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.