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

摘要

Abstract: The iridium-catalyzed highly enantioselective hydrogenation of -aryloxy and -alkoxy-substitutedn,u0001-unsaturated carboxylic acids was developed. By using chiral spiro phosphino-oxazoline ligands, thenhydrogenation proceeded smoothly to produce various -aryloxy- and -alkoxy-substituted carboxylic acidsnwith extremely high enantioselectivities (ee up to 99.8%) and reactivities (TON up to 10 000) under mildnconditions. The hydrogenation of -benzyloxy-substituted ,u0001-unsaturated acids provided an efficientnalternative for the synthesis of chiral -hydroxy acids after an easy deprotection. A mechanism involvingna catalytic cycle between IrnInand IrnIIInwas proposed on the basis of the coordination model of the unsaturatednacids with the iridium metal center. The rationality of the catalytic cycle, with an olefin dihydride complexnas the key intermediate, was supported by the deuterium-labeling studies. The X-ray diffraction analysis ofnthe single crystal of catalyst revealed that the rigid and sterically hindered chiral environment created bynthe spiro phosphino-oxazoline ligands is the essential factor that permits the catalyst to obtain excellentnchiral discrimination. A chiral induction model was suggested on the basis of the catalyst structure and thenproduct configuration.