2.3- Diaza-Dewar benzene

The product in the case of (4) would be 2,3-diaza-Dewar benzene (5). Masamune et al. have presented evidence for the transient formation of (5) followed by a reverse Diels-Alder reaction to form cyclobutadiene (6). Thus mild oxidation of... 

Diaza-Dewar benzene, 56 3H-1,2-Diazepines, 599 Diaziridines, 253 a-Diazoacetophenone, 577 Diazoalkanes, 514, 577... 

Examples of the photoisomerization of pyrazoles to imidazoles have been reported by Nishiwaki et al. In the photoisomerization of perfluoroalkyl-pyridazines to the corresponding pyrazines, Chambers et al. have shown that an intermediate diaza Dewar-benzene is involved rather than a diazaprismane. Ogata and Takagi have observed a novel photoisomerization of 2-pyridylaceto-nitrile to u-aminobenzonitrile, possibly via a Dewar-intermediate. 

Oxidation of tricarbonylcyclobutadieneiron in the presence of l,2-bis-(p-tosyl-ethoxycarbonyl)diazene gave the diazabicyclo[2,2,0]hexene (232a), which was easily converted into 2,3-diazabicyclo[2,2,0]hexene (232b).Mild oxidation of (232) was expected to give diaza-Dewar-benzene (233). This compound could not be isolated, as it loses nitrogen to give cyclobutadiene, which was detected by trapping with dienophiles or by the formation of its dimer. This sequence affords a useful route to cyclobutadiene from transition-metal-free precursors. 

In contrast to thermal isomerization, photolysis of pyridazines yields mainly pyrazines. The photochemical reorganization of the 1,2-diazine framework presumably involves intermediates of the diaza-Dewar benzene type 1 and 2. 

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