Enantioselective hydrogenation of alpha-aryloxy and alpha-alkoxy alpha,beta-unsaturated carboxylic acids catalyzed by chiral spiro iridium/phosphino-oxazoline complexes.
The iridium-catalyzed highly enantioselective hydrogenation of alpha-aryloxy and alpha-alkoxy-substituted alpha,beta-unsaturated carboxylic acids was developed. By using chiral spiro phosphino-oxazoline ligands, the hydrogenation proceeded smoothly to produce various alpha-aryloxy- and alpha-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 alpha-benzyloxy-substituted alpha,beta-unsaturated acids provided an efficient alternative for the synthesis of chiral alpha-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.
Li
Shen
S
State Key Laboratory and Institute of Elemento-organic Chemistry, Nankai University,Tianjin 300071, China.
Zhu
Shou-Fei
SF
Xie
Jian-Hua
JH
Song
Song
S
Zhang
Can-Ming
CM
Zhou
Qi-Lin
QL
D002264:Carboxylic Acids / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D018360:Crystallography, X-Ray; D006865:Hydrogenation; D007495:Iridium / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000737:chemistry*; D010080:Oxazoles / Q000737:chemistry*; D010720:Phosphines / Q000737:chemistry*; D013141:Spiro Compounds / Q000737:chemistry*; D013237:Stereoisomerism
D002264:Carboxylic Acids; D008024:Ligands; D009942:Organometallic Compounds; D010080:Oxazoles; D010720:Phosphines; D013141:Spiro Compounds; D007495:Iridium; C044646:phosphine