Asymmetric Hydrogenation of Isoxazolium Triflates with a Chiral Iridium Catalyst

摘要: The iridium catalyst [IrCl(cod)]2 -phosphine-I2 (cod=1,5-cyclooctadiene) selectively reduced isoxazolium triflates to isoxazolines or isoxazolidines in the presence of H2 . iridium-catalyzed hydrogenation proceeded high-to-good enantioselectivity when an optically active phosphine-oxazoline ligand was used. 3-substituted 5-arylisoxazolium salts were transformed into 4-isoxazolines with up 95:5 enantiomeric ratio (e.r.). Chiral cis-isoxazolidines obtained 89:11 e.r., no formation their trans isomers, substrates had a primary alkyl substituent at 5-position. mechanistic studies indicate that hydridoiridium(III) species prefers deliver its hydride C5 atom isoxazole ring. attack leads chiral isoxazolidine via 3-isoxazoline intermediate. Meanwhile, selective 4-isoxazolines, may be obstructed by steric hindrance from 5-aryl substituent.