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

The thermal and acid-catalyzed rearrangement mechanisms of N-methyl-N-nitroanilines were theoretically investigated via density functional theory (DFT) calculations for all possible proposed mechanisms. The results indicate that the thermal rearrangement of N-methyl-N-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, N-methyl-N-nitroanilines are first protonated on the nitrogen atom of their aniline moiety and then generate protonated N-methyl-O-nitroso-N-phenylhydroxylamines through a three-membered spirocyclic oxadiaziridine transition state. The N-protonated N-methyl-O-nitroso-N-phenylhydroxylamines favor homolytic dissociation to generate N-methylaniline cationic radical and nitrogen dioxide complexes, which further combine together and aromatize to afford protonated N-methyl-o-nitroanilines and N-methyl-p-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.