Heck reaction in ionic liquids and the in situ identification of N-heterocyclic carbene complexes of palladium

The Heck reaction was found to proceed markedly more efficiently in the ionic liquid 1-butyl-3-methylimidazolium bromide ([bmim][Br]) than in the analogous tetrafluoroborate salt ([bmim][BF4]). In relation to this observation, 1-butyl-3-methylimidazol-2-ylidene (bmiy) complexes of palladium [PdBr(mu-Br)(bmiy)](2) (1) and [PdBr2(bmiy)(2)] (2) have been isolated in [bmim][Br] but not in [bmim][BF4] under conditions similar to those employed for the Heck reaction. In both the ionic liquid [bmim][Br] and molecular solvents, complex 2 was formed via the intermediate 1. 2 has been shown to be active in the Heck reaction in [bmim][Br] but much less so in [bmim][BF4] due to transformation by the latter into less active species.