N-Heterocyclic Carbene Conjugated with Poly(ethylene glycol) for Palladium-Catalyzed Suzuki–Miyaura Coupling in Aqueous Solvents

Here we report a type of N-heterocyclic carbene (NHC)- and phosphine-chelated palladium catalysts with poly(ethylene glycol) (PEG) chain for Suzuki–Miyaura cross-coupling reactions. 1-(2-Diphenylphosphinoferrocenyl)-ethyl-3-imidazolium iodides conjugated with MeO–PEG400 and MeO–PEG750, respectively, have been synthesized and characterized. It was demonstrated that the salts bearing with PEG chain could act as NHC precursors successfully under the catalytic condition to form NHC-supported palladium complexes joined by phosphine. The formed palladium complexes are highly efficient for Suzuki–Miyaura coupling of aryl bromides with phenylboronic acid at the palladium loading of 0.1 mol% in both organic and aqueous solvents. A type of N-heterocyclic carbene (NHC)- and phosphine-chelated palladium catalysts with different length of poly(ethylene glycol) (PEG) chain has been developed and shown to be highly efficient for Suzuki–Miyaura cross-coupling reactions.