Catalytic Asymmetric Reduction of Prochiral Ketones with Chiral β-Amino Alcohol N-Boranes and the Corresponding Tris(oxazaborolidine)borazines

Abstract

Chiral β-amino alcohol-derived N-borane catalysts, namely noncyclic (2S)-and (2R)-2-amino-2-phenylethanol N-borane and their corresponding cyclic trimeric borazine derivatives, were synthesized and their catalytic activities in the asymmetric reduction of prochiral ketones were examined. Both the noncyclic and cyclic catalysts successfully catalyzed this reaction, giving the desired secondary alcohols in up to 82% isolated yield and with up to 80% enantioselectivity. The noncyclic catalyst was stable in aqueous and organic solvents, whereas the polycyclic borazine was stable only in nonprotic dry organic solvents.