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 Cs 2 CO 3 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)