Gold(I) or Gold(III) as Real Intermediate Species in Gold-Catalyzed Cycloaddition Reactions of Enynal/Enynone?

Metal-bound
isobenzopyrylium species are proposed as reactive intermediates
in transition-metal-catalyzed [3 + 2] or [4 + 2] cyclizations of o-alkynylbenzaldehydes (enynals) or o-alkynylarylketones
(enynones). Because of their high reactivity, isolation and structural
characterization of such intermediates have remained elusive. Herein,
we report the isolation of such gold­(I)- and gold­(III)-bound isobenzopyrylium
intermediates, and two of them have been characterized by X-ray diffraction
analysis. When reacting with reaction partners, such as styrene and
phenyl acetylene, the Au­(I) intermediate exhibited a reaction rate
much faster than that of the Au­(III) species. A degradation pathway
of active Au­(III)–carbene intermediate was observed, leading
to release of Au­(I) species and consequent formation of a dichloride
compound. In other related catalytic reactions, the Au­(I) intermediate
could react well with a range of cyclization partners, whereas the
Au­(III) intermediate did not work. The above observations indicate
that Au­(I) species is probably the real catalyst in such gold-catalyzed
transformations of o-alkynylbenzaldehydes and o-alkynylarylketones.