Remarkably Facile Borane-Promoted, Rhodium-Catalyzed Asymmetric Hydrogenation of Tri- and Tetrasubstituted Alkenes

Oxime-directed catalytic asymmetric hydroboration is diverted to catalytic asymmetric hydrogenation (CAR) upon the addition of a proton source, such as MeOH, or by running the reaction under a hydrogen atmosphere. A borane (e.g., pinacolborane) is required to promote CAH. Tri- and tetrasubstituted alkenes, including the challenging all-alkyl tetrasubstituted alkenes, undergo CAH with enantiomer ratios (er) as high as 99:1. The mild reaction conditions, i.e., ambient temperature, moderate reaction times, and the need for only a slight excess of H-2, contrast those used in most state-of-the-art catalysts for related substrates.