Functionalization of Phenyl Rings by Imidoylnitrenes. 2. Cycloaddition or Electrophilic Aromatic Substititution?

The thermal decomposition of N ‘-[(cyanoaryl)oxy]imidoyl azides with various substituted benzenes (OCH 3, NHCOCH 3, C(CH 3 ) 3, p -xylene, CH 3, H, Cl, CO 2 CH 3, NO 2 ) has been studied in detail. With electron-rich aromatic nuclei, the addition of a nitrene yields a benzenonium ion ( 3 ) intermediate. Benzaziridine ( 1 ) and/or 3 intermediate is formed from benzene. Electron-poor aromatic nuclei generated only intermediate 1, in which no benzenonium diradical is formed. Intermediate 3 is stabilized by extended conjugation through an extra resonance structure. For electron-rich aromatic compounds at 65−120 °C, the thermodynamically-controlled products, N -arylisoureas (( Z )-NH-Ar), were produced in very high yield. Azepenes (the kinetically-controlled products) were detected by neither 1 H-NMR nor a trapping experiment using tetracyanoethylene (TCNE). In the case of benzene 95% azepene was produced, which at 80 °C converted to isourea ( k = 7.0 × 10 - 5 s - 1 ). The ortho/para ratio resembles that of electrophilic aromatic substitution reactions. The Hammett plot of log k X / k H or log f P versus σ + -values showed excellent correlation, with ρ + -values of −2.80 and −3.10, respectively (poor correlation resulted with σ-values). The Hammett ρ + - or ρ-values are inconsistent with a radical mechanism for these reactions.