Fragmentation of Hydroperoxides

Methyl ketones are obtained by thermally induced P-oxidation followed by a decarboxylation reaction (Fig. 3.37). Aldehydes are obtained from the fragmentation of hydroperoxides by a P"Scission mechanism (Fig. 3.38) occurring nonselectively at elevated temperatures (compare the difference with 3.7.2.1.9). 

The final possible mode of action for an antioxidant is as a peroxide decomposer. In the sequences that lead to photodegradation of a polymer the ready fragmentation of the hydroperoxide groups to free radicals is the important step. If this step is interfered with because the peroxide has undergone an alternative decomposition this major source of initiation is removed. The additives which act by decomposing hydroperoxide groups include compounds containing either divalent sulfur or trivalent phosphorus. The mechanism involves... 

Extensive studies in vitro from many groups have confirmed that exposure of LDL to a variety of pro-oxidant systems, both cell-free and cell-mediated, results in the formation of lipid hydroperoxides and peroxidation products, fragmentation of apoprotein Bioo, hydrolysis of phospholipids, oxidation of cholesterol and cholesterylesters, formation of oxysterols, preceded by consumption of a-tocopherol and accompanied by consumption of 8-carotene, the minor carotenoids and 7-tocopherol. 

We next examine the possible fates of the alkoxy radical produced as a result of hydroperoxide fragmentation. It should be noted that the other fragment produced in this process, an hydroxy radical (not shown), would be an extremely reactive species. Since it is not attached to a polymer chain end, it is also capable of more readily diffusing through the polymer matrix than most of the radicals discussed to this point. This also makes the photo-oxidation of the glycol potentially more destructive. 

Fragmentation of the hydroperoxide can then lead to chain shortening, as illustrated. 

Fragmentation of a-alkoxy hydroperoxides. Schrciber1 has reported a short synthesis ol I be mucrolide ( + )-recileiolide (2) in which the key step involves Irngmeutnliim of llie /(-alkoxy hydroperoxide 1. available in two steps from... 

Compound 403 is readily reduced with sodium borohydride at -78°C and yields the monoalcohol 405 (115). It also reacts with potassium t -butyl hydroperoxide at -20°C and gives the cis-enone-perester carbonate 406 in high yield (116). This last transformation can be explained by retro-Claisen fragmentation of intermediate 407 followed by the elimination of methoxide ion from 408. It is also possible that 407 undergoes a direct stereoelectronically controlled Grob type fragmentation to compound 406. 

A great tendency towards fragmentation is displayed by alkoxy] radicals which are formed as primary species of hydroperoxides or peroxides decomposition or during self-interaction of peroxyl radicals. 

One of these (kA) is attack by hydroxide on 3. This might entail nucleophilic attack by hydroxide on the methyl group, with subsequent or concomitant fragmentation of the rj2-peroxide to form terminal oxos.35 However, the system will also have some free H202, which would be deprotonated by the base (p a H202 = 11.62, vs 15.74 for H20).36 The hydroperoxide anion could then attack free MTO ... 

The methoxycarbonylmethyl radical I is obtained by Kolbe electrolysis from methyl-malonate [Eq. (4, path a)], and in homogeneous solution by reductive fragmentation of the hydroperoxide of dimethyl acetone dicarboxylate [Eq. (4, path b)]. Radical I forms with styrene the adduct II, which reacts by disproportionation to III and IV, by coupling with I to V and by dimerization to VI. The yields and product ratios for the electrolysis and the reductive fragmentation correspond surprisingly well, and no extensive polymerization was found for the homogeneous reaction. 

Figure 6 Short-chain aldehydic products resulting from the oxidative fragmentation of lipid hydroperoxides.

The mechanisms of the photooxidation of polyethylene and polypropylene have been discussed in depth with particular emphasis on the importance of hydroperoxides as the precursor to free radical formation . Both the kinetics and nature of the photooxidation products of the polymers are markedly controlled by these species especially polypropylene. On the other hand the density of polyethylene has been found to play an important role on the photooxidation rate of the polymer . Here the photostability of the polymer decreased with decreasing film density indicating that oxygen diffusion is impaired by the crystallites and therefore improves stability. In fact, other workers have found that the crystalline regions of polyethylene are unaffected by irradiation in air . These workers also found new crystalline regions are formed on irradiation due to the smaller polymer fragments... 

The main product is pinane hydroperoxide (2) which can be reduced to 2-pinanol (3), an important intermediate in the industrial synthesis of linalool [1, 2, 3]. Pinocampheol (4), verbanol (5), pinocamphone (6) and verbanone (7) are also formed in minor amounts. When the reaction is carried out at temperatures higher than room temperature, pinane hydroperoxide is decomposed. It yields mainly the monocyclic products bearing the cyclobutyl ring (9 - 12) (Figure 1) [1], These products result from the fragmentation of the 2-pinanyloxy radicals which in turn result from the decomposition of 2-pinane hydroperoxide [1]. 

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