Autoxidation and photo-oxygenation

Autoxidation and photo-oxygenation are two aspects of the non-enzymic reaction between oxygen and unsaturated fatty acids. The enzymic reactions are discussed in Section 10.3. Oxidation of lipids during storage and handling, involving complex substrates and ill-defined reaction conditions, proved difficult to understand. This difficulty is enhanced by the fact that the primary oxidation products are labile and readily converted to secondary oxidation products of several kinds. Understanding of these processes has come from studies of simpler substrates such as methyl oleate or methyl linoleate under clearly defined reaction conditions. 

Recent developments in this field have resulted from improved methods of separation, especially high-performance liquid chromatography, and from additional spectroscopic information, particularly mass spectrometry and and NMR spectroscopy. 

The nature of the initiation step remains unclear. Once formed, hydroperoxides readily decompose-especially in the presence of metals such as copper or iron-to give chain-initiating radicals. There is some evidence that photo-oxygenation may be responsible for the first-formed hydroperoxides and thermal initiation is possible in heated samples. The structure of the radical R influences both the composition of the product and the rate of the reaction (see below). 

This sequence of reactions is affected by prooxidants and by anti-oxidants. The former-frequently metals or radical-producing species-encourage initiation whilst anti-oxidants usually shorten the propagation sequence by promoting the termination reactions. Other compounds (synergists), added to anti-oxidants to enhance their activity, are mainly metal-chelating compounds such as citric acid, phosphoric acid, ascorbic acid or ethylenediamine tetra-acetic acid which inhibit the metal-catalysed initiation. Anti-oxidants include natural compounds like the tocopherols or synthetic materials such as butylated hydroxy-anisole (BHA), butylated hydroxytoluene (BHT), propyl gallate or tertiary butyl hydroquinone. 

A more detailed description of the reaction products follows in (c) to (e). 

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Table 10.2 Monohydroperoxides of methyl linolenate formed during autoxidation and photo-oxygenation...

It is difficult to establish a clear distinction between autoxidation and photo-oxidation, since it is not always reported in the literature whether the uptake of oxygen by pyrroles required the presence of natural and/or artificial light. Therefore, studies carried out without specific use of light sources are reported in this section, despite the fact that, in some cases, the structures of the products seemed to suggest an attack by singlet oxygen. 

The free NH group is of course required for the autoxidation described above, and photo-oxygenation [14] of the vinyl indoles (13) took the route shown. No products deriving from the addition of O2 to the 2,3-bond of the indole nucleus were observed, and the peroxide (14) apparently rearranges in protic solvents to the dioxetan (15). 

V-Formylkynurenine is one of the 16 autoxidation products of tryptophan (51) (Fig. 8). The dye-sensitized photo-oxygenation of tryptophan in sodium carbonate - acetic acid buffer (pH 7) gave TV-for my Iky n urenine as the major product (52). This is also the oxidation product of tryptophan with hydrogen peroxide (53) and with ozone (54). This is an interesting case the same degradation impurity can be obtained in different ways, probably... 

The ozone concentration in the troposphere during the daytime is typically about 1 pphm (parts per hundred million parts of air by volume) [20], Values up to 100 pphm were measured in some photochemical smog areas. The molecular mechanism of the ozone aging of diene based elastomers was studied in detail and is well understood [19,21], Products or intermediates different from those arising in autoxidation or photo-oxidation of polymers were identified ozonides (3), zwitterions (4), diperoxides (5), polyperoxides (6), polymeric ozonides (7) and terminal aldehydes (8). Reactivity of aminic antiozonants (AOZ) with these species accounts for the protection of rubbers against atmospheric 03. AOZ must also possess antioxidant properties, because the free radical processes are concerted with ozonation due to the permanent presence of oxygen. 

In summaiy, organic ozone chemistry is an active research area and will continue to see new developments both from theoretical and industrial viewpoints. In particular, we shall likely see the relationship between ozone chemistry, autoxidation, photo-oxidation, and singlet oxygen chemistry more clearly defined, with a noteworthy contribution being made at this symposium. 

Olefins containing at least one allylic hydrogen are suitable substrates and are of special importance and interest with regard to the intrinsic mechanism involved in their reactions with singlet oxygen. Allylic hydroperoxides are formed, but the mechanism of their formation is clearly distinct from that by which allylic hydroperoxides are produced in thermal or photochemically initiated (see example above for a Type I process) autoxidation reactions. This has unequivocally been shown with optically active limonene as a substrate, which gives rise to different products in free radical and Type II photo-oxygenation reactions (22, 57, 61). 

In the studies cited above, only gross reaction equations are given. However, with modem knowledge we can speculate about aqueous oxygen chemistry and photo-catalytic-enhanced redox processes (Chapter 5.3.5) that reactive oxidants such as OH, O3 and H2O2 will elementarily react with HS (in the autoxidation process all these species are slowly produced from O2), similar to the sulfite oxidation (Chapter 5.5.2.2). 

Photo-oxygenation is a quicker reaction than autoxidation and the relative rates for oleate, linoleate and linolenate are shown in Table 10.1. In autoxidation the methylene-interrupted diene system is significantly more reactive than the isolated double bond of a monoene but in photooxygenation the relative rates of oxidation are more closely related to the number of double bonds. On the basis of these values photo-oxygenation of methyl oleate can be 30000 times quicker than autoxidation and for the polyenes photo-oxygenation can be 1000-1500 times quicker. 

Oxidation of methyl oleate has been extensively studied and is considered to be typical of all monoene acids/esters. Photo-oxygenation produces only two products - the 9-hydroperoxide (AlOt) and the 10-hydroperoxide (A8t) - in equal amounts. Reaction is confined to the olefinic carbon atoms and is accompanied by double-bond migration and stereomutation. As shown in Scheme 10.5 autoxidation of methyl oleate forms eight monohydroperoxides of which two (9-OOH A 10c and lO-OOH A8c) are only minor products. This range of compounds arises from initial attack at either allylic carbon atom to give a delocalized radical with oxygen finally attached to any one of four carbon atoms. 

The monohydroperoxides produced from this triene ester by photo-oxygenation and by autoxidation are set out in Table 10.2. In addition, bishydroperoxides, hydroperoxy peroxides and hydroperoxy diperoxides are formed. Typical structures are indicated but in all... 

The ability of hydroperoxides from linoleate and linolenate to give cyclic peroxides is summarized in Table 10.3. It will be noted that some of the monohydroperoxides giving rise to these more complex products are not formed during autoxidation but only by photo-oxygenation. 

Photo-oxygenation. Autoxidation of linoleic acid gives only the 9- and 13-hydroperoxides whereas photo-oxygenation gives four hydroperoxides (9-,10-,12-, and 13-), of which the 10- and 12-hydroperoxides are 3 to a double bond and can react further to give a hydroperoxy peroxide (such as (52) from the 12-hydroperoxide). Some... 

A short review of photo-oxidation of a range of polymers by Faucitano and co-workers [30] summarised the understanding at the time of PET photolysis as a splitting of C-0 bonds in the ester gronps, with formation of acyl and carboxyl radicals, which themselves can lose carbon oxides to produce phenyl or alkyl radicals, or abstract hydrogen to produce aldehydes and carboxylic acids. When oxygen is present, the authors state that the autoxidation chain reaction will lead to formation of anhydrides and aldehydes, and to hydroxy-substituted phenyl species. 

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