Oxidation reactions autooxidation

In this context, homolytical oxidation reactions (autooxidation) proceeding by the free-radical mechanism seem to be the most suitable model. In autooxidation reactions ROOH dissociation catalysis (in the most reduced case, H202) plays the key role, but does not eliminate selectivity. This is the most urgent problem of homolytical oxidation. 

We call this type of reaction autooxidation because it is a an autocatalytic process (the reaction generates radical intermediates that propagate chain reactions) and it is an oxidation that converts alkanes into alkyl peroxides. 

Figure 12. Direct P-450 type oxidation, Reaction 5 TPP Mn autooxidation, Reaction 6...

When an oxidation reaction involves molecular oxygen, the reaction occurs spontaneously under mild conditions. It is known as autooxidation. In an autooxidation process, free radicals, formed by thermal or photolytic cleavage of chemical bonds (e.g., peroxide, ROOH) or redox processes with metal ions present in raw material impurities, are involved... 

Moreover, formation of radical transients with S.-.O bonds is kinetically preferred, but on longer time scale they convert into transients with S.-.N bonds in a pH dependent manner. Ultimately transients with S.-.N bonds transform intramolecularly into C-centred radicals located on the C moiety of the peptide backbone. Another type of C-centred radicals located in the side chain of Met-residue, a-(aikylthio)alkyl radicals, are formed via deprotonation of MetS +. C-centred radicals are precursors for peroxyl radicals (ROO ) that might be involved in chain reactions of peptide and/or protein oxidation. Stabilization of MetS +through formation of S.-.O- and S.-.N-bonded radicals might potentially accelerate oxidation and autooxidation processes of Met in peptides and proteins. Considering that methionine sulfoxide, which is the final product coming from all radicals centred on sulphur, is restored by the enzyme methionine sulfoxide reductase into MetS, stabilization of MetS +appears as a protection against an eventual peroxidation chain that would develop from a carbon centred radical. 

The mechanism in Scheme 8 was proposed for the oxidation reaction. In the first step, the Cu(II) salt, which is formed in the autooxidation of cuprous chloride, forms a complex with the amine. This is followed by a rate-determining electron transfer from the amine to the Cu(II) species giving Cu(I) and an aminium radical. The subsequent steps were considered to be fast. The authors accounted for the secondary hydrogen-deuterium kinetic isotope effect by suggesting that there was hyperconjugative electron release to the aminium ion nitrogen that forms in the slow step of the reaction. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

Synthetic melanins are obtained by biomimetic oxidation reactions using known precursors. So far, four different methods for melanin synthesis have been reported, i.e. in vitro enzymatic, autooxidative, electrochemical, and photochemical methods. Of these, the first two have been generally used, for large scale preparations of the pigment polymers and have been reviewed elsewhere (70, 211). The latter two methods which are discussed here, have been used effectively to understand the mechanism of the melanization process in biological systems. 

Autooxidation. Liquid-phase oxidation of hydrocarbons, alcohols, and aldehydes by oxygen produces chemiluminescence in quantum yields of 10 to 10 ° ein/mol (128—130). Although the efficiency is low, the chemiluminescent reaction is important because it provides an easy tool for study of the kinetics and properties of autooxidation reactions including industrially important processes (128,131). The light is derived from combination of peroxyl radicals (132), which are primarily responsible for the propagation and termination of the autooxidation chain reaction. The chemiluminescent termination step for secondary peroxy radicals is as follows ... 

Also autooxidation or auto-oxidation. A slow, easily initiated, self-catalyzed reaction, generally by a free-radical mechanism, between a substance and atmospheric oxygen. Initiators of autoxidation include heat, light, catalysts such as metals, and free-radical generators. Davies (1961) defines autoxidation as interaction of a substance with molecular oxygen below 120°C without flame. Possible consequences of autoxidation include pressure buildup by gas evolution, autoignition by heat generation with inadequate heat dissipation, and the formation of peroxides. 

Figure 10-4 Reaction steps to make propylene oxide and either isobutylene or styrene by autooxidation.

Brandt, C., I. Fabian, and R. van Eldik, Kinetics and Mechanism of the Iron(III)-Catalyzed Autooxidation of SulfuKIV) Oxides in Aqueous Solution—Evidence for the Redox Cycling of Iron in the Presence of Oxygen and Modeling of the Overall Reaction Mechanism, Inorg. Chem., 33, 687-701 (1994). 

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