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, 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 RuCl 3 ·3(H 2 O) in the presence of Zn and 1,5-cyclooctadiene (cod) afforded a ZnCl 4 -bridged (Phebox)Ru dimer [( dm -Phebox- R )RuCl(CO)] 2 (ZnCl 2 ) ( 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 RuCl 3 ·3(H 2 O) 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).