Enantioselective Hydrogenation of α-Aryloxy and α-Alkoxy α,β-Unsaturated Carboxylic Acids Catalyzed by Chiral Spiro Iridium/Phosphino-Oxazoline Complexes

Abstract

The iridium-catalyzed highly enantioselective hydrogenation of α-aryloxy and α-alkoxy-substituted α,β-unsaturated carboxylic acids was developed. By using chiral spiro phosphino-oxazoline ligands, the hydrogenation proceeded smoothly to produce various α-aryloxy- and α-alkoxy-substituted carboxylic acids with extremely high enantioselectivities (ee up to 99.8%) and reactivities (TON up to 10 000) under mild conditions. The hydrogenation of α-benzyloxy-substituted α,β-unsaturated acids provided an efficient alternative for the synthesis of chiral α-hydroxy acids after an easy deprotection. A mechanism involving a catalytic cycle between Ir^I and Ir^III was proposed on the basis of the coordination model of the unsaturated acids with the iridium metal center. The rationality of the catalytic cycle, with an olefin dihydride complex as the key intermediate, was supported by the deuterium-labeling studies. The X-ray diffraction analysis of the single crystal of catalyst revealed that the rigid and sterically hindered chiral environment created by the spiro phosphino-oxazoline ligands is the essential factor that permits the catalyst to obtain excellent chiral discrimination. A chiral induction model was suggested on the basis of the catalyst structure and the product configuration.