Asymmetric reduction of prochiral aromatic ketones by borane-amine complexes in the presence of chiral amine-BF < inf > 3 < /inf > 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-phenylethyl)-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′-diethylaniline-BH3 to give 1-phenylethyl alcohol in 51% e.e. A transition state consisting of a chiral amine·BF 3·BH3 complex and the ketone is proposed to explain the transformation.