Copper complexes reaction with peroxides

From the known chemical properties of superoxide free radicals and hydrogen peroxide, it is unlikely that these two species will react directly with the range of biomolecules found in synovial fluid. It is more likely, particularly for superoxide radicals, that they will instead participate in redox reactions with complexes of metal ions such as iron and copper, although reaction with phenolic compounds cannot be excluded. It has been proposed therefore that synovial fluid, in particular hyaluronic acid, can be degraded in vivo through an iron-catalysed Haber-Weiss reaction. 

A mechanism has been proposed for the oxidation of copper(i) [and copper(n)] bipyridyl complexes [CuL3]"+ by HgOa in the presence of alcohols. The initial reaction with peroxide is considered to produce a copper(ra) intermediate which acts as an oxidant ... 

During the oxidative copperizing process, an o-hydroxyazo compound reacts with a copper(II) salt in the presence of hydrogen peroxide. Both methods broaden the scope of metal complexation reactions by extending the selection of suitable diazo components [8,9]. 

In 2003, Velusamy and Punniyamurthy reported on a copper(II)-catalyzed C—H oxidation of alkylbenzenes and cyclohexane to the corresponding ketones with 30% hydrogen peroxide (Scheme 131). The reaction was catalyzed by the copper complex 192a depicted in Scheme 131 and yields were high in the case of alkylbenzenes (82-89%) whereas cyclohexanone was obtained with a low yield of 18%. Chemoselectivity was very high in every case neither aromatic oxidation nor oxidation at another position of the alkyl chain was observed. 

A side-on p,-Tq2 Tq2-peroxo dicopper(II) complex. A very important development in copper-dioxygen chemistry occurred in 1989 with the report by Kitajima et al. [10,108] that another Cu202 species could be prepared and structurally characterized by using copper complexes with a substituted anionic tris(pyrazolyl)borate ligand. This intensely purple compound, Cu[HB(3,5-iPr2pz)3] 2(02) (5), was prepared either by reaction of Cu[HB(3,5-iPr2pz)3] (4) with 02 or by careful addition of aqueous hydrogen peroxide to the p-dihydroxo... 

Turning to the complexes of copper(II), copper(IV) is not stable and heterolysis of the 0—0 bond of the peroxide to form the copper(IV) oxo complex does not occur. In addition, the Lewis acidity of the copper(II) ion is not high enough to enhance the electrophilicity of the coordinated alkyl- or acylperoxide to promote direct oxo-incorporating reactions. With these points in mind, the inert activity of alkyl- and acylperoxo copper(II) complexes, experimentally observed, is understandable, and it is quite unlikely that the mechanism of copper monooxygenase parallels that of cytochrome P-450. 

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... 

Not many reduction potentials are known for copper complexes. That of the Cu2+/Cu+ couple is 0.16 V Since lo(Cu+/Cu°) is 0.52 V, the disproportionation of Cu+ to Cu° and Cu2+ is favourable. This reaction does indeed occur, which makes is impossible to study stable copper(I) solutions. Reduction potentials of copper(II)-/copper(I)-(l,10-phenanthroline)2 and a few derivatives have been calculated from a kinetic analysis of appropriate rate constants values range from 108 mV for the 5-methyl-l, 10-phenanthroline complex to 219 mV for the complex with a nitro group at the 5 position [52], Values of 0.17 V and 0.12 V are given by Phillips and Williams [53] for the phenanthroline and bipyridine complexes, respectively. Such complexes can thermodynamically catalyse both the superoxide dismutation and the one-electron reduction of hydrogen peroxide (see below). 

Copper in trace amounts can distort the kinetics of superoxide reactions with other compounds, because it effectively catalyses the dismutation reaction [143], Complexation by edta prevents this, as well as the reduction of hydrogen peroxide. Thus, edta protects the bases of DNA against copper-mediated damage [144]. However, edta will not protect if iron is involved, since rates of reaction between this metal complex and oxygen, superoxide or hydrogen peroxide are appreciable. 

The reaction of dioxygen with laccase or ascorbate oxidase was reviewed in Section IX and in Messerschmidt et al. (74), where the possible binding modes of dioxygen to binuclear and trinuclear copper centers are discussed. A novel mode of dioxygen binding to a binuclear copper complex was found in a compound synthesized by Kitajima et al. (165). The complex contains peroxide in the mode, i.e., side-on between... 

Catalysis of the decomposition by several metal ions has been reported . The order in peroxomonosulphate was found to be one, except for reaction with cobalt(II) and cerium(IV) (both second-order in the sulphate) and manga-nese(II) (half-order). Formation of a complex with cerium(IV) and peroxomonosulphate has been both asserted and denied . Catalysis by copper(II) is not significant unless manganese is also added, at least in the presence of some hydrogen peroxide ... 

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