Palladium-Catalyzed Suzuki Cross-Coupling Reactions in Ambient Temperature Ionic Liquids: Evidence for the Importance of Palladium Imidazolylidene Complexes

Palladium-catalyzed Suzuki cross-coupling reactions have been conducted in a range of ambient temperature ionic liquids. The ionic liquids give excellent reactivities for bromoarenes in addition to easy product isolation. The preferred reaction conditions are explored and the effect of changing the ionic liquid components is investigated. The in situ formation of mixed phosphine/imidazolylidene palladium complexes is demonstrated in all of the catalytically active solutions, but in none of the inactive solutions. Formation of 1,3-dialkyl-2-arylimidazolium salts is demonstrated, but it is also shown that these salts can act as a source of arene for the reaction; that is, both reductive elimination and oxidative addition of 1,3-dialkyl-2-arylimidazolium salts from/to the palladium are occurring. The implications of this for the use of ionic liquids as solvents for Suzuki reactions are discussed.