Transient aminoxyl radicals

It is appropriate to summarize here relevant data of both the aminoxyl radicals and their precursors that are useful for the ensuing discussion. Hydroxylamine (H2NOH) is the precursor of the archetypal aminoxyl radical (HaNO ). Upon one-electron oxidation of H2NOH by a suitable oxidant, e.g. Ce, the formation of H2NO (plus one proton) has been documented by the first step being the formation of the transient radical... 

Another transient aminoxyl radical has been generated , and employed in H-abstraction reactivity determinations" . Precursor 1-hydroxybenzotriazole (HBT, Table 2) has been oxidized by cyclic voltammetry (CV) to the corresponding >N—O species, dubbed BTNO (Scheme 9). A redox potential comparable to that of the HPI —PINO oxidation, i.e. E° 1.08 V/NHE, has been obtained in 0.01 M sodium acetate buffered solution at pH 4.7, containing 4% MeCN". Oxidation of HBT by either Pb(OAc)4 in AcOH, or cerium(IV) ammonium nitrate (CAN E° 1.35 V/NHE) in MeCN, has been monitored by spectrophotometry , providing a broad UV-Vis absorption band with A-max at 474 nm and e = 1840 M cm. As in the case of PINO from HPI, the absorption spectrum of aminoxyl radical BTNO is not stable, but decays faster (half-life of 110 s at [HBT] = 0.5 mM) than that of PINO . An EPR spectrum consistent with the structure of BTNO was obtained from equimolar amounts of CAN and HBT in MeCN solution . Finally, laser flash photolysis (LFP) of an Ar-saturated MeCN solution of dicumyl peroxide and HBT at 355 nm gave rise to a species whose absorption spectrum, recorded 1.4 ms after the laser pulse, had the same absorption maximum (ca 474 nm) of the spectrum recorded by conventional spectrophotometry (Scheme 9)59- 54... 

In the absence of either HPI or Co(acac)2, no appreciable conversion into products was obtained. EPR evidence for the formation of an aminoxyl radical intermediate was acquired, and a KIE of 3.8 determined . Generation of PINO in situ as the reactive intermediate was postulated . A subtle alternative enables the functionalization of hydrocarbons through the formation of carbocations as transient intermediates whenever PINO is formed and reacts in the presence of NO (e.g. 1 atm) . 

In the photoinduced cleavage of N-alkoxyamines, where the transient and the persistent radical are an alkyl and an aminoxyl radical, Fischer et al. used time-resolved CIDNP to clarify the mechanism of cross-termination and to determine the rate constant of the recombination. They concluded that disproportionation plays no role in this system, and that no unknovm side reactions occur. Later, Ananchenko and Fischer compared the recombinations of the transient ferf-butyl and 2-hydroxy-... 

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