Gold(I)-Catalyzed Intramolecular [4+2] Cycloadditions of Arylalkynes or 1,3-Enynes with Alkenes: Scope and Mechanism

The cyclizations of enynes substituted at the alkyne gives products of formal [4+2] cyclization
with Au(I) catalysts. 1,8-Dien-3-ynes cyclize by a 5-exo-dig pathway to form hydrindanes. 1,6-Enynes with
an aryl ring at the alkyne give 2,3,9,9a-tetrahydro-1H-cyclopenta[b]naphthalenes by a 5-exo-dig cyclization
followed by a Friedel−Crafts-type ring expansion. A 6-endo-dig cyclization is also observed in some cases
as a minor process, although in a few cases, this is the major cyclization pathway. In addition to cationic
gold complexes bearing bulky biphenyl phosphines, a gold complex with tris(2,6-di-tert-butylphenyl)phosphite
is exceptionally reactive as a catalyst for this reaction. This cyclization can also be carried out very efficiently
with heating under microwave irradiation. DFT calculations support a stepwise mechanism for the
cycloaddition by the initial formation of an anti-cyclopropyl gold(I)-carbene, followed by its opening to form
a carbocation stabilized by a π interaction with the aryl ring, which undergoes a Friedel−Crafts-type reaction.