Homolytic cleavage, hydroperoxides

FIGURE 6 Speculative mechanism of Crl hydrocarbon biosynthesis from fatty acid hydroperoxides in algae. Homolytic cleavage of the hydroperoxide is assumed to give an allyl radical, which cyclizes to the thermolabile (1S,2R)-cyclopropane. The sequence is terminated by transfer of a hydrogen radical from C(16) to the -X-0 function. The cyclopropane rearranges to (6S)-ectocarpene as shown in Figure 4. 

The proposed mechanism (Scheme 1) involves the mixed-valence compounds [Rh2" " ( Ji-cap)4(OH)] and [Rh2 (p.-cap)4(OOt-Bu)] formed from the homolytic cleavage of t-BuOOH. The t-BuOO radicals in the medium promote a selective hydrogen abstraction from the alkene to give the allylic alkenyl radical. This species traps the peroxide in [Rh2 (p.-cap)4 (OOt-Bu)] to produce the alkenyl hydroperoxide, which rapidly decomposes to the isolated products, thus regenerating the catalyst. 

The autoxidation of polyunsaturated fatty acids (cf. Porter et al. 1981) is usually monitored by the formation of malonaldehyde using the 2-thiobarbituric acid essay. This is carried out under rather severe conditions which decomposes its precursor. This malonaldehyde-like product is obviously formed via a cycliza-tion reaction of a peroxyl radical, followed by other processes such as further cyclization and hydroperoxide formation [reactions (21)-(23)]. The resulting hydroperoxides may eliminate malonaldehyde upon a homolytic cleavage of the endoperoxidic intermediate (Pryor and Stanley 1975). 

Epoxidation of cyclooctene with hydrogen peroxide, catalysed by the methoxide-ligated form of iron(III) tetrakispentafluorophenyl [F20TPPFe(III)] porphyrin, is proposed to involve a reaction of F20TPPFe(in) with hydrogen peroxide to form an iron(III) hydroperoxide species, which then undergoes both heterolytic and homolytic cleavage to form iron(IV) n -radical cations and iron(IV) oxo species, respectively. 

Free radicals may be generated by oxidation, reduction, or by homolytic cleavage of one or more covalent bonds, such as C—C bonds e.g. dimers of triarylmethyl radicals), N—N bonds e.g. tetrasubstituted hydrazines), O—O bonds e.g. hydroperoxides, dialkyl and diacyl peroxides, peroxycarboxylic esters), C—N bonds e.g. dialkyl azo compounds), and N—O bonds (as in the thermolysis of nitrogen pentoxide O2N— O—NO2). Two typical examples, which have been investigated in different solvents, are given in Eqs. (5-56) and (5-57) cf. also reaction (5-39a) in Section 5.3.2. 

In the absence of the activating second carbonyl functionality, it is necessary to use more ingenious methods to produce the same net effect. These procedures more often than not involve radical reactions. Among them is the thermolysis of tert-butyl esters of peroxyacids 437, which are readily synthesized in a standard esterification of tert-butyl hydroperoxide with an acid chloride. Decarboxylation proceeds via an initial homolytic cleavage of the 0-0 bond, elimination of CO2, and reduction of the incipient alkyl radical by an added hydrogen atom donor such as 438 (Scheme 2.143). Examples showing the exceptional synthetic importance of this decarboxylation procedure will be presented later. 

Hydroperoxides of unsaturated fatty acids formed by autoxidation are very unstable and break down into a wide variety of volatile flavor compounds as well as nonvolatile products. It is widely accepted that hydroperoxide decomposition involves homolytic cleavage of the -OOH group, giving rise to an alkoxy radical and a hydroxy radical (5). 

The high oxidation rates of EPA and DHA and the instability of their hydroperoxides caused the rapid formation of secondary products such as volatile aldehydes and other compounds, which, in turn, impart flavor reversion in fish oils (56). The hydroperoxides produced from autoxidation of EPA (73) and DHA (74) have been identified but not quantified. They form eight and ten isomers, respectively. Noble and Nawar (75) analyzed the volatile compounds in autoxidized DHA and identified a number of aldehydes. Most of the aldehydes identified could be explained by the p-scission of alkoxy radicals generated by the homolytic cleavage of each isomer of the hydroperoxides as shown in Figure 9. 

Common error alert Hydrogen peroxide (H2O2) and alkyl hydroperoxides (ROOH) are not free-radical initiators. The OH radical is too high in energy to be produced by homolytic cleavage of these compounds at ordinary temperatures. 

A problem with hydroperoxides is that they can also form peroxidic radicals (RO) by homolytic cleavage of the peroxidic 0-0 bond or other pathways as advocated by the groups of Mansuy and Bruice [550,551]. This can lead to unwanted side reactions, including oxidative degradation of the porphyrin. 

Alkylidenephosphoranes (23) react with t-butyl hydroperoxide to give an adduct which subsequently undergoes homolytic cleavage of the 0—0... 

While most peroxide reactions involve homolytic cleavage of the O—O bond, generating free radicals (sec. 13.3), H2O2 and its monosubstituted derivatives can react with alkenes via either a concerted or ionic mechanism.244 Three categories of peroxides are used for epoxidation H2O2, alkyl hydroperoxides (155),... 

Oxy radicals Heat and single electron reduction by transition metal ions are among the more important ways lipid oxy radicals are formed from homolytic cleavage of hydroperoxides. 

Oxidation is initiated by radicals present in living organisms (e.g., hydroperoxide H0 2, hydroxide OH, peroxide ROO, alcoxyl RO, alkyl L ) or by thermal or photochemical homolytic cleavage of an R-H bond. 

One of the most damaging species in the oxidation process is the hydroperoxide. Under elevated temperature hydroperoxides decompose via hemol5flic cleavage to yield two free radicals. This step demonstrates the catalytic nature of autoxidation. The destruction of hydroperoxides, which continually build up in the polymer, is essential in protecting the polymer (Salamone 1996). Phosphites prevent further formation of free radicals by decomposing unstable hydroperoxides prior to their homolytic cleavage. Instead, the unstable hydroperoxide forms a stable product (Zhu Shi 2001) as illustrated in Figure 13. 

As outlined in a simplified mechanisms in Fig. 4.1, degradation proceeds through a radical chain mechanism (2,3). Initiation typically occurs through exposure to heat generated during production. Trace metal impurities such as copper or iron accelerates radical formation. Reactive hydroperoxides are formed after reaction of the carbon-centered radical with oxygen. Thermally induced homolytic cleavage of hydroperoxides leads to additional reactive radical formation and subsequent polymer chain scission. 

With autoxidized linoleate, 2,4-decadienal and methyl octanoate are produced by homolytic cleavage A on the 9-hydroperoxide, and 3-nonenal and 9-oxononanoate by cleavage B hexanal, pentane, 1-pentanol, and pentanal are produced from the 13-hydroperoxide (Figure 4.12). With linoleate subjected to photosensitized oxidation, the 9- and 13-hydroperoxides produced less 2,4-decadienal, methyl octanoate and pentane, and similar amounts of hexanal. The unique unconjugated 10-hydroperoxide produced 9-oxononanoate and methyl lO-oxo-8-decenoate, and the 12-hydroperoxide produced a significant amount of 2-heptenal (Figure 4.13). 

In contrast to thermal and metal-catalysed decomposition of hydroperoxides, which proceed by homolytic cleavage, decomposition under acid conditions... 

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