Dispersed Gold Nanoparticles Supported in the Pores of Siliceous Mesocellular Foam: A Catalyst for Cycloisomerization of Alkynoic Acids to [gamma]-Alkylidene Lactones

A versatile approach for the production of dispersed thiol-stabilized gold nanoparticles in the pores of siliceous mesocellular foam (MCF) is described. The reported method is based on an electrochemical oxidation of a gold surface generating oxidative AuIII species, which give rise to a surface-confined redox reaction yielding MCF-supported AuI thiolates. By reducing the corresponding AuI-S-MCF species with sodium borohydride, thiol-stabilized gold nanoparticles in the size range of 1-8 nm were obtained as determined by transmission electron microscopy. Elemental analysis indicated an Au loading of 3% (w/w) on the MCF. The surface-confined Au nanoparticles were used to catalyze the cycloisomerization of alkynoic acids to the corresponding [gamma]-alkylidene lactones in high efficiency and complete 5-exo-dig selectivity under mild reaction conditions.