Cationic Gold Catalysis with Pyridine-Tethered Au(III) NHC-Carbenes: An Experimental and DFT Computational Study

A novel strategy is developed to produce catalytically active cationic Au­(III) species by pyridine-tethered NHC complexes. The catalytic properties of two NHC complexes, equipped with distinct pyridine moieties, were studied for two catalytic benchmark reactions, the cycloisomerization of alkynylfurans to isobenzofuranoles and o-alkynylanilines to indoles. Optimal catalytic results were obtained employing a complex tethered with an electron-rich, p-methoxy-substituted pyridine. The catalysis showed high selectivity, yielding clean product conversions, while the catalytic TONs achieved in the studied reactions are well comparable with the ones reported in the literature for silver salt-activated Au­(I) NHC carbene catalysts. Computational results at the DFT level support the hypothesis that equilibrating ionic complexes are the plausible catalytically active species. Computational inspection of the energetics and molecular orbital involved in the reaction reveals that the ionic state is more favored in the AuCl₃NHC complex equipped with the electron-rich pyridine.