Expanded-Ring N-Heterocyclic Carbenes Efficiently Stabilize Gold(I) Cations, Leading to High Activity in Π-Acid-Catalyzed Cyclizations

A series of six- and seven-membered expanded-ring N-heterocyclic carbene (er-NHC) gold(I) complexes has been synthesized using different synthetic approaches. Complexes with weakly coordinating anions [(er-NHC)AuX] (X-=BF4-, NTf2-, OTf-) were generated in solution. According to their (CNMR)-C-13 spectra, the ionic character of the complexes increases in the order X-=Cl-