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 CH 2 . 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.