Synthesis of rhodium(I) and iridium(I) complexes of chiral N-heterocyclic carbenes and their application to asymmetric transfer hydrogenation

Abstract

Rhodium and iridium complexes of chiral NHC–phenolimine and NHC–amine ligands have been prepared and studied for asymmetric transfer hydrogenation. X-ray and NMR spectroscopy show that for NHC–phenolimine complexes abstraction of chloride results in a change in ligand coordination from NHC only to chelating NHC–imine. Complexes of NHC–amines are inactive for transfer hydrogenation, whereas complexes of NHC–phenolimines are active at room temperature for a range of aryl containing ketones. Enantioselectivity is very sensitive to the NHC N-substituent resulting in a switch in the predominant enantiomer.