Cyclobutene vs 1,3-Diene Formation in the Gold-Catalyzed Reaction of Alkynes with Alkenes: The Complete Mechanistic Picture

The intermolecular gold­(I)-catalyzed
reaction between arylalkynes
and alkenes leads to cyclobutenes by a [2 + 2] cycloaddition, which
takes place stepwise, first by formation of cyclopropyl gold­(I) carbenes,
followed by a ring expansion. However, 1,3-butadienes are also formed
in the case of ortho-substituted arylalkynes by a
metathesis-type process. The corresponding reaction of alkenes with
aryl-1,3-butadiynes, ethynylogous to arylalkynes, leads exclusively
to cyclobutenes. A comprehensive mechanism for the gold­(I)-catalyzed
reaction of alkynes with alkenes is proposed on the basis of density
functional theory calculations, which shows that the two pathways
leading to cyclobutenes or dienes are very close in energy. The key
intermediates are cyclopropyl gold­(I) carbenes, which have been independently
generated by retro-Buchner reaction from stereodefined 1a,7b-dihydro-1H-cyclopropa­[a]­naphthalenes.