Oxidation and acid‐catalyzed cyclization of aldehyde 2‐aminophenylhydrazones. Alternative syntheses for 1,2,4‐benzotriazines and benzimidazoles

Aldehyde 2‐aminophenylhydrazones are easily oxidized by air to 3‐substituted 1,2,4‐benzotriazines. In two cases the intermediate 3,4‐dihydro‐1,2,4‐benzotriazines were isolated. In the absence of oxygen, acids catalyze the formation of 2‐substituted benzimidazoles with loss of ammonia. It is suggested that both reactions proceed via 1,2,3,4‐tetrahydro‐1,2,4‐benzotriazines (III), formed by addition of the amino group on the hydrazone double bond. Such cyclic forms (III) predominate over the open chain structures (II) in the cases of aliphatic and arylalkylaldehyde 2‐aminophenylhydrazones, as supported by nmr spectra, while benzaldehyde 2‐aminophenylhydrazone exists mainly or exclusively in the open chain form (II).