Transfer hydrogenation reactions catalyzed by chiral half-sandwich Ruthenium complexes derived from Proline

Abstract

Chiral ruthenium half-sandwich complexes were prepared using a chelating diamine made from proline with a phenyl, ethyl, or benzyl group, instead of hydrogen on one of the coordinating arms. Three of these complexes were obtained as single diastereoisomers and their configuration identified by X-ray crystallography. The complexes are recyclable catalysts for the reduction of ketones to chiral alcohols in water. A ruthenium hydride species is identified as the active species by NMR spectroscopy and isotopic labelling experiments. Maximum enantio-selectivity was attained when a phenyl group was directly attached to the primary amine on the diamine ligand derived from proline. Transfer hydrogenation of ketones in water can be carried out using water soluble half-sandwich complexes of ruthenium with chiral ligands derived from proline with better enantioselectivity when phenyl groups are on the proline based chiral ligand.