Asymmetric Hydrogenation of Isoxazolium Triflates with a Chiral Iridium Catalyst

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. The iridium‐catalyzed hydrogenation proceeded in high‐to‐good enantioselectivity when an optically active phosphine–oxazoline ligand was used. The 3‐substituted 5‐arylisoxazolium salts were transformed into 4‐isoxazolines with up to 95:5 enantiomeric ratio (e.r.). Chiral cis‐isoxazolidines were obtained in up to 89:11 e.r., with no formation of their trans isomers, when the substrates had a primary alkyl substituent at the 5‐position. The mechanistic studies indicate that the hydridoiridium(III) species prefers to deliver its hydride to the C5 atom of the isoxazole ring. The hydride attack leads to the formation of the chiral isoxazolidine via a 3‐isoxazoline intermediate. Meanwhile, in the selective formation of 4‐isoxazolines, hydride attack at the C5 atom may be obstructed by steric hindrance from the 5‐aryl substituent. Optically active isoxazolines or isoxazolidines were obtained with high stereoselectivity by hydrogenation of isoxazolium triflates with a chiral iridium catalyst (see scheme). The undesirable N−O bond cleavage that commonly accompanies the reaction of oxazoles with H2 was completely avoided by using the iridium catalyst.