Tris(pentafluorophenyl)borane adducts of substituted imidazoles:: conformational features and chemical behavior upon deprotonation

Tris(pentaflrophenyl) borane adds to the donor nitrogen atom of 1,4,5-trimethyl- and 1-methyl-4,5-diphenylimidazole (5a,b) to yield the corresponding adducts 6a and 6b, respectively. Treatment of 1-methylbenzimidazole with B(C6F5)(3) or B(C6H5)(3) gave the related adducts 6c and 7, respectively. All four adducts were characterized by X-ray diffraction. In solution, the compounds 6 exhibit dynamic F-19 NMR spectra, each featuring 15 separate F-19 NMR resonances at low temperature. With increasing temperature a coalescence of the o-, m- and p-F signals of only a pair of -C6F5 signals is observed, leaving the set of five resonances of the third -C6F5 group unchanged. It required a further increase of the monitoring temperature to eventually observe the coalescence of the respective signals of all three -C6F5 groups. A DFT study revealed no specific intramolecular interactions of the F-C(Ar) substituents with other moieties of the molecules 6; a topological control is thus likely to have caused this remarkably specific dynamic behavior. Deprotonation of the compounds 6a and 6c at carbon atom C2 was achieved by treatment with methyl-lithium. The expected "Arduengo carbene anions" (8) are, however, not stable under the reaction conditions but rapidly react by an intramolecular nucleophilic aromatic substitution at one of the adjacent -C6F5 groups to yield the heterotricyclic products 9. The respective benzimidazole-derived compound 9c was also characterized by an X-ray crystal structure analysis.