Sulfonated N-Heterocyclic Carbenes for Pd-Catalyzed Sonogashira and Suzuki-Miyaura Coupling in Aqueous Solvents

The reactions of the N,N'-diarylimidazolium and N,N'-diarylimidazolinium salts with chlorosulfonic acid result in the formation of the respective disulfonated N-heterocyclic carbene (NHC) precursors in reasonable yields (46-77%). Water-soluble palladium catalyst complexes, in situ obtained from the respective sulfonated imidazolinium salt, sodium tetrachloropalladate (Na(2)PdCl(4)) and potassium hydroxide (KOH) in water, were successfully applied in the copper-free Sonogashira coupling reaction in isopropyl alcohol/water mixtures using 0.2 mol% catalyst loading. The preformed (disulfonatedNHC)PdCl(cinnamyl) complex was used in aqueous Suzuki-Miyaura reactions at 0.1 mol% catalyst loading. The coupling protocol reported here is very useful for Sonogashira reactions of N- and S-heterocyclic aryl bromides and chlorides with aryl- and alkylacetylenes.