Chiral Amino-acids Derivatives-Ruthenium Catalyzed Asymmetric Transfer Hydrogenation of Ketones

Benzimidazoles derived from chiral amino acids and benzimidazoles derived from dipeptides were prepared and proved to be efficient ligands in the ruthenium catalyzed asymmetric transfer hydrogenation of acetophenone. Employing ligand 2 in the reduction of acetophenone resulted in better activity and enantioselectivity (with 15400 h TOF and 77% ee). Through structure-activity correlations, the corresponding metal-ligand bifunctional mechanism was speculated. In addition, the catalytic system of [RuCl2(p-cymene)]2/2 exhibited reversed temperature effects, which explained through kinetics data. The enantioselectivity was increased from 38% to 43% when LiCl was added in dipeptides derived benzimidazoles 7/[RuCl2(p-cymene)]2 catalyzed asymmetric transfer hydrogenation.