Asymmetric reduction of prochiral aromatic ketones by borane–amine complexes in the presence of chiral amine–BF3 catalysts

Abstract

The borane complexes of (S)-(–)-N,N-dibenzyl-1-phenylethylamine 1. (S,S)-(–)-N,N-bis(1-phenylethyl)-1-phenylethylamine 2, (–)-2-phenyl-1-(1-phenylethyl)pyrrolidine 3, (–)-7-(1-phenylthyl)-4,5-dihydro-3H-dinaphth[2,3-c;2′,3′-e]azepine 4 and 1-(1-phenylethyl-3,4-diphenyl-pyrrolidine 5 have been prepared and used to reduce prochiral aromatic ketones to alcohols in 10–57% e.e. in the presence of BF₃·OEt₂. The complex 4·BF₃ catalyses asymmetric reduction of acetophenone by N,N′-diethylaniIine–BH₃ to give 1-phenylethyl alcohol in 51% e.e. A transition state consisting of a chiral amine·BF₃·BH₃ complex and the ketone is proposed to explain the transformation.