Autoxidation metal-catalyzed

A review by Brandt and van Eldik provides insight into the basic kinetic features and mechanistic details of transition metal-catalyzed autoxidation reactions of sulfur(IV) species on the basis of literature data reported up to the early 1990s (78). Earlier results confirmed that these reactions may occur via non-radical, radical and combinations of non-radical and radical mechanisms. More recent studies have shown evidence mainly for the radical mechanisms, although a non-radical, two-electron decomposition was reported for the HgSC>3 complex recently (79). The possiblity of various redox paths combined with protolytic and complex-formation reactions are the sources of manifest complexity in the kinetic characteristics of these systems. Nevertheless, the predominant sulfur containing product is always the sulfate ion. In spite of extensive studies on this topic for well over a century, important aspects of the mechanisms remain to be clarified and the interpretation of some of the reactions is still controversial. Recent studies were... 

A prolific author, Professor van Eldik has been responsible for some 580 papers in refereed journals, and four books as editor or co-editor. His current research intrests are the application of high pressure techniques in mechanistic studies metal-catalyzed autoxidation processes and bioinorganic studies. As such he is eminently qualified to edit the prestigious Advances in Inorganic Chemistry. We are confident that he is a worthy successor to Professor Geoff Sykes and that he will maintain the high standards for which the series is known. 

The last work to be considered from our standpoint on the metal-catalyzed autoxidation of organic ligands, is the autoxidation of a, a -dipiperidyl to a, oi -bis A4-piperidine in alkaline solutions (66), which is most probably cataly d by Fe+2 ions. 

The formation of ligated transition metal ions at unstable high states of oxidation, its implications in the mechanisms of metal-catalyzed autoxidation, and the effect of configuration of a metal-ligand system on its redox stability have been pointed out. These considerations may be helpful in interpreting more complex metal-ligand systems including metal-enzyme reactions. 

The work of Fallab and his collaborators has shown how the coordination act may bring the reactants together in autoxidation reactions. In several instances coordination furnishes a catalytic path for these reactions. Specific examples include the autoxidation of Fe+2 in the presence of sulfosalicylic acid (28), the autoxidation of 1-hydrazinophthalazine by iron (II) (27, 83), and the autoxidation of a formazyl-zinc complex (11). It is probable that the importance of this kind of a mechanism will be more widely realized as more and more detailed kinetic studies are made on metal-catalyzed autoxidation reactions. Some other... 

Bawn and co-workers carried out detailed investigations of metal-catalyzed autoxidations of acetaldehyde313,314 and benzaldehyde.315 316a b The rate of chain initiation in the autoxidation of benzaldehyde catalyzed by cobalt acetate in acetic acid was equal to the rate of reaction of Co(III) with benzaldehyde in acetic acid in the absence of oxygen. Moreover, the onset of oxygen absorption coincided with the conversion of Co(II) to Co(III). The catalyst was maintained... 

Depending on the conditions, metal-catalyzed autoxidation of acetaldehyde can be utilized for the manufacture of either acetic acid or peracetic acid.321 In addition, autoxidation of acetaldehyde in the presence of both copper and cobalt acetates as catalysts produpes acetic anhydride in high yield.322 b The key step in anhydride formation is the electron transfer oxidation of acetyl radicals by Cu(II), which competes with reaction of these radicals with oxygen ... 

In the preceding sections, we discussed the various interactions that may be implicated in metal-catalyzed autoxidations. In a particular system, several reactions may be occurring simultaneously. The overall influence due to environmental factors affecting each reaction is difficult to predict. Some general points concerning the influence of the factors on metal-catalyzed autoxidations will be treated separately, although they are all interrelated. 

With purified hydrocarbons where such trace impurities have been scrupulously removed, a long induction time precedes metal-catalyzed autoxidation. The mechanism of formation of initial R02H in the absence of a radical chain is not known in any detail, but metal ions do not seem to be involved. The most important point to note in this context is that various types of metal-dioxygen complexes have been isolated and fully characterized. However, such complexes do not seem to play any role in metal ion initiated autoxidation reactions. 

What are the main products of metal-catalyzed autoxidation of methyl cyclohex-2-ene Why is cyclohexene more susceptible to autoxidation than cyclohexane ... 

Classical (metal-catalyzed) autoxidation of olefins is facile but not synthetically useful owing to competing oxidation of allylic C-H bonds and the olefinic double bond, leading to complex product mixtures [105]. Nonetheless, the synthetic chemist has a number of different tools for the allylic oxidation of olefins available. 

This mechanism appears to be most commonly reported for transition-metal-catalyzed autoxidation of organic substrates. However, at least a few other possibilities exist (23). The effect of oxygen absorption and subsequent oxidation and cleavage of cellulosic chains on the permanence as well as the appearance of paper can be very significant. The absorption of even the smallest amounts of oxygen is known to produce a substantial loss in mechanical properties of polymeric materials. In the case of paper, oxygen absorption leads to a decrease in the degree of polymerization, color reversion, and a loss in mechanical properties. 

One of the largest industrial-scale applications of homogeneous catalysis is represented by the oxidation of hydrocarbons, especially the transition metal-catalyzed autoxidation of p-xylene to terephthalic acid or its esters (cf. Section 2.8.1.2, [1], (eq. (1)). 

However, in spite of broad knowledge of metal-catalyzed autoxidation of aromatic compounds, the nature of the major chain-propagating steps is still not totally understood, nor are the relative rates of the dozens of single reaction steps. Unpredictable couplings of chemical and physical aspects make the reactions complicated sometimes oscillatory or even chaotic behavior occurs. Due to manifold back-coupling effects, often solely empirical research techniques can be applied to lead successfully to the desired oxidations. 

In comparison, metal-catalyzed autoxidation and oxidation of S(IV) with O3 tend to proceed more slowly with decreasing pH (36). 

Model calculations indicate that metal-catalyzed autoxidations may contribute significantly to the overall sulfate formation rate in atmospheric droplets, particularly in the range of Fe and Mn concentrations observed in urban fog (, 23,, 37-39)... 

For the metal-catalyzed autoxidation of S(IV), there is considerable ambiguity about the mechanism(s) of reaction. First-row transition-metal species can catalyze the reaction of aquated sulfur dioxide and 0 through four distinctly different pathways as described by HoTfmann and Boyce ( ) and Hoffmann and Jacob (37). These mechanisms include a thermally-initiated free radical chain processes involving a sequence of one-electron transfer steps,... 

The role of NHPI is analogous to that of HBr in metal-catalyzed autoxidation processes. The proposed reaction sequence for cyclohexylbenzene is shown in Scheme 5. NHPI efficiently traps the intermediate alkylperoxy radicals (reaction... 

The autoxidation of carbonyl compounds can be performed under a variety of conditions, the choice of which depends on the substrate and the desired product. Metal-catalyzed autoxidation proceeds via intermediate enols to generate (x-hydroperoxides or other products resulting from hydroperoxide decomposition [21-23], Autoxidation under basic conditions also requires generation of an enol or enolate [3] under photochemical or protic conditions, tautomerism of the carbonyl compound to an enol may be observed prior to autoxidation [24-25]. Both on the basis of detection of enol radicals as discrete intermediates in certain cases [24] and on kinetic studies... 

Although liquid-phase oxidations of alkanes can be carried out even in the absence of any metal derivative (the role of an inihator of chain radical process can be played by a non-metal compound), derivahves of transition metals are often used in these reactions. Metal-catalyzed autoxidation will be considered in Chapter IX. 

Kamiya, Y. Ingold, K. U. The Metal-Catalyzed Autoxidation of Tetralin. IV. The Effect of Solvent and Temperature. Can. J. Chem. 1964, 42, 2424-2433. Starks, C. M. Liotta, C. L. Halpem, M. Phase-Trantfer Catalysis Fundamentals, Applications, and Industrial Perspectives-, Chapman Hall New York, 195M. 

Sercheli, R., Ferreira, A., Baptistella, L., et al. (1997). Transition-metal Catalyzed Autoxidation of cis- and trans-Pmaste to a Mixture of Diastereoisomeric Pinanols, J. Agric. Food Chem., 45, pp. 1361-1364. 

Black, J.F. Metal-catalyzed autoxidation. The unrecognized consequences of metal-hydroperoxide complex formation. Journal of the American Chemical Society, 100 (1978) 2, p. 527 - 535... 

---