New Bis(oxazolinyl)phenyl−Ruthenium(II) Complexes and Their Catalytic Activity for Enantioselective Hydrogenation and Transfer Hydrogenation of Ketones

Abstract

Synthesis and characterization of ruthenium(II) complexes having a meridional bis(oxazolinyl)benzene (dm-Phebox-R)H (1a: R = ip, 1b: R = ph, 1c: R = dm; ip = isopropyl, dm = dimethyl, ph = phenyl) ligand and their applications to enantioselective hydrogenation and transfer hydrogenation of simple ketones are described. Reaction of (dm-Phebox-R)H with RuCl3·3(H2O) in the presence of Zn and 1,5-cyclooctadiene (cod) afforded a ZnCl4-bridged (Phebox)Ru dimer [(dm-Phebox-R)RuCl(CO)]2(ZnCl2) (2a, 2b, and 2c) in 73%, 58%, and 83% yield, respectively, through C−H bond activation. Treatment of 2a−c with an excess amount of sodium acetylacetonate gave the corresponding acetylacetonato complexes (dm-Phebox-R)Ru(CO)(acac) (3a−c) in high yields. Such acetylacetonato complexes (3) were prepared by a one-pot reaction of 1 with RuCl3·3(H2O) in the presence of Zn and cod under reflux in ethanol, followed by treatment of acetylacetone at room temperature. The reaction of 2 generated in situ with CO (1 atm) also provided th...