Experimental and theoretical characterization of the aromatization, epimerization, and fragmentation reactions of bi-2H-azirin-2-yls prepared from 1,4-diazidobuta-1,3-dienes.

1,4-Diazidobuta-1,3-dienes (Z,Z)-10, 17, and 21 were photolyzed and thermolyzed to yield the pyridazines 13, 20, and 23, respectively. To explain these aromatic final products, the generation of highly strained bi-2H-azirin-2-yls 12, 19, and 22 and their valence isomerization were postulated. In the case of meso- and rac-22, nearly quantitative formation from diazide 21, isolation as stable solids, and complete characterization were possible. On the thermolysis of 22, aromatization to 23 was only a side reaction, whereas equilibration of meso- and rac-22 and fragmentation, which led to alkyne 24 and acetonitrile, dominated. Prolonged irradiation of 22 gave mainly the pyrimidine 25. The change of the configuration at C-2 of the 2H-azirine unit was observed not only in the case of bi-2H-azirin-2-yls 22 but also for simple spirocyclic 2H-azirines 29 at a relatively low temperature (75 °C). The fragmentation of rac-22 to give alkyne 24 and two molecules of acetonitrile was also studied by high-level quantum chemical calculations. For a related model system 30 (methyl instead of phenyl groups), two transition states TS-30-31 of comparable energy with multiconfigurational electronic states could be localized on the energy hypersurface for this one-step conversion. The symmetrical transition state complies with the definition of a coarctate mechanism.