Synthesis and characterization of novel chiral bidentate P,N-containing ligands and ruthenium(II) complex. The application in asymmetric transfer hydrogenation of ketones

Novel chiral bidentate P,N-containing ligands have been easily synthesized by Schiff-base condensation of o-(diphenylphosphino)benzaldehyde and modified chiral diamine, (R,R)-2-(2,5-dimethyl-pyrrol-1-yl)-cyclohexylamine, further reduction with NaBH4. The chiral ruthenium(II) complex could be successfully prepared from the reaction between chiral bidentate aminophosphine ligand and RuCl2(PPh3)3. The chiral bidentate P,N-containing ligands and ruthenium(II) complex were fully characterized by NMR, IR, HRMS and single-crystal X-ray diffraction studies. In the presence of KOH, the asymmetric transfer hydrogenation (ATH) of various ketones catalyzed by the chiral ruthenium(II) complex proceeded smoothly under mild conditions, affording corresponding chiral secondary alcohols with up to 99% conversion and up to 60% ee. Additive such as NH4I was found to be helpful to promoting the enantioselectivity.