Unfunctionalized olefins ketone-catalyzed epoxidation

Shi asymmetric epoxidation Chiral-ketone catalyzed epoxidation of unfunctionalized olefins. 410... 

Asymmetric epoxidation of olefins is an effective approach for the synthesis of enan-tiomerically enriched epoxides. A variety of efficient methods have been developed [1, 2], including Sharpless epoxidation of allylic alcohols [3, 4], metal-catalyzed epoxidation of unfunctionalized olefins [5-10], and nucleophilic epoxidation of electron-deficient olefins [11-14], Dioxiranes and oxazirdinium salts have been proven to be effective oxidation reagents [15-21], Chiral dioxiranes [22-28] and oxaziridinium salts [19] generated in situ with Oxone from ketones and iminium salts, respectively, have been extensively investigated in numerous laboratories and have been shown to be useful toward the asymmetric epoxidation of alkenes. In these epoxidation reactions, only a catalytic amount of ketone or iminium salt is required since they are regenerated upon epoxidation of alkenes (Scheme 1). 

Epoxidation of olefins was catalyzed by the ruthenium(II) complex of the above perfluorinated y3-diketone in the presence of 2-methylpropanal (Scheme 50). Unfunctionalized olefins were epoxidized with a cobalt-containing porphyrin complex, and epoxidation of styrene derivatives was catalyzed by chiral salen manganese complexes (248) (Scheme 50). In the latter case, chemical yields were generally high, however, the products showed low enantiomeric excess with the exception of indene (92% ee). [Pd(C7Fi5COCHCOC7Fi5)2] efficiently catalyzed the oxidation of terminal olefins to methyl ketones with f-butylhydroperoxide as oxidant in a benzene-bromoperfluoro-octane solvent system (Scheme 50). In all these reactions, the product isolation and efficient catalyst recycle was achieved by a simple phase separation. 

Epoxides are very versatile intermediates, and asymmetric epoxidation of olefins is an effective approach to the synthesis of enantiomericaUy enriched epoxides [1-3]. Great success has been achieved for the epoxidation of allyhc alcohols [1], the metal-catalyzed epoxidation of unfunctionalized olefins (particularly conjugated cis- and tri-substituted) [2], and the nucleophilic epoxidation of electron-deficient olefins [3]. In recent years, chiral dioxiranes have been shown to be powerful agents for asymmetric epoxidation of olefins. Dioxiranes can be isolated or generated in situ from Oxone (potassium peroxymonosulfate) and ketones (Scheme 3.1) [4,5]. When the di-oxirane is used in situ, the corresponding ketone is regenerated upon epoxidation. Therefore, in principle, a catalytic amount of ketone can be used. When a chiral ketone is used, asymmetric epoxidation should also be possible [6]. Extensive studies have been carried out in this area since the first chiral ketone was reported by Curd in 1984 [7]. This chapter describes some of the recent progress in this area. 

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