3‐Methylcarboxy‐1H‐indazole. Theoretical study of its formation via intramolecular aliphatic diazonium coupling and x‐ray crystal structure

The methyl ester of 1H‐indazole‐3‐carboxylic acid (2‐Me) is formed in the diazotization of o‐aminophenylacetic acid to o‐diazoniumphenylacetic acid (1) in an intramolecular aliphatic diazonium coupling. 2‐Me was identified and characterized by single‐crystal x‐ray diffraction and found to crystallize as hydrogen‐bonded trimers of crystallographically independent molecules. The methylcarboxy groups are rotated slightly out of the best plane of the trimer resulting in only a pseudo‐threefold axis. The crystal structure of 2‐Me is compared with other indazoles and pyrazole. Regioselective electrophilic diazonium ion addition to the enol tautomer of 1 and subsequent hyrazone–azo tautomerization are proposed as a possible mechanism for indazole formation under acidic conditions. The tautomerization equilibrium between acetic acid and its enol 1,2‐dihydroxyethene was studied and the effects of phenyl and o‐diazoniumphenyl substitution on this equilibrium were explored with semi‐empirical quantum mechanical methods. The performance of the semi‐empirical method was assessed by comparison with ab initio and/or experimental data. It was found that the enol of o‐diazoniumphenylacetic acid is stabilized greatly owing to extended conjugation and push–pull interactions in the enol form. These results suggest that the enol tautomer might be a viable reactive intermediate that warrants considerations in discussions of reaction mechanisms.