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.