Autoxidation metal catalysed

Table 7.5 Catalytic and deactivating effects of ligands on metal-catalysed autoxidation of petroleum. (After Pedersen )...

The metal-catalysed autoxidation of alkenes to produce ketones (Wacker reaction) is promoted by the presence of quaternary ammonium salts [14]. For example, using copper(II) chloride and palladium(II) chloride in benzene in the presence of cetyltrimethylammonium bromide, 1-decene is converted into 2-decanone (73%), 1,7-octadiene into 2,7-octadione (77%) and vinylcyclohexane into cyclo-hexylethanone (22%). Benzyltriethylammonium chloride and tetra-n-butylammo-nium hydrogen sulphate are ineffective catalysts. It has been suggested that the process is not micellar, although the catalysts have the characteristics of those which produce micelles. The Wacker reaction is also catalysed by rhodium and ruthenium salts in the presence of a quaternary ammonium salt. Generally, however, the yields are lower than those obtained using the palladium catalyst and, frequently, several oxidation products are obtained from each reaction [15]. 

This review is concerned with the quantitative aspects of metal-catalysed oxyradical reactions. As such one will find discussions of structures of metal complexes, rate constants and reduction potentials, not unlike our review of 1985 [34], Two areas related to the role of transition metals in radical chemistry and biology have been reviewed recently these are the metal-ion-catalysed oxidation of proteins [35] and the role of iron in oxygen-mediated toxicities [36]. These topics will not be discussed in detail in this review. Related to this work is a review on the role of transition metals in autoxidation reactions [37]. Additional information can be obtained from Afanas ev s two volumes on superoxide [38,39], This subject is also treated in a more general and less quantitative manner by Halliwell and Gutteridge [40],... 

Martell, A. E., 1980, Dioxygen complexes as intermediates in metal-catalysed oxidation of organic substances, in Autoxidation in Food and Biological Systems (M. G. Simic and M. Karel, eds.), pp. 89-118, Plenum Press, New York and London. 

In addition to a general review, specific reviews of autoxidation reactions, and oxidation of C=C and C=N groups have been published. By considering the consequences of initial metal-hydroperoxide complex formation in metal-catalysed autoxidation reactions it has proved possible to predict the critical metal concentration level at which the catalyst-inhibitor transition occurs. Epoxides are de-oxygenated by Na-Hg in the presence of C5HB)aXa] (M = Mo or W, Xg = Clg or O) the active catalyst is believed to be MCpa (Scheme 4). ... 

A review of recent advances in transition-metal-catalysed oxidations by molecular oxygen has highlighted the scope and limitations, as well as the meehanisms of these reactions. " " An overview of the fundamental studies on a new method of synthesis of nicotinic acid by the gas-phase catalytic oxidation of -picoline by oxygen has been presented. The reactivity of vanadium species has been considered in order to discover the nature of the active catalyst. Kinetic equations for -picoline oxidation on vanadia-titania catalysts have been discussed. " The effect of quaternary ammonium salts or macrocyclic ethers on the autoxidation of ethylbenzene or the decomposition of the a-phenylethyl hydroperoxide intermediate catalysed by Ni(II) or Fe(III) acetylacetonates has been reviewed. ... 

Pedersen on the effect of various chelating agents upon the autoxidation of petroleum (a useful model system) catalysed by various transition metals. The deactivating efficiency is the percentage restoration of the original induction period observed with a control sample of petroleum without metal or deactivator. Only iV" -tetrasalicylidenetetra(aminomethyl)methane was effective... 

Generation of a radical through an oxidative process probably occurs in the initiation of the autoxidation of benzaldehyde (p. 319), which is catalysed by a number of heavy metal ions capable of one-electron transfers, e.g. Fe3 ... 

Aldehydes, and particularly aromatic ones, are highly susceptible to autoxidation thus benzaldehyde (97) is rapidly converted into benzoic acid (98) in air at room temperature. This reaction is catalysed by light and the usual radical initiators, but is also highly susceptible to the presence of traces of metal ions that can act as one-electron oxidising agents (cf. p. 306), e.g. Fe3 , Co3 , etc ... 

The actual schemes of these reactions are very complicated the radicals involved may also react with the metal ions in the system, the hydroperoxide decomposition may also be catalysed by the metal complexes, which adds to the complexity of the autoxidation reactions. Some reactions, such as the cobalt catalysed oxidation of benzaldehyde have been found to be oscillating reactions under certain conditions [48],... 

The browning of proteins by glucose is catalysed by transition metal ions and by air. The AGEs in tissue proteins best characterised are CML and pentosidine, products of a combination of glycation and oxidation of hexoses or ascorbate. In view of facts such as these, the AGE hypothesis evolved to accommodate a role for oxidative stress.399 Multiple autoxidative mechanisms are involved in the formation of AGEs, as illustrated in Scheme 8.4 for the formation of CML from glucose, including ... 

Iron and copper catalyse the formation of oxyradicals. Three reactions are relevant in this context (1) Autoxidation of metal complexes may yield the superoxide radical which by itself is not very reactive, but is a precursor of more reactive radical species. (2) The one-electron reduction of hydrogen peroxide -the Fenton reaction - results in hydroxyl radicals via a higher oxidation state of iron [2]. (3) A similar reaction with organic peroxides leads to alkoxyl radicals, although a recent report alleges that hydroxyl radicals are also formed [3]. There is a fourth radical, the formation of which does not require mediation by a metal complex. This is the alkyldioxyl radical, ROO , which is formed at a... 

The initial step of both the above reactions, the formation of an acyl radical, is catalysed by light and metal ions that are capable of a one-electron reduction transition (e.g. Fe3+ — Fe2+). Thus, the autoxidation of aldehydes can be greatly slowed down by keeping the compounds in the dark and by very careful purification. However, the most efficient method is the addition of antioxidants, such as phenols and aromatic amines that react preferentially with any radicals that may be present. 

The mechanism of metal phthalocyanine catalysed oxidation by molecular oxygen -isobutyraldehyde system is not established at this stage. The iron[14], manganese[15] and cobalt tetrasulphonato-[16] phthalocyanines are known to form superoxo complexes with dioxygen and are known to catalyse autoxidation reactions[13]- The acyl radical formation thus can be initiated by interaction of metal phthalocyanine-dioxygen superoxo complex with isobutyraldehyde. The acyl radical in presence of oxygen can yield acylperoxy radical or peracid as the oxidising speceis[17]. 

The situation outlined above is subject to dramatic changes in the presence of some transition metal ions. Therefore, as seen for many other phenolics, in the presence of small concentrations of copper or iron ions, a steep increase in the autoxidation rate for HA could be expected. The mechanisms by which copper and iron catalyse the autoxidation of phenolics do not follow the same routes [62], and so, for example in the case of 1,2,4-benzenetriol (a toxic benzene metabolite in the liver), iron(III) was found to be a comparatively inefficient autoxidation catalyst in comparison with copper(II). It is worth noting, however, that in the case of that substrate the addition of SOD shows an inhibitory effect towards autoxidation, evidently because superoxide compulsorily participates as... 

Reaction [215] represents the initial event in thiyl radical formation as proposed for the so-called spontaneous autoxidation of thiols catalysed by metal ion complexes (Misra 1974, Saez et al. 1982, Harman et al. 1984). The propagation (reactions [216] and [217]) leads to thiol oxidation and contemporary superoxide radical formation. The latter might be reduced to hydrogen peroxide by reacting with (EDTA)2-Mn " to yield back the oxidised complex and close the catalytic cycle. 

These break down to give shorter-chain products, including free radicals, which then attack other fatty adds much more readily than does the original oxygen. More free radicals are produced, with the result that the speed of the oxidation increases exponentially. Eventually the concentration of free radicals becomes such that they react with each other and the reaction is terminated. Such a reaction, in which the products catalyse the reaction, is described as autocatalytic. This particular reaction is an autoxidation. The formation of the free radicals is catalysed by ultraviolet hght and certain metal ions, particularly copper, and the presence of either increases the rate of oxidation dramatically. 

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