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

摘要

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 based on 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. The 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 the gold vinylidene formation.