Theoretical mechanistic insights on the thermal and acid-catalyzed rearrangements of N -methyl- N -nitroanilines

The thermal and acid-catalyzed rearrangement mechanisms of -methyl- -nitroanilines were theoretically investigated density functional theory (DFT) calculations for all possible proposed mechanisms. The results indicate that the thermal rearrangement of -methyl- -nitroanilines undergoes a radical pair complex mechanism through the homolysis of their N-N bond to generate a radical pair complex and the recombination of the radical pairs followed by aromatization. For the acid-catalyzed rearrangements, -methyl- -nitroanilines are first protonated on the nitrogen atom of their aniline moiety and then generate protonated -methyl- -nitroso- -phenylhydroxylamines through a three-membered spirocyclic oxadiaziridine transition state. The -protonated -methyl- -nitroso- -phenylhydroxylamines favor homolytic dissociation to generate -methylaniline cationic radical and nitrogen dioxide complexes, which further combine together and aromatize to afford protonated -methyl- -nitroanilines and -methyl- -nitroanilines, respectively. The radical pair complexes are more stable than the corresponding solvent-caged radical pairs. The thermal rearrangements require higher activation energy than the corresponding acid-catalyzed rearrangements.