Experimental and computational evidence for gold vinylidenes: generation from terminal alkynes via a bifurcation pathway and facile C-H insertions.

Facile cycloisomerization of (2-ethynylphenyl)alkynes is proposed to be promoted synergistically by two molecules of BrettPhosAuNTf(2), affording tricyclic indenes in mostly good yields. A gold vinylidene is most likely generated as one of the reaction intermediates on the basis of both mechanistic studies and theoretical calculations. Different from the well-known Rh, Ru, and W counterparts, this novel gold species is highly reactive and undergoes facile intramolecular C(sp(3))-H insertions as well as O-H and N-H insertions. The formation step for the gold vinylidene is predicted theoretically to be complex with a bifurcated reaction pathway. A pyridine N-oxide acts as a weak base to facilitate the formation of an alkynylgold intermediate, and the bulky BrettPhos ligand in the gold catalyst likely plays a role in sterically steering the reaction toward formation of the gold vinylidene.