Enantioselective Reduction of Ketones - A New Alternative to the CBS Reduction?

Significance

Falck and co-workers report the highly enantioselective reduction of aromatic and aliphatic ketones promoted by the thiourea organocatalyst 1. Using catecholborane as the reducing agent the desired alcohols were generally obtained in high yields and high optical purities. The proposed mechanism involves activation of the ketone by hydrogen bonding and Lewis base activation of the reducing agent by the secondary amine functionality. Thus, the substrate and the reagent are simultaneously activated and preco­ordinated to undergo the desired transformation.

Review: E. J. Corey, C. J. Helal Angew. Chem. Int. Ed. 1998, 37, 1986-2012.