Hydroperoxides in autoxidation

A number of mechanisms have been advanced to explain the formation of all eight cis and trans isomers of 8-, 9-, 10- and 11-hydroperoxides in autoxidized methyl oleate. In one mechanism, the delocalized radicals formed by abstraction of the hydrogens allylic to the double bond of oleate lose their... 

Chan, H.W.-S., and Coxon, D.T. Lipid hydroperoxides, in Autoxidation of Unsaturated Lipids, pp. 17-50 (1987) (edited by H.W.-S. Chan), Academic Press, London. 

Hydroperoxides have been obtained from the autoxidation of alkanes, aralkanes, alkenes, ketones, enols, hydrazones, aromatic amines, amides, ethers, acetals, alcohols, and organomineral compounds, eg, Grignard reagents (10,45). In autoxidations involving hydrazones, double-bond migration occurs with the formation of hydroperoxy—azo compounds via free-radical chain processes (10,59) (eq. 20). 

The peioxy free radicals can abstract hydrogens from other activated methylene groups between double bonds to form additional hydroperoxides and generate additional free radicals like (1). Thus a chain reaction is estabhshed resulting in autoxidation. The free radicals participate in these reactions, and also react with each other resulting in cross-linking by combination. 

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. 

In the preceding paragraph peroxides were described as key intermediates in autoxidation chemiluminescence. In most cases hydroperoxides were involved. The majority are well-defined compounds (e.g. cumene hydroperoxide), but autoxidation reactions are rather complex and peroxides are only one, though very important type of compound involved. 

The reaction of ions with peroxyl radicals appears also in the composition of the oxidation products, especially at the early stages of oxidation. For example, the only primary oxidation product of cyclohexane autoxidation is hydroperoxide the other products, in particular, alcohol and ketone, appear later as the decomposition products of hydroperoxide. In the presence of stearates of metals such as cobalt, iron, and manganese, all three products (ROOH, ROH, and ketone) appear immediately with the beginning of oxidation, and in the initial period (when ROOH decomposition is insignificant) they are formed in parallel with a constant rate [5,6]. The ratio of the rates of their formation is determined by the catalyst. The reason for this behavior is evidently related to the fast reaction of R02 with the... 

The setereochemistry of the products isolated in this reaction is related to that of the hydroperoxides produced in autoxidation of free cod. 

Butenes were subjected to photosensitized reaction with molecular oxygen in methanol. 1-Butene proved unreactive. A single hydroperoxide, l-butene-3-hydroperoxide, was produced from 2-butene and isolated by preparative gas chromatography, Thermal and catalyzed decomposition of pure hydroperoxide in benzene and other solvents did not result in formation of any acetaldehyde or propionaldehyde. The absence of these aldehydes suggests that they arise by an addition mechanism in the autoxidation of butenes where they are important products. l-Butene-3-hydroperoxide in the absence of catalyst is converted predominantly to methyl vinyl ketone and a smaller quantity of methyl vinyl carbinol —volatile products usually not detected in important quantities in the autoxidation of butene. 

The observed half life at 100°C. of 23 hours for a dilute solution of hydroperoxide in benzene indicates that significant decomposition may occur in the autoxidation of butene, depending on reaction conditions. No reliable evaluation can be made because of the known complications introduced on hydroperoxide decomposition by the effect of the solvent, the hydroperoxide concentration (2), the presence of oxygen (12), and the possibility of a strong acceleration in rate in the presence of oxidizing olefin, observed in at least one system (8). However, using the data reported by Bateman for a benzene solvent at 100 °C. in the presence of air (2), l-butene-3-hydroperoxide decomposes 13 times faster than cyclohexene hydroperoxide, a product which may be formed in extremely high yield by the oxidation of cyclohexene. 

Peroxide Decomposition Mechanism. Since virtually no work has been reported which concerns only the mechanism by which zinc dialkyl di-thiophosphates act as peroxide decomposers, it is pertinent to discuss metal dialkyl dithiophosphates as a whole. The mechanism has been studied both by investigating the products and the decomposition rates of hydroperoxides in the presence of metal dithiophosphates and by measuring the efficiency of these compounds as antioxidants in hydrocarbon autoxidation systems in which hydroperoxide initiation is significant. 

Many of the compounds derived from enzyme-catalysed oxidative breakdown of unsaturated fatty acids may also be produced by autoxidation [23]. While the enzymatically produced hydroperoxides in most cases yield one hydroperoxide as the dominant product, non-enzymatic oxidation of unsaturated fatty acids yields a mixture of hydroperoxides which differ in the position of the peroxide group and in the geometrical isomerism of the double bonds [24]. As the number of double bonds increases, the number of oxidation and oxygen-addition sites increases proportionally and thus the number of possible volatile... 

At elevated temperature alkyl hydroperoxides undergo thermal decomposition to alcohols [Eqs. (9.9)—(9.11)]. This decomposition serves as a major source of free radicals in autoxidation. Because of side reactions, such as p scission of alkylperoxy radicals, this process is difficult to control. Further transformation of the alkoxy... 

Autoxidation of alkanes may be carried out by metal catalysis.2,14 17 Although metal ions participate in all oxidation steps, their main role in autoxidation is not in their ability to generate free radicals directly by one-electron oxidation [Eq. (9.14)] but rather their activity to catalyze the homolytic decomposition of the intermediate hydroperoxide according to Eqs. (9.15) and (9.16). As a result of this decomposition, metal ions generate chain-initiating radicals. The overall reaction is given in Eq. (9.17) ... 

The observed lag phase of activity, seen in Figure C4.2.2, is variable in duration and may not be noticed. The lag is generally thought to be due to the time required to convert inactive native Fe2+-LOX into active Fe3+-LOX. Thus, some amount of fatty acid hydroperoxide product is required to prime the pump. As a consequence, relatively long lag phases are often due to either low LOX concentrations, highly purified substrates containing no hydroperoxides from autoxidation, or both. 

Epoxides can also be formed from the oxidation of alkenes by molecular oxygen via in situ generation of hydroperoxides by autoxidation.251,252 An interesting example is the direct stereoselective oxidation of cyclohexene by 02 to syn-l,2-epoxycyclohexan-3-ol catalyzed by CpV(CO)4 with a 65% yield and 99% stereoselectivity (equation 78).253... 

The aldehydes and ketones are least abundant of all the compounds found which may be considered as derived from the fat. The carbonyl compounds are probably produced by an indirect route, which is most likely similar to that involved in autoxidation of a fat. The alkyl free radical can absorb oxygen, form a hydroperoxide, and then follow the many decomposition paths which are familiar in the oxidation chemistry of fats. The more abundant aldehydes found are unsaturated, which further agrees with the hypothesis that they are derived from the decomposition of hydro-... 

It is interesting to consider the concentrations of free radicals that result from lipid hydroperoxides in an in vitro model system. For example, my group has been studying the autoxidation of linoleic acid in SDS micelles at 37°C. We initiate the autoxidation by the decomposition of an initiator, as shown in Equation 3. 

Thermal decomposition of alkyl hydroperoxides represents a major source of free radicals in autoxidation reactions. Unless hydrocarbons are rigorously purified before use, the trace amounts of hydroperoxides present can lead to erroneous results in kinetic studies, especially when there are no added initiators. 

There is an extensive literature dealing with metal-catalyzed decompositions of peroxides.67,68 For the purposes of this article we will concentrate primarily on the reactions of metal complexes with hydrogen peroxide, alkyl hydroperoxides, and peracids, since these are the usual peroxidic intermediates in autoxidations. 

Rhodium and iridium, which are in the same group as cobalt in the Periodic Table, are also expected to effect reactions analogous to Eqs. (95) and (96) this view is supported by recent studies of autoxidations catalyzed by Rh and Ir complexes136-140 (see Section II.B.2). Complexes of these metals rapidly decompose hydroperoxides in a catalytic reaction.141... 

However, a recent kinetic study188 has shown unequivocally that chain initiation proceeds via the usual metal-catalyzed decomposition of the hydroperoxide. Thus, the rate of initiation of the autoxidation of cumene was, within experimental error, equal to the rate of production of radicals in the (Ph3P)4Pd-catalyzed decomposition of tert-butyl hydroperoxide in chlorobenzene at the same temperature and catalyst concentration. Moreover, long induction periods were observed (in the absence of added tert-butyl hydroperoxide), when the cumene was purified by passing it down a column of basic alumina immediately prior to use. Autoxidation of cumene purified by conventional procedures showed only short induction periods. These results further demonstrate the necessity of using highly purified substrates in kinetic studies. 

The first reaction (346) consists of hydroperoxide formation by a typical autoxidation process, and the second represents selective epoxidation by the hydroperoxide. In the absence of the autoxidation catalyst, no reaction is observed under these conditions due to efficient removal of chain-initiating hydroperoxide molecules by reaction (347). Optimum selectivities obtain when the autoxidation catalyst is of low activity, which implies a low total activity of the catalytic system. The molybdenum complexes related to Mo02(oxine)2 are among the most effective catalysts for epoxidation.496 Although the autoxidation catalysts were limited to two types (phosphine complexes of noble metals and transition metal acetylacetonates), there is no reason, a priori, why other complexes such as naphthenates should not produce similar results. 

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