Gold versus Palladium: A Regioselective Cycloisomerization of Aromatic Enynes

Aromatic enynes can be transformed into arylnaphthalenes or benzofulvenes depending on the reaction conditions. Under gold(I) catalysis, exclusive or major 6-endo-dig cyclization took place leading to arylnaphthalenes. However, a catalytic system based on palladium iodide/1,3-bis(diphenylphosphino)propane, in the presence of cesium carbonate as a base was necessary to furnish exclusively 5-exo-dig cyclization pattern, regardless of the electronic effects of the substituents. In the latter transformation, a mechanistic study (kinetic isotopic effect, density functional theory) involving a CH activation is suggested for the exclusive formation of benzofulvenes.