Cyclization of 1,6-Enynes Catalyzed by Gold Nanoparticles Supported on TiO2: Significant Changes in Selectivity and Mechanism, as Compared to Homogeneous Au-Catalysis

Gold nanoparticles supported on TiO2 (1.2 mol %) catalyze, for the first time under heterogeneous conditions, the cycloisomerization of a series of 1,6-enynes in high yields. In several cases, the product selectivity differs significantly as compared to homogeneous Au(I)-catalysis. Based on product analysis and stereoisotopic studies it is proposed that the major or exclusive pathway involves a 5-exo cyclization mode to form stereoselectively gold cyclopropyl carbenes that undergo a single cleavage pathway, in contrast to homogeneous Au-catalysis where the double cleavage pathway operates substantially.