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 tetraalkylammo- Introduction A problem has emerged in today’s environmentally conscious world, namely that many of the solvents traditionally used in organic reactions, such as chlorinated hydrocarbons, acetonitrile, DMF, DMA and NMP, to name but a few, are currently on the “environmental blacklist”. Furthermore, toxic and air-sensitive phosphanes are commonly used as ligands in transition-metal-catalysed reactions, therefore most industrial organic syntheses need rethinking. An approach to resolve these drawbacks may be the replacement of traditional solvents with ionic liquids (ILs), together with the discovery of new ligands for the metals. The advantages of using ILs would be manifold: in addition to facilitating recovery of the catalyst and reaction products, they exhibit high thermal and air stability and have a negligible vapour pressure. For these reasons, ILs are nowadays finding an increasing use for different reactions.[1] However, few studies have been dedicated to exploring the influence of different ILs on the rate and selectivity of organic reactions. The objective of our research concerns the study of the effects exerted by ILs on the rate and selectivity of C–C coupling reactions. [a] Department of Chemistry, University of Bari, Via Orabona 4, 70126 Bari, Italy [b] Lab. for Repressione frodi alimentari, Via Cavedone, 18, 41100 Modena, Italy E-mail: calo@chimica.uniba.it MICROREVIEWS: This feature introduces the readers to the authors’ research through a concise overview of the selected topic. Reference to important work from others in the field is included. Eur. J. Org. Chem. 2006, 3791–3802 © 2006 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim 3791 nium 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.