Anionic peroxo species

Phase Transfer Catalysis (PTC) with tungsten anionic peroxo species for epoxidation of la... 

Snbsequent detailed kinetic stndies revealed that the reaction mechanism for the hydroxy-lation of arenes is mnch more complicated than that indicated above Furthermore, the active intermediate is likely an anion radical species formed upon interaction of two molecules of the vanadium peroxo complex. The sequence of the various steps is indicated in equations 17-24. The steps indicated in equations 17-21 refer to a radical chain which accounts for decomposition of the peroxo complex to form dioxygen, whereas the subsequent steps are those required for the functionalization of the substrate. 

The first step consists of the formation of the dioxygen adduct which can have either a superoxo structure (1) if the metal is a potential one-electron donor, or a peroxo structure (2) if the metal is a potential two-electron donor. These superoxo or peroxo complexes can be considered as the formal, but not chemical, analogs of the superoxide 02 and peroxide 022- anions. The superoxo complex (1) can further react with a second reduced metal atom to give the /x-peroxo species (3), which can cleave itself into the oxo species (4), which may be hydrolyzed to give the hydroxo species (6) or react with a second metal atom to give the p.-oxo species (5). The alkylperoxo (7) and hydroperoxo (8) species can result from the alkylation or protonation of the peroxo species (2), or from anion exchange from metal salts by alkyl hydroperoxides or hydrogen peroxide. 

It should be emphasized that even trapped electrons and holes can rapidly recombine on the particle surface (eqn.2). To prevent recombination of holes and electrons, the latter carrier is scavenged by pre-adsorbed (and photoadsorbed) molecular oxygen to give the superoxide radical anion, 02 (ads) (eqn.8) that can be reduced further to the peroxide dianion, 02 "(ads) Alternatively, surface peroxo species can be formed [22]. 

The mechanisms of decomposition of peroxomono-sulphates and -phosphates in alkaline media have been investigated, the rates being first-order with respect to hydroxide ion and the peroxo-species. The reaction is considered to proceed via a one-electron transfer from the OH to the peroxo-anion,... 

Likewise, within mechanism B, the /r-peroxo intermediate may be susceptible to reversible one-electron reduction to anionic [(dipor)Co202], which may become important only at potentials <0.5 V. There is some indication that the formally peroxo adducts, [(dipor)Co2 02 )], formed by addition of O2 to fully reduced (dipor)Co2, may undergo reversible reduction. Protonation of the anionic species may be followed by its hydrolysis, releasing H2O2. 

Fig. 14. Manifold of reactive species produced from the reaction of a heme group with oxygen and two reducing equivalents. The rate of conversion of A to B limits the lifetime (and therefore reactivity) of the Fe peroxo anion. The rate of formation of the ferryl species C via the Fe -OOH complex B competes with the intramolecular hydroxylation reaction to give hydroxyheme. Reactions of the Fe -hydroperoxy complex B with exogenous electrophilic substrates must compete with conversion of the intermediate to both C and meso-hydroxyheme. The Fe -OOH complex B can also be formed directly with H2O,.

A less common reactive species is the Fe peroxo anion expected from two-electron reduction of O2 at a hemoprotein iron atom (Fig. 14, structure A). Protonation of this intermediate would yield the Fe —OOH precursor (Fig. 14, structure B) of the ferryl species. However, it is now clear that the Fe peroxo anion can directly react as a nucleophile with highly electrophilic substrates such as aldehydes. Addition of the peroxo anion to the aldehyde, followed by homolytic scission of the dioxygen bond, is now accepted as the mechanism for the carbon-carbon bond cleavage reactions catalyzed by several cytochrome P450 enzymes, including aromatase, lanosterol 14-demethylase, and sterol 17-lyase (133). A similar nucleophilic addition of the Fe peroxo anion to a carbon-nitrogen double bond has been invoked in the mechanism of the nitric oxide synthases (133). 

The synthesis of peroxo Nb species may be obtained with the general method indicated in eqnation 2, and also by replacing peroxo groups with polyaminocarboxylato ligands from a tetraperoxoniobate anion. 

A few peroxochloro derivatives of the [M(02)C15]2 391 and [M(02)C14(H20)] 431 anions are known. Addition of /3-diketones to solutions of the latter afforded [Nb(02)Cl3(diket)]- the stereorigidity of these heptacoordinated species in solution was assigned to the bidentate peroxo moiety.431... 

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... 

Some of the early transition metals are known to form mononuclear peroxo complexes. These complexes are not formed by reacting a metal complex in a low oxidation state with dioxygen, as this usually results in the formation of metal oxo species, but rather by reaction of a metal complex in a high oxidation state, eg. TiIV, Nbv or MoVI, with the peroxide anion. This frequently leads to more than one peroxide ligand per metal centre. 

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