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

Abstract

Tris(pentafluorophenyl)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 ¹⁹F NMR spectra, each featuring 15 separate ¹⁹F 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 methyllithium. 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.