Enantioselective Epoxidations of Olefins with Various Oxidants on Bioinspired Mn Complexes: Evidence for Different Mechanisms and Chiral Additive Amplification

It has been demonstrated that chiral manganese aminopyridine complexes [LMnII(OTf)(2)] are able to epoxidize olefins with excellent enantioselectivities (up to 99% ee, several examples) with various terminal oxidants (H2O2, alkyl hydroperoxides, peroxyacids, and iodosylarenes). The mechanisms of enantioselective olefin epoxidations on aminopyridine manganese(II) complexes [LMnII(OTf)(2)] with different oxidants are considered. The analysis of reaction products and epoxidation enantioselelctivity, Hammett correlations, and isotopic (O-18) labeling studies bear evidence in favor of the formation of different oxidizing species in the presence of different terminal oxidants. The addition of cocatalytic additives (carboxylic acid or H2O) dramatically changes the catalytic behavior of catalyst systems Mn complex/hydrogen peroxide and Mn complex/alkyl hydroperoxide; in the presence of additives, the catalyst systems with H2O2 and alkyl hydroperoxides demonstrate very similar behaviors. Remarkably, in the presence of chiral additive Boc-protected (L)-proline, achiral Mn aminopyridine complexes catalyze the epoxidation of chalcone in an enantioselective fashion (with up to 60% ee), representing a rare example of chiral environment amplification.