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

摘要

We report on the use of an in situ system for the catalytic Suzuki cross-coupling reaction of chlorobenzenes and ben-zeneboronic 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-di-isopropylphenyl-substituted calixarene-imidazolium salts.Among the bases tested Cs_2CO_3 and CsF were the most effective.The catalytic protocol established for organic solvents could also be used for the Suzuki cross-coupling reac-tion 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.