Enantioselective borohydride reduction catalyzed by optically active β-ketoiminato cobalt complexes

Optically active β-ketoiminato cobalt complexes were recently developed as high performance catalysts for enantioselective borohydride reduction of aromatic ketones or imines to afford the optically active alcohols or amines with high enantiomeric excesses. This enantioselective reduction is carried out using a precisely pre-modified borohydride with alcohols such as tetrahydrofur-furyl alcohol, ethanol and methanol, and high optical yields are realized by choosing the appropriate alcohol as modifiers and a suitable β-ketiominato ligand of the catalyst. The enantioselective borohydride reduction has been successfully applied to the efficient preparations of optically active 1,3-diaryl-1,3-propanediols, the stereoselective reduction of diacylferrocenes, and the dynamic and/or kinetic resolution of 1,3-dicarbonyl compounds to afford the corresponding optically active anti-aldol compounds. In the presence of a catalytic amount of the cobalt complex and modified borohydride, the asymmetric 1,4-reduction of β,β-disubstituted α,β-unsaturated carboxamides proceeded with high enantioselectivity, and the 1,4-reduction of the α-substituted α,β-unsaturated carboxamides followed by enantioselective in situ protonation smoothly proceeded to afford the optically active α-substituted carboxamides with good enantiomeric excesses.