Thioarylation of\nAlkynes to Generate Dihydrothiopheniums\nthrough Gold(I)/(III)-Catalyzed Cyclization–Cross-Coupling

A thioarylation method is developed for the synthesis\nof 2,3-dihydrothiopheniums\nthrough an electrophilic-cyclization–cross-coupling mechanism,\nharnessing the gold(I)/(III) cycle of the recently developed MeDalPhosAuCl\ncatalyst. Single-crystal X-ray crystal structural analysis of the\ndihydrothiophenium products characterized the antiaddition of the\nsulfur and Csp<sup>2</sup> group to the alkyne and a preference for\n5-endo dig cyclization. The dihydrothiophenium products are demonstrated\nas synthetic building blocks for stereodefined acyclic tetrasubstituted\nalkenes upon ring-opening reaction with amines. Intramolecular competition\nexperiments show the favorability of Csp<sup>3</sup> tether cyclizations\nover Csp<sup>2</sup> tethers, preferentially generating dihydrothiopheniums\nover thiopheniums. Intermolecular competition experiments of alkyne\naryl groups and an intermolecular aryl iodide competition suggest\na rate-determining reductive elimination step in the gold(I)/gold(III)\ncatalytic cycle. This rate-determining step is further supported by\nHRMS analysis of reaction intermediates that identify the catalyst\nresting state under turnover conditions. Catalyst poisoning experiments\nprovide evidence of substrate inhibition, further consistent with\nthese conclusions.