Theoretical mechanistic study of TangPhos-catalyzed asymmetric γ addition of thiols to allenoates

Abstract

TangPhos-catalyzed asymmetric γ addition of thiols to allenoates has been investigated according to density functional theory. The uncatalyzed addition occurs at β-carbon via a process which involves C–S bond formation and proton transfer from S to γ-carbon. The β-thioester is generated. In TangPhos-catalyzed case, the nucleophilic thiol attacks γ-carbon after the addition of TangPhos to β-carbon. The proton transfers firstly from P of TangPhos to carbonyl O and then to β-carbon. The γ-thioester is obtained. Step1 is rate-limiting. As nucleophilic catalyst, P2 forms strong covalent bond with β-carbon which shifts the positive charge of C2, leaving C3 as the electrophilic center for γ addition. The regioselectivity is consequently altered. As Lewis base, P1 deprotonates thiol enhancing the nucleophility of S and facilitates the proton transfer to β-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.