Ru()--cymene complexes containing esters of chiral /-phenylalanine derived aroylthiourea ligands for enantioselective reduction of pro-chiral ketones

A series of new chiral aroylthiourea ligands was derived from unprotected d / l -phenylalanine: ( R )/( S )-2-(3-benzoylthioureido)-3-phenylpropanoic acid ( L1 / L2 ), ( R )/( S )-2-(3-(thiophene-2-carbonyl)thioureido)-3-phenylpropanoic acid ( L3 / L4 ) and ( R )/( S )-2-(3-(furan-2-carbonyl)thioureido)-3-phenylpropanoic acid ( L5 / L6 ). Chiral Ru( ii ) complexes ( 1-6 ) were obtained from the reactions between the chiral ligands ( L1-L6 ) and [RuCl 2 ( p -cymene) 2 ] 2 through in situ catalytic esterification of the ligand in the presence of methanol solvent. The ligands and complexes were characterized by analytical and spectral ( 1 H NMR, 13 C NMR, Mass, FT-IR, electronic) techniques. The molecular structure of the ligand L1 showed the presence of an unprotected acid group and that of the representative complexes confirmed the conversion of acid to ester. The X-ray structure of two of the complexes ( 3 and 6 ) revealed the sulfur only monodentate coordination of the aroylthiourea ligands. All the chiral complexes turned out to be efficient catalysts for the enantioselective reduction of aromatic pro-chiral ketones in the presence of 2-propanol and NaOH to produce chiral alcohols in excellent conversions (up to 99%) and enantiomeric excesses (up to 99%) within 10-12 h. The chiral Ru( ii )- p -cymene complexes are efficient catalysts for the enantioselective reduction of ketones to chiral alcohols.