Asymmetric Hydrogenation of Ketones with Ruthenium Complexes of rac- and Enantiopure (S,S)-1,2-Bis((diphenylphosphino)methyl)cyclohexane: A Comparative Study with rac- and (R)-BINAP

Ruthenium(II) complexes of the type trans-[RuCl2{1,2-bis((diphenylphosphino)methyl)cyclohexane}(diamine)] based on the inexpensive and easy-to-prepare rac- and (S,S)-1,2-bis((diphenylphosphino)methyl)cyclohexane form highly active and enantioselective catalysts for the asymmetric hydrogenation of a wide range of aryl and heteroaryl ketones, in most cases giving ee's that exceed those obtained with their BINAP counterparts. Although precatalysts based on 1,2-bis((diphenylphosphino)methyl)cyclohexane slowly isomerize in solution to afford the thermodynamically favored isomer with a cis arrangement of chlorides, catalysts generated from both isomers afford similar enantioselectivities.