On the validity of Au-vinylidenes in the gold-catalyzed 1,2-migratory cycloisomerization of skipped propargylpyridines

Gold(I)-catalyzed hydroarylation reaction of aryl (3-iodoprop-2-yn-1-yl) ethers: synthesis of 3-iodo-2H-chromene derivatives

An efficient entry to the preparation of elusive 4-unsubstituted-3-iodo-2H-chromenes has been accomplished as result of a catalytic cyclization. Thus, upon exposition of [(3-iodoprop-2-yn-1-yl)oxy]arenes to IPrAuNTf₂ (3 mol %), in 1,4-dioxane at 100 °C, the desired heterocyclic motif is readily assembled. This process nicely tolerates a variety of functional groups and, interestingly, it is compatible with the presence of strong electron-withdrawing groups attached to the arene. The overall transformation can be termed as a new example of a migratory cycloisomerization and, formally, it involves well-blended 1,2-iodine shift and hydroarylation steps.

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.