4-Halomethyl-Substituted Imidazolium Salts: A Versatile Platform for the Synthesis of Functionalized NHC Precursors.

N,N'-Diarylimidazolium salts containing haloalkyl functional groups that are reactive with various nucleophiles are considered to be promising reagents for the preparation of functionalized N-heterocyclic carbene (NHC) ligands, which are in demand in catalysis, materials science, and biomedical research. Recently, 4-chloromethyl-functionalized N,N'-diarylimidazolium salts became readily available via the condensation of N,N'-diaryl-2-methyl-1,4-diaza-1,3-butadienes with ethyl orthoformate and MeSiCl, but these compounds were found to have insufficient reactivity in reactions with many nucleophiles. These chloromethyl salts were studied as precursors in the synthesis of bromo- and iodomethyl-functionalized imidazolium salts by halide anion exchange. The 4-ICH-functionalized products were found to be unstable, whereas a series of novel 4-bromomethyl functionalized N,N'-diarylimidazolium salts were obtained in good yields. These bromomethyl-functionalized imidazolium salts were found to be significantly more reactive towards various N, O and S nucleophiles than the chloromethyl counterparts and enabled the preparation of previously inaccessible heteroatom-functionalized imidazolium salts, some of which were successfully used as NHC proligands in the preparation of Pd/NHC and Au/NHC complexes.