Azodioxide radical cations

Article abstract of DOI:10.1021/ja00147a007

This report provides the first examples of solution-stable azodioxide radical cations and describes their direct spectroscopic observation and, in one case, their thermal chemistry. The formal oxidation potentials, E^o′, for N,N′-dioxo-2,3-diazabicyclo[2.2.2]oct-2-ene (3), N,N′-dioxo-2,3-diazabicyclo[2.2.1]hept-2-ene (4), and N,N′-dioxo-1,1′-azobis(norbornane) (5) are 1.65, 1.68, and 1.54 V vs SCE, respectively. ESR spectroscopy shows the intermediate cations to be π radicals. Radical cation 5^?+ (red, λ_m 510 nm) has a five-line ESR spectrum of a(2N) 1.1 G, while 3^?+ (bronze) has a nine-line ESR spectrum simulated as a(4H) 0.86 and a(2N) 1.22 G. Both 3^?+ and 5^?+ decay in seconds to minutes at room temperature. Thermal decomposition of 5^?+ results in C,N and N,N bond cleavage, yielding 1-norbornyl cation (trapped by solvent) and NO⁺ (trapped in low yield by the oxidant under chemical oxidation conditions). Two viable mechanisms are presented for 5^?+'s thermal decay, both of which invoke nitrosoalkane monomer 5m as an intermediate. In a related study, oxidation of nitrosoalkane 2m is found to mediate its facile denitrosation. This work affords the first examples of electron-transfer-mediated C,N bond cleavage of azodioxides and of nitrosoalkanes. Substantial bond weakening is shown to accompany electron loss from these substrates. For 5, π oxidation leads ultimately to σ C,N bond activation.