Heck coupling reactions catalysed by Pd particles generated in silica in the presence of an ionic liquid

Silica-supported Pd catalysts were synthesized in the presence of the ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate. Two samples with extremely low Pd loadings, 0.35Pd and 0.08Pd, with Pd contents 0.35 and 0.08%, respectively, were subjected to further investigations. Structural characterization was performed by ICP-AES and Raman measurements. Raman spectra indicated the presence of the ionic liquid in the Pd-silica samples. The samples were tested as catalysts in the Heck coupling reactions of methyl acrylate and styrene, with substituted bromoarenes and chloroarenes. Both samples proved to be highly efficient catalysts and displayed excellent activities and selectivities for the reactions of activated haloarenes including chloroarenes, which could be efficiently transformed without applying harsh reaction conditions. As expected, the presence of an electron withdrawing group (EWG) on the aromatic ring of the haloarene was found to increase both the conversion and the selectivity to an appreciable extent. For the transformations of bromoarenes, the sample with the lowest Pd loading proved to be a more efficient catalyst. Upon recycling of the catalysts, a considerable activity loss was detected, which was attributed to an extensive leaching of Pd into the solution, as confirmed by hot filtration measurements.