Functionalization of phenyl rings by imidoylnitrenes .2. Cycloaddition or electrophilic aromatic substitution?

The thermal decomposition of N'-[(cyanoaryl)oxy]imidoyl azides with various substituted benzenes (OCH3, NHCOCH3, C(CH3)(3), p-xylene, CH3, H, Cl, CO2CH3, NO2) 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 degrees 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 H-1-NMR nor a trapping experiment using tetracyanoethylene (TCNE). In the case of benzene 95% azepene was produced, which at 80 degrees C converted to isourea (k = 7.0 x 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 sigma(+)-values showed excellent correlation, with rho(+)-values of -2.80 and -3.10, respectively (poor correlation resulted with sigma-values). The Hammett rho(+)- or rho-values are inconsistent with a radical mechanism for these reactions.