Revised Mechanism of Gold-Catalyzed Thioallylation of Propiolates

Abstract

Gold-catalyzed enantioselective thioallylation of propiolates proved effective in delivering highly enantio-enriched α-allyl-β-thioacrylates. In this work, we report a revised mechanism for this process based on the new mechanistic experiments and kinetic data in the presence of a competitive inhibitor. The employment of thioethers as nucleophiles inevitably involves their competitive binding to the only catalytic site of the Au(I) catalyst, which may inhibit the activity. We developed a modified Hammett plot in the presence of a dummy thioether inhibitor, which revealed a true kinetic profile, excluding the effect of inhibition. A revised mechanism suggested that the conjugate addition of thioethers to the Au(I)-activated alkynes is the turnover-limiting step, and the subsequent [3,3]-rearrangement occurs quickly, suggesting the efficacy of the sulfonium-based approach in accelerating Claisen rearrangement. In addition, the enantioselectivity was suggested to be determined during the sigmatropic rearrangement by discriminating the prochiral olefin faces of the allyl group in the σ-bound Au(I) complex.