Hydroperoxide decomposition catalyzed chain reactions

Fig. 8-4. Examples of radical chain reaction initiated by oxygen and decomposition of hydroperoxides catalyzed by transition metal ions. R denotes an organic residue.

Thermal oxidation is also autocatalytic and considered as metal-catalyzed because it is very difficult to eliminate trace metals (from fats and oils or food) that act as catalysts and may occur as proposed in Equation 4. Redox metals of variable valency may also catalyze decomposition of hydroperoxides (Scheme 2, Equations [6] and [7]). Direct photooxidation is caused by free radicals produced by ultraviolet radiation that catalyzes the decomposition of hydroperoxides and peroxides. This oxidation proceeds as a free radical chain reaction. Although there should be direct irradiation from ultraviolet light for the hpid substrate, which is usually uncommon under normal practices, the presence of metals and metal complexes of oxygen can become activated and generate free radicals or singlet oxygen. 

It is well recognized that the oxidation of cyclohexene with O2 in the presence of several transition metal catalysts is a free-radical chain reaction giving cyclohexenyl hydroperoxide as an intermediate (eq. 2). Some metal complexes are known to catalyze the formation of the hydroperoxide, while others catalyze the decomposition of the peroxide to the oxygenated products 1-3. 

Radical reactions are characterized by complex product distributions since oxygen exhibits high reactivity towards organic reactants, metal centers, and many ligands. Metals play an important role as initiators for radical chain reactions. Radicals are often generated by metal-catalyzed decomposition of organic hydroperoxides [15],... 

This fact illustrates the point where the functions of metal salt catalysts become apparent. If oxidation to the alcohol, ketone or carboxylic acid (i.e. beyond the hydroperoxide stage) is the objective, metal catalysts should be used to promote decomposition of the hydroperoxide. The metal ion (complex) catalyzed decomposition of hydroperoxides is responsible for the sustained and rapid formation of radicals participating in a chain reaction. The most effective are metals with at least two accessible oxidation states. Both components of a redox couple may be capable of reacting with alkyl hydroperoxides ... 

Cyclohexene oxidations in the presence of a variety of acetylacetonates [442] were found to be free radical chain reactions having the same homogeneous propagation steps and yielding as the principle primary product, cyclohexenyl hydroperoxide. The metal catalyzed decomposition of the primary product appeared to give rise to varying amounts of the principle stable monomeric products of oxidation 2-cyclohexene-l-one, 2-cyclohexene-l-ol and cyclohexene oxide. 

In metal-catalyzed auto-oxidation the role of the metal ion is to initiate the radical chain. Reactions 8.3.1.6 and 8.3.1.7 show the initiation steps when metal ions are present. The initial hydroperoxide required for metal-catalyzed decomposition, reactions 8.3.1.6 and 8.3.1.7, is normally present in trace quantities in most hydrocarbons. 

It is clear from a recent review of the mechanisms of metal-catalyzed oxidations of hydrocarbons (89) that by far the most extensive studies have been on the oxidation of alkenes and aromatic compounds relatively little work on alkane oxidation is to be found. The studies on these reactions show that, if the reactivity is enhanced by a hard metal, it is often because the metal becomes involved in the free-radical reactions and generates further free radicals by the chain decomposition of hydroperoxides (39) ... 

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) ... 

Cobalt compounds are generally the more effective catalysts and consequently have received the most attention.34,112 131c Much information has been gained from studies of cobalt-catalyzed decompositions of alkyl hydroperoxides under nonautoxidizing conditions. One important point to be borne in mind in these studies is that radical-induced chain decomposition of the hydroperoxide, via reactions (55) and (56), is always in competition with decomposition via the foregoing cycle. 

As mentioned above, catalytic oxidation of olefins via coordination catalysis with an intermediate such as LnM (olefin) 02 seemed an attractive possibility, and Collman s group (45) tentatively invoked such catalysis in the 02-oxidation of cyclohexene to mainly 2-cyclo-hexene-1-one promoted by IrI(CO)(PPh3)2, a complex known to form a dioxygen adduct. Soon afterwards (4, 46, 47) such oxidations involving d8 systems generally were shown to exhibit the characteristics of a radical chain process, initiated by decomposition of hydroperoxides via a Haber-Weiss mechanism, for example Reactions 10 and 11. Such oxidations catalyzed by transition-metal salts such as... 

The rate constant of the reaction of cobalt(III) acetate with benzaldehyde in the absence of dioxygen was determined in independent experiments. It turned out to be virtually the same as the rate constant of the chain initiation in the oxidation reaction in the presence of O2- However, the contribution of chain initiation to the radical formation is insignificant in the developed oxidation process. The radicals are mainly formed in the reactions of the intermediates in the process of degenerate chain branching. These reactions are also catalyzed by transition metal ions. Especially well studied is the acceleration of radical decomposition of intermediately formed hydroperoxides (see, e.g., [10]). 

In mammalian tissues, cytochrome P-450 may catalyze the generation of free radicals via the decomposition of hydroperoxides (Horton and Fairhurst, 1987). The reaction in skeletal muscle microsomes is dependent on the presence of NADH or NADPH, ADP and iron (Rhee, 1988). Also, the NADPH dehydrogenase of the respiratory chain may initiate lipid peroxidation via the generation of superoxide radicals (Horton and Fairhurst, 1987). Furthermore, peroxidases, cyclooxygenases, prostaglandin synthase, methemoglobin and microsomal oxidases in animal tissues have been implicated in the initiation and promotion of lipid peroxidation in muscle tissues (Kanner and Harel, 1985 Hsieh and KinseUa, 1989). 

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