Mild thermal route to phthalimidonitrene and its reaction with activated benzenes to give 2H- and 3H-azepines; X-ray crystal structure analysis of an isolable 2H-azepine

Abstract

At 80 °C 1a-acetyl-l-phthalimido-1a,6b-dihydrobenzofuro[2,3-b]azirine 1(X = Ac) transfers phthalimidonitrene to a series of traps: Me₂SO, 2-methoxynaphthalene, indene, methyl methacrylate, methyl crotonate, mesityl oxide, cis- and trans-stilbene and cyclohexene. Oxidation of N-aminophthalimide with lead tetraacetate in the presence of these traps gives the same major products (ref. 2a). With 1,3-dimethoxybenzenes, the reagent 1(X = Ac) provides 2H- and 3H-azepines e.g.25a and 24a as the major products with N-phthalimidoanilines e.g.18 formed to a lesser extent. On the other hand Pb(OAc)₄ oxidation of N-aminophthalimide in the presence of 1,3-dimethoxybenzenes gives N-phthalimidoanilines and no detectable 2H- or 3H-azepines. Small quantities of acetic or benzoic acid present during the dissociation of 1(X = Ac) in the presence of 1,3-dimethoxybenzenes cause marked increases in the formation of N-phthalimidoanilines compared with azepines. The unusually stable and isolable 2H-azepines 25 rearrange to 3H-azepines 24 only on heating and 5,7-dimethoxy-3-methyl-2-phthalimido-2H-azepine 25d is characterised by X-ray crystal structure analysis.