Promoters hydroperoxide decomposition

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. 

In addition to hydroperoxide decomposition and peroxy-radical reduction, manganese-ion catalysts have a pronounced tendency to promote the rapid oxidation of carbonyl-containing intermediates via enol mechanisms [49, 52-56] ... 

True antioxidants can be classed as deinitiators and chain terminators. Deinitiators discourage the formation of hydroperoxides or promote their decomposition in such a way that fewer free radicals are formed. Chain terminators enter the kinetic chain and terminate the propagating free radicals. 

Certain sulfur compoimds also convert hydroperoxides to alcohols, while undergoing oxidation to form sulfoxides. The sulfoxides can oxidise further to form acidic catalysts that are capable of promoting further hydroperoxide decomposition. 

The rate of autoxidation is thus proportional to the square root of the metal ion concentration. However, this rate does not hold when the concentration of metal initiator is too small to measure. In many food and biological systems, metal catalysts may be coordinated with ligands as complexes or may exist as dimers or higher molecular weight compounds. Other chelating materials may form strong complexes with metals and inactivate their catalytic effects in promoting hydroperoxide decomposition (Chapter 4). 

Some stabilizers appear to provide protection through more than one mechanism. For example, stabilization by HALS is generally attributed to radical scavenging but it has been suggested that in addition HALS may promote hydroperoxide decomposition, though this has been disputed by some researchers. 

However, the activity of a heme(in) protein towards hydroperoxides is influenced by its steric accessibility to fatty acid hydroperoxides. Hydroperoxide binding to the Fe-porphyrin moiety of native catalase and peroxidase molecules is obviously not without interferences. The prosthetic group is free to promote hydroperoxide decomposition only after heat denaturation of the enzymes. Indeed, a model experiment with peroxidase showed that the peroxidation of linoleic acid increased by a factor of 10 when the enzyme was heated for 1 minute to 140 °C. As expected, the enzymatic activity of peroxidase decreased and was only 14%. Similar results were obtained in reaction systems containing catalase. 

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. 

Depending on the peroxide class, the rates of decomposition of organic peroxides can be enhanced by specific promoters or activators, which significantly decrease the energy necessary to break the oxygen—oxygen bond. Such accelerated decompositions occur well below the peroxides normal appHcation temperatures and usually result in generation of only one usehil radical, instead of two. An example is the decomposition of hydroperoxides with multivalent metals (M), commonly iron, cobalt, or vanadium ... 

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. 

The di(hydroxyaLkyl) peroxide (2) from cyclohexanone is a soHd which is produced commercially. The di(hydroxyaLkyl) peroxide (2) from 2,4-pentanedione (11, n = 1 X = OH) is a water-soluble soHd which is also produced commercially (see Table 5). Both these peroxides are used for curing cobalt-promoted unsaturated polyester resins. Because these peroxides are susceptible to promoted decomposition with cobalt, they must exist in solution as equihbrium mixtures with hydroperoxide stmctures (122,149). 

Materials that promote the decomposition of organic hydroperoxide to form stable products rather than chain-initiating free radicals are known as peroxide decomposers. Amongst the materials that function in this way may be included a number of mercaptans, sulphonic acids, zinc dialkylthiophosphate and zinc dimethyldithiocarbamate. There is also evidence that some of the phenol and aryl amine chain-breaking antioxidants may function in addition by this mechanism. In saturated hydrocarbon polymers diauryl thiodipropionate has achieved a preeminent position as a peroxide decomposer. 

Zinc dithiophosphates act as anti-oxidants by promoting the decomposition of hydroperoxides. The mechanism of this reaction is complicated involving hydroperoxides and peroxy radicals192,193 and is also affected by the other additives present in the lubricant oil.194 However the first step is thought to be a rapid initial reaction of the zinc dithiophosphate and hydroperoxide to give a basic compound [Zn4(/i4-0)(S2P(0R)2)6] (Equation 88 Figure 9).141... 

Antioxidants act so as to interrupt this chain reaction. Primary antioxidants, such as hindered phenol type antioxidants, function by reacting with free radical sites on the polymer chain. The free radical source is reduced because the reactive chain radical is eliminated and the antioxidant radical produced is stabilised by internal resonance. Secondary antioxidants decompose the hydroperoxide into harmless non-radical products. Where acidic decomposition products can themselves promote degradation, acid scavengers function by deactivating them. 

The oxidation of polymers can be represented by an autocatalytic mechanism involving the intermediate formation of hydroperoxides (Scheme 1) (B-79MIU501). The termination steps involving reactions of peroxy radicals are thought to predominate in most cases. Metal ion impurities accelerate degradation by promoting decomposition of intermediate hydroperoxides (B-79MI11502). 

Figure 4 Scheme of ozone decomposition mechanism in water. P = promoter (e.g., ozone, methanol). S = scavenger or inhibitor (i.e., /-butanol, carbonate ion). I = initiators (e.g., hydroxyl ion and hydroperoxide ion). 

A method for introducing remote double bonds by the ferrous sulfate-cupric acetate-promoted decomposition of certain alkyl hydroperoxides has recently been reported,108 e.g.,... 

The metal ion-promoted decomposition of alkyl hydroperoxides can be employed as a method for introducing the alkylperoxy group into various substrates673 b 103,109 ... 

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