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Isolable sulfonium-ylide stabilized palladium carbene complexes were synthesized through palladium(II)-induced cyclization of 1,2-alkynylarylsulfanes. X-ray crystallographic analysis characterized the maintenance of a palladium +2 oxidation state, with carbene-carbon–palladium bond lengths of 1.95 Å, indicating partial double bond character. These endocyclic sulfonium ylide carbenes represent the first characterized and/or isolable examples of this ligand class; such groups were previously proposed as reaction intermediates during cyclization–carbonylation reactions. A variety of palladium(II) complexes bearing sulfonium ylide carbene ligands, with differing substituents, were synthesized and the structure and stability of these complexes in solution were analyzed by 1H and 13C NMR spectroscopy, revealing reversibility and a stability dependence on substituents. The chirality of the sulfur heteroatom and the overall properties these ligands provide a potential electronic and steric alternative to existing carbene ligands, which could facilitate the future development of complementary metal-based reactivity. 

A thioarylation method is developed for the synthesis of 2,3-dihydrothiopheniums through an electrophilic-cyclization–cross-coupling mechanism, harnessing the gold(I)/(III) cycle of the recently developed MeDalPhosAuCl catalyst. Single-crystal X-ray crystal structural analysis of the dihydrothiophenium products characterized the anti-addition of the sulfur and Csp2 group to the alkyne and a preference for 5-endo dig cyclization. The dihydrothiophenium products are demonstrated as synthetic building blocks for stereodefined acyclic tetrasubstituted alkenes upon ring-opening reaction with amines. Intramolecular competition experiments show the favorability of Csp3 tether cyclizations over Csp2 tethers, preferentially generating dihydrothiopheniums over thiopheniums. Intermolecular competition experiments of alkyne aryl groups and an intermolecular aryl iodide competition suggest a rate-determining reductive elimination step in the gold(I)/gold(III) catalytic cycle. This rate-determining step is further supported by HRMS analysis of reaction intermediates that identify the catalyst resting state under turnover conditions. Catalyst poisoning experiments provide evidence of substrate inhibition, further consistent with these conclusions.