid. bOnly conversion was reported.

Figure 22. Juliá−Colonna catalyst system.

>10 equiv), gradual degradation of the solid catalysts under basic conditions, and long reaction times (2−5 days). Some practical improvements were introduced to the oxidation protocol. Instead of the original three-phase reaction mediatoluene (or CCl4 or CH2Cl2)/aqueous H2O2/solid catalysttwo-phase systems were proposed, based on organic solvent (THF, EtOAc, DME, tBuOMe) and a solid H2O2containing oxidant (urea hydroperoxide, Na2CO3·1.5H2O2): this replacement significantly shortened the reaction times to only several hours. The catalyst degradation was suppressed by replacing aqueous NaOH with 1,8-diazabicyclo[5.4.0]undec-7ene, DBU (Table 9).185 To facilitate the catalyst recyclability, the polypeptides were supported on solid surfaces.186−188,190−194 With sodium percarbonate (Na2CO3· 1.5H2O2) as the oxidant, there was no need for an external base.188,189,192,193 An inconvenience of the original system was the need for a prolonged preactivation period, which could be avoided by using the high-temperature polypeptide preparation procedure developed by Geller and co-workers; moreover, the resulting catalyst exhibited very high activities, so that the reaction (under triphasic conditions) required only 7−30 min in the presence of phase-transfer agent tetra-n-butylammonium bromide.195,196 Also, the oxidant excess was reduced to only 1.3 equiv.196,197 A scaled-up epoxidation procedure (100 g scale) was proposed.198 A recyclable catalyst (based on imidazoliummodified poly-L-leucine), requiring no preactivation, and recoverable by filtration, was developed.199 Roberts and co-workers and Berkessel and co-workers considered the effect of the primary structure of the polypeptide catalyst and of its helicity on the enantioselectivity

Figure 21. Chiral ketone- and iminium salt-based catalysts for asymmetric epoxidations with H2O2.

the latter afforded benzeneperseleninic acid, which in turn converted the iminium precatalysts of the type 71 into the active oxaziridinium salt.177 Good enantioselectivities (80−86% ee) were reported for the epoxidation of 1-phenylcyclohexene. Moreover, the corresponding chiral amine 72 demonstrated the same catalytic behavior.177 2.6.2. Polypeptide Catalysts. Polypeptide-based catalysts have been known since the early 1980s, when Juliá and Colonna reported the epoxidation of chalcone and related electron-deficient olefins with basic hydrogen peroxide in the presence of poly-L-alanine (73a) (Figure 22).178−182 The catalyst system operated at room temperature in the ambient atmosphere, and demonstrated high enantioselectivities (Table 9). The authors screened a series of “synthetic enzymes” polypeptides (such as poly-L-alanine, poly-L-leucine, and poly-Lisoleucine)and found that poly-L-leucine (73b) (with n = 10) showed the highest enantioselectivity (97% ee for chalcone epoxidation).182 In addition to enones, the method was found applicable to enynones, enediones, unsaturated ketoesters,183 and geminally disubstituted and trisubstituted enones.184 The drawbacks of the system are the high excess of H2O2 (typically 11424


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Table 9. Polypeptide-Catalyzed Asymmetric Epoxidation of α,β-Unsaturated Ketones with H2O2 and Its Derivatives

a

DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene. bConversion reported. cPoly(L-neopentylglycine). dPoly(styrene-co-diviny1benzene)-supported poly-Lleucine. eWith phase-transfer catalysis tetrabutylammonium bromide. fReaction performed at a 100 g substrate loading. gImidazolium-modified. h DIC = diisopropylcarbodiimide (2.0 equiv), and DMAP = N,N-dimethyl-4-aminopyridine (phase-transfer agent).

of epoxidation of enones.200−203 The enone was concluded to predominantly bind to the poly-L-leucine near the Nterminus,201−203 which suggests that even short polypeptides should be enantioselective epoxidation catalysts. Indeed, a series of oligo-L-leucines, soluble in various organic solvents, showed good chalcone epoxidation enantioselectivities (up to 94% ee) with UHP/DBU.204 Demizu, Kurihara, and co-workers synthesized helical L-leucine-based heptapeptides of the type 74 (Figure 23), which showed excellent enantioselectivities in chalcone epoxidation with UHP in the presence of DBU.205 In a series of practice-related works, polyethylene glycol- and polystyrene-bound oligo-L-leucines were prepared;206−208 the resulting (homogeneous) catalyst systems, using UHP as the oxidant, in some cases exhibited high catalytic activities (reaction times of 15−60 min). A continuous-flow reactor was constructed, in which the catalyst was maintained by a nanofiltration membrane.207 Kudo and co-workers developed a library of resin-supported peptides containing non-natural amino acid L-3-(1-pyrenyl)alanine that demonstrated enantioselectivity opposite that of the original Juliá−Colonna system.209 Modifications of the Juliá−Colonna process had

been utilized in the syntheses of biologically active chiral compounds, such as (+)-clausenamide,187 diltiazem and the paclitaxel side chain,210 naturally occurring styryl lactones,211 etc. Miller and co-workers developed an aspartate-derived tripeptide catalyst, 75, which (at 5−10 mol % loadings) catalyzed the epoxidation of 1-substituted cyclohexenones and other trisubstituted olefins with 2.5 equiv of H2O2 with in most cases moderate to good enantioselectivities (76−92% ee).212 Although DMAP was used as a phase-transfer catalyst, the reactions took up to 3 days. Functional analysis of the aspartatebased catalyst 75 revealed the crucial role of the Pro-D-Val moiety for achieving high enantioselectivity.213 It was shown that even dipeptides of the types 76 and 77 (Figure 23) are capable of conducting the Juliá−Colonna-type epoxidation of chalcone in a stereoselective fashion, the cyclic structures 76 exhibiting much higher enantioselectivities than the linear dipeptide 77 (89% ee vs 15% ee).214 More recently, Miller’s group developed an elegant strategy for site-selective and enantioselective oligopeptide-catalyzed epoxidation of polyunsaturated allylic alcohols such as farnesol 11425


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2.6.3. Phase-Transfer Catalysts. The phase-transfercatalyzed enantioselective epoxidations of α,β-unsaturated ketones have a long history; as early as in 1986, Wynberg and co-workers published a pioneering paper on the asymmetric epoxidation of trans-chalcones and naphthoquinones with 30% H2O2 in toluene in the presence of a 2 mol % loading of cinchona alkaloid-derived quaternary ammonium chloride salt 80 (Figure 25).218 Moderate enantioselectivities

Figure 23. Examples of well-defined oligopeptide catalysts. Trt = trityl, and Bn = benzyl.

and its derivatives.215−217 For hexapeptide 78, the hydroxyldirecting mechanism led mainly to 2,3-epoxyfarnesol, while pentapeptide 79 afforded predominantly 6,7-epoxy alcohol (Figure 24). Low to good enantioselectivities were reported,

Figure 25. Phase-transfer catalysts for the enantioselective epoxidation of enones.

Figure 24. Site-selective epoxidation of farnesol in the presence of peptides 76 and 77.

were reported for the reactions in biphasic media (aqueous H2O2/CCl4, with addition of the basic cocatalytic additive NaOH, Table 10). In a series of subsequent publications, the authors examined the effect of the solvent, and considered a broader range of substrates, reporting improved enantioselectivities (up to 55% ee).219−223 In 1998, Arai, Shiori, and co-workers examined the effect of electron-withdrawing substituents at the para-position of the aryl susbstituent R of alkaloids 81a−81e (Figure 25) on the

ranging from 10% ee (for the 6,7-epoxide) to 86% ee (for the 2,3-epoxide).217 A similar site-selectivity trend was reported for the epoxidation of geranylgeraniol, containing four double bonds.215 For several monounsatureted allylic alcohols, peptide 78 (10 mol %) demonstrated enantioselectivities up to 97% ee (Table 9) and moderate yields (9−79%, obtained within 748 h at 4 °C).217 11426


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Table 10. Asymmetric Epoxidation of α,β-Unsaturated Ketones with H2O2-Mediated Phase-Transfer Catalysts

aliphatic α,β-unsaturated ketones, in the form of trichloroacetic or trifluoroacetic salts. A 1.5 equiv portion of H2O2 was enough to achieve synthetically useful yields (55−90%) and high enantioselectivities (90−99% ee).229 The authors proposed a plausible reaction mechanism, assuming nucleophilic conjugate addition of H2O2 to the activated enone, with formation of a βperoxyenamine intermediate.229 Various salts of chiral amines were examined as catalysts in the epoxidation of cyclic enones, and 85b and 86 showed the highest enantioselectivities in the series, affording chiral epoxides with opposite absolute configurations.230 The use of a chiral amine salt with (R)Mosher’s acid (catalyst 87) was reported to substantially improve the enantioselectivity of epoxidation of 2-cyclopentenones.231 A summary of the catalytic activity of cinchona alkaloid-derived amines 85−87, together with some mechanistic suggestions, was published.232 Various modifications of the cinchona alkaloid-based catalysts were reported. Tanaka and Nagasawa prepared a guanidine−urea bifunctional organocatalyst of the type 88 (Figure 25), which was found to conduct chalcone epoxidation with up to 96% ee.233 Shibata and co-workers developed a highly enantioselective (96−99% ee) aerobic epoxidation of βtrifluoromethyl β,β-disubstituted enones in a system consisting of the cinchona alkaloid-based salt 81g (Figure 25), air (1 atm), and H2NHMe.234 It was presumed that the real oxidant was

epoxidation enantioselectivity.224,225 Iodine-substituted catalyst 81c (5 mol %) showed the best chalcone epoxide yield (97%) and enantioselectivity (84% ee). LiOH was found to result in better ee’s than NaOH. For substituted chalcones, enantioselectivities up to 92% ee were reported.224,225 Structure 81f turned out to be the most enantioselective for the epoxidation of substituted naphthoquinines (up to 76% ee).225 Even higher enantioselectivity (up to 84% ee) was reported for the epoxidation of 2-isopropyl-1,4-naphthoquinone, using quaternary salts of structural analogues of cinchona alkaloids of the type 8.226 Besides the cinchona alkaloids, BINOL-derived quaternary ammonium salts of the type 83 were examined as catalysts, exhibiting generally moderate ee’s; the enantioselectivity level was dependent on the length of the alkyl substituents.227 Jew and co-workers proposed to add surfactants, such as Triton X-100 and Span 20 (sorbitan monolaurate), into the reaction mixture,228 which resulted in significant improvement of the epoxidation enantioselectivity in the presence of dimeric cinchona phase-transfer catalyst 84 (1 mol %). Span 20 ensured the highest ee’s for the epoxides of substituted chalcones (up to >99% ee), along with high yields (>94%), and allowed the reduction of the excess of H2O2 (from 30 to 5 equiv). List and co-workers tested cinchona alkaloid-derived amines of the type 85 as catalysts of epoxidation of simple acyclic 11427


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the epoxidation of cinnamic aldehyde and other α,βunsaturated aldehydes with H2O2, UHP, and Na2CO3· 1.5H2O2 in different solvents.240,241 Hayashi and co-workers reported the asymmetric organocatalytic epoxidation of αsubstituted acroleins, and identified diphenylprolinol diphenylmethylsilyl ether (92a) as the most active and stereoselective (up to 94% ee) catalyst (20 mol %) for this reaction.242 List and co-workers applied catalysts 93, combining the chiral cinchonaderived amine with the BINOL-derived chiral phosphoric acid, for the enantioselective epoxidation of α,β-substituted α,βunsaturated aldehydes and α-substituted acroleins.243 The authors proposed that the primary cinchona-derived amine activated the enal substrates via iminium ion, which underwent conjugate addition by hydrogen peroxide and ring closure via an enamine intermediate. Gilmour and co-workers reported the fluorinated catalyst 92b (Figure 26), which exhibited good to excellent enantioselectivities in the epoxidation of β-substituted α,β-unsaturated aldehydes.244,245 Hayashi and co-workers studied the epoxidation of 2-oxindol-3-ylidene acetaldehydes in the presence of chiral diarylprolinol silyl esters (of the types 90 and 92), and selected structure 94 on the basis of balance between the epoxide yield and enantioselectivity.246 Good diastereoselectivities were reported for the epoxidation of a mixture of E/Z isomers of the 2-oxoindolin-3-ylidene acetaldehydes, with preferential formation of one diastereomer, having good to high optical purity (90−98% ee). Kudo and Akagawa prepared resin-supported peptidecontaining unnatural amino acids that catalyzed the asymmetric epoxidation of α,β-unsaturated aldehydes with H2O2 with up to 95% ee, and demonstrated that the catalyst can be reused at least three times without loss of enantioselectivity.247 Terada and Nakano applied a sterically encumbered BINOL-derived chiral guanidine base of the type 95 as the catalyst (10 mol %) for the epoxidation of chalcone with H2O2 (5 equiv), with up to 65% ee.248 Tu and co-workers tested spiropyrrolidine-derived organocatalyst 96 (20 mol %), which demonstrated previously unachievable diastereoselectivities (up to >20:1) and enantioselectivities (up to 99% ee) in the epoxidation of substituted cinnamaldehydes with H2O2 (3 equiv).249 The generally accepted mechanism of chiral amine-catalyzed epoxidations invokes the condensation of the catalyst with the substrate, to form the iminium intermediate I, followed by the nucleophilic addition of H2O2 to the β-carbon of the iminium species, with subsequent epoxide formation via attack of the αcarbon on the electrophilic oxygen atom of the enamine intermediate II (Figure 27). The epoxide product and the amine catalyst are evolved after the hydrolysis step.250 Jørgensen and co-workers also concluded that the peroxyhydrate of the epoxy aldehyde serves as a phase-transfer agent, thus accounting for the observed increase in rate in the course of the reaction.250 Despite some practical drawbacks (slow reactions, high catalyst loadings, typically 10−20 mol %, and in some cases a high excess of H2O2), amine-catalyzed epoxidations have demonstrated high synthetic utility at the laboratory scale for designing complex synthetic procedures such as tandem organocatalytic asymmetric synthesis of 1,2,3-triols,251 tandem epoxidation−Wittig and epoxidation−Mannich reactions,241 and enantioselective synthesis of trans-2,3-dihydroxy aldehydes and their derivatives252 β-hydroxy esters,253,254 2,3-epoxy alcohols,249 electron-deficient 2-(hydroxyalkyl)- and 2(aminoalkyl)furans,255 α,β-epoxy esters,256 etc.

hydrogen peroxide, formed in situ from H2NHMe and O2; a possible reaction mechanism was discussed. The highly enantioselective epoxidation of (E)-4,4,4-trifluoro-1,3-diphenylbut-2-en-1-one (with up to 99% ee) was also successfully achieved by Chen and co-workers, who used H2O2 as the oxidant and pentafluorophenyl-substituted quaternary ammonium salt 81h (3 mol %) as catalyst in chloroform.235 Siva and co-workers prepared bis quaternary ammonium bromides of the type 89 (Figure 25), which demonstrated higher enantioselectivities in the epoxidation of α,β-unsaturated ketones than their monomeric prototypes (e.g., 81i).236,237 The authors screened a variety of basic cocatalysts, and Cs2CO3 appeared as ensuring the highest yields and enantioselectivities.237 2.6.4. Amine Catalysts. The chiral amine-based olefin epoxidation catalyst systems with H2O2 are mostly applicable to the epoxidation of α,β-unsaturated aldehydes. In 2005, Jørgensen and co-workers reported the first organocatalytic experimental procedure of this type, relying on the chiral amine catalyst 90a (Figure 26), used at a 10 mol % loading.238 The

Figure 26. Chiral amine-based catalysts of enantioselective epoxidations with H2O2.

reaction proceeded in CH2Cl2 within a few hours at room temperature (2 h for H2O2 and 5 h for UHP), with excellent enantioselectivities (96−98% ee, Table 11). In water−methanol solutions, catalyst 80a exhibited somewhat lower enantioselectivities (91−93% ee for cinnamic aldehyde; cf. 96% ee in CH2Cl2), and required a higher excess of the oxidant (3 equiv vs 1.3 equiv).239 Córdova and co-workers undertook screening of a library of proline-derived chiral amines as epoxidation catalysts, which revealed compounds 90b and 91 as the most enantioselective in 11428


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Table 11. Asymmetric Organocatalytic Epoxidations in the Presence of Chiral Amine Catalysts

a

Conversion reported. bProduct isolated as the chiral 2,3-epoxy alcohol after reduction of the epoxy aldehyde with NaBH4. cNot reported. dee for the minor/major diastereomer.

Figure 27. Proposed active species of enantioselective epoxidation of α,β-unsaturated aldehydes in the presence of amine catalysts.

3. ASYMMETRIC SULFOXIDATION REACTIONS 3.1. Vanadium Systems

The first vanadium-based catalysts for the asymmetric sulfoxidations with H2O2 were reported in 1995 by Bolm and Bienewald, who combined vanadyl acetylacetonate with a 1.5fold excess of a Schiff base ligand of the types 97a−c (Figure 28) to form the catalytically active species in situ.257 The catalyst system was highly efficient, vanadium loadings from 1.0 down to 0.01 mol % being sufficient for achieving high conversions; H2O2 was taken in 10% excess. Moderate to good enantioselectivities (53−76% ee) were reported for the oxidation of simple alkyl aryl sulfides (Table 12).257,258 The scope of this catalyst system included dithioacetals and dithioketals; monooxidation of 2-phenyl-1,3-dithiolane with up to 85% ee was reported. 257,259 The experimental observations of ligand acceleration of the reaction,257 and the absence of chiral amplification,257 suggested that the active species should contain one chiral ligand per vanadium. Two

Figure 28. Vanadium-based catalyst system of Bolm and Bienewald.

types of monoperoxovanadium(V) complexes (existing in comparable concentrations, and exhibiting simultaneous decay after the addition of sulfide) were detected in this catalyst system by multinuclear NMR spectroscopy.260,261 Subsequently, the nature of the catalytically active species was extensively studied by several research groups.262−267 11429


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Table 12. Asymmetric Sulfoxidations with H2O2 in the Presence of Vanadium Schiff Base Catalysts

a

Overall yield of bis-sulfoxides ((R,R) + (S,S) + meso), having 60% de. bSulfide conversion reported. cMonosulfoxide yield.

The ground-breaking contribution by Bolm and Bienewald inspired an extensive search for novel ligand structures. Skarzewski and co-workers proposed ligand 97d (Figure 28), which was most effective for the oxidation of bis(arylthio)alkanes into the corresponding mono- and bis-sulfoxides; the latter were highly optically pure (up to >95% ee).268,269 Vetter and Berkessel prepared several tert-leucinol Schiff base ligands, featuring additional elements of central, planar, or axial chirality (98−100, Figure 29).270 Ligand 98 ensured the highest enantioselectivity (78% ee at 0 °C) in thioanisole oxidation and 97% sulfoxide yield. Ellman’s groups reported the monooxygenation of tert-butyl disulfide in the presence of VO(acac)2/97b, with up to 91% ee.271 Katsuki and co-workers designed a series of Schiff bases bearing additional elements of axial chirality (of the type 101, Figure 29).272 Ligand 101a showed the highest enantioselectivity in the series (up to 93% ee for methyl 2-naphthyl sulfide). A library of Schiff base ligands, originating from substituted salicylaldehydes and chiral β-amino alcohols, was reported by Somanathan and coworkers; vanadium-catalyzed sulfoxidations in the presence of those ligands showed moderate enantioselectivities (up to 65% ee).273,274

Anson, Jackson, and co-workers found that halogensubstituted Schiff bases of the types 102 and 103 (Figure 29) ensured excellent enantioselectivities in the vanadium-catalyzed oxidation of alkyl aryl sulfides (89−97% ee).275 One should notice that the high enantioselectivities were apparently achieved in a tandem asymmetric oxidation of sulfides/ oxidative kinetic resolution of the resulting sulfoxides, at the expense of reduced sulfoxide yields. For the second process, stereoselectivity factors (krel) of 10.5 to >30 were reported.276 On the basis of these findigs, a combined stepwise VO(acac)2/ 102-catalyzed enantioselective oxidation (at 0 °C)/kinetic resolution (at 20 °C) process was designed, affording alkyl aryl sulfoxides in >70% yield and up to 99.5% ee.277 Zeng and co-workers synthesized vanadium complexes of the type 104, which displayed good to excellent enantioselectivities (76−99% ee) but moderate yields (31−78%) in the oxidation of aryl methyl sulfides and benzyl phenyl sulfide, accompanied by oxidative kinetic resolution of the corresponding sulfides.278 The oxidation of allyl and cinnamyl aryl sulfides was examined, using a vanadium complex with ligand 102 as the catalyst, to afford the corresponding sulfoxides with moderate yields (43− 78%) and enantioselectivities ranging from 33.5% to 97.3% 11430


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ligand 102 as the most efficient source of chirality.280,281 The resulting bulky sulfoxides, having 56−99% ee optical purity, were obtained in CH2Cl2, with moderate chemical yields, generally not exceeding 50%. Sun and co-workers examined a series of 3-aryl-substituted chiral ligands of the type 105 in vanadium-catalyzed oxidations of aryl methyl sulfides, and reported moderate to good enantioselectivities (53−92% ee).282,283 tert-Leucinol-derived ligands were shown to afford better asymmetric induction than valine- or phenylalanine-derived structures. The catalyst system VO(acac)2/106 exhibited enantioselectivities up to 77% ee in oxidation of aryl methyl sulfides in acetone.284 Gau and co-workers isolated and X-ray characterized a series of vanadium(V) complexes having general structure 107 (Figure 28), which showed moderate to high yields (61− 80%) and enantioselectivities (26−98% ee) in the oxidation of methyl phenyl sulfide.285 The optical purity of the sulfoxide was improved by lowering the temperature to −20 °C, and by using a CH2Cl2/toluene mixture as the solvent instead of CH2Cl2: this replacement apparently increased the contribution of kinetic resolution to the resulting enantiomeric excess. Wang, Sun, and co-workers synthesized a family of tridentate ligands of the types 108 and 109, derived from Br- and Isubstituted hydroxynaphthaldehydes.286 For the oxidation of a series of aryl methyl sulfides in toluene, enantioselectivities up to 99% ee were reported; the low yields (50−60%) hinted at the significant contribution of the oxidative kinetic resolution of sulfides. Li and co-workers prepared a series of chiral Schiff bases of the type 110, with the amino alcohol part bearing two stereogenic centers.287 One of those (110a) appeared to be a good chiral inducer for the tandem asymmetric sulfoxidation/ kinetic resolution process, to afford a series of aryl methyl sulfides with >80% yield, having 98 to >99% ee, which is among the best results reported for the asymmetric oxidation of alkyl aryl sulfoxides. Another ligand, 111, also featuring two stereogenic centers at the amino alcohol moiety, appeared to be very efficient in the monooxidation of 2-aryl-1,3-dithianes.288 A series of BINOL-derived Schiff base ligands of the types 112 and 113 were reported by Ahn and co-workers;289,290 112a showed excellent enantioselectivities in the oxidation of aryl benzyl sulfides (94−99% ee), along with good yields (78− 90%).289 Aryl methyl sulfides were oxidized with lower enantioselectivities, not exceeding 87% ee. Khiar, Fernandez, and co-workers studied the monooxygenation of disulfides in the presence of VO(acac)2/114; tert-butyl disulfide was oxidized with 85% ee.291 In a series of publications, Romanowski and co-workers prepared and X-ray characterized a series of dimeric oxovanadium(V) complexes of the general formula [L*(O)V−O−V(O)L*] (where L* is a chiral Schiff base of the type 115),292−295 which exhibited generally moderate enantioselectivities in the oxidation of methyl phenyl sulfide and benzyl phenyl sulfide at room temperature; at −20 °C, the former could be oxidized with up to 90% yield and 94% ee.294 Various types of other ligand structures were examined, with varying degree of success. Pati and co-workers synthesized trimeric Schiff bases with a central linker.296 Several Schiff bases featuring carbohydrate-derived chiral diamine moieties were reported.297,298 Schiff base ligands prepared from α-pinene- and 3-carene-derived amino alcohols were also reported.299,300 Aydin proposed novel thiophene-derived Schiff bases of the type 116 (Figure 29), and their enantiomers, which appeared to

Figure 29. Schiff base ligands and complexes used in vanadiumcatalyzed asymmetric sulfoxidations with H2O2.

ee.279 The concomitant kinetic resolution of sulfoxides was exploited for the preparation of aryl benzyl sulfoxides with high optical purity from the corresponding aryl benzyl sulfides, using 11431


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be efficient chiral inducers in vanadium -atalyzed oxidation of thioanisole and its substituted analogues.301 Various heterogenized vanadium Schiff base catalysts have been reported, relying on the immobilization of chiral Schiff bases (via covalent bonding) to resins,275,302 polystyrene or polyacrylate,303 modified SBA-15,304 or modified MCM-41.305 In none of those reports, however, were synthetically useful enantioselectivities reported, with the highest 61% ee achieved for thioanisole oxidation.303 Vanadium-based catalysts with ligands other than amino alcohol-derived tridentate Schiff bases have been rarely reported. Bryliakov and Talsi reported low enantioselectivities (not exceeding 37.5% ee) in the oxidation of aryl methyl sulfides with H2O2 in the presence of VO(OnBu)3 and chiral salan (tetrahydrosalen) ligands.306 Correia, Pessoa, and coworkers reported the catalytically active oxovanadyl complexes with chiral salan and salalen ligands, which catalyzed the oxidation of methyl phenyl sulfide with up to 47% ee.307 To summarize, vanadium tridentate Schiff base catalyst systems have demonstrated good to excellent enantioselectivities in the oxidation of thioethers with H2O2. These systems are easy to handle and require relatively low catalyst loadings (typically 1 mol %) and a small oxidant excess vs the substrate. Drawbacks of those systems are their relatively low activity, which sometimes requires reaction times up to several days, and limited substrate scope, mostly restricted to simple aryl methyl sulfides, disulfides, and 1,3-dithianes (whereas the oxidation of bulky sulfides in high yield and chemo- and enantioselectivities remains a challenge). So far, attempts to apply vanadium Schiff base systems for the syntheses of biologically active sulfoxides have had limited success.20,308,309

Figure 30. Iron complexes and ligands used in Fe-catalyzed asymmetric sulfoxidations with H2O2.

3.2. Iron Systems

catalyzed the oxidation of several aryl methyl sulfides with H2O2 (1.5 equiv) with moderate to good yields (21−99%) and with up to 96% ee.317 The reactions were successfully conducted in water, without added organic solvents. Optimization of the reaction conditions, with respect to temperature, catalyst loading, and amount of water solvent, was reported.318 Yang and co-workers reported the encapsulation of catalyst 120c in modified mesoporous SiO2: the resulting supported catalyst demonstrated high sulfoxide yields, but the enantioselectivities were lower than for the same iron salan complex under homogeneous conditions.319 Abdi and co-workers reported a series of chiral Schiff base ligands of the type 121, and the latter (3 mol %) were reacted with Fe(acac)3 (2 mol %) to afford in situ the catalysts of oxidation of alkyl aryl sulfides with H2O2 (1.2 equiv).320 For substituted thioanisoles, enantioselectivities up to 96% ee were reported, while bulkier substrates (PhSMe, PhSBn) showed lower ee’s. The addition of a 2 mol % loading of acid 119a did not significantly affect the sulfoxide yield but susbtantially improved the enantioselectivity. In 2011, Le Maux and Simonneaux reported the chiral porphyrin-catalyzed enantioselective sulfoxidation with H2O2.321 The porphyrin complex 122 (1 mol %) conducted the oxidation of substituted thioanisoles in up to 90% ee (for PhSMe). The addition of N-methylimidazole improved the enantioselectivity but deteriorated the substrate conversion (Table 13). To date, iron-catalyzed enantioselective sulfoxidation with H2O2 remains a narrow area; although enantioselectivities exceeding 90% ee were reported in several cases, their limited substrate scope (essentially it is aryl methyl sulfides), relatively

In 1997, Fontecave and co-workers reported the first iron-based system, capable of conducting the asymmetric sulfoxidations with H2O2.310 Under the high substrate excess conditions (Fe:sulfide:H2O2 = 1:600:10), the catalyst 117 (Figure 30) afforded p-bromophenyl methyl sulfoxide in 90% yield (based on the oxidant). The authors concluded that the oxidation proceeded via nucleophilic attack of the sulfide on the active peroxoiron species.311,312 In 2003, Bolm and Legros reported a system based on the catalyst formed in situ from β-amino alcohol-derived chiral Schiff bases of the type 118 (Figure 30) and Fe(acac)2, 4 mol % each.313 Using 1.2 equiv of H2O2, several aryl methyl sulfides were oxidized to the corresponding sulfoxides in generally low yield (15−44%) and enantioselectivities ranging from 13% to 90% ee. The oxidation of bulkier sulfides (PhSEt, PhSBn) occurred with low enantioselectivities. Subsequently, the authors reported the use of cocatalytic additives of the type 119 (Figure 30), substantially improving the sulfoxide yield and enantioselectivity.314,315 Upon the use of a 1 mol % loading of the additives, the Fe(acac)2 loading was reduced to 2 mol %, while the chiral ligand was introduced at a 4 mol % loading. To explain the effect of the additive, the authors hypothesized that monocarboxylate-bridged dinuclear iron species might be involved in the asymmetric oxygen transfer.314,315 Using this catalyst system, both enantiomers of the nonsteroidal antiinflammatory drug sulindac were synthesized in up to 71% yield and up to 92% ee.316 A highly enantioselective catalyst system was reported by the group of Katsuki, who prepared BINOL-derived iron(III) complexes 120 (Figure 30). These complexes (1 mol %) 11432


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Table 13. Enantioselective Oxidation of Sulfides with H2O2 in the Presence of Iron Catalysts

a

Fe catalyst enacapsulated in modified silica.

oxidation of substituted thioanisoles proceeded with low to moderate enantioselectivities (up to 43% ee), while benzyl phenyl sulfoxide was formed with 65% ee. Somanathan and coworkers synthesized a family of chiral tridentate ligands of the type 128, featuring one or two centers of chirality at the amino alcohol part.274 Thioanisole oxidation enantioselectivities up to 64% ee were reported, which was slightly higher than those for vanadium-based catalysts under the same conditions. A series of dicumenyl-substituted tridentate Schiff bases of the type 129 were prepared and tested as ligands in titanium-catalyzed oxidation of aryl methyl sulfides.327 With ligand 129b, the most efficient chiral inducer of the series, formation of methyl phenyl sulfoxide with 73% ee was achieved. Abdi and co-workers prepared a series of ligands 130 with two asymmetric centers at the amino alcohol moieties; with ligand 130a, p-NO2substituted thioanisole was oxidized in up to 98% ee, whereas the oxidation of unsubstituted thioanisole, as well as bulkier sulfides, showed lower enantioselectivities.328 Bryliakov and Talsi demonstrated that dinuclear titanium salan complexes of the type 131, analogous to those previously studied by Katsuki in asymmetric epoxidations (section 2.4), can also conduct the enantioselective oxidation of sulfides with high efficiency (at least 500 turnovers).306 The observed enantioselectivity was improved by the kinetic resolution to afford sulfoxides with up to 98.5% ee. The highest enantioselectivities were documented for the oxidation of bulky substrates (aryl benzyl sulfides).329 Titanium complexes with 3-halogen-substituted ligands 131d−131f exhibited higher enantioselectivities for the oxidation of smaller sulfides than the parent catalyst 131a.330 A set of titanium salan complexes of the type 132 were prepared, which appeared to be moderately stereoselective thioanisole oxidation catalysts.331 Barman and

high catalyst loadings (typically 1−4 mol %), and unexplored functional group tolerance so far limit their potential significance. 3.3. Titanium Systems

Historically, titanium catalyst systems have been some of the earliest transition-metal-based systems applied to the enantioselective sulfoxidations with H2O2. The first example dates back to 1986, when Pasini and co-workers reported chiral titanium salen catalyst 123 (presumably polymeric, Figure 30), capable of promoting the oxidation of thioanisole in aqueous methanol or dichloromethane with low enantioselectivities (20:1 dr. cFormation of monosulfoxide “with high diastereoselectivity” was reported.

Gao and co-workers applied the “porphyrin-inspired” structures formed in situ from Mn(OTf)2 and ligands of the type 145 (Figure 34).356 In the presence of AcOH as the additive, the resulting catalysts (1 mol %) were highly enantioselective in CH2Cl2 (15 examples with >99% ee) for the enantioselective oxidation of alkyl aryl and benzyl aryl sulfides, along with generally good yields (typically 70−90%). The optical yields reported for the oxidations of model simple alkyl aryl sulfides are among the highest ever reported, owing to the concomitant stereoconvergent oxidative kinetic resolution of sulfoxides (with krel > 7). At the same time, the oxidation of bulky omeprazole sulfide in the presence of Mn(OTf)2/145b afforded (S)-omeprazole in 82% yield and only 90% ee.356 Another potential drawback is the high excess of oxidant (2 equiv for simple sulfides and 3 equiv for omeprazole sulfide). The replacement of the acetic acid additive with 1adamantanecarboxylic acid (ACA; Figure 7) and the dichloromethane solvent with acetonitrile/2-propanol in most cases improved the oxidant economy and sulfoxide yields but

deteriorated their optical purities, probably due to the suppression of the kinetic resolution process.357 Using the Mn(OTf)2/145b/ACA catalytic combination, the asymmetric sulfoxidation procedure under continuous-flow conditions was developed.358 Heterogeneous manganese-based systems were reported by Sun and co-workers, who synthesized a “triply immobilized” Jacobsen-type complex, which catalyzed the oxidation of methylthioarenes with up to 89% ee, yet with low oxidant efficiency (a 10-fold excess of H2O2 was used).359 There was a report by Mukaiyama on the aerobic enantioselective oxidation of several aryl methyl sulfoxides with up to 72% ee in the presence of an excess of the coreductant pivalaldehyde and a 12−18 mol % loading of a Mn catalyst of the type 146 (Figure 34).360,361 An interesting example of aerobic sulfoxidation was reported by Katsuki and co-workers, who used the ruthenium salen complex 147 for the oxygenation of aryl methyl sulfides and 1,3-dithianes under visible light irradiation without coreductants.115 High to 11437


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excellent enantioselectivities (84−98% ee), along with 18−98% yields, were achieved within 48 h upon the use of a 5 mol % loading of the ruthenium catalyst. Fontecave and co-workers reported the “chiral-at-metal” cis-[Ru(dmp)2(CH3CN)2][PF6]2 (dmp = 2,9-dimethyl-1,10-phenanthroline) complexes (Λ- and Δ-enantiomers), which promoted the oxidation of several simple alkyl aryl sulfides with low enantioselectivities (up to 18% ee).362 Several molybdenum-based catalysts could be mentioned. Romanowski and Kira prepared a series of chiral dioxidomolybdenum(VI) complexes with tridentate Schiff bases of the type 115 (Figure 29), which catalyzed the oxidation of methyl phenyl sulfide with low ee (not exceeding 17%).363 Galindo and co-workers reported a molybdenum complex prepared in situ from [MoO(O2)2(H2O)n] (2.5 mol %), [PPh4]Br (1.25 mol %), and chiral imidazolium-based dicarboxylate 148 (Figure 34).364 Low to moderate thioanisole sulfoxidation enantioselectivity (up to 42% ee) was reported at 93% conversion; the use of a 1.6-fold excess of H2O2 resulted in an ee enhancement to 83% ee, in ca. 40% yield, owing to the oxidative kinetic resolution of the sulfoxides. Hirao and Tan developed a procedure for the asymmetric oxidation of aryland (methylaryl)thioacetic acid esters, based on the use of the bisguanidinium dinuclear oxodiperoxomolybdosulfate [149]2+[(μ-SO4)Mo2O2(μ-O2)2(O2)2]2− ion pair, prepared from Na2MoO4.365 Good to high enantioselectivities (up to 96% ee in one case), along with good yields, were reported for the target class of sulfides; enantioselective synthesis of a direct precursor of the commercial drug (R)-modafinil was performed with 91% yield and 91% ee. The authors ascribed the observed stereoselectivity to ion pairing interaction and other noncovalent interactions in the 149−Mo ion pair.365 Using a different metal sourceAg2WO4the enantioselective oxidation of alkyl aryl sulfides was achieved with the aid of similar ion pair catalysts, with up to 99% ee.366 Using this approach, the (S)-stereoisomer of the commercial proton pump inhibitor (R)-lansoprazole was prepared in 81% yield and 90% ee. These values are lower than those achieved with titanium salalen and titanium salen catalysts;335,336 other drawbacks are the relatively high catalyst loading (2 mol % Ag2WO4), long reaction time (48 h), and necessity to use 3% aqueous H2O2 instead of the commercial 30% solution. Nevertheless, this work is evidence that the synthetic potential of other metal-based asymmetric sulfoxidation catalysts is considered underexplored, and novel practice-related catalyst systems are likely to appear in the near future.

Figure 35. Organocatalysts studied in the asymmetric sulfoxidations with H2O2.

demonstrated higher activities, efficiencies, and enantioselectivities, affording methyl phenyl sulfoxide in high chemical yield and 80% optical yield within only 10 min, using an only 1 mol % concentration of the catalyst in buffered aqueous solutions.370 The catalyst loading was further reduced to 0.2 mol % to achieve the maximum efficiency of 395 TNs. An even higher enantioselectivity of 91% ee was reported for the oxidation of tert-butyl methyl sulfide.371 Subsequently, the authors extensively studied the effect of the structure of the conjugates on the sulfoxidation enantioselectivity,372−374 but the enhancement of enantioselectivity or efficiency was not achieved. In the study by Yashima and co-workers, optically active riboflavin-derived polymer 154 was synthesized, which catalyzed the oxidation of three model sulfide substrates with higher enantioselectivity than the monomeric analogue; methyl p-tolyl sulfoxide was obtained in 39% yield and 60% ee using 5 mol % 154.375 Tao, Wang, and co-workers developed the enantioselective sulfoxidation procedure in the presence of chiral BINOLderived phosphoric acids of the type 155 (10 mol %).376 The best catalyst of the series, 155a, mediated the oxidation of aryl methyl sulfides with moderate to high enantioselectivities (up to 82% ee) and mostly good yields; however, under the optimal conditions (CHCl3, −40 °C, 1.5 equiv of 50% H2O2), the reaction required as long as 48−144 h. A series of 2-arylsubstituted 1,3-dithianes were oxidized to the corresponding monosulfoxides with high yields (60−99%) and high diastereoselectivities (88:12 to >99:1 dr) but moderate enantioselectivities (56−70% ee). List and co-workers evaluated a series of “confined” phosphoric acids of the type 156 (Figure 35) and reported high sulfoxide yields and good to excellent enantioselectivities (up to 99% ee) for a series of

3.5. Metal-Free Systems

The first organocatalyzed asymmetric sulfoxidation with hydrogen peroxide was reported in 1988 by Shinkai and coworkers, who used planar chiral flavinium salt 150 (Figure 35) in methanolic solutions.367 The catalyst performed up to eight turnovers, affording aryl methyl sulfoxides in up to 65.4% ee (Table 16). The authors invoked chiral flavinium hydroperoxide as the plausible active species.367 Mirahashi prepared a structurally related planar chiral amine, 151 (Figure 35), which ensured a higher level of stereocontrol in the oxidation of methyl 2-naphthyl sulfoxide (up to 72% ee).368 Cibulka and co-workers synthesized the nonbridged chiral flavinium salts of the type 152, which demonstrated lower stereoselectivities (4−54% ee) in the oxidation of several alkyl aryl sulfides in water−methanol mixed solvent.369 Flavin−βcyclodextrin conjugates of the type 153 (Figure 35) 11438


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Table 16. Organocatalyzed Asymmetric Sulfoxidations with H2O2

a

Eight catalytic turnovers reported. bNot specified. cConversion reported. dYield of monosulfoxide, having a >99:1 dr.

Figure 36. Pt-catalyzed Baeyer−Villiger oxidation with H2O2 reported by Strukul and co-workers and the proposed oxidation mechanism. The abbreviation “r.d.s.” indicates the proposed rate-determining step.

4. ASYMMETRIC BAEYER−VILLIGER OXIDATIONS

sulfides; aryl methyl sulfides showed the highest enantioselection.377 The addition of MgSO4 (probably acting as a moisture absorbent) substantially shortened the reaction time. Using the proposed approach, the key oxidation step of the synthesis of the nonsteroidal anti-inflammatory drug (R)sulindac was conducted in 95% yield and 98% ee. There seem to be no efficient asymmetric sulfoxidation organocatalysts other than flavin derivatives and chiral phosphoric acids; the camphor-derived N-sulfonyl imines of the types 157 and 158 ensured good enantioselectivities only if used in stoichiometric amounts.378,379 The catalyst systems reported so far demonstrate moderate enantioselectivities, reactivities, and catalytic efficiencies; it is only the catalyst system of List377 which can compete with the metal-based catalysts in these respects. The advantages of organocatalyzed sulfoxidations are the high sulfoxide selectivity (typically no overoxidation takes place) and high diastereo- and chemoselectivities for the oxidation of 1,3-dithianes.

4.1. Metal-Based Systems

One of the first metal-based catalyst systems for the Baeyer− Villiger oxidation with H2O2 as the terminal oxidant was reported by Strukul and co-workers, who studied platinum complex 159.380 The catalyst conducted the oxidative kinetic resolution of cyclic 2-alkyl-substituted cyclopentanones (in neat substrate) to form chiral lactones with low to moderate optical purities (up to 58% ee), along with residual enantiomerically enriched substrate (Figure 36 and Table 17), performing 40− 90 catalytic turnovers. The authors proposed the plausible catalytic cycle for the oxidation (Figure 36). A series of chiral platinum complexes of the type 160 (Figure 37), bearing various chiral diphosphine moieties, were examined as catalysts in the oxidative desymmetrization of substituted meso-cyclohexanones, to afford the corresponding ketones with higher optical yield (up to 80% ee).381,382 Later, the authors studied 11439


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Table 17. Asymmetric Baeyer−Villiger Oxidations with H2O2 and O2

a Yield and ee of the minor lactone given in parentheses. bA 0.5 equiv portion of pivalaldehyde. cWith a 1.5-fold excess of PA, 62% substrate conversion was reported. dYield “normal”/yield “abnormal”, ee “normal”/ee “abnormal”; see Figure 37. eDMAP = N,N-dimethyl-4-aminopyridine, and DIC = diisopropylcarbodiimide. fBTCN = benzene-1,2,4,5-tetracarbonitrile. gYield not reported.

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Figure 37. Catalysts used for enantioselective Baeyer−Villiger oxidations with H2O2 and O2.

with urea hydroperoxide into the corresponding lactones in up to 94% yield and 73−83% ee; tricyclic cyclobutanone was oxidized with >99% stereoselectivity.385 Malkov and co-workers adapted the same synthetic protocol for the oxidation of substituted cyclobutanones, using terpene-derived chiral ligands of the type 163 (Figure 37).386 Good to excellent yields (up to >99%) and moderate to good ee’s (up to 81%) were reported. Simultaneously with the works of Strukul, Bolm and coworkers pioneered Cu-catalyzed Baeyer−Villiger oxidation of 2arylcylcohexanones, using complexes of the type 164 (Figure 38) in benzene solutions and O2 as the oxidant, together with 0.5 equiv of pivaladehyde as the sacrificial reductant.387 In benzene solutions, the reaction proceeded within 16−20 h at room temperature or within up to 5 days at 6 °C. The oxidation

BINOL-derived platinum diphosphine complexes of the type 161 (Figure 37), which exhibited high yields (>99%) and good enantioselectivities (up to 56% ee) in the enantioselective oxidation of prochiral substituted cyclobutanones in aqueous media in the presence of surfactants; substututed cyclohexanones were oxidized with low yields (typically not higher than 15% even at 5 mol % catalyst loadings) but with higher enantioselectivities (up to 92% ee).383 In CH2Cl2, this and similar catalysts demonstrated comparable yields but lower stereoselection.384 An example of palladium-catalyzed asymmetric Baeyer− Villiger oxidation was contributed by Katsuki and co-workers, who reported that cationic palladium(II) complex 162 (Figure 37) catalyzed the oxidation of 3-aryl-substituted cyclobutanones 11441


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enantioselective oxidation of 3-aryl-substituted cyclobutanones with hydrogen peroxide (1.5 equiv), the resulting chiral lactones having up to 74% ee.397 The reaction was conducted in a mixture of trifluoroethanol/methanol/water (6:3:1) at −30 °C, in the presence of a 25 mol % concentration of catalytic additive NaOAc. In spite of the high catalyst loading (10 mol %), the reaction proceeded slowly (within 6 days), the yields being moderate (17−67%, Table 17). Ding and co-workers discovered that a series of chiral BINOL-derived phosphoric acids of the type 155 (Figure 34) and H8-BINOL-derived phosphoric acids of the type 171 (Figure 37), in addition to the asymmetric sulfoxidations, are also capable of mediating enantioselective Baeyer−Villiger oxidations of 3-substituted cyclobutanones with hydrogen peroxide in chloroform.398,399 With a 10 mol % concentration of the catalyst of choice (171), the reaction took 18−36 h at −40 °C to achieve quantitative ketone conversion and afford the optically active lactone in 65−99% yield and 55−93% ee. At a 1 mol % catalyst loading, a 95% yield was achieved within 80 h, without deterioration of the ee.398 The addition of electronacceptor additives (such as p-quinones or benzene-1,2,4,5tetracarbonitrile) in some cases improved the enantioselectivity by a few percent.399 The authors conducted the asymmetric oxidation of several bi- and tricyclic ketones, typically with low enantioselectivities for the “normal” lactones (7−38% ee) and much higher enantioselectivities for the “abnormal” one (74 to >99% ee).400 On the basis of the conducted computations, the authors ruled out the formation of an active peroxophosphate intermediate and proposed a two-step mechanism in which the chiral phosphoric acid was presumed to activate the ketone and H2O2 in a synergistic manner through partial proton transfer.401 Peris and Miller reported two examples of oxidative desymmetrization of cyclic ketones with H2O2 (12.5 equiv) in the presence of oligopeptide catalyst 172 (Figure 37). The resulting chiral lactones had low to moderate optical purities (30% and 42% ee).402 Extensive catalyst and substrate screening revealed the specific combinations demonstrating better enentioselecitvities for the formation of abnormal lactone (44−88% ee).403 Interestingly, the abnormal lactone prevailed in most cases. An example of this kind is reported in Table 17 for catalyst 173. Unfortunately, the conversions were rather low, 12−36% within 18 h, at a catalyst loading of 10 mol % and a 3.8-fold excess of H2O2.403 At present, metal-based and metal-free catalyst systems for the enantioselective Baeyer−Villiger oxidation with H2O2 and O2 are not numerous; known systems have rather limited scope, mostly restricted to very specific model substrates, such as reaction-prone substituted cyclobutanones or polycyclic ketones. The reactivities of the majority of the systems, particularly organocatalytic, are not high, which requires high catalyst loadings and long reaction times. Overall, this area needs further investigations prior to considering synthetic applications.

Figure 38. Bolm’s Cu-based catalyst system for aerobic Baeyer− Villiger oxidation.

of cyclobutanones proceeded with higher enantioselectivities, the “abnormal” oxidation products demonstrating higher optical purity.388 3-Substituted prochiral cyclobutanones were oxidized with moderate stereoselection, the resulting lactones having up to 47% ee,389 while a tricyclic prochiral cyclobutanone afforded the corresponding lactone with 91% ee.258 Feng and Jiang contributed a series of copper oxazoline complexes of the type 165 (Figure 37), which catalyzed the oxidation of 2-phenylcyclohexanone with O2 (with 1.5 equiv of sacrificial aldehyde) with up to 26% ee.390 Plausibly, the dioxygen activation in the aerobic Baeyer−Villiger oxidations proceeded through the formation of the peroxycarboxylic acid upon the interaction of O2 with the sacrificial reductant. Uchida and Katsuki showed that cobalt salen complexes of the type 166 (5 mol %) mediated the oxidation of 3-phenylsubstituted cyclobutanone with H2O2 and UHP, demonstrating good yields (72−92%) and moderate to good enantioselectivities (50−77% ee).391,392 Ethanol was identified as the preferred reaction solvent, and UHP was shown to afford chiral lactones with higher optical yields than H2O2. Strukul and co-workers conducted the oxidation of cyclobutanones with H2O2 in water in the presence of cobalt salen complex 167 (1 mol %); for the solubilization of the hydrophobic substrate and catalyst, micelles were used.393 Sandaroos and Goldani found that supported catalysts of the type 168 exhibited higher enantioselectivity in the oxidation of 3-arylcyclobutanones than their homogeneous analogues.394 Katsuki’s second-generation zirconium salen complex 169a, which was used at a 5 mol % loading, catalyzed the oxidation of 3-phenylcyclobutanone with urea hydroperoxide in chlorinated organic solvents with up to 87% ee.395 With bicyclic and tricyclic cyclobutanones, the enantioselectivity was much higher (>99% ee). The analogous hafnium complex 169b showed a slightly lower product yield and enantioselectivity.396

5. TRANSITION-METAL-CATALYZED ASYMMETRIC OLEFIN CIS-DIHYDROXYLATION REACTIONS Designing methods for catalyzed asymmetric cis-dihydroxylation may be of high synthetic potential, 1,2-diols being common motifs in many natural products and intermediates.404,405 There have been several reports on bioinspired iron and manganese complexes capable of mediating enantioselective cis-dihydroxylation with hydrogen peroxide. When modeling Rieske dioxygenases with synthetic non-heme iron

4.2. Metal-Free Systems

There has been a limited number of organocatalytic catalyst systems for the asymmetric Baeyer−Villiger oxidations; they are mostly focused on the oxidation of prochiral cyclobutanones. In 2002, Murahashi and co-workers found that planar chiral bisflavinium perchlorate 170 (Figure 37) catalyzed the 11442


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More recently, Che and co-workers contributed complex 177 (also exhibiting a cis-α-coordination topology), which appeared to be capable of cis-dihydroxylating (E)-olefins in highly enantioselective fashion (87 to >99% ee) and in synthetically useful yields (Figure 40 and Table 18) under practical

complexes, Que and co-workers found that complex 174 (Figure 39) converted various olefins into nonracemic cis-1,2-

Figure 40. Che’s iron-based catalyst system for the enantioselective cisdihydroxylation of (E)-olefins with H2O2.

conditions, not requiring a high excess of olefin and H2O2.408 With (Z)-olefins, the enantioselectivities were lower (22−83% ee for reported examples); nonconjugated terminal olefins demonstrated good ee levels (81−92%), while substituted styrenes gave 1,2-diols with poor optical purity (15−37% ee). On the basis of 18O-labeling data, the authors proposed the plausible cis-dihydroxylation mechanism, likely involving a ferric hydroperoxo complex (Figure 41), which is consistent with the earlier prediction by Chen and Que.409

Figure 39. Metal complexes used in asymmetric cis-dihydroxylations of olefins.

diols (having 3−82% ee, Table 18) with H2O2.406 The yields of diols were very moderate, not exceeding 11.2 mmol of diol/ mmol of catalyst, under the following loadings: 2.1 μmol of catalyst 174, 2.1 mmol of olefin, and 14.7−29.4 μmol of H2O2. In a subsequent study, iron complexes 175 and 176 were reported and afforded chiral 1,2-diols demonstrating somewhat lower efficiencies (1.1−7.5 TNs), but in some cases with higher enantioselectivities (up to 97% ee).407 Besides 1,2-diols, all catalysts afforded epoxides as byproducts, with the diol:epoxide ratios ranging from 1.1:1 to >75:1.

Figure 41. Proposed key active ferric hydroperoxo species of ironcatalyzed cis-dihydroxylation.

Table 18. Asymmetric cis-Dihydroxylations with H2O2 Catalyzed by Metal Complexes

a

cis-Diol yield (mmol of diol/mmol of catalyst). bA 55% conversion was reported. 11443


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accounted for by ion pair formation with the substrate and the chiral catalyst.411 Excess P(OEt)3 was added to the reaction mixture for the in situ reduction of the labile hydroperoxide intermediate to the target α-hydroxy carbonyl compound. A series of structural analogues of 179 were compared in the oxidation of 2-ethyltetralone, exhibiting moderate enantioselectivities (not exceeding 59% ee).226 Chiral crown ether 180 (Figure 42) showed slightly lower stereoselectivity levels under similar conditions (10 mol % catalyst loading), but the latter were achieved at lower temperatures within 24 h of oxidation.412 A model of the transition state, assuming the oxidation of the enolate form of the ketone, was proposed.412 Itoh and co-workers screened a series of quaternary ammonium salts as catalysts, and developed a procedure for the asymmetric oxidation of oxindoles with air in the presence of catalyst 181 (Figure 42).413 At 20 mol % catalyst loadings, high conversions (typically >90%), along with generally good to high ee’s (up to 93%) were achieved at −20 °C within 2.5− 75 h depending on the substrate. Substrates of this class can also be oxidized with O2 in the presence of 5 mol % pentanidium catalyst 182 (Figure 42).414 The best enantioselectivity was achieved at −60 °C; at this temperature, the reaction was accomplished within 40 h. More recently, Zhao and co-workers developed the more elaborate phase-transfer catalyst 183 (Figure 42), which ensured moderate to good yields and enantioselectivities up to 98% ee in the oxidation of both cyclic (2-alkyltetralones and 2-alkylindanones) and acyclic ketones.415 With 5 mol % catalyst loadings, the reaction required 18−72 h at −10 °C. Meng and co-workers focused on the oxidation of 1indanone-derived β-keto esters, which was achieved with moderate enantioselectivities (39−75% ee) using catalyst 184 (5 mol %) under visible light irradiation within 0.5−3.5 h at −18 °C, and proposed a 1O2-driven mechanism for the transformation of the initially formed substrate hydroperoxide intermediate into the final product.416 Subsequently, modified phase-transfer catalyst 185 showed higher enantioselectivities, approaching 90% ee, and high yields (89−98%) at room temperature.417 In addition, the possibility of hydroxylation of indanone-derived β-keto amides (with 5−66% ee) was demonstrated.417 Irradiation was also used by Córdoba and co-workers to achieve singlet oxygen asymmetric incorporation at the αposition of aldehydes in the presence of 20 mol % L-proline (186; Figure 44).418,419 With subsequent in situ reduction of the intermediate hydroperoxide with NaBH4, the novel synthetic route to optically active terminal 1,2-diols was proposed; good yields (72−95%) and low to moderate ee’s (14−66%) were reported. Subsequently, protected diarylprolinols of the type 187 were examined and exhibited much higher enantioselectivities, affording the 1,2-diols with up to 98% optical purity, albeit in moderate yields (50−76%).419 Other amino acids were also tested as catalysts of asymmetric α-hydroxylation of ketones under UV irradiation;420 valine and alanine showed higher stereoselection than proline. The reaction can be conducted without an external reductant (such as P(OEt)3), affording α-hydroxy ketones with moderate to good yields and moderate enantioselectivities (up to 72% ee).420 Triplet oxygen did not lead to hydroxylated products. Overall, organocatalyzed α-hydroxylations of carbonyl compounds are not numerous and at present suffer from a narrow substrate scope, low catalytic activities and efficiencies, and in most cases the need for an external coreductant.

At present, there are no other metal-based catalysts capable of competing with the Fe complex 177 in terms of the stereoselectivity of cis-1,2-dihydroxylation with H2O2. There was a report by Feringa and co-workers on the use of dinuclear manganese trimethyltriazacyclononane complex 178; the latter showed moderate enantioselection (up to 54% ee).410

6. ASYMMETRIC C−H OXIDATIONS 6.1. Organocatalyzed α-Hydroxylation of Carbonyl Compounds

The enantioselective α-hydroxylation of aldehydes and ketones, affording optically active α-hydroxy carbonyl compounds, can be achieved with the aid of organocatalysts, mostly phasetransfer catalysts. The first example of this kind was reported in 1988 by Shiori and co-workers, who used cinchona alkaloidderived quaternary ammonium chloride salt 179 (Figure 42)

Figure 42. Phase-transfer catalysts used for the enantioselective αhydroxylation of carbonyl compounds.

for the oxidation of 2-alkyltetralones and 2-alkylindanones with molecular oxygen at room temperature (Figure 43 and Table

Figure 43. Organocatalyzed α-hydroxylation of 2-alkyltetralones.

19).411 Good yields (55−98%) and moderate to good enantioselectivities (27−79% ee) were achieved using a 5−10 mol % concentration of the organocatalyst; sodium hydroxide was added as an additive. The reaction proceeded via aerobic oxidation of the tertiary C−H group to the corresponding hydroperoxide, the generation of asymmetric induction being 11444


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Table 19. Asymmetric Organocatalyzed α-Hydroxylation of Carbonyl Compounds with O2

a

TPP = tetraphenylporphyrin.

Figure 44. Synthetic route to chiral 1,2-diols. Tetraphenylporphyrin is used as a sensitizer.

6.2. Direct Asymmetric Oxidation of C−H Groups

So far, there have been only four reported examples of direct enantioselective oxidation of prochiral C−H groups with H2O2. In 2012, the group of Simonneaux reported the benzylic oxidations of several (cyclic and acyclic) arylalkanes in water− methanol solutions with 5 equiv of H2O2 in the presence of chiral manganese porphyrin 188 (Figure 45 and Table 20).89 The system required the use of an imidazole additive; remarkably, the corresponding chiral alcohols were formed in synthetically useful yields, being the major oxidation products (which rules out a possible contribution of oxidative kinetic resolution to the resulting optical purity). The highest enantioselectivity (57% ee) was obtained with 4-ethyltoluene, for which a 93:7 alcohol:ketone ratio was reported. Substitution of the SO3− groups of 188 with H, NO2, or NMe2 deteriorated both the alcohol selectivity and the enantioselectivity.90 Bryliakov and co-workers found that bioinspired manganese aminopyridine complex 189 (Figure 46) also exhibited enantioselection in the oxidation of substituted ethylbenzenes

Figure 45. Transition-metal-catalyzed C−H hydroxylation with H2O2.

(Table 20) with H2O2 in the presence of carboxylic acid additives.421 The hydroxylation was highly efficient (up to 250 TNs); however, significant amounts of ketones formed, which required an excess of substrate to obtain the chiral alcohol as the major product. With 2-ethylhexanoic acid, the enantioselectivity was generated mostly (if not exclusively) at the oxidation stage only, and approached 50% for p-methyltoluene. Very recently, the use of chiral carboxylic acid N-Boc-L-proline (Boc-L-Pro; Figure 7) in combination with catalyst ent-189 has afforded several chiral 1-arylethanols with much higher 11445


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Table 20. Asymmetric C−H Hydroxylations with H2O2

a

In moles of alcohol per mole of catalyst.

Figure 46. Non-porphyrin manganese-based catalysts for the direct asymmetric C−H hydroxylation.

enantiomeric excess (up to 86% ee, Table 20) due to the chiral additive amplification effect.422 The optical purity of the 1phenylethanol has been found to slightly increase over the reaction course (from 71% to 83% ee),422 thus indicating a minor contribution of oxidative kinetic resolution to the enantioselectivity. Hua and co-workers developed a new class of poly(Nvinylpyrrolidinone)s containing an asymmetric center at the C5 position of the pyrrolidinone ring; the latter were used to stabilize Cu/Au nanoparticles that appeared to be capable of promoting the oxidation of prochiral cycloalkanes with H2O2 in generally good yields (87−98% within 7 days at a 1 mol % catalyst loading) and high enantioselectivities (81−93% ee, five examples).423 The cyclic substrates were preferentially oxidized at the third position to form ketone functionality; the resulting product possessed chirality at the tertiary or quaternary carbon (Figure 47). A serious drawback of this procedure is the high excess of the oxidant, typically 2 mL of 30% H2O2 (i.e., ca. 20 mmol) was required to oxidize 0.7 mmol of the susbtrate. Milan, Bietti, and Costas proposed a complementary, truly homogeneous catalytic oxidation of substituted cyclohexanes with H2O2 in the presence of bioinspired manganese complexes of the type 191 (Figure 46).424 Again the oxidation proceeded at the third position to ketones, the latter possessing chirality at the tertiary carbon (Figure 48). Cyclopropanecarboxylic acid was identified as the best (in terms of enantioselectivity) catalytic additive; enantioselectivities up to 94% ee (two examples) and 96% ee (one example) were reported.424 Although the substrate scope has been very specific so far,

Figure 47. Oxidation of cycloalkanes with H2O2 in the presence of chiral polymer-supported bimetallic nanoparticles.

Figure 48. Highly enantioselective ketonization of substituted cyclohexanes.

the extraordinary regio- and stereoselectivities hold considerable promise for future developments.

7. CONCLUSIONS AND OUTLOOK The progress of asymmetric catalysis, achieved since the 1970s, has been one of the major landmarks in the area of catalysis in general. Nowadays, catalytic asymmetric synthesis is a mature field, undergoing the transition from the laboratory to practice. Catalytic asymmetric oxidation is a relatively small but important branch of asymmetric catalysis. The major current 11446


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Overall, despite certain growing pains, the area of catalyzed asymmetric oxygenations with environmentally benign oxidants H 2 O 2 and O 2 is facing its maturity in good shape, demonstrating rapid progress in both quantitative and qualitative respects. Apart from the well-developed subareas, such as asymmetric epoxidations and sulfoxidations, and previously active but now relatively abandoned niches, such as Baeyer−Villiger oxidations, there are emerging branches (e.g., direct asymmetric oxidation of C−H groups, which is one of the major focuses of our current research), witnessing selfmaintained evolution of the whole area. The future of catalytic asymmetric oxygenations with environmentally benign oxidants seems to be bright, and on completion of this review, I am full of positive expectancies.

trend in catalytic asymmetric oxidations, dictated by the steadily toughening environmental and economic restrictions, is the shift to more green and environmentally sustainable catalyst systems, essentially to those exploiting cheap, safe, and oxygenrich oxidants such as H2O2 and O2. The late 1990s and first decades of the 21st century have become an epoch-making time for the asymmetric oxygenations with H2O2 and O2, during which period the area experienced rapid growth and took the current state. Within the variety of oxygenation reactions with H2O2 and O2, asymmetric epoxidations are most abundant, and are currently being most extensively studied. In these studies, biomimetic catalysts are widely involved, similar to those reported to mimic the catalytic behavior of naturally occurring metalloenzymes (mainly mono- and dioxygenases), combined with detailed mechanistic studies aimed at the understanding of the nature of the active sites and the asymmetric oxygentransfer mechanism at the atomistic level. Such a situation may reflect a convergence between the chemical and biological pictures of catalyzed asymmetric oxygenations, which is believed to mutually enrich both disciplines. Among asymmetric oxygenation catalysts, transition-metal complexes occupy a dominating position, while organocatalysts are less represented, their major drawback being relatively low activities and efficiences, sometimes many orders of magnitude lower than those for metal-based catalysts. At the same time, from the synthetic perspective, organocatalysts may be considered as complementary to metal catalysts, capable of conducting otherwise difficult (or impossible) asymmetric processes, e.g., epoxidation of α,β-unsaturated aldehydes, of trisubstituted olefins with carbamate functionalities, or aerobic α-hydroxylations of ketones and aldehydes. We have to notice that, with the priority development of the catalytic asymmetric oxidations in the past years, there has occurred a gap between the laboratory and industry. From the practical perspective, catalytic asymmetric oxidations with environmentally benign oxidants H2O2 and O2 have been relatively undeveloped compared with catalyst systems that use traditional oxidants (such as peroxyacids, iodosylarenes, bleach, oxone, pyridine N-oxides, etc.), having so far a narrower reported substrate scope or serious substrate limitations (those issues are briefly mentioned at the end of the corresponding sections) and few preparative-scale application examples. From an industrial perspective, few of the hereinbefore considered catalyst systems can be of interest: in most cases, higher catalytic activities and efficiencies are expected, as well as stereoselectivities of at least 95% ee. Another general problem is the low oxidant efficiency: in many cases, the target asymmetric oxidation reaction competes with unproductive hydrogen peroxide disproportionation; the latter pathway may even be prevalent, thus deteriorating the process economics. Moreover, the industry prefers supported catalysts, since immobilization facilitates transportation, as well as charging and recycling procedures.425,426 For me, it has been gratifying to survey some of the latest publications focused on immobilization of homogeneous catalysts of asymmetric oxidative transformations here. My vision of the current shape and practical perspectives of catalytic asymmetric oxidations with H2O2 and O2 has been summarized in the recent monograph (Chapter 8 of ref 10). I share the generally optimistic opinion427 that the industrial use of such processes will grow in the medium- and long-term perspective.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. ORCID

Konstantin P. Bryliakov: 0000-0002-7009-8950 Notes

The author declares no competing financial interest. Biography Konstantin Bryliakov graduated from Novisibirsk State University in 1999 and obtained the Cand. Chem. Sci. degree (Ph.D.) in chemical physics from the Institute of Chemical Kinetics and Combustion (Novosibirsk, 2001) and the Dr. Chem. Sci. degree (Habilitation) in catalysis from the Boreskov Institute of Catalysis (Novosibirsk, 2008), where he currently occupies the Leading Researcher position. In 2016, he was elected Honorary Professor of the Russian Academy of Sciences. His research interests encompass single-site olefin polymerizations, as well as stereo- and chemoselective transition-metalcatalyzed oxidative transformations, particularly those relying on green oxidants.

ACKNOWLEDGMENTS This work was conducted within the framework of Budget Project 0303-2016-0005 for the Boreskov Institute of Catalysis. Financial support from the Russian Foundation for Basic Research (Project 16-29-10666) is gratefully acknowledged. ABBREVIATIONS Ac acetyl ACA 1-adamantanecarboxylic acid Ada 1-adamantyl BINOL 1,1′-bi-2-naphthol BTCN benzene-1,2,4,5-tetracarbonitrile H2dipic 2,6-dipicolinic acid HOTf trifluoromethanesulfonic acid CLAMPS cross-linked (aminomethyl)polystyrene DABCO 1,4-diazabicyclo[2.2.2]octane DBU 1,8-diazabicyclo[5.4.0]undec-7-ene D-CPA D-camphoric acid DIC diisopropylcarbodiimide DMAP N,N-dimethyl-4-aminopyridine DMBA 2,2-dimethylbutyric acid DME 1,2-dimethoxyethane DMSO dimethyl sulfoxide de diastereomeric excess dr diastereomeric ratio 11447


Chemical Reviews EDTA ee EHA IBA IBU Imd MA MTO N-Bu-Imd N-Hex-Imd NLE N-Me-Imd NMO N-Oct-Imd NPha-ILE PA Py-N-O (S)-IBU TN TPP UHP

Review

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ethylenediaminetetraacetic acid enantiomeric excess 2-ethylhexanoic acid isobutyraldehyde (S)-ibuprofen imidazole maleic anhydride methyltrioxorhenium N-n-butylimidazole N-n-hexylimidazole nonlinear effect N-methylimidazole N-methylmorpholine N-oxide N-n-octylimidazole Npha-protected isoleucine pivalic aldehyde pyridine N-oxide (S)-(+)-ibuprofen turnover number tetraphenylporphyrin urea hydroperoxide

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