Methoxycarbonylation of Iodobenzene in Ionic Liquids. A Case of Inhibiting Effect of Imidazolium Halides

The palladium(II) complexes, PdCl 2 (cod) (1), PdCl 2 [P(OPh) 3 ] 2 (2), [bmim] 2 [PdCl 4 ] (3), and [bmpy] 2 [PdCl 4 ] (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, BF 4 , PF 6 ) (76-100% yield). In methoxycarbonylation reactions carried out in imidazolium salts, [bmim]BF 4 and [bmim]PF 6 , 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.