Enantioselective α-amination of ketones mediated by chiral nitridomanganese(V) complexes using ammonia as the terminal nitrogen source

Abstract

A series of chiral nitridomanganese(V)salen complexes have been prepared, characterized and investigated as the source of the nitrogen fragment for chiral α-amination of ketones. The chiral nitridomanganese(V)salen complexes are formed by treatment of manganese(III)salen complexes with ammonia under oxidative reaction conditions, and these nitridomanganese(V) complexes have been characterized by spectroscopic methods and X-ray analysis. The structures of the different complexes are discussed and the ability of the chiral nitridomanganese(V)salen complexes to act as chiral nitrogen sources activated by trifluoroacetic anhydride for α-amination of silyl enol ethers has been investigated. The yield and enantiomeric excess of the α-amino ketones are very dependent on the nature of the chiral nitridomanganese(V)salen complex and the silyl enol ether, and the reaction has been investigated under various reaction conditions. The α-amino ketones can be formed in up to 83% yield and with up to 75% ee and the ee can be improved to 93% by a single recrystallization. The perspectives and mechanism for the α-amination of the silyl enol ethers by chiral nitridomanganese(V)salen complexes are discussed.