In Situ Brønsted Acid Activated Cyclometalated N-Heterocyclic Carbene-Au(III)-Sulfonamide Complexes as Precatalysts for Alkyne Activation in Benzofuran Synthesis

Cyclometalated N-heterocyclic carbene (NHC)-NTs-AuCl₂ complexes of type [(NHC)Au(N)Cl₂] are presented as a stable precatalyst, easily activated in situ by an acid (HX) for cationic [(NHC)AuCl₂]X catalysis. The “ethyl-and propyl-tosylamide arm” tethered NHC-Au(III)Cl₃ complexes can be smoothly converted to cyclometalated (-HCl) analogues by K₂CO₃ treatment, and, via the addition of a stoichiometric amount of Brønsted acid (HX), can be further converted to [NHC-Au(III)Cl₂]X carrying various counteranions (X = CCl₃CO₂^–, CF₃CO₂^–, MsO^–, TsO^–, BF₄^–, TfO^–, and Tf₂N^–). We studied this concept in catalysis using the cycloisomerization of 2-alkynylphenol to benzofurans as a probe reaction. Excellent yields were obtained in 2 h with an equimolar 1 mol % loading of the Mes-NHC-propyl-NTs-AuCl₂ precatalyst and MsOH as the acid additive under ambient conditions in CHCl₃. The single crystal structure for a cyclometalated NHCAu(III)-complex was characterized by X-ray analysis. ¹H NMR monitoring of reaction kinetics suggests that the in situ formed Au(III) complex is the catalytically active species.