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A novel cascade radical reaction is described involving aryl isothiocyanates and 2-cyanoaryl radicals. The mechanism entails the formation of an alpha-(arylthio)imidoyl radical, a 5-exo-dig cyclization onto a cyano group, and a final 6-membered ring closure of an iminyl radical. The competitive 5-membered spiro-cyclization of the iminyl, leading to an isomeric product, was only observed in the case of a disubstituted aryl isothiocyanate. The whole process involves a rare example of [3 + 2] radical annulation and allows the one-pot synthesis of tetracondensed nitrogen heterocycles in good yields.

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It was shown by EPR spectroscopy that N-aryl-imines undergo hydrogen atom abstraction by tert-butoxyl radicals to generate imidoyl radicals in competition with addition to the C᎐ ᎐ N double bond to generate aminyl radicals. The latter were subsequently oxidised to nitroxides (aminoxyls) and also fragmented to release tert-butyl radicals and iminyl ethers. The main products identified were imines formed by combination of the imidoyl radicals with other radicals in the system. Thus, hydrogen abstraction is probably the most important homolytic pathway for the precursor imines. A similar reactivity pattern was found for N-tert-butyl-imines. DFT computations (B3LYP with a 6-31G* basis set) indicated that α-scissions of imidoyl radicals, ArN᎐ ᎐ C ؒ R (R = t-Bu or Me), to produce isonitriles, ArNC, were too endothermic to occur under the present experimental conditions. tert-Butoxyl radicals were shown to add to 1-isocyano-4-chlorobenzene to generate N-arylimidoyl radicals that fragmented to afford 4-chloro-1isocyanatobenzene.

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