Radical-chain autoxidation

A question which inevitably arises on surveying the enormous sucess of the Amoco catalyst is why the combination Co/Mn/Br in acetic acid In order to answer this question we must first examine the mechanism of free radical chain autoxidations of alkylaromatics (ref. 4). 

This oxidative process has been successful with ketones,244 esters,245 and lactones.246 Hydrogen peroxide can also be used as the oxidant, in which case the alcohol is formed directly.247 The mechanisms for the oxidation of enolates by oxygen is a radical chain autoxidation in which the propagation step involves electron transfer from the carbanion to a hydroperoxy radical.248... 

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

A large number of Group VIII metal-dioxygen complexes catalyze the oxidation of phosphine to phosphine oxide or isocyanides to isocyanates by molecular oxygen.6,12,56,140,141 146,184 However, their use as catalysts for the oxidation of alkenes generally leads to the same products as those obtained from free radical chain autoxidations.184,196-198... 

It may be concluded from the preceding discussion that at this juncture there is no bona fide evidence for the initiation of autoxidations by direct hydrogen transfer between metal-dioxygen complexes and hydrocarbon substrates. Although such a process may eventually prove feasible, in catalytic systems it will often be readily masked by the facile reaction of the metal complex with hydroperoxide. The choice of cumene as substrate by many investigators is somewhat unfortunate for several reasons. Cumene readily undergoes free radical chain autoxidation under mild conditions and its hydroperoxide readily decomposes by both homolytic and heterolytic processes. 

We have mentioned in Section II.B.2 studies of the oxidation of olefins by molecular oxygen in the presence of low-valent Group VIII metal complexes, with the expectation of effecting homogeneous, nonradical oxidation processes. However, these reactions were shown to involve the usual free radical chain autoxidation, and no direct transfer of oxygen from a metal-dioxygen complex to an olefin was demonstrated. 

Vanadyl salen complexes epoxidized cyclohexene presumably through intermediate hydroperoxides formed by radical chain autoxidation <1997105927, 1998TL5923>. Reactions using vanadium complexes have been carried out in liquid carbon dioxide <19990M4916>. 

One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily imdergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). 

The holy grail of oxidation chemistry is the design of catalytic systems capable of mediating oxygen transfer from dioxygen, without the need for a sacrificial reductant, i.e. a Mars-van Krevelen mechanism [18] in the liquid phase. Indeed, the confinement of substrate molecules in the micropores of molecular sieves might be expected to create quasi gas phase conditions conductive to such a mechanism at the expense of free radical chain autoxidation (we note, however, that a mechanism involving two metal centres as shown in eq. 12 would be difficult to achieve in a molecular sieve). 

TBA-I catalyzed the oxidation of cyclohexane with 1 atm molecular oxygen at 365 K. The main products wa e cyclohexanol and cyclohexanone and an induction period was observed. The selectivities changed little with time. A small amount of dicyclohexyl, which is formed by the reaction of two cyclohexyl radicals, was observed. Neither acids nor oxoesters were observed. The induction period and the formation of dicyclohexyl suggest that the reaction involves a radical-chain autoxidation mechanism. The... 

This mechanism involves the reduction of Fe(III) followed by the addition of dioxygen to produce dioxo species IX-1. This species reacts with a second molecule of catalyst (intermediate IX-2) giving two molecules of the monooxo complex IX-3, which is capable of oxidizing the alkane. Species IX-3 abstracts a hydrogen atom from the alkane to generate an alkyl radical and the hydroxy derivative IX-4. However, the alternative mechanism does not consider Fe(II)-02 species and assumes the conventional radical-chain autoxidation [30d]. Such a mechanism is depicted in Scheme 1X.3. 

Scheme DC.3. The tilleniative mechanism for the air oxidation of alkanes assuming a radical-chain autoxidation.

The anion of 2-nitropropane in EtOH/EtOLi will undergo a free radical chain autoxidation (Russell, 1953). However, the anion under these conditions does not undergo an electron transfer reaction with molecular O2. The reaction can be initiated by the presence of a nitroaromatic or by the unionized 2-nitropropane according to (1) and (2) (Russell et al., 1965). The initiation step can be accelerated by photolysis whereby the electron accep-... 

A number of reactions involving alkane photooxidation by polyoxometalates have been run under aerobic conditions. Those by PWi2O40 and Wio032 are summarized at the end of Table I (see section 5). Unfortunately these reactions, collectively, are rather uninformative with respect to mechanism. Radical chain autoxidation processes are undoubtedly present and few if any of the reported papers have addressed the requisite experiments to differentiate autoxidation from other oxidation pathways. A similar general limitation exists in the literature on photocatalytic aerobic oxidation of organic compounds by semiconductor materials [66]. 

Since the effects of heavy metals increase the amount of free radicals in the lipid phase, not only do the rates of initiation and propagation reactions increase, but also the rate of termination reaction increases. Heavy metals therefore also change the composition of the reaction products. At high concentrations of free radicals, the termination reaction may dominate and metals then act as the inhibitors of autoxidation. Autoxidation reaction can also be inhibited by metals when they are present at higher concentrations. It is assumed that the reason is the oxidation and reduction of free hydrocarbon radicals to anions and cations by ions of Fe and Cu and the formation of complexes of free radicals. Other complexes are also formed with Co. All these reactions interrupt the radical chain autoxidation reaction. Reactions with Fe ions are given as examples. 

The mechanism of the antioxidant effect of vitamin E is similar to the effect of other lipophilic antioxidants. Tocopherols react with a number of free radicals including active oxygen species. One tocopherol molecule can react with two hydroperoxyl radicals. Autoxi-dation of lipids is inhibited by reaction of tocopherols (abbreviated as T-OH) with hydroperoxyl lipid radicals (R-O-O ) with the formation of hydroperoxides (R O-OH) and radicals of tocopherols (tocopheroxyl radicals, T O ). This reaction interrupts the radical chain autoxidation reaction of Hpids during the propagation phase ... 

Ascorbic acid, its isomers and derivatives can react with free radicals that cause oxidation of lipids and other oxylabile food components. It inhibits the radical chain autoxidation reaction and effectively acts as an antioxidant. The reaction of ascorbic acid with hpid (fatty acid) peroxyl radical (ROO ) or with alkoxyl radical (RO ) can be... 

ROO") produced by lipid oxidation, or with alkoxyl radicals (RO ) arising by decomposition of lipid hydroperoxides. They provide these radicals with hydrogen, thereby interrupting the radical chain autoxidation reaction. The resulting products are phenoxyl (aryloxyl) radicals of antioxidants ... 

There are two general mechanisms for this oxidation which have been considered. One is radical chain autoxidation in which the propagation step involves... 

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