Oxidation of unsaturated

Reactions in which a product remains in the him (as above) are complicated by the fact that the areas of reactant and product are not additive, that is, a nonideal mixed him is formed. Thus Gilby and Alexander [310], in some further studies of the oxidation of unsaturated acids on permanganate substrates, found that mixed hlms of unsaturated acid and dihydroxy acid (the immediate oxidation product) were indeed far from ideal. They were, however, able to ht their data for oleic and erucic acids fairly well by taking into account the separately determined departures from ideality in the mixed hlms. 

VI,14. OXIDATION OF UNSATURATED COMPOUNDS WITH OZONISED OXYGEN (OZONOLYSIS)... 

The scope of oxidation chemistry is enormous and embraces a wide range of reactions and processes. This article provides a brief introduction to the homogeneous free-radical oxidations of paraffinic and alkylaromatic hydrocarbons. Heterogeneous catalysis, biochemical and hiomimetic oxidations, oxidations of unsaturates, anodic oxidations, etc, even if used to illustrate specific points, are arbitrarily outside the purview of this article. There are, even so, many unifying features among these areas. 

JS/oble Metals. Noble or precious metals, ie, Pt, Pd, Ag, and Au, are ftequendy alloyed with the closely related metals, Ru, Rh, Os, and Ir (see Platinum-GROUP metals). These are usually supported on a metal oxide such as a-alumina, a-Al202, or siUca, Si02. The most frequently used precious metal components are platinum [7440-06-4J, Pt, palladium [7440-05-3] Pd, and rhodium [7440-16-6] Rh. The precious metals are more commonly used because of the abiUty to operate at lower temperatures. As a general rule, platinum is more active for the oxidation of paraffinic hydrocarbons palladium is more active for the oxidation of unsaturated hydrocarbons and CO (19). 

CoA dehydrogenase shows absolute specificity for the L-hydroxyacyl isomer of the substrate (Figure 24.16). (o-Hydroxyacyl isomers, which arise mainly from oxidation of unsaturated fatty acids, are handled differently.)... 

FIGURE 24.23 )3-Oxidation of unsaturated fatty acids. In the case of oleoyl-CoA, three /3-oxidation cycles produce three molecules of acetyl-CoA and leave m-AAdodecenoyl-CoA. Rearrangement of enoyl-CoA isomerase gives the tran.s-A species, which then proceeds normally through the /3-oxidation pathway. 

An Isomerase and a Reductase Facilitate the /3-Oxidation of Unsaturated Fatty Acids... 

Monoalkylthallium(III) compounds can be prepared easily and rapidly by treatment of olefins with thallium(III) salts, i.e., oxythallation (66). In marked contrast to the analogous oxymercuration reaction (66), however, where treatment of olefins with mercury(II) salts results in formation of stable organomercurials, the monoalkylthallium(III) derivatives obtained from oxythallation are in the vast majority of cases spontaneously unstable, and cannot be isolated under the reaction conditions employed. Oxythallation adducts have been isolated on a number of occasions (61, 71,104,128), but the predominant reaction pathway which has been observed in oxythallation reactions is initial formation of an alkylthallium(III) derivative and subsequent rapid decomposition of this intermediate to give products derived by oxidation of the organic substrate and simultaneous reduction of the thallium from thallium(III) to thallium(I). The ease and rapidity with which these reactions occur have stimulated interest not only in the preparation and properties of monoalkylthallium(III) derivatives, but in the mechanism and stereochemistry of oxythallation, and in the development of specific synthetic organic transformations based on oxidation of unsaturated systems by thallium(III) salts. 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

Figure 22-4. Sequence of reactions in the oxidation of unsaturated fatty acids, eg, linoleic acid. A -c/s-fatty acids or fatty acids forming A -c/s-enoyl-CoA enter the pathway at the position shown. NADPH for the dienoyl-CoA reductase step is supplied by intramitochondrial sources such as glutamate dehydrogenase, isocitrate dehydrogenase,and NAD(P)H transhydrogenase.

OXIDATION OF UNSATURATED FATTY ACIDS OCCURS BY A MODIFIED P-OXIDATION PATHWAY... 

Oxidation of unsaturated alcohols by Mn(III) pyrophosphate is also faster than that of saturated alcohols. ... 

The NO2/N2O4 system is a particularly dangerous oxidant of unsaturated and... 

Of particular interest are oxidations of unsaturated alcohols, for example, oxidation of cinnamyl alcohol to cinnamaldehyde,74,75 and special promoters have been added to increase selectivity (Fig. 6.13).75 Although the functions of these promoters are still not fully undestood, some authors attribute their increased selectivity to physical blocking of reaction sites. This blocking reduces the size of the active site ensemble and suppresses the tendency for alcohols to strongly adsorb and dissociate on Pt.75... 

Lipoxygenases, which catalyse the oxidation of unsaturated fatty acids containing the cis,cis-l,4-pentadiene moiety to the corresponding 1 -hydroperoxy-f rans,ds-2,4-diene (Table 2.3), are widely distributed in plants and animals. The mammalian... 

N.A. Porter, S.E. Caldwell, K.A. Mills, Mechanisms of free radical oxidation of unsaturated lipids, Lipids, 30, 277 290 (1995). 

A substance which results in the chemical inactivation of a metal. The catalytic effect of heavy metals, mainly copper and manganese, on the oxidation of unsaturated compounds such as rubber, results in very rapid deterioration. Chelating agents convert the metal into a chelate co-ordination compound and thus render the metal inactive. The term sequestering agents has been applied to chelating agents but this infers that the metal has been removed and not merely inactivated. 

The attack of peroxyl radicals on 0-CH2 groups produces the same functional groups (hydroperoxyl, hydroxy, oxo) as in the case of subsequent hydrocarbon oxidation. The oxidation of unsaturated acids proceeds similarly to the oxidation of olefins [4,7]. 

NA Prilezhaev. Organic Peroxides and their Application for Oxidation of Unsaturated Compounds. Warsaw, 1912 [in Russian]. 

It is well known that neutrophils, monocytes, macrophages, and other phagocytes produce superoxide upon activation with various stimuli and therefore, are potential initiators of lipid peroxidation. In 1985, Carlin and Arfors [75,76] showed that leukocytes initiate the oxidation of unsaturated lipids. Surprisingly, the leukocyte-initiated peroxidation of linoleic acid was not inhibited by SOD and, therefore, apparently was not initiated by superoxide, while liposome peroxidation was mediated by superoxide. No convincing explanations were given. 

In the last decade numerous studies were dedicated to the study of biological role of nonenzymatic free radical oxidation of unsaturated fatty acids into isoprostanes. This task is exclusively difficult due to a huge number of these compounds (maybe many hundreds). Therefore, unfortunately, the study of several isoprostanes is not enough to make final conclusions even about their major functions. F2-isoprostanes were formed in plasma and LDL after the treatment with peroxyl radicals [98], It is interesting that their formation was observed only after endogenous ascorbate and ubiquinone-10 were exhausted, despite the presence of other antioxidants such as urate or a-tocopherol. LDL oxidation was followed by... 

Besides the oxidation of unsaturated acids such as arachidonic and linoleic acids, LOXs are able to oxidize other substrates. One of the most important oxidative processes catalyzed by LOXs is the oxidation of low-density lipoproteins (LDL). (Nonenzymatic LDL oxidation has been discussed in detail in Chapter 25.) As already mentioned, the oxidation of LDL in the arterial intimal space is an important step in the development of atherogenesis. Now, we will consider the involvement of LOXs in this process. In 1989, Parthasarathy et al. [25] found that... 

LOX-dependent superoxide production was also registered under ex vivo conditions [55]. It has been shown that the intravenous administration of lipopolysaccharide to rats stimulated superoxide production by alveolar and peritoneal macrophages. O Donnell and Azzi [56] proposed that a relatively high rate of superoxide production by cultured human fibroblasts in the presence of NADH was relevant to 15-LOX-catalyzed oxidation of unsaturated acids and was independent of NADPH oxidase, prostaglandin H synthase, xanthine oxidase, and cytochrome P-450 activation or mitochondrial respiration. LOX might also be involved in the superoxide production by epidermal growth factor-stimulated pheochromo-cytoma cells [57]. 

Thus, LOX-catalyzed oxidative processes are apparently effective producers of superoxide in cell-free and cellular systems. (It has also been found that the arachidonate oxidation by soybean LOX induced a high level of lucigenin-amplified CL, which was completely inhibited by SOD LG Korkina and TB Suslova, unpublished data.) It is obvious that superoxide formation by LOX systems cannot be described by the traditional mechanism (Reactions (1)-(7)). There are various possibilities of superoxide formation during the oxidation of unsaturated compounds one of them is the decomposition of hydroperoxides to alkoxyl radicals. These radicals are able to rearrange into hydroxylalkyl radicals, which form unstable peroxyl radicals, capable of producing superoxide in the reaction with dioxygen. 

In most palladium-catalyzed oxidations of unsaturated hydrocarbons the reaction begins with a coordination of the double bond to palladium(II). In such palladium(II) olefin complexes (1), which are square planar d8 complexes, the double bond is activated towards further reactions, in particular towards nucleophilic attack. A fairly strong interaction between a vacant orbital on palladium and the filled --orbital on the alkene, together with only a weak interaction between a filled metal d-orbital and the olefin ji -orbital (back donation), leads to an electrophilic activation of the alkene9. 

V.C.8.1. Alkenes and Alcohol Functions. Although TS-1 and other titanosi-licates oxidize alcohols to the corresponding aldehydes and ketones, the rates are suppressed in the presence of compounds containing C=C bonds. CH3OH, for example, is not oxidized at all during epoxidations of alkene reactants. Higher alcohols, however, are partially oxidized. The oxidation of unsaturated alcohols in the presence of TS-1 is shown in Table XVII (193). 

Oxidation of unsaturated alcohols in the presence ofTS-1 effect of alkene structure on selectivity... 

Odor and color stability problems were also related to the alkyl chains used for SAI. These could be traced to the oxidation of unsaturated carbons, such as oleic acid (Ci8 fatty acid with a single double bond between carbon 9 and 10, i.e. bond position 9 counted from the carboxyl carbon), linoleic acid (Cis fatty acid with two double bonds at position 9 and 12), and linolenic acid (Cis fatty acid with three double bonds at position 9, 12, and 15). Natural coconut fatty acid contains about 6% oleic acid, about 3% linoleic acid, and less than 1% linolenic acid. Tallow fatty acid contains nearly 44% oleic and about 6% of other unsaturates [20]. Partial hydrogenation of the coconut fatty acid used in the manufacture of SCI served to eliminate linoleic and linolenic acids for improved odor stability, while not eliminating oleic acid, which is important for good lather. 

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