Benzimidazoles as Ligands in the Ruthenium-Catalyzed Enantioselective Bifunctional Hydrogenation of Ketones

A series of Cl2Ru(diphosphane)L2 (II) complexes in which L = N1-alkylated benzimidazoles, bonding to the metal through nitrogen, have been synthesized and characterized. In the case of 1-methylbenzimidazole, the resulting complexes exist as statistical mixtures of all possible conformational isomers. When the size of the substituent on the benzimidazole was increased to complexes could be prepared that exist as a single diastereomer. All complexes possessing benzimidazole ligands bound to the ruthenium center are active for the mild and chemoselective hydrogenation of ketones in the presence of alkenes. Catalysts that exist as a single diastereomer, prepared with enantiomerically pure diphosphanes, catalyze the hydrogenation of prochiral ketones with moderate levels of enantioselectivity that are significantly improved relative to catalysts existing in several conformations.