Deconstructing Selectivity in the Gold-Promoted Cyclization of Alkynyl Benzothioamides to Six-Membered Mesoionic Carbene or Acyclic Carbene Complexes

We demonstrate that the experimentally observed switch in selectivity from 5-exo-dig to 6-endo-dig cyclization of an alkynyl substrate, promoted by AuI and AuIII complexes, is connected to a switch from thermodynamic to kinetic reaction control. The AuIII center pushes alkyne coordination toward a single Au–C­(alkyne) σ-bond, conferring carbocationic character (and reactivity) to the distal alkyne C atom.