Peroxy from unsaturated hydroperoxides

We developed a method for the study of peroxy radical cycli-zation based on the generation of peroxy radicals from unsaturated hydroperoxides (jS, T) Thus, treatment of the hydroperoxide with free radical initiators such as DBPO ( ) led to the formation of monocyclic peroxides that could be isolated by liquid chromatography and characterized by standard techniques. 

The arachidonic acid cascade is a biological free radical oxidation of unsaturated fatty acids leading to formation of the prostaglandins (equation 102). Cyclization of a peroxy radical intermediate 66 leading to endoperoxide 67 was proposed as a pathway for this process, and this was demonstrated in chemical model systems, in which the peroxyl radical 66 was generated by hydrogen abstraction from the hydroperoxide corresponding to 66. 

However, the ratios of the unsaturated materials to the saturated materials and of the ketones to the alcohols (Table I) indicate that the yields of unsaturated materials are higher than those of saturated products, especially cyclohexenol. [The excess alcohol might come from 2ROO - 2RO- + 02 however, its importance in the gas phase is unknown.] It was suggested from the above that some cyclohexenol and possibly cyclo-hexenone may be formed from cyclohexenyl hydroperoxide which is produced from chain reactions initiated by the cyclohexyl peroxy radical and cyclohexenyl peroxy radical as shown below. 

The mechanisms behind lipid oxidation of foods has been the subject of many research projects. One reaction in particular, autoxida-tion, is consistently believed to be the major source of lipid oxidation in foods (Fennema, 1993). Autoxidation involves self-catalytic reactions with molecular oxygen in which free radicals are formed from unsaturated fatty acids (initiation), followed by reaction with oxygen to form peroxy radicals (propagation), and terminated by reactions with other unsaturated molecules to form hydroperoxides (termination O Connor and O Brien, 1994). Additionally, enzymes inherent in the food system can contribute to lipid oxidization. 

Reagents which effect epoxidation of the enol ether unsaturation effect a-hydroxylation comparable to the peracid approach. Thus a combination of molybdenum hexacarbonyl and r-butyl hydroperoxide converts the substrates to a-silyloxy derivatives. The peroxide generate in situ from benzonitrile, potassium carbonate and hydrogen peroxide can also perform the oxidation. Molybdenum-peroxy complexes, including MoOPH, could presumably also effect this transformation. Lastly, dimethyldioxirane has been used to epoxidize alkenes and it is likely that application of this useful, debris free, organic peroxide to these reactions will soon emerge. 

Peroxy radicals ROO are key species in the mechanisms of oxidative and combustion systems. At the same time they have been among the most difficult radicals to study experimentally. There are only limited or no thermochemical information available for unsaturated alkylperoxy and hydroperoxide species. An explanation for this paucity of data could be the fact that these species are unstable and short-lived, and therefore difficult to study and characterise by experimental methods. The difficulty arises in part from the lability of these radicals towards reversible unimolecular dissociation into R + O2 and then to reversible isomerization into hydroperoxy alkyl radicals ROOH, both of these reactions occurring at comparable rates at temperatures below 450 K [2]. 

As with the stable hydroperoxide molecules above, the isodesmic reactions for the unsaturated alkoxy and peroxy radicals resulting from C—OOH, CO—OH and COO—H bond cleavage reveal that enthalpies on a number of vinyl and ethynyl, alkoxy and peroxy radicals are needed for use as reference species. Limited data are available for these radical species enthalpies [47] and this data is listed Appendix A. 

Autoxidation is the process of oxidation at room temperature. Autoxidation of fatty acids occurs in a series of steps including the formation of free radicals. The process is very complicated and can be roughly divided into three phases initiation, propagation and termination. In the initiation phase, hydrogen is abstracted from the a-methylenic carbon of fatty acids to yield a free radical (equation (11.1)). The presence of a free radical initiator or catalyst is needed for the reaction to take place. Once a free radical is formed, it may form peroxy radicals through reaction with atmospheric oxygen (equation (11.2)). Also, these free radicals can abstract hydrogen from another unsaturated molecule to form a hydroperoxide and a new free radical (equation (11.3)). 

The non-specific lipoxygenases can cooxidize carotenoids and chlorophyll and thus can degrade these pigments to colorless products. This property is utilized in flour bleaching (cf. 15.4.1.4.3). The involvement of LOX in cooxidation reactions can be explained by the possibility that the peroxy radicals are not as rapidly and fully converted to their hydroperoxides as in the case of specifically reacting enzymes. Thus, a fraction of the free peroxy radicals are released by the enzyme. It can abstract an H-atom either from the unsaturated fatty acid present (pathway 2a in Fig. 3.30) or from a polyene (pathway 2b in Fig. 3.30). 

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