Reduction of nitro- and nitroso-compounds by tervalent phosphorus reagents. Part X. Ring expansion to give 2-diethylamino-3H-azepines
Reactions of tervalent phosphorous reagents [(EtO)2PMe, Ph2, Ph2POEt] with nitroarenes (XC6H4·NO2; X = H, o-, m-, or p-Me, m- or p-CO2Et, m- or p-Cl, m- or p-Br, m-MeO, or 2·4.6-Me3) in diethylamine give 2-diethylamino-3H-azepines in 36–78% yields, together with smaller amounts of the corresponding anilines. m-Nitroarenes give mixtures of 4- and 6-substituted 2-diethylamino-3H-azepines, the isomeric compositions of which are identical, within experimental error, with those obtained by photolysis, in Pyrex, of the corresponding m-substituted aryl azides in diethylamine, thus providing strong confirmatory evidence for the existence of a common intermediate, the 7-azabicyclo[4.1.0]heptatriene (2), in equilibrium with the arylnitrene, in these reactions. In the above reactions of diethyl methylphosphonite with nitroarenes, the ratio of 3H-azepine to aniline produced varies with substituent. Reactions of m- and p-chloronitrobenzenes with tris(diethylamino)phosphine in t-butyl alcohol gave the corresponding 2-diethylamino-3H-azepines.
In the photoconversion of aryl azides into 3H-azepines, benzophenone enhances the formation of anilines, the triplet-derived products, but pyrene does not exhibit noticeable singlet sensitiser behaviour, in contrast to previously reported results obtained from related systems.