Synthesis and characterization of carbometallated palladacycles over 3-hydroxypropyltriethoxysilyl-functionalized MCM-41

Heterogeneous carbometallated palladacycle catalysts have been synthesized by treatment of palladation reagents with 3-hydroxypropyltriethoxysilane-incorporated MCM-41 (OH-M) under mild reaction conditions. Palladation over organo-functionalized MCM-41 and organo-functionalized silica has been performed and it was found that the spatial confinement induced by the pore walls of organo-functionalized MCM-41 forces the propyl group to cyclize in the presence of an electrophilic metal center which is found to be much lower in organo-functionalized silica. Optimization studies, including change in Pd concentration in solution, different palladation reagents in palladation reactions such as Li2PdCl4, Pd(OAc)2, effect of base, palladation temperature, and different solvents (methanol, chloroform, and acetone used as reaction medium), have been performed over organo-functionalized MCM-41 for obtaining better results of palladation. A number of characterization techniques have been exploited to support the formation of a palladacycle complex inside the pores of mesoporous materials. The Heck alkenylation reaction, which is universally accepted as a sharpening stone of palladium catalysts, was set as a model reaction to evaluate the catalytic activity of all the catalysts investigated in this study. The heterogeneity of the catalyst has also been studied.