Synthesis and Crystal Structures of Ru(II) Complexes Containing Chelating (Phosphinomethyl)oxazoline P,N-Type Ligands and Asymmetric Catalytic Transfer Hydrogenation of Acetophenone in Propan-2-ol

A series of new Ru(II) complexes containing two chelating P, N ligands of the type (2-oxazolin-2-ylmethyl)diphenylphosphine (PCH 2 -oxazoline I ), (2-oxazolin-4,4-dimethyl-2-ylmethyl)diphenylphosphine (PCH 2 -oxazoline-Me 2 II ), and (2-oxazolin-4-( S )-isopropyl-ylmethyl)diphenylphosphine (PCH 2 -oxazoline- i Pr III ) have been prepared and characterized by IR, 1 H, 13 C{ 1 H}, and 31 P{ 1 H} NMR spectroscopy and by X-ray diffraction studies in the cases of cis, cis, cis -[RuCl 2 (PCH 2 -oxazoline) 2 ] ( 1 ) and cis, cis, trans -[RuCl 2 (PCH 2 -oxazoline-Me 2 ) 2 ] ( 3 ). Four different types of octahedral coordination geometries are found for these complexes depending on the ligand. With I, cis, cis, cis and trans, trans, trans geometries were characterized in complexes 1 and 2, respectively, whereas with ligand II only the cis, cis, trans isomer 3 was formed. Reaction of the enantiomerically pure ligand III with [RuCl 2 (COD)] n afforded a mixture of isomers, and complex trans, cis, cis- [RuCl 2 (PCH 2 -oxazoline- i Pr) 2 ] 4 was fully characterized. This complex was used for catalytic transfer hydrogenation of acetophenone in propan-2-ol and led to 96% conversion and 72% ee (3 h, 82 °C, [ketone] = 0.1 M, Ru:ketone:base = 1:200:5). For comparison the crude mixture of complexes gave a higher turnover frequency but a lower enantioselectivity than pure 4 .