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.