The novel water-soluble chiral PNNP-type ligand for the enantioselective reduction of ketones in aqueous media

Abstract

The condensation of o-(diphenylphosphino)benzaldehyde and ( R, R)-1,2-diaminocyclohexane in dichloromethane gives a diiminodiphosphine ligand, which is reduced with excess NaBH 4 in refluxing ethanol to afford the corresponding diaminodiphosphine ligand [( R, R)-C 6P 2(NH) 2]. The novel water-soluble PNNP-type tetradentate diaminodiphosphine ligand [( R,R)-C 6P 2(NH) 2(SO 3Na) 4] has been prepared by the sulfonation of the chiral ligand [( R,R)-C 6P 2(NH) 2] and also characterized by IR, NMR and CD. The water-soluble iridium catalyst is generated in situ from [IrCl(COD)(Ph 3P)] and water-soluble ligand [( R,R)-C 6P 2(NH) 2(SO 3Na) 4] in a mixture solvent of 2-propanol and water. This water-soluble iridium catalytic system has been examined for asymmetric transfer hydrogenation of various aromatic ketones in aqueous media, giving the corresponding optically active alcohols in high yield and excellent enantioselectivity. Even those ketones having a great bulkiness of the alkyl group, such as isobutyrophenone, phenyl cyclohexyl and 1,1-diphenylacetone, are smoothly converted to optically active alcohols in up to 99% ee.