Experimental and Computational Evidence for Gold Vinylidenes: Generation from Terminal Alkynes via a Bifurcation Pathway and Facile C–H Insertions

Abstract

Facile cycloisomerization of (2-ethynylphenyl)alkynes is proposed to be promoted synergistically by two molecules of BrettPhosAuNTf₂, 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(sp³)–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.