Zwitterionic Palladium Complexes: Room-Temperature Suzuki-Miyaura Cross-Coupling of Sterically Hindered Substrates in an Aqueous Medium

A series of new imidazolium chlorides were straightforwardly prepared from the reactions between chloroacetone and imidazole derivatives. Deprotonation of the methylene proton next to the ketone group in these salts by pyridine led to the formation of a monodentate ligand that coordinated to palladium, readily forming zwitterionic anionic palladium pyridine complexes bearing a formal positive charge on the ligand ancillary. The pyridine ligand in the zwitterionic complexes can be facilely replaced by phosphine ligands. Seven of these new complexes were successfully characterized by X-ray crystallography. The zwitterionic phosphine complexes were highly efficient in catalyzing room-temperature SuzukiMiyaura reactions between sterically hindered aryl chlorides and arylboronic acids in an aqueous medium.