Hydroxy radicals from alkyl hydroperoxides

Figure 17.2 Lipid peroxidation scheme. LH, a polyunsaturated fatty acid LOOM, lipid hydroperoxide LOH, lipid alcohol L, lipid radical LOO, lipid hydroperoxyl radical LO, lipid alkoxyl radical. Initiation the LH hydrogen is abstracted by reactive oxygen (e.g. lipid alkyl radical, lipid alkoxy radical, lipid hydroperoxyl radical, hydroxy radical, etc.) to produce a new lipid alkyl radical, L. Propagation the lipid alkyl, alkoxyl or hydroperoxyl radical abstracts hydrogen from the neighbouring LH to generate a new L radical.

Treatment of a-iodo lactone (45) with triethylborane under oxygen atmosphere gives the corresponding a-hydroxy lactone (46), via a-lactone radical species. This reaction comprises of SH2 reaction by Ef on the iodine atom of a-iodo lactones, reaction of the formed a-lactone radical with molecular oxygen, and subsequent hydrogen-atom abstraction from the solvent to form alkyl hydroperoxide (ROOH). Finally, by the addition of dimethyl sulfide for the reduction of the peroxide, the corresponding a-hydroxy lactone is obtained (eq. 2.24) [58]. 

The use of additives to control lubricant degradation requires a focus on alkyl radicals (R), alkylperoxy radicals (ROO) and hydroperoxides (ROOH). Primary alkoxy radicals (RCH2O) and hydroxy radicals (HO) rapidly abstract hydrogen from the substrate. It is therefore very unlikely that they can be deactivated by natural or synthetic antioxidants. In practice, three additive types have proven to be successful in controlling the degradation of lubricating oils ... 

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