Methoxycarbonylation of iodobenzene in ionic liquids. A case of inhibiting effect of imidazolium halides

The palladium(II) complexes, PdCl2(cod) (1), PdCl2[P(OPh)(3)](2) (2), [bmim](2)[PdCl4] (3), and [bmpy](2)[PdCl4] (4) (bmim = 1-butyl-3-methylimidazolium cation, bmpy = 1-butyl-4-methylpyridinium cation), were found to be active catalysts for the methoxycarbonylation of iodobenzene in ionic liquid (IL) media. The best results were obtained in pyridinium salts. [bmpy]X (X = Cl, Br, BF4, PF6) (76-100% yield). In methoxycarbonylation reactions carried out in imidazolium salts, [bmim]BF4 and [bmim]PF6, the yield of benzoic acid methyl ester was slightly lower (50-78% yield), whereas in [bmim]X (X = Cl, Br) the reaction was totally retarded. The inhibiting effect was not observed when a Me group was present at C-2 (instead of a proton) on the imidazole ring. A palladium-carbene complex, Pd(bmimy)(2)Br-2 (5), obtained in the reaction of a palladium catalyst precursor with [bmim]Cl, presented much lower activity than precursors 1-4. Its formation may explain the observed inhibition of methoxycarbonylation in a [bmim]Cl medium.