Oxidation 10.1
Epoxidation of Olefins
The asymmetric epoxidation of CbC double bonds provides access to enantiomerically enriched epoxides. The latter materials are of great practical value, in particular as intermediates in the production of enantiomerically pure pharmaceuticals. Recent years have seen the dramatic development of methods for this purpose. In the field of metal-catalyzed epoxidations, in particular, the titanium-catalyzed asymmetric epoxidation of allylic alcohols (Sharpless epoxidation) has found immense application in synthesis . Similarly, manganese-salen catalysts have proven their potential for asymmetric transformation of non-functionalized olefins (Jacobsen-Katsuki epoxidation) . For metal-free asymmetric epoxidation current methodology relies mainly on the development of chiral ketones A for catalytic generation of dioxiranes B as epoxidizing agents (Scheme 10.1, a). Potassium persulfate (KHSO 5 ) -in the form of its triple salt with K 2 SO 4 and KHSO 4 (''Oxone'', ''Curox'' etc.) -usually serves as the final oxidizing agent. Similarly, it has been reported that chiral iminium ions C enable generation of oxaziridinium cations D (Scheme 10.1, b). In these reactions either potassium persulfate or hydrogen peroxide served as the source of oxygen.
It should be noted that the related imine-oxaziridine couple E-F finds application in asymmetric sulfoxidation, which is discussed in Section 10.3. Similarly, chiral oxoammonium ions G enable catalytic stereoselective oxidation of alcohols and thus, e.g., kinetic resolution of racemates. Processes of this type are discussed in Section 10.4. Whereas perhydrates, e.g. of fluorinated ketones, have several applications in oxidation catalysis [5], e.g. for the preparation of epoxides from olefins, it seems that no application of chiral perhydrates in asymmetric synthesis has yet been found. Metal-free oxidation catalysis -achiral or chiral -has, nevertheless, become a very potent method in organic synthesis, and the field is developing rapidly [6].
10.1.1
Chiral Dioxiranes
The idea of using chiral ketones as catalysts for asymmetric epoxidation of olefins was first addressed by Curci et al. in the middle of the 1980s [7]. In this initial ex-