Hydroperoxide decomposition transition metal-promoted

Eithei oxidation state of a transition metal (Fe, Mn, V, Cu, Co, etc) can activate decomposition of the hydiopeioxide. Thus a small amount of tiansition-metal ion can decompose a laige amount of hydiopeioxide. Trace transition-metal contamination of hydroperoxides is known to cause violent decompositions. Because of this fact, transition-metal promoters should never be premixed with the hydroperoxide. Trace contamination of hydroperoxides (and ketone peroxides) with transition metals or their salts must be avoided. 

Transition metal-promoted hydroperoxide deconposition is inqiortant to the oxidative stability and quality of foods for several reasons. First, the abstraction of hydrogen from an unsaturated fatty acid results in the formation of a single alkyl radical. Followii hydrogen abstraction, oxygen adds to the alkyl radical to form a peroxyl radical and subsequent abstraction of a hydrogen from another fatty acid or antioxidant to form a lipid hydroperoxide (Figure 1). These reactions by themselves do not result in an increase in free radical numbers. If these reactions were tiie only steps in tiie lipid oxidation reactions, the rapid exponential increase in oxidation that is commonly observed in lipids would not occur. Transition metal-promoted decomposition of lipid hydroperoxides results in the formation of additional radicals (e.g. alkoxyl and peroxyl) which exponentially increase oxidation rates as they start to attack otiier unsaturated fatty acids. 

We studied the oxidation of cyclohexene at 70°C in the presence of cyclopentadienylcarbonyl complexes of several transition metals. As with the acetylacetonates, the metal center was the determining factor in the product distribution. The decomposition of cyclohexenyl hydroperoxide by the metal complexes in cyclohexene gave insight into the nature of the reaction. With iron and molybdenum complexes the product profile from hydroperoxide decomposition paralleled that observed in olefin oxidation. When vanadium complexes were used, this was not the case. Variance in product distribution between the cyclopentadienylcarbonyl metal-promoted oxidations as a function of the metal center were more pronounced than with the acetylacetonates. Results are summarized in Table V. 

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... 

At low temperature, decomposition of a hydroperoxide initiator may require promotion. Transition-metal ions such as Fe2+, Co2+, Ag+, etc., that can easily switch valence states can serve this purpose. A likely mechanism of such promotion by an ion Me2+ is the Haber-Weiss redox cycle [2] ... 

PINO generation being a key step of the overall process, in most cases, the use of NHPI was proposed in combination with different cocatalysts or initiators. Several examples report the beneficial effect of transition metal salts and complexes for this purpose [4]. In this context, the main role of metal salts (including Mn, Co, Cu, V, and Fe salts) is not only to accelerate classical autoxidation, by promoting the decomposition of the intermediate hydroperoxides (Scheme 16.3a), but also to bind oxygen (Scheme 16.3b), leading to the formation of PINO radical without requiring thermal treatment (Scheme 16.3c). 

Once a small amount of hydroperoxides is formed, the transition metals can promote the decomposition of the preformed hydroperoxides due to their unpaired electrons in the 3d and 4d orbitals. A metal, capable of existing in two valence states typically acts as ... 

However, because of the high temperature nature of this class of peroxides (10-h half-life temperatures of 133-172°C) and their extreme sensitivities to radical-induced decompositions and transition-metal activation, hydroperoxides have very limited utility as thermal initiators. The acid-promoted decomposition to produce radicals has been reported (122). The oxygen-hydrogen bond in... 

It is well documented that transition metals such as chromium, copper, iron and vanadium can catalyse the degradation of polymers [132, 139, 144]. These metals promote the decomposition of hydroperoxides, which are important in the degradation mechanism of most polymers. 

Transition metals will promote oxidative reactions by hydrogen abstraction and by hydroperoxide decomposition reactions that lead to the formation of free radicals. Prooxidative metal reactivity is inhibited by chelators. Chelators that exhibit antioxidative properties inhibit metal-catalyzed reactions by one or more of the following mechanims prevention of metal redox cycling occupation of all metal coordination sites thus inhibiting transfer of electrons formation of insoluble metal complexes stearic hinderance of interactions between metals and oxidizable substrates (e.g., peroxides). The prooxidative/antioxidative properties of a chelator can often be dependent on both metal and chelator concentrations. For instance, ethylene diamine tetraacetic acid (EDTA) can be prooxidative when EDTAiiron ratios are <1 and antioxidative when EDTAiiron is >1. The prooxidant activity of some metal-chelator complexes is due to the ability of the chelator to increase metal solubility and/or increase the ease by which the metal can redox cycle. 

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