Peroxo-carbonate

In a similar fashion, the homoleptic complex [Pd(ITmt) ] lb readily reacts with O2 to form the corresponding peroxo-complex 2b (Scheme 10.1). This complex, npon exposure to CO, leads to the peroxo-carbonate complex 3b [10]. Under the same reaction conditions, the formation of 3a does not occur, presumably due to the larger steric hindrance of the Mes ligand. 

Recently it has been shown that simple manganese sulfate in the presence of sodium bicarbonate is reasonably effective in promoting the epoxidation of alkenes with aqueous H202 using DMF or t-BuOH as solvents [69]. In this system peroxo-carbonate is formed in situ, thus minimizing the catalase activity of the Mn salt. Following this discovery, Chan and coworkers introduced an imidazole-based ionic... 

There are numerous species where —O— is replaced by —O—O—. The peroxo carbonate ion 02C00H occurs inNaHC04H20 and alkali metal salts of stoichiometry M2Q06 exist in the solid state. In solution in aqueous H202 there appear to be the reactions ... 

Scheme 2. Mechanism of formation of peroxo-carbonates from a dioxygen complex andCC>2.

Only a few examples are reported in the literature of the direct carbonation of olefins, namely the direct functionalization of propene [112, 113] and styrene [108]. Such an approach has a drawback represented by the addition of dioxygen across the C=C double bond with formation of aldehydes or of the relevant acids. Using Rh-complexes the active species in the epoxidation of the olefin has been shown to be the peroxocarbonate moiety [114]. Using differently labeled O2 molecules such as i 08)q 18(16)q possible to build peroxo carbonates... 

Fig. 23.5 [13] shows current efficiencies for the production of peroxides, i.e., peroxo carbonate, in sodium carbonate solution obtained on BDD and platinum electrodes. The efficiency for the former is 80%, which is approximately three times higher than that for the latter. 

As is already well known, BDD is a very promising electrode material for water treatment technologies and their markets due to its outstanding features. This may be associated with the production of the hydroxyl radical, which may also be responsible for the production of ozone, peroxo-disulfate, peroxo-carbonate, hydrogen peroxide and their derivatives, which are powerful oxidants in naturally mineralized water. Since most surface waters contain some bicarbonates and sulfates, which can be transformed into peroxide compounds, and chlorides, which can be transformed into hypochlorite, using the BDD electrode, water itself can help industrial water treatments. 

The above proposed process can be expected to easily put into practice as ammonia is abuandant as the main feedstock for fertilizer. Nevertheless, there is also a problem that Co(NH3)6 is apt to be oxidized to Co(NH3)6 which is unable to form the peroxo binuclear complex and ineffective to O2 solubility enhancement, thus reaction (4) is inhibited. But Co will be relatively stable, and Co may be reduced to Co " by H2O [12]. As a result, a regenration method has also been proposed by using the activated carbon as the catalyst[7], in which Co(NH3)6 dissociation into Co " and NH3 occurs on the activated carbon surface followed by reduction of Co with H2O into Co, O2 and H. ... 

The key factor is the action of the metal on the peroxo group making one oxygen atom electrophilic. Whether or not the metal is bonded to carbon in the intermediate is not known, but also considered unlikely naturally this will depend on the particular substrate and catalyst. Epoxidation will be discussed in Chapter 14, with special emphasis on asymmetric epoxidation with chiral metal catalysts. 

S.3 Cytochrome P450 Model Compounds Functional. Ferric-peroxo species are part of the cytochrome P450 catalytic cycle as discussed previously in Section 7.4.4. For instance, these ferric-peroxo moieties are known to act as nucleophiles attacking aldehydic carbon atoms in oxidative deformylations to produce aromatic species.An example of this work, establishing the nucleophilic nature of [(porphyrin)Fe (02)] complexes, was achieved for alkene epoxidation reactions by J. S. Valentine and co-workers. The electron-deficient compound menadione (see Figure 7.18) yielded menadione epoxide when reacted with a [(porphyrin)Fe X02)] complex. 

Fig. 12. Proposed mechanism for the HO-1-catalyzed conversion of Fe a-meso-hydroxyheme (shown deprotonated) to Fe verdoheme. In an equally good variant of this mechanism, the oxygen molecule binds to the Fe" before it binds to the carbon of the porphyrin to give the same peroxo-bridged intermediate.

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). 

Numerous d cobalt(III) complexes are known and have been studied extensively. Most of these complexes are octahedral in shape. Tetrahedral, planar and square antiprismatic complexes of cobalt(lII) are also known, but there are very few. The most common ligands are ammonia, ethylenediamine and water. Halide ions, nitro (NO2) groups, hydroxide (OH ), cyanide (CN ), and isothiocyanate (NCS ) ions also form Co(lII) complexes readily. Numerous complexes have been synthesized with several other ions and neutral molecular hgands, including carbonate, oxalate, trifluoroacetate and neutral ligands, such as pyridine, acetylacetone, ethylenediaminetetraacetic acid (EDTA), dimethylformamide, tetrahydrofuran, and trialkyl or arylphosphines. Also, several polynuclear bridging complexes of amido (NHO, imido (NH ), hydroxo (OH ), and peroxo (02 ) functional groups are known. Some typical Co(lll) complexes are tabulated below ... 

The oxidation of sulfides to sulfoxides (1 eq. of oxidant) and sulfones (2 eq. of oxidant) is possible in the absence of a catalyst by employing the perhydrate prepared from hexafluoroacetone or 2-hydroperoxy-l,l,l-trifluoropropan-2-ol as reported by Ganeshpure and Adam (Scheme 99 f°. The reaction is highly chemoselective and sulfoxidation occurs in the presence of double bonds and amine functions, which were not oxidized. With one equivalent of the a-hydroxyhydroperoxide, diphenyl sulfide was selectively transformed to the sulfoxide in quantitative yield and with two equivalents of oxidant the corresponding sulfone was quantitatively obtained. 2-Hydroperoxy-l,l,l-fluoropropan-2-ol as an electrophilic oxidant oxidizes thianthrene-5-oxide almost exclusively to the corresponding cw-disulfoxide, although low conversions were observed (15%) (Scheme 99). Deprotonation of this oxidant with sodium carbonate in methanol leads to a peroxo anion, which is a nucleophilic oxidant and oxidizes thianthrene-5-oxide preferentially to the sulfone. 

One of the most important peroxo complexes synthesized after 1983 is the rhenium species formed from methyltrioxorhenium (MTO) precursor. The synthesis of this complex is achieved in the way indicated in equation 2, by reacting hydrogen peroxide with MTO . The isolated peroxo complex 1 contains in the coordination sphere two /7 -peroxide bridges, a direct metal carbon bond and a molecule of water. The crystal structure of the peroxo rhenium derivative, however, was obtained by substitution of the water molecule with other ligands " more details on this aspect are enclosed in the structural characterization paragraph. 

Based on the analysis of the reactions in Scheme 3 and on previous studies (46, 47), a mechanism for the reaction was proposed in which the /x-peroxo complex, 16, may simultaneously abstract two hydrogen atoms from iso-propyl groups on the pyrazolyl ligands. Alternatively, because of the weak 0—0 bond, 16 may homolytically dissociate to form two Tp"Co(0-) oxo-radical moieties, and these species would then abstract hydrogen from the iso-propyl groups. In either case, the resulting carbon-centered radical can either react with solvent, as was observed for the Tp complex (46), or with another carbon-centered radical so as to regenerate the Tp"Co(OH) complex and produce a derivative of the Tp" complex with an iso-propenyl substituent, 18. Ultimately, either route would produce the (/x-OH)2 complex, 17. 

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