Oxygen radicals, -cleavage

The reaction endothermicity establishes a minimum for the activation energy whereas abstraction of a hydrogen atom from carbon is a feasible step in a chain process, abstraction of a hydrogen atom from a hydroxyl group is unlikely. Homolytic cleavage of an O-H bond is likely only if the resulting oxygen radical is stabilized, such as in phenoxy radicals formed from phenols. 

The decomposition of aliphatic peroxides produces oxygen radicals too unstable for paramagnetic measurement. These radicals initiate the polymerization of olefins and give the complex mixtures of decomposition products associated with radical mechanisms. On the other hand, aliphatic peroxides are also capable of polar decomposition reactions, a subject to be taken up in Chapter VIII. The characteristic reactions of the less stable oxygen free radicals are /3-cleavage to form... 

Kharasch has pointed out that there are two very similar and not easily distinguished reactions of oxygen radicals, /9-cleavage and rearrangement.111 The difficulty is that they can often lead to the same final product. 

The decomposition of t.ert-bvA.y triarylmethyl peroxides is particularly well suited to the measurement of migration aptitudes in oxygen radicals because the nature of the product leaves no doubt that the reaction is in fact a rearrangement rather than a /9-cleavage. 

Although hydroxyl radical is commonly assumed to be the most toxic of the oxygen radicals (with little direct evidence), other direct reactions are more likely to be important for understanding the cytotoxicity of peroxynitrite. A second oxidative pathway involves the heterolytic cleavage of peroxynitrite to form a nitronium-like species (N02 ), which is catalyzed hy transition metals (Beckman et al., 1992). Low molecular weight metal complexes as well as metals bound in superoxide dismutase and other proteins catalyze the nitration of a wide range of phenolics, including tyrosine residues in most proteins (Beckman et al., 1992). 

Carbon-oxygen bond cleavage may occur by /1-scission (Fig. 21) but only significantly in cases where relatively stable radicals are the resulting intermediates, such as in the fluorination of f-butylmethyl ether (Fig. 22) [78]. 

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