Suzuki Coupling Reactions in Ether-Functionalized Ionic Liquids: The Importance of Weakly Interacting Cations
Imidazolium- and pyridinium-based ionic liqs. with ether/polyether substituents have been evaluated as solvents for palladium-catalyzed Suzuki C-C coupling reactions. In general, reactions proceed more efficiently in these solvents compared to other ionic liqs., which is believed to be due to better stabilization of the palladium catalyst, involving weak interactions with the ether groups. The position and the no. of oxygen atoms in the ether side chain strongly influence the outcome of the coupling reactions in the imidazolium-based ionic liqs., whereas for the pyridinium-based liqs. no influence is obsd. Carbene derivs., generated from the imidazolium-based ionic liqs., are believed to play a role by terminating the catalytic cycle, and representative species have been isolated and characterized from stoichiometric reactions.