Ruthenium(II)-Catalyzed Asymmetric Transfer Hydrogenation Using Unsymmetrical Vicinal Diamine-Based Ligands: Dramatic Substituent Effect on Catalyst Efficiency

The use of unsymmetrical vicinal diamines as ligands for Ru-catalyzed asymmetric transfer hydrogenation is described. With a SmI2-mediated cross-coupling protocol, a series of enantiomerically pure unsymmetrical vicinal diamines were readily prepared and examined in the asymmetric transfer hydrogenation. It was found that an aromatic substituent on the carbon bearing the -NHTs group and a bulky alkyl substituent on the other side, are both very important for the effectiveness of the ligand, suggesting that the substituent has a dramatic effect on the catalyst efficiency. With ligand 8, excellent enantioselectivities that are comparable to N-tosyl-1,2-diphenylethane-1,2-diamine (TsDPEN) were achieved. The results provide some helpful information on the mechanism of Ru-catalyzed asymmetric transfer hydrogenation.