Piano-stool Ru(II)-benzene complexes bearing D/L-alanine derived chiral aroylthiourea ligands for asymmetric transfer hydrogenation of ketones in water

Abstract

The new water soluble chiral Ru(II)-benzene complexes of the type [ $$\hbox {RuCl}_{2}(\eta ^{6}\hbox {-C}_{6}\hbox {H}_{6})\hbox {L}$$ ] were obtained from the reactions between $$[\hbox {RuCl}_{2}(\eta ^{6}\hbox {-C}_{6}\hbox {H}_{6})]_{2 }$$ and the chiral aroylthiourea ligands (L) derived from unprotected D/L-alanine and characterized. The solid-state structure of representative complexes was confirmed by single crystal X-ray diffraction technique. The Ru(II)-benzene complexes catalyzed the asymmetric transfer hydrogenation (ATH) of aromatic ketones to their enantiopure secondary alcohols. The reactions were carried out in the presence of formic acid–triethylamine mixture in water, and the product alcohols were obtained with excellent conversions (up to 99%) and enantiomeric excesses (up to 99%). The scope of the catalytic system was extended to various aromatic ketones. The catalytic activity of the present water-soluble Ru-benzene complexes toward enantioselective reduction of ketones was considerably higher than that of p-cymene analogues in water. The water-soluble chiral Ru(II)-benzene complexes were produced from the reactions between $$[\hbox {RuCl}_{2}(\eta ^{6}\hbox {-C}_{6}\hbox {H}_{6})]_{2 }$$ and the chiral aroylthiourea ligands derived from unprotected D/L-alanine. The catalytic activity of the Ru(II)-benzene complexes toward enantioselective reduction of ketones was found to be good in water medium.