The Thermolysis of the Cycloadducts between Aryl Azides and Hexamethyl‐ Dewar ‐Benzene Revisited

The reactions of 1‐azido‐2,4,6‐trimethyl‐ and 1‐azido‐2,6‐dimethylbenzene with hexamethyl‐ Dewar ‐benzene (HMDB) give rise to the pentamethyl(iminoethyl)‐4 H ‐1,2‐diazepines 4 and 5 , respectively. The X‐ray crystal‐structure analysis of 5 unequivocally established these unexpected structures, which motivated us to re‐investigate the thermolysis of the adduct 1a of HMDB and azidobenzene. Previously, the triazonine 2 was reported to be the major product, but this interpretation could not be corroborated now, and, instead, the 1‐aminopyrrole 6 was identified as the true compound. The structure of the analogous compound 7 , obtained by thermolysis of the (1‐azido‐4‐nitrobenzene)‐HMDB adduct 1b , was confirmed by X‐ray diffraction. In the original literature investigation, a second product had been isolated, in addition to the alleged 2 , by chromatographic purificiation of the thermolysis mixture, and structure 3 had been ascribed to it. It turns out now that this compound is not a valence isomer of the major product, but a hydrolysis product, i.e. , the ketone 8 . The formation of the rather different compounds 4 and 5 on the one hand, and 6 and 7 on the other hand, can be rationalized straightforwardly by a sequence of pericyclic reactions leading to 3 H ‐1,2‐diazepines 11 as common intermediates, which are converted to either 4 and 5 , or 6 and 7 , depending on the nature of the aromatic group.