Synthesis of Azo Dyes from Mesoionic Carbenes and Nitrous Oxide

Covalent adducts of imidazole-based mesoionic carbenes and nitrous oxide (N₂ O, "laughing gas") can be converted into azo dyes by reaction with arenes in the presence of AlCl₃ or HCl. The azo coupling can be achieved with electron-rich aromatic compounds such as mesitylene, trimethoxybenzene, azulene, or dibenzo-18-crown-6. The latter coupling reaction allows for the easy preparation of a colorimetric sensor for potassium or sodium ions. As a putative intermediate of the reaction with acid, we were able to isolate and structurally characterize a rare diazohydroxide. Using imidazolium compounds with two N₂ O groups, it was possible to prepare amine-substituted azo dyes or dyes with two azoarene groups attached to the heterocycle. A triazole-based mesoionic carbene was also found to form a stable covalent adduct with N₂ O. The adduct could be used to prepare novel azo triazolium dyes in good yields.