Journal of Molecular Catalysis A-Chemical, Vol.339, No.1-2, 99-107, 2011
Theoretical mechanistic study of TangPhos-catalyzed asymmetric gamma addition of thiols to allenoates
TangPhos-catalyzed asymmetric gamma addition of thiols to allenoates has been investigated according to density functional theory. The uncatalyzed addition occurs at beta-carbon via a process which involves C-S bond formation and proton transfer from S to gamma-carbon. The beta-thioester is generated. In TangPhos-catalyzed case, the nucleophilic thiol attacks gamma-carbon after the addition of TangPhos to beta-carbon. The proton transfers firstly from P of TangPhos to carbonyl 0 and then to beta-carbon. The gamma-thioester is obtained. Step1 is rate-limiting. As nucleophilic catalyst, P2 forms strong covalent bond with beta-carbon which shifts the positive charge of C2, leaving C3 as the electrophilic center for gamma addition. The regioselectivity is consequently altered. As Lewis base, P1 deprotonates thiol enhancing the nucleophility of S and facilitates the proton transfer to beta-carbon as a medium. Among four competitive pathways, ER path is the most favorable one with smallest rotation of the single bond linking two chiral rings in TangPhos. The primary domination on enantioselectivity of chiral rings is assisted by t-butyl group, which also prefers ER path with the least steric hindrance. Our conclusion is supported by NBO analysis and the predicted ee values according to the experiment. (C) 2011 Published by Elsevier B.V.