Asymmetric transfer hydrogenation of ketones catalyzed by chiral macrocyclic cobalt(II) complexes

Abstract

Using easily available CoBr2 and chiral cyclic PxNy-type ligands as starting materials, novel chiral cyclic cobalt(II) complexes could be conveniently prepared. Furthermore, we obtained the single crystals suitable for X-ray diffraction to confirm the structures of these cobalt(II) complexes. The asymmetric transfer hydrogenation (ATH) of ketones catalyzed by these well-designed cobalt(II) complexes was investigated. Among them, cobalt(II) complex containing chiral macrocyclic iminophosphine ligand CyP2N4 exhibited high catalytic activity and enantioselectivity (up to 99% ee). Density functional theory (DFT) calculations suggested that cobalt(II) complex (R,R,R',R')-CyP2N4-Co(II) (C1) is easier to form metal hydride, which is the key intermediate during the enantioselective transfer hydrogenation reaction. Study results also revealed that the unique macrocyclic structure of complex C1 could form a special microenvironment around cobalt ion. Therefrom the substrate coordinated with the central metal along this specific reaction channel during the asymmetric catalytic reaction, resulting high enantioselectivity.