Hydrogen abstraction persistent radicals

The persistence of alkoxamidyls (164) probably facilitates rebound capture of acyloxyl radicals in competition with their decarboxylation and hydrogen abstraction reactions. 

Dihydro-l, 2,3,5-thiatriazole 1-oxide radicals were generated from the thiatriazole oxides upon hydrogen abstraction in the reaction with thermally formed bis(4-methylphenyl)aminyl (4.20.5.1). Only radicals with aryl substituents in positions two and four are persistent, radicals with alkyl groups in these positions could not be detected. The radicals were investigated by ESR, ENDOR, and general triple resonance spectroscopy <92MRC84>. 

Citation of the classic chain reaction for lipid oxidation persists even though, as product analysis and studies of mechanisms have become more sophisticated, there is now considerable evidence that only Reactions 1, 2, and 5 (and perhaps also 6) of Figure 1 are always present. Research has shown that, although hydrogen abstraction ultimately occurs, it is not always the major fate of the initial peroxyl or alkoxyl radicals. Indeed, lipid alcohols from H abstraction are relatively minor products of lipid oxidation. There are many competing alternative reactions for LOO and LO that propagate the radical chain but lead to different kinetics and different products than expected from the classic reaction sequence (5, 6, 21). A more detailed consideration of each stage shows how this basic radical chain sequence portrays only a small part of the lipid oxidation process and products, and a new overall reaction scheme for lipid oxidation is needed. 

Irradiation of 2-oxooctanoic acid 5 in water causes hydrogen abstraction and coupling of the persistent radicals. In this way, a single tailed surfactant is transformed into a double tailed surfactant 6, and this... 

Although some radical species may persist for prolonged periods, most are generally unstable and will attempt to donate their unpaired electron to a nearby molecule or to remove or abstract a second electron, usually in the form of a hydrogen atom, from a neighboring molecule to pair with their free electron. Free-radical reactions are intrinsic to a majority of the metabolic and... 

Formation of both adduct 106, mesitylene adduct 109 and heterocycle 104 can be ascribed to homolysis (Scheme 21, pathway (i)). Decarboxylation of acyloxyl radical gives radicals, R , which either combine with the persistent free radical 102 giving 106 or, by abstracting hydrogen from solvent, give 3,5-dimethylbenzyl radicals, which combine with 102 to give 109. 

Atom abstraction from the cr-carbon atom in spin adducts, which leads to nitrones, and thence in some instances to spin adducts of these nitrones (e.g. Scheme 9), is undoubtedly a general phenomenon. For this sequence to be observed it is necessary that some, or all, of the following conditions are fulfilled. (0 The nitrone is more reactive towards radical addition than is the original spin trap and/or the latter is substantially consumed in the reaction. (h) The initial spin adduct is particularly susceptible to hydrogen-atom loss (and probably reacts this way by disproportionation). ( 7) The secondary spin adduct is particularly persistent. 

The utility of SmT-generated ketyl radicals in selective organic synthesis can be ascribed in part to their persistence. Thus ketyl radical anions have been postulated to form reversibly in the presence of Sml2 [9]. With the possible exception of aromatic carbonyls, the radical anions are not further reduced under the reaction conditions to the corresponding dianions. In addition, ketyls are not particularly prone to quenching by hydrogen atom abstraction from the solvent, from disproportionation, or related processes [52],... 

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