A simple approach to pyrrolylimidazole derivatives by azirine ring expansion with imidazolium ylides†
A domino reaction of 2H-azirines with 1-alkyl-3-phenacyl-1H-imidazolium bromides in the presence of Et3N provides a facile route to 1-alkyl-3-(1H-pyrrol-3-yl)-1H-imidazol-3-ium bromides. 1-Benzyl derivatives can be reduced to 1-(1H-pyrrol-3-yl)-1H-imidazoles with HCO2NH4 on Pd/C. The action of KOH on pyrrolylimidazolium salts leads to a new type of stable ylide, 3-(1H-imidazol-3-ium-3-yl)-pyrrol-1-ides, which can, in principle, be in tautomeric equilibrium with the corresponding N-heterocyclic carbene. Although, according to the DFT B3LYP/6-31G(d) calculations in vacuo, electron-donating substituents in the 2-aryl-group cause the tautomeric equilibrium to shift to the carbene side, the investigated compounds exist in the ylide form in solution and in the solid state, which is in agreement with the relative stabilities of the species calculated with the PCM solvent model.