ASYMMETRIC EPOXIDATION USING CHIRAL SALEN COMPLEXES

Asymmetric epoxidation of unfunctionalized olefins is of current interest in synthetic organic chemistry and many interesting methodologies have so far been reported for this reaction. Still, there is no sufficient methodology in terms of enantioselectivity and generality. However, optically active (salen) manganese (III) complexes were recently found to be efficient catalysts for epoxidation of this class of olefins by us and Jacobsen et al, respectively. With these (salen) manganese complexes as catalysts, high enantioselectivity of >95% ee has been realized, especially in the epoxidation of cis-olefins. For example, 2,2-dimethylchromene derivative 14 was converted into the corresponding expoxide of 95.5% ee. Although trans-disubstituted olefins are not very good substrates for this salen complex-catalyzed epoxidation, moderate enantioselectivity of 62.5% ee was observed in the epoxidation of trans-stilbene. Quite recently, (salen)manganese(III) complexes were also found to catalyze the asymmetric aziridination of styrene. In this paper, we describe these new developments in salen chemistry mainly on the basis of our recent study.