Intermolecular [2 + 2] Cycloaddition of Alkyne-Alkene Catalyzed by Au(I) Complexes. What Are the Catalytic Sites Involved?

Bulky Au(I) biphenylphosphine complexes form with phenylacetylene isolable digold complexes under conditions of the room-temperature intermolecular [2 + 2] cycloaddition of phenylacetylene and α-methylstyrene. Single-crystal X-ray diffraction (XRD) of two digold complexes show the presence of Au atoms connected to the C≡C triple bond of a phenylacetylene subunit through a σ and a π bond. The two Au atoms are fluxional and undergo exchange even at −80 °C. These digold complexes exhibit as catalysts almost complete selectivity toward the intermolecular cycloaddition and higher final yield to the corresponding cyclobutene than the corresponding mono Au(I) complex precursor. The difference in selectivity between the commercial mono Au(I) complex and the corresponding digold-phenylacetylene complex was found to be due to the generation of Brönsted acids of the counteranion [HSbF6 or HN(CF3SO2)2 in the cases studied] that are formed by replacement of the C≡C–H by a C≡C–Au bond. This Brönsted acid causes α-methylstyrene dimerization and degradation of the cyclobutene, two processes that do not occur when the reaction is promoted by the digold complex.