Development of 4,4′-substituted-xylBINAP ligands for highly enantioselective hydrogenation of ketones

Abstract

(Chemical Equation Presented) A family of 4,4′-substituted-xylBINAPs was synthesized in multistep sequences and characterized by NMR spectroscopy and mass spectrometry. Ru(diphosphine)(diamine)Cl2 complexes based on these 4,4′-substituted-xylBINAPs and chiral diamines (DPEN and DAIPEN) were synthesized by treatment of [(benzene)RuCl2]2 with 4,4′-substituted-xylBINAP followed by chiral diamine, and characterized by 1H and 31P NMR spectroscopy and mass spectrometry. These Ru complexes were used for asymmetric hydrogenation of aromatic ketones in a highly enantioselective manner with complete conversion. With 0.1% catalyst loading, complete conversion and enantioselectivity greater than 99% were obtained for most of the aromatic ketones examined. These Ru catalysts thus gave the highest ee for asymmetric hydrogenation of aromatic ketones among all of the catalysts reported in the literature. A single-crystal X-ray diffraction study of Ru[(R)-L4] [(R,R)-DPEN]Cl2 indicated that the 4-methyl group of the naphthyl ring and the methyl groups of the two xylyl moieties form a fence on the opposite side of the DPEN ligand of the Ru center. These three methyl groups will have significant repulsive interactions with the bulky aryl ring of the hydrogen-bonded aromatic ketone in the disfavored transition state. These results support our hypothesis of combining dual modes of enantiocontrol (i.e., the substituents on 4,4′-positions of the binaphthyl framework and the methyl groups on the bis(xylyl)phosphino moieties) to achieve higher stereoselectivity in the hydrogenation of aromatic ketones.