Aminoxyl radicals reactivity studies

III. REACTIVITY STUDIES A. H-abstraction by Persistent Aminoxyl Radicals... 

Another facet of the reactivity of aminoxyl radicals has been brought to attention by recent studies of the oxidation of substrates endowed with low redox potential. Some aminoxyl radicals, depending on the reduction potential value of the couple >N—O / >N—O, can behave as moderate one-electron abstractor towards substrates endowed with appropriate oxidation potential. This rather unprecedented reactivity feature, outlined in Scheme 10, has been substantiated by the oxidation of aniline or phenol derivatives, whose redox potential is conveniently located in the 0.4-1.0 V/NHE range ". 

As opposed to such a consistent body of evidence in favour of the H-abstraction route with aminoxyl radical intermediates, the reactivity features of the oxoammonium ion , as a derivative of the aminoxyl radical (TEMPO), are somewhat baffling (Section III.E). In spite of the many studies from the literature, a lack of uniformity emerges whenever the Hammett and KIE parameters are investigated and compared. The possible interplay of different mechanistic routes has been suggested, and more experimental work is needed before satisfactory conclusions can be drawn. Certainly, this does not undermine the synthetic value of the procedure, as we will see below, even though care must be exerted when comparing results obtained by the use of different primary oxidants. 

As expected, the parameters for stabilization and steric effects are positive, in good agreement with the decrease of the fee value with the inaease in both the stabilization and the bulkiness of the aminoxyl radical. It is shown that the major effect is due to the congestion around the aminoxyl moiety ( 90%) with a lower contribution of the stabilization effect ( 10%). The fec,o parameter has a value (2.2 x 10 °M" s" eqn [18]), typical of the diffusion-controlled rate constant that should correspond to the scavenging of alkyl radicals by the hypothetically more reactive dimethylaminoxyl radical. These studies showed that only the four a,a -substituents flanking the aminoxyl function governed the steric hindrance around the radical center. As a consequence, the fee value is independent of the type of nitroxide, provided they have the same substituents and the same stabilization effect. 

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