Intermolecular [2 + 2] cycloaddition of alkene catalyzed by Au(I) complexes. What are the catalytic sites involved?

Abstract

: Bulky Au(I) biphenylphosphine complexes form with phenylacetylene isolable digold complexes under conditions of the room-temperature intermolecular [2 + 2] cycloaddition of phenylacetylene and ¿-methylstyrene. Singlecrystal X-ray di¿raction (XRD) of two digold complexes show the presence of Au atoms connected to the CtC triple bond of a phenylacetylene subunit through a¿ and a ¿ bond. The two Au atoms are ¿uxional and undergo exchange even at 80 C. These digold complexes exhibit as catalysts almost complete selectivity toward the intermolecular cycloaddition and higher¿nal yield to the corresponding cyclobutene than the corresponding mono Au(I) complex precursor. The di¿erence in selectivity between the commercial mono Au(I) complex and the corresponding digold-phenylacetylene complex was found to be due to the generation of Br¿onsted acids of the counteranion [HSbF6 or HN(CF3 SO2 ) 2 in the cases studied] that are formed by replacement of the CtC H by a CtC Au bond. This Br¿onsted acid causes¿-methylstyrene dimerization and degradation of the cyclobutene, two processes that do not occur when the reaction is promoted by the digold complex.