Autoxidation scheme

A likely biological function for the superoxide dismutase proteins (SOD) is to remove Oa -, and thereby preclude formation of HOO- [Eq. (5-18)] and prevent initiation of lipid peroxidation and autoxidation (Scheme 5-2). An SOD model... 

In the course of the synthesis of quaternary triazolopyrimidinium salts, such as 35 (Section II,A,5, Scheme 10), betains 99 are formed along with the salts by autoxidation (Scheme 27) (01CHE338). There are contributions of several 6-oxo resonance structures. 

This autoxidative scheme can be verified by ESR studies. The 17-line spectrum reported in the literature to be the alkyl (4) radical R formed during polypropylene irradiation at room temperature is shown in Figure 1(a) (3, 5). When the polypropylene irradiated in vacuum is exposed to oxygen, ESR spectra change progressively, as illustrated in Figure 1. After several days the peroxy radical (6) [Figure 1(d)] is the dominant species. If the sample is irradiated and then allowed to decay in vacuum, the peroxy radical does not form, as illustrated in... 

The basic autoxidation scheme of polymers that results in OL has been presented by Stivala et al. (160). This scheme involves the following reactions ... 

Polyethylene a) Product yield Basic autoxidation scheme and secondary oxidations of products. Percentages of major products formed. Same for (ethylene-ketone) copolymers. [118-121]... 

Gillen K.T., Wise J., Clough R.L. (1995). General solution for the basic autoxidation scheme. Polymer Degradation and Stability, 47 (1), 149-161. 

Figure 29 Simplified autoxidation scheme of polymers with dimerization of two peroxy radicals followed by the formation of alcohol, oxygen, and a carbonyl moiety in an excited singlet state that deactivates under photon emission.

Complex reactions occur on the autoxidation of pyrroles (see Section 3.05.1.4) predictably, susceptibility to autoxidation increases with increasing alkyl substitution, llie photosensitized reaction of pyrrole and oxygen yields 5-hydroxy-A -pyrrolin-2-one, probably by way of an intermediate cyclic peroxide (Scheme 28) (76JA802). 

From the preceding examples it can be seen that oxidants and electrophilic reagents attack pyrroles and furans at positions 2 and 5 in the case of indoles the common point of attack is position 3. Thus autoxidation of indoles e.g. 99) gives 3-hydroperoxy-3H-indoles (e.g. 100). Lead tetraacetate similarly reacts at the 3-position to give a 3-acetoxy-3H-indole. Ozone and other oxidants have been used to cleave the 2,3-bond in indoles (Scheme 30) (81BCJ2369). 

The Apomorphine-derived alkaloid PO-3 (129) was isolated as violet needles after crystallization from acetone and ether from Papaver orientale (66MI2), but was not found in the green solutions of autoxidized apomorhine hydrochloride (62M941, 68HCA683) (Scheme 51). No anion was detected by elemental analysis. The pA"a of PO-3 is 3.88 0.02 in 50% ethanol. The IR spectrum displays no carbonyl absorption between 1650 and 1700 cm (69MI2). The UV absorption maxima of PO-3 are in agreement with the formulation of a mesomeric betaine [T-max (EtOH) = 310... 

Bateman, Gee, Barnard, and others at the British Rubber Producers Research Association [6,7] developed a free radical chain reaction mechanism to explain the autoxidation of rubber which was later extended to other polymers and hydrocarbon compounds of technological importance [8,9]. Scheme 1 gives the main steps of the free radical chain reaction process involved in polymer oxidation and highlights the important role of hydroperoxides in the autoinitiation reaction, reaction lb and Ic. For most polymers, reaction le is rate determining and hence at normal oxygen pressures, the concentration of peroxyl radical (ROO ) is maximum and termination is favoured by reactions of ROO reactions If and Ig. 

The development of the autoxidation theory, in which the propagating radicals, alkyl, and alkylperoxyl (R ROO ), and the hydroperoxide (ROOH) are the key intermediates, has therefore led to a comprehensive theory of antioxidant action Scheme 2 shows the two major... 

It should be pointed out that not all benzoin derivatives (75) are suitable for use as photoinitialors. Benzoin esters (75, R=aeyl) undergo a side reaction leading to furan derivatives. Aryl ethers (75, R=aryl) undergo (3-seission to give a phenoxy radical (an inhibitor) in competition with a-scission (Scheme 3.54). Benzoin derivatives with a-hydrogens (75 R-Il) are readily autoxidized and consequently can have poor shelf lives. 

The Diels-Alder reaction of 2-vinylfurans 73 with suitable dienophiles has been used to prepare tetrahydrobenzofurans [73, 74] by an extra-annular addition these are useful precursors of substituted benzofurans (Scheme 2.29). In practice, the cycloadditions with acetylenic dienophiles give fully aromatic benzofurans directly, because the intermediate cycloadducts autoxidize during the reaction or in the isolation procedure. In the case of a reaction with nitro-substituted vinylbenzofuran, the formation of the aromatic products involves the loss of HNO2. 

The blue dye (47), formed from the autoxidation of 4-/V,TV-di methyl-amino-2-hydroxyaniline, is the oxygen analogue of methylene blue. The autoxidation of 1,2,4-trihydroxybenzene, carried out in the presence of ammonia, gives the hydroxyphenoxazinone dye (48) via a 2,4-dihydroxyani-line intermediate (Scheme 17). Many types of phenoxazines, phenazines, and phenoxazinium salts can be obtained by autoxidation of polyhydroxyben-zenes and their amino derivatives. Some autoxidative dyes may give poly-... 

Inhibit autoxidation of organic materials by interfering with free radical reactions that lead to incorporation of oxygen into macromolecules in a chain mechanism consisting of two interacting cyclical processes (Scheme II.1). 

Recently, we have demonstrated another sort of homogeneous sonocatalysis in the sonochemical oxidation of alkenes by O2. Upon sonication of alkenes under O2 in the presence of Mo(C0) , 1-enols and epoxides are formed in one to one ratios. Radical trapping and kinetic studies suggest a mechanism involving initial allylic C-H bond cleavage (caused by the cavitational collapse), and subsequent well-known autoxidation and epoxidation steps. The following scheme is consistent with our observations. In the case of alkene isomerization, it is the catalyst which is being sonochemical activated. In the case of alkene oxidation, however, it is the substrate which is activated. 

The type I mechanism is a radical process, and involves the excited state of the photosensitizer in electron-transfer processes, as indicated in Scheme 1. The reactions there are essentially photochemically stimulated autoxidation processes. 

We propose that the first step in the formation of quinones, as shown in Scheme 3 for BP, involves an electron transfer from the hydrocarbon to the activated cytochrome P-450-iron-oxygen complex. The generate nucleophilic oxygen atom of this complex would react at C-6 of BP in which the positive charge is appreciably localized. The 6-oxy-BP radical formed would then dissociate to leave the iron of cytochrome P-450 in the normal ferric state. Autoxidation of the 6-oxy-BP radical in which the spin density is localized mainly on the oxygen, C-l, C-3 and C-12 (19,20) would produce the three BP diones. 

Since the substituted hydroquinones and quinone dioximes are better electron donors than hexamethylbenzene (as established by cyclic voltammetric studies), donor-induced disproportionation (to generate NO+ NOf) is even more favored. Furthermore, either two successive one-electron oxidations of hydro-quinone (or quinone dioxime) by NO + followed by the loss of two protons from the dication or two sequential oxidation/deprotonation steps complete the oxidative transformation in equation (97). Importantly, the ready aerial oxidation of NO to NO provides the basis for the nitrogen oxide catalysis of hydroquinone (or quinone dioxime) autoxidation as summarized in Scheme 26. 

Moreover, the efficiency of catalytic autoxidation in Scheme 26 is also attributed to the short lifetime of the hydroquinone cation radical (t < 10 10 s-1)254, which renders the sequential electron-transfer/proton-trans-fer cycles extremely efficient. 

R. F. Vasil ev and coworkers 14> suggested that within the well-known general scheme of radical chain autoxidation reactions ... 

Phosphites can react not only with hydroperoxides but also with alkoxyl and peroxyl radicals [9,14,17,23,24], which explains their susceptibility to a chain-like autoxidation and, on the other hand, their ability to terminate chains. In neutral solvents, alkyl phosphites can be oxidized by dioxygen in the presence of an initiator (e.g., light) by the chain mechanism. Chains may reach 104 in length. The rate of oxygen consumption is proportional to v 1/2, thus indicating a bimolecular mechanism of chain termination. The scheme of the reaction... 

The common element of Schemes 1-3 is that they each postulate direct interaction between the metal center and dioxygen. Although it is not stated explicitly, Eqs. (3) and (11) most likely proceed via an inner-sphere mechanism. Thus, the metal-dioxygen interaction implies spin pairing between the reactants when the metal ion is paramagnetic. As a consequence, the formation of the M-O2 type intermediates circumvents the restriction posed by the triplet to singlet transition which seems to be the major kinetic barrier of autoxidation reactions (5). 

It should be emphasized that clear-cut situations described in Schemes 1-3 are uncommon and typically the combination of these models needs to be considered for kinetic and mechanistic description of a real system. However, even when one of the limiting cases prevails, each of these models may predict very different formal kinetic patterns depending on where the rate determining step is located. For the same reason, different schemes may be consistent with the same experimental rate law, i.e. thorough formal kinetic description of a reaction and the analysis of the rate law may not be conclusive with respect to the mechanism of the autoxidation process. 

The model shown in Scheme 2 indicates that a change in the formal oxidation state of the metal is not necessarily required during the catalytic reaction. This raises a fundamental question. Does the metal ion have to possess specific redox properties in order to be an efficient catalyst A definite answer to this question cannot be given. Nevertheless, catalytic autoxidation reactions have been reported almost exclusively with metal ions which are susceptible to redox reactions under ambient conditions. This is a strong indication that intramolecular electron transfer occurs within the MS"+ and/or MS-O2 precursor complexes. Partial oxidation or reduction of the metal center obviously alters the electronic structure of the substrate and/or dioxygen. In a few cases, direct spectroscopic or other evidence was reported to prove such an internal charge transfer process. This electronic distortion is most likely necessary to activate the substrate and/or dioxygen before the actual electron transfer takes place. For a few systems where deviations from this pattern were found, the presence of trace amounts of catalytically active impurities are suspected to be the cause. In other words, the catalytic effect is due to the impurity and not to the bulk metal ion in these cases. 

Searching for other oscillatory autoxidation reactions led Druliner and Wasserman to use cyclohexanone as a substrate instead of benzalde-hyde (168). Unlike the simple stoichiometry found for the benzaldehyde reaction, the ketone gives at least six or more products, and the relative amounts of these vary substantially with the experimental conditions (Scheme 7). 

The ability of organoboron compounds to participate in free radical reactions has been identified since the earliest investigation of their chemistry [1-3]. For instance, the autoxidation of organoboranes (Scheme 1) has been proven to involve radical intermediates [4,5]. This reaction has led recently to the use of triethylborane as a universal radical initiator functioning under a very wide range of reaction conditions (temperature and solvent) [6,7]. 

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