Trapping of the cyano radical: Does it ever happen?

The possible trapping of the cyano radical by alpha-phenyl-N-tert-butylnitrone (PBN) has been probed by a new method Tor the generation of CN., photolysis of N-cyanosuccinimide with PBN. Only in 1,1,1,3,3,3-hexafluoroprppan-2-ol was EPR spectral evidence obtained for the formation of a true spin adduct, namely the isocyano adduct of PBN. In other solvents, like dichloromethane or acetonitrile. the photolysis reaction took an entirely different course and gave the N-cyano-3-succinimidyl adduct of PBN and a second aminoxyl of unknown structure. The same pair of adducts could be obtained by photolysis of N-cyanomaleimide and PBN in a reductive coupling reaction. Maleimide gave a similar but clearly different pair of spectra. The possibility that the cyano or isocyano PBN adduct might undergo spontaneous hydrolysis to give the aminoformyl or formylamino adduct, respectively, was explored. It was shown that the authentic cyano adduct does not give the aminoformyl adduct under hydrolytic conditions. Since there is no method to prepare the isocyano adduct for such experiments, the problem of its hydrolytic reactivity is still open. The aminoformyl adduct of PBN was prepared by addition of the formamide anion to PBN, followed by oxidation by nickel peroxide, and characterized by its EPR spectrum.