Copper catalyzed autoxidation

The kinetic results reported by Jameson and Blackburn (11,12) for the copper catalyzed autoxidation of ascorbic acid are substantially different from those of Taqui Khan and Martell (6). The former could not reproduce the spontaneous oxidation in the absence of added catalysts when they used extremely pure reagents. These results imply that ascorbic acid is inert toward oxidation by dioxygen and earlier reports on spontaneous oxidation are artifacts due to catalytic impurities. In support of these considerations, it is worthwhile noting that trace amounts of transition metal ions, in particular Cu(II), may cause irreproducibilities in experimental work with ascorbic acid (13). While this problem can be eliminated by masking the metal ion(s), the masking agent needs to be selected carefully since it could become involved in side reactions in a given system. 

In non-aqueous solution, the copper catalyzed autoxidation of catechol was interpreted in terms of a Cu(I)/Cu(II) redox cycle (34). It was assumed that the formation of a dinuclear copper(II)-catecholate intermediate is followed by an intramolecular two-electron step. The product Cu(I) is quickly reoxidized by dioxygen to Cu(II). A somewhat different model postulated the reversible formation of a substrate-catalyst-dioxy-gen ternary complex for the Mn(II) and Co(II) catalyzed autoxidations in protic media (35). 

The effect of non-participating ligands on the copper catalyzed autoxidation of cysteine was studied in the presence of glycylglycine-phosphate and catecholamines, (2-R-)H2C, (epinephrine, R = CH(OH)-CH2-NHCH3 norepinephrine, R = CH(OH)-CH2-NH2 dopamine, R = CH2-CH2-NH2 dopa, R = CH2-CH(COOH)-NH2) by Hanaki and co-workers (68,69). Typically, these reactions followed Michaelis-Menten kinetics and the autoxidation rate displayed a bell-shaped curve as a function of pH. The catecholamines had no kinetic effects under anaerobic conditions, but catalyzed the autoxidation of cysteine in the following order of efficiency epinephrine = norepinephrine > dopamine > dopa. The concentration and pH dependencies of the reaction rate were interpreted by assuming that the redox active species is the [L Cun(RS-)] ternary complex which is formed in a very fast reaction between CunL and cysteine. Thus, the autoxidation occurs at maximum rate when the conditions are optimal for the formation of this species. At relatively low pH, the ternary complex does not form in sufficient concentration. 

The main features of the copper catalyzed autoxidation of ascorbic acid were summarized in detail in Section III. Recently, Strizhak and coworkers demonstrated that in a continuously stirred tank reactor (CSTR) as well as in a batch reactor, the reaction shows various non-linear phenomena, such as bi-stability, oscillations and stochastic resonance (161). The results from the batch experiments can be suitably illustrated with a two-dimensional parameter diagram shown in Pig. 5. 

Trace metals, especially copper, catalyze autoxidation by reacting with hydroperoxides to create new free radicals and initiate new chain reactions. 

Recently Jameson and Blackburn (14,15,16) have suggested an alternate mechanism for the copper-catalyzed autoxidation of ascorbic acid, involving the formation of a binuclear Cu(II) complex (17) of the ascorbate anion, and the subsequent formation of an intermediate peroxo type Cu( II)-dioxygen-ascorbate complex (18). Their kinetic data suggested a variety of rate behavior depending on the nature of the supporting electrolyte. Formula 17, which was postulated for nitrate... 

The copper-catalyzed autoxidation is light-inhibited because of the... 

The effect of Cl" ion on the copper-catalyzed autoxidation of ascorbic acid has been examined. It is pointed out that the rate dependence on [O2] can be explained by a mechanism involving a copper(I) intermediate rather than the copper(III) proposed earlier. 

Iron(III)-catalyzed autoxidation of ascorbic acid has received considerably less attention than the comparable reactions with copper species. Anaerobic studies confirmed that Fe(III) can easily oxidize ascorbic acid to dehydroascorbic acid. Xu and Jordan reported two-stage kinetics for this system in the presence of an excess of the metal ion, and suggested the fast formation of iron(III) ascorbate complexes which undergo reversible electron transfer steps (21). However, Bansch and coworkers did not find spectral evidence for the formation of ascorbate complexes in excess ascorbic acid (22). On the basis of a combined pH, temperature and pressure dependence study these authors confirmed that the oxidation by Fe(H20)g+ proceeds via an outer-sphere mechanism, while the reaction with Fe(H20)50H2+ is substitution-controlled and follows an inner-sphere electron transfer path. To some extent, these results may contradict with the model proposed by Taqui Khan and Martell (6), because the oxidation by the metal ion may take place before the ternary oxygen complex is actually formed in Eq. (17). 

Fig. 3. Decay of the H202 concentration versus time during the anaerobic oxidation reaction with cysteine in the presence of CuS04. First stage of constant rate (first-order in [Cu]) during the period of oxidation, second stage of increasing rate after completion of the oxidation of cysteine to cystine. Reprinted from Journal of Molecular catalysis, vol. 11, Zwart, J. van Wolput, J. H. M. C. van der Cammen, J. C. J. M. Koningsberger, D. C. Accumulation and Reactions of H202 During the Copper Ion Catalyzed Autoxidation of Cysteine in Alkaline Medium, p. 69, Copyright (2002), with permission from Elsevier Science.

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 ... 

Edetic acid and edetates are primarily used as antioxidant synergists, sequestering trace amounts of metal ions, particularly copper, iron, and manganese, that might otherwise catalyze autoxidation reactions. Edetic acid and edetates may be used alone or in combination with true antioxidants the usual concentration employed being in the range 0.005-0.1% w/v. Edetates have been used to stabilize ascorbic acid ... 

One aspect which sets oxidation apart from other reactions, e.g. hydrogenation and carbonylation is the fact that there is almost always a reaction (free radical chain autoxidation) in the absence of the catalyst (Reactions 1-3). Moreover, (transition) metal ions which readily imdergo a reversible one-electron valence change, e.g. manganese, cobalt, iron, chromium, and copper, catalyze this process by generating alkoxy and alkylperoxy radicals from RO2H (Reactions 4-6). 

The presence of the above-mentioned metal ions increases the decomposition rate of hydroperoxides and the overall oxidation rate in the autoxidation of a hydrocarbon to such an extent that even in the presence of antioxidants, the induction period of oxygen uptake is drastically shortened. In such a case, sterically hindered phenols or aromatic amines even at rather high concentrations, do not retard the oxidation rate satisfactorily. A much more efficient inhibition is then achieved hy using metal deactivators, together with antioxidants. Metal deactivators are also known as copper inhihitors, because, in practice, the copper-catalyzed oxidation of polyolefins is by far of greatest importance. This is due to the fact that polyolefins are the preferred insulation material for communication wire and power cables, which generally contain copper conductors. 

Hamilton and Al-Arab have studied the copper(II) catalyzed autoxidation of diaminouracils 31 [52]. The primary product is imine 33, which is further converted by hydrolysis to alloxan and NH. The... 

The copper ion catalyzed autoxidation of thiols has long been believed to occur via a redox mechanism [51-57] involving free thiyl and superoxide radicals, the catalytic effect of copper being due to the redox couple Cu(I)/Cu(II). The function of 0 would merely be the... 

A variety of transition metal complexes, especially those of cobalt and copper, catalyze the oxidation of phenols. Autoxidations of 2,6-disubstituted phenols in organic solvents produce mainly the 2,6-disubstituted-1,4-benzoquinone and the 3,5,3, 5 -tetrasubstituted-4,4 -diphenoquinone with Co catalysts and certain copper catalysts (eq. Poly(2,6-... 

Copper salt-amine catalyzed autoxidation 28,130 -(I) stearate 26,155 -(II) sulfate 6, 251 suppl. 27 ... 

However, it is highly unlikely that the active oxygenation species is a discrete copper-dioxygen adduct in this system. In fact, the authors suggest that it is probably a Cu(n)-catalyzed autoxidation reaction that leads to the oxygenation of a methine position in the ligand. 

There is specificity of the antioxidant action in the presence of heterogeneous catalyst. The kinetics of ionol retarding action on the oxidation of fuel T-6 catalyzed by the copper powder and homogeneous catalyst copper oleate was studied in Ref. [12]. Copper oleate appeared to be very active homogeneous catalyst it was found to catalyze the autoxidation of T-6 in such small concentration as 10 6 mol L-1 (T = 398 K). The kinetics of autoxidation catalyzed by copper salt obeys the parabolic law (see Chapter 4) ... 

We observed a more complicated behavior in the study of retarding action of amines (IV-benzyl-IV -phenyl-l,4-benzenediamine and 4-hydroxyphenyl-2-naphtalenamine) on fuel T-6 oxidation catalyzed by the copper powder [13]. Both antioxidants appeared to retard the autoxidation of T-6 very effectively. They stop chain oxidation during the induction period in concentrations equal to 5 x 10 5mol L 1 and higher. The induction period was found to be the longer, the higher the concentration of the antioxidant and lower the amount of the copper powder introduced in T-6. 

Copper complexes were used as efficient catalysts for selective autoxidations of flavonols (HFLA) to the corresponding o-benzoyl salicylic acid (o-BSH) and CO in non-aqueous solvents and at elevated temperatures (124-128). The oxidative cleavage of the pyrazone ring is also catalyzed by some cobalt complexes (129-131). 

An oxygen activation mechanism is favored by Ochiai for the autoxidation of cyclohexene catalyzed by copper phthalocyanine.197a,b Kamiya also proposed that the CuPc—02 complex initiates the autoxidation of a-methylstyrene by an addition mechanism196 ... 

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