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

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, din = 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 center dot 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 center dot 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 the corresponding dicarbonyl chloro Ru complexes (dm-Phebox-R)RuCl(CO)(2) (4) in moderate yields. New (Phebox)Ru complexes were found to exhibit catalytic activity toward hydrogenation of simple aromatic ketones in the presence of NaOMe under 30 atm of a hydrogen atmosphere to provide the corresponding secondary alcohols with high enantioselectivity (up to 98% ee). Transfer hydrogenation of bulky ketones was also catalyzed by 2 and 3 with high enatioselectivity (up to 97% ee).