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

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• 作者：Roman V. Ottenbacher ; Denis G. Samsonenko ; Evgenii P. Talsi ; Konstantin P. Bryliakov
• 刊名：ACS Catalysis
• 出版年：2016
• 出版时间：February 5, 2016
• 年：2016
• 卷：6
• 期：2
• 页码：979-988
• 全文大小：486K
• ISSN：2155-5435

文摘

It has been demonstrated that chiral manganese aminopyridine complexes [LMn^II(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 [LMn^II(OTf)₂] with different oxidants are considered. The analysis of reaction products and epoxidation enantioselelctivity, Hammett correlations, and isotopic (¹⁸O) 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.