Thermal Rearrangements of (Arylimino)diaziridines by Simultaneous Cascades of Pericyclic Reactions

(Arylimino)diaziridines rearrange in several cascade reactions at temperatures 60–100 °C. Those that possess unoccupied ortho positions yield fluorescent 3‐amino‐2H‐indazoles and 2‐amino‐1H‐benzimidazoles. If both ortho positions are blocked by methyl groups, indazoles are not formed and deeply yellow 2‐imino‐2,3‐dihydro‐3aH‐benzimidazoles are formed, which partly dimerize through Diels–Alder reaction or regenerate the aromatic system by formal loss of CH2. In addition, one of the methyl groups of 2,6‐dimethylphenyl rings is involved in a [1,7] H shift affording orthoquinonoid intermediates which undergo 1,6‐electrocyclization to furnish 2‐amino‐3,4‐dihydroquinazolines. The formation of five‐membered ring heterocycles is interpreted in terms of valence isomerization by [1,3] N shift to yield elusive 1‐aryl‐3‐iminodiaziridines as first step. These immediately experience triaza‐Cope rearrangement to benzimidazole derivatives or electrocyclic opening of the N–C bond to generate conjugated azomethine imines (1,5‐dipoles), followed by their 1,5‐electrocyclization to indazoles. First‐order rate constants of the decay of (arylimino)diaziridines refer to the [1,3] N shifts as rate‐determining steps. They are larger than the corresponding rate constants for alkylsubstituted iminodiaziridines. Depending on the substituents of the aryl rings, thermal rearrangements of (arylimino)diaziridines yield 3‐amino‐2H‐indazoles, 2‐amino‐1H‐benzimidazoles, 2‐imino‐2,3‐dihydro‐3aH‐benzimidazoles, 2‐amino‐3,4‐dihydroquinazolines, and isocyanides plus azo compounds.