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 (PCH2-oxazoline I), (2-oxazolin-4,4-dimethyl-2-ylmethyl)diphenylphosphine (PCH2-oxazoline-Me-2 II), and (2-oxazolin-4-(S)-isopropyl-ylmethyl)diphenylphosphine (PCH2-oxazoline-Pr-i III) have been prepared and characterized by IR, H-1, C-13(H-1), and P-31(H-1) NMR spectroscopy and by X-ray diffraction studies in the cases of cis,cis,cis-[RuCl2(PCH2-oxazoline)(2)] (1) and cis,cis,trans-[RuCl2(PCH2-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 IT only the cis,cis;trans isomer 3 was formed. Reaction of the enantiomerically pure ligand III with [RuCl2-(COD)](n) afforded a mixture of isomers, and complex trans,cis,cis-[RuCl2(PCH2-oxazoline-Pr-i)(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 degrees 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.