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-((1 R,2 S,5 R )-menthoxy)-1-(2-pyridyl)ethane ( 2R ), ( S )-1-(diphenylphosphino)-2-((1 R,2 S,5 R )-menthoxy)-1-(2-pyridyl)ethane ( 2S ), and ( S )-(phenyl(2-anisyl)phosphino)(2-pyridyl)methane ( 3 ) have been synthesized, as well as the corresponding RuCl 2 (PPh 3 )(L) complexes. The complexes RuCl 2 (PPh 3 )( 2R ) ( 5 ) and RuCl 2 (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 η 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 RuCl 2 (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%.