Effects of Ionic Liquids on Pd-Catalysed Carbon-Carbon Bond Formation

A comparative study of the effects exerted by different ionic liquids on catalyst stability, reaction rates and regio‐ and stereoselectivity in carbon–carbon coupling reactions points to the superiority of the quaternary ammonium halides over pyridinium‐ and imidazolium‐derived ionic liquids. The influence of ionic liquids on the Heck and Suzuki reactions emerges as being associated with the Coulombic interaction between the cations and anions in ILs as well as the nucleophilicity of the anions. In the tetraalkylammonium halides and acetate, the bulkiness of tetrahedral tetraalkylammonium cations, which forces the anions away from the cation, renders these anions available for a good activity and stability of the palladium catalysts. On the contrary, the planar structures of imidazolium and pyridinium cations, due to a strong Coulombic interaction that binds the anions tightly, decrease their availability for stabilisation and activity of the catalysts.(© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)