Oxidation with peroxo compounds

If the electron exchange is accompanied by a break of a covalent bond, the redox reaction is slow. This is the case with peroxo compounds such as hydrogen peroxide and oxidant oxanions such as S208. ... 

The oxidation of S(IV) to S(VI) is of environmental importance and a very interesting paper on the reaction with peroxo compounds in aqueous solution has appeared from the Darmstadt group. For reaction with HOOH,... 

Due to the limited extent of this paper, we mention only some positive results achieved in the orientation to two main directions i.e. to oxidative principles and to alcoholates in aprotic solvents. The research of oxidative principles (see e.g. [9]), involved the use of compounds with active chlorine, active iodine, peroxo-compounds, various mixtures of these compounds, as well as the use of oxidative agents with enhancing solubility and thus decontamination efficiency by adding detergents. It can... 

In a redox system consisting of a peroxo compound and an iron(II) salt, the initiating radicals are formed by electron transfer from Fe " to the peroxo compound, causing the peroxy link to be cleaved, with simultaneous formation of a radical and an anion. In a second step, the oxidized metal reacts with another hydroperoxide to form a peroxy radical and a proton ... 

Apart from the above-mentioned interest in m-octahedral species [M(bipy)2(anion)2] from the point of view of high-spin to low-spin transitions,72 this geometry is also of interest for the formation of reactive species, leading to, for example, dinuclear species with catalytic properties, such as the cobalt-peroxo compound shown in Figure 21. This compound is active in oxidative phenol coupling.132 With Cu the stoichiometry Cu(bipy)2X2 results in a variety of five-coordinate and distorted six-coordinate structures (see Figure 3).21... 

Titanium containing materials have been investigated for various reactions, but selective oxidations with H202 as the oxidant have attracted the most interest. For these reactions, the formation of surface titanium peroxo compounds with H202 and the subsequent transfer of the peroxidic oxygen to the organic reactants have been proposed to explain the mechanism by which titanium participates in the catalytic cycle (Notari, 1988). 

Concerted mechanisms have been proposed on the basis of work carried out with soluble Movl, Wvl and TiIV peroxo compounds. The experimental evidence is consistent with the hypothesis that these compounds act as oxidants in stoichiometric epoxidations and that the reactions involve electrophilic attack of the peroxo compound on the organic molecule or, what is equivalent, a nucleophilic attack of the organic molecule on the peroxidic oxygen, in a butterfly transition state. The reaction product is formed and, after desorption, the peroxo compound is regenerated by reaction of TiIV with H202 this accounts for the catalytic nature of the reaction (Amato et al, 1986). The same type of mechanism... 

The reaction of the fully reduced enzyme with 02 does not follow the pathway described above. Instead, loss of compound A is followed by the production of compound B through the intermediate formation of another compound (II), which may be equivalent to the peroxo compound C, but having reduced cytochrome a and CuA. The transient nature of this species II compared to compound C would result from the possibility of electron transfer from cytochrome a and CuA, to give compound B. The fact that cytochrome a and CuA are at least partially oxidized in compound B is shown by the appearance of the ESR signal (in 30-50% intensity) characteristic of the fully oxidized enzyme. Intermediates beyond compound B in the oxidation sequence have also been described. In one of these, the CuB2+ is ESR-detectable. 

Finally, a mention should be made about the one peroxo system which will become more and more dominant the organometallic oxides of rhenium(VII). Such compounds have been found to be of outstanding catalytic activity for a number of oxygen transfer reactions with hydrogen peroxide.92 The best studied complex is methyltrioxorhenium(VII) (MTO) and its congeners. Figure 2.32 illustrates its synthesis. Epoxidation, aromatic oxidation and halide oxidation with these complexes have been studied with hydrogen peroxide and shown to be remarkably efficacious. 

It is somewhat surprising that Mn compounds with 02 moieties appear to have such limited existence peroxo compounds of the higher oxidation states are an important feature of the earlier transition metals (and we shall notice a few, poorly characterized, examples for Mn in Section 41.5.3) and the metals which follow manganese—Fe and Co—have an extensive and important chemistry of reversible combination with the oxygen molecule. It is perhaps pertinent to recall that some Mn11 species are very efficient catalysts for the decomposition of hydrogen peroxide (Section 41.3.3.7). 

Like other transition metals, notably Ti, V, Nb, Ta, Mo, and W, chromium forms peroxo compounds in the higher oxidation states. Most of them are unstable, and in the solid state some of them are dangerously explosive or flammable in air. Bis(peroxo)chromium(IV) complexes with nitrogen ligands are remarkably stable, e.g., (17-C-XII).40... 

However, the similarity in bond strengths of the peroxide linkage to molecular 02, the ease with which the known -peroxo Cobalt complexes liberate 02 (in contrast to /x-oxo bipyridyl Mn dimers) on photolysis, kinetic barriers on ju-oxo to peroxo dimer conversions led Sawyer et al.47 -49) to suggest peroxo binuclear complexes as the most probable intermediates. More studies with model compounds are needed to elucidate this point. Various mechanisms proposed for water oxidations are variations of these two principal types. 

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