The Suzuki Coupling of Aryl Chlorides in Aqueous Media Catalyzed by in situ Generated Calix[4]arene-Based N-Heterocyclic Carbene Ligands

We report on the use of an in situ system for the catalytic Suzuki cross-coupling reaction of chlorobenzenes and benzeneboronic acid to yield biphenyls. Calix[4]arene-based imidazolium salts were used as precursors of N-heterocyclic carbene ligands and Pd(OAc)2 as the palladium source. In dioxane as the organic medium, the steric demands of the substituents on the calixarene skeleton and on the imidazolium moiety parallel the catalytic activity; the best catalytic results were obtained by using dimesityl- or 2,6-diisopropylphenyl-substituted calixarene-imidazolium salts. Among the bases tested Cs2CO3 and CsF were the most effective. The catalytic protocol established for organic solvents could also be used for the Suzuki cross-coupling reaction in aqueous solution. In dioxane/water (50:50) the activity is nearly the same as in pure dioxane and in pure water, an environmentally friendly and cheap solvent, a 60 % level of reactivity was maintained; here the calixarene ligand precursor 3a showed the highest activity. Nonmacrocyclic compounds used for comparison showed considerably lower catalytic ability proving that calix[4]arene-based imidazolium salts are attractive supramolecular skeletons for the N-heterocyclic carbene ligands used in selective Suzuki cross-coupling reactions of aryl chlorides in aqueous solution. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006)