Ruthenium(II) complexes containing optically active hemilabile P,N,O-tridentate ligands. Synthesis and evaluation in catalytic asymmetric transfer hydrogenation of acetophenone by propan-2-ol

The trifunctional ligands (R)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (2R), (S)-1-(diphenylphosphino)-2-((1R,2S,5R)-menthoxy)-1-(2-pyridyl)ethane (25), and (S)-(phenyl(2-anisyl)phosphino)(2-pyridyl)methane (3) have been synthesized, as well as the corresponding RuCl2(PPh(3))(L) complexes. The complexes RuCl2(PPh(3))(2R) (5) and RuCl2(PPh(3))(2S) (6) were isolated as mixtures of two isomers, 5a and 5b and 6a and 6b, respectively. In each of these isomers, the ligands 2 are eta(3)-(P,N,O) bound. They differ by the position of the triphenylphosphine, which is either in a trans position relative to the pyridyl ring (5a or 6a) or in a trans position relative to the ether function (5b or 6b). Variable temperature NMR experiments have shown that the hemilabile character in solution of the ligands 2 is through their pyridyl arm in the isomers a or their ether arm in b. The complexes 6b and RuCl2(PPh(3))(3) (9) were characterized by X-ray diffraction. The complexes 5, 6, and 9 are very active catalysts for the transfer hydrogenation of acetophenone by propan-2-ol in basic media, the higher activity being observed for 9 (turnovers frequency 48 900 h(-1)). The enantioselectivity is modest and dependent on the reactions conditions. The best result has been observed for 5, with an ee of 60%.