Synthesis and characterization of pyrazolyl-functionalized imidazolium-based ionic liquids and hemilabile (carbene)palladium(II) complex catalyzed Heck reaction

Neat reactions of 1-(pyrazolylmethyl)imidazole with an excess of alkyl or polyfluoroalkyl halides at 100 degrees C followed by subsequent metathetical reactions with LiN(SO2CF3)(2) or KPF6, at 25 degrees C gave rise to a series of monoquaternary salts 3a-3k. These salts can be also prepared through treatment of 1-alkylimidazole with 1-(chloromethyl)pyrazole hydrochloride in the presence of base, followed by anion exchange with LiN(SO2CF3)(2) or KPF6. Their phase-transition temperature, thermal stability, density and solubility in common solvents have been investigated. Most of the bis(trifluoromethanesulfon)amide salts are room-temperature ionic liquids. The effect of anions and of alkyl substituents bonded to the imidazolium cation on the physicochemical properties was examined. Using 3-butyl-1-(pyrazolylmethyl)imidazolium chloride (2d), the precursor of 3-butyl-1-(pyrazolylmethyl)imidazolium bis(trifluoromethanesulfon)amide (3d), as a reactant, a hemilabile (N-heterocyclic carbene)palladium(II) complex 4 was synthesized through a (carbene) silver (1) transfer reagent. It was characterized by single-crystal X-ray diffraction analysis. The catalytic activity and recyclability of 4 in 3d were preliminarily evaluated by consecutive Heck reactions using different substrates. (C) Wiley-VCH Verlag GmbH & Co.