Nonmetal oxidation catalysts have gained much attention in recent years. The reason for this surge in activity is 2-fold: On one hand, a number of such catalysts has become readily accessible; on the other hand, such catalysts are quite resistant toward self-oxidation and compatible under aerobic and aqueous reaction conditions. In this review, we have focused on five nonmetal catalytic systems which have attained prominence in the oxidation field in view of their efficacy and their potential for future development; stoichiometric cases have been mentioned to provide overview and scope. Such nonmetal oxidation catalysts include the alpha-halo carbonyl compounds 1, ketones 2, imines 3, iminium salts 4, and nitroxyl radicals 5. In combination with a suitable oxygen source (H2O2, KHSO5, NaOCl), these catalysts serve as precursors to the corresponding oxidants, namely, the perhydrates I, dioxiranes II, oxaziridines III, oxaziridinium ions IV, and finally oxoammonium ions V. A few of the salient features about these nonmetal, catalytic systems shall be reiterated in this summary. The first class entails the alpha-halo ketones, which catalyze the oxidation of a variety of organic substrates [figure: see text] by hydrogen peroxide as the oxygen source. The perhydrates I, formed in situ by the addition of hydrogen peroxide to the alpha-halo ketones, are quite strong electrophilic oxidants and expectedly transfer an oxygen atom to diverse nucleophilic acceptors. Thus, alpha-halo ketones have been successfully employed for catalytic epoxidation, heteroatom (S, N) oxidation, and arene oxidation. Although high diastereoselectivities have been achieved by these nonmetal catalysts, no enantioselective epoxidation and sulfoxidation have so far been reported. Consequently, it is anticipated that catalytic oxidations by perhydrates hold promise for further development, especially, and should ways be found to transfer the oxygen atom enantioselectively. The second class, namely, the dioxiranes, has been extensively used during the last two decades as a convenient oxidant in organic synthesis. These powerful and versatile oxidizing agents are readily available from the appropriate ketones by their treatment [figure: see text] with potassium monoperoxysulfate. The oxidations may be performed either under stoichiometric or catalytic conditions; the latter mode of operation is featured in this review. In this case, a variety of structurally diverse ketones have been shown to catalyze the dioxirane-mediated epoxidation of alkenes by monoperoxysulfate as the oxygen source. By employing chiral ketones, highly enantioselective (up to 99% ee) epoxidations have been developed, of which the sugar-based ketones are so far the most effective. Reports on catalytic oxidations by dioxiranes other than epoxidations are scarce; nevertheless, fructose-derived ketones have been successfully employed as catalysts for the enantioselective CH oxidation in vic diols to afford the corresponding optically active alpha-hydroxy ketones. To date, no catalytic asymmetric sulfoxidations by dioxiranes appear to have been documented in the literature, an area of catalytic dioxirane chemistry that merits attention. A third class is the imines; their reaction with hydrogen peroxide or monoperoxysulfate affords oxaziridines. These relatively weak electrophilic oxidants only manage to oxidize electron-rich substrates such as enolates, silyl enol ethers, sulfides, selenides, and amines; however, the epoxidation of alkenes has been achieved with activated oxaziridines produced from perfluorinated imines. Most of the oxidations by in-situ-generated oxaziridines have been performed stoichiometrically, with the exception of sulfoxidations. When chiral imines are used as catalysts, optically active sulfoxides are obtained in good ee values, a catalytic asymmetric oxidation by oxaziridines that merits further exploration. The fourth class is made up by the iminium ions, which with monoperoxysulfate lead to the corresponding oxaziridinium ions, structurally similar to the above oxaziridine oxidants except they possess a much more strongly electrophilic oxygen atom due to the positively charged ammonium functionality. Thus, oxaziridinium ions effectively execute besides sulfoxidation and amine oxidation the epoxidation of alkenes under catalytic conditions. As expected, chiral iminium salts catalyze asymmetric epoxidations; however, only moderate enantioselectivities have been obtained so far. Although asymmetric sulfoxidation has been achieved by using stoichiometric amounts of isolated optically active oxaziridinium salts, iminium-ion-catalyzed asymmetric sulf-oxidations have not been reported to date, which offers attractive opportunities for further work. The fifth and final class of nonmetal catalysts concerns the stable nitroxyl-radical derivatives such as TEMPO, which react with the common oxidizing agents (sodium hypochlorite, monoperoxysulfate, peracids) to generate oxoammonium ions. The latter are strong oxidants that chemoselectively and efficiently perform the CH oxidation in alcohols to produce carbonyl compounds rather than engage in the transfer of their oxygen atom to the substrate. Consequently, oxoammonium ions behave quite distinctly compared to the previous four classes of oxidants in that their catalytic activity entails formally a dehydrogenation, one of the few effective nonmetal-based catalytic transformations of alcohols to carbonyl products. Since less than 1 mol% of nitroxyl radical is required to catalyze the alcohol oxidation by the inexpensive sodium hypochlorite as primary oxidant under mild reaction conditions, this catalytic process holds much promise for future practical applications.

Synthetic applications of nonmetal catalysts for homogeneous oxidations.

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.

Catalytic Asymmetric Oxygenations with the Environmentally Benign Oxidants H2O2 and O2.

Oxidation of sulfides to sulfoxides. Part 1: Oxidation using halogen derivatives

Heterocyclic hydroperoxides in selective oxidations.

Through the oxidation of sulfides useful elements of stereocontrol in asymmetric synthesis and important targets of pharmaceutical interest can be obtained. A number of metal-free methods have been developed over recent years for sulfoxidation reactions. The results published after 1998 through the beginning of 2010 have been reviewed with a focus being placed on sulfoxidations with hypervalent iodine compounds; iminium- and oxaziridinium derivatives; organic hydroperoxides and organocatalysts. In addition, autocatalytic and catalyst-free variants are discussed. Although, the asymmetric organocatalytic sulfoxidation is still in its infancy, the reported results show the high potential and versatility of this metal-free methodology.

Recent advances in sulfoxidation reactions: a metal-free approach

[(3,3-Dimethoxycamphoryl)sulfonyl]oxaziridine oxidizes sulfides to sulfoxides cleanly and efficiently in up to 98% ee.

Asymmetric sulfoxidation using [(3,3-Dimethoxycamphoryl)sulfonyl]oxaziridine

A new system for catalytic asymmetric oxidation of sulfides using a hydrogen peroxide based reagent

Catalytic enantioselective oxidation of a range of sulfides is achieved in very high yields by treatment with hydrogen peroxide in conjunction with enantiomerically pure sulfonylimines under basic conditions. Enantioselectivities of up to ca. 98% or better were obtained using acetals of oxocamphorsulfonylimine as mediators. This process is clean, inexpensive and simple, requires minimum work up and no close monitoring. The reactive intermediate is currently formulated as an α-hydroperoxyamine.

Asymmetric Sulfur Oxidation Mediated by Camphorsulfonylimines

In homogeneous methanol solution containing potassium carbonate, hydrogen peroxide and acetonitrile, the active oxidant in the conversion of p-substituted-phenyl methyl sulfides into the sulfoxides is selective (ρ=–1.0 at 0 °C) but the oxidation of the related sulfoxides at 0 and 24 °C shows low selectivity, which, taken in conjunction with direct kinetic data, throws new light on the active oxidizing species in these nitrile-mediated reactions.

Jag P. Heer

Papers

Asymmetric sulfoxidation using [(3,3-Dimethoxycamphoryl)sulfonyl]oxaziridine

A new system for catalytic asymmetric oxidation of sulfides using a hydrogen peroxide based reagent

Asymmetric Sulfur Oxidation Mediated by Camphorsulfonylimines

Reactivity and selectivity in the oxidation of aryl methyl sulfides and sulfoxides by hydrogen peroxide mediated by acetonitrile

Sulfur Oxidation Mediated by Imine Derivatives