Oxidation two-electron

As shown in equation 12, the chemistry of this developer s oxidation and decomposition has been found to be less simple than first envisioned. One oxidation product, tetramethyl succinic acid (18), is not found under normal circumstances. Instead, the products are the a-hydroxyacid (20) and the a-ketoacid (22). When silver bromide is the oxidant, only the two-electron oxidation and hydrolysis occur to give (20). When silver chloride is the oxidant, a four-electron oxidation can occur to give (22). In model experiments the hydroxyacid was not converted to the keto acid. Therefore, it seemed that the two-electron intermediate triketone hydrate (19) in the presence of a stronger oxidant would reduce more silver, possibly involving a species such as (21) as a likely reactive intermediate. This mechanism was verified experimentally, using a controlled, constant electrochemical potential. At potentials like that of silver chloride, four electrons were used at lower potentials only two were used (104). 

FIGURE 18.30 The physiological effects of ascorbic acid (vitamin C) are the result of its action as a reducing agent. A two-electron oxidation of ascorbic acid yields dehy-droascorbic acid. 

K Fe(CN)6 oxidation Compound F is stoichiometrically inactivated by oxidation with K.3Fe(CN)6 (Shimomura and Johnson, 1967) thus, it is possible to estimate the molecular extinction coefficient (e) of the 388-390 nm absorption peak by titrating F with K.3Fe(CN)6- The e value obtained by the titration in 50% ethanol was 15,400 (assuming the reaction to be one-electron oxidation) or 30,800 (assuming two-electron oxidation). Two other methods of lesser precision were used to determine the true s value 1) the dry weight of the ethyl acetate extract of an acidified solution of F gave an e value of 14,100 2) the comparison of NMR signal intensities gave a value of 11,400 2,000 in water (H. Nakamura, Y. Oba, and A. Murai, 1995, personal... 

An extensive chemistry is developing of dinuclear gold(III) complexes with phosphorus ylid ligands (Figure 4.41). As mentioned in section 4.19, gold(I) compounds can undergo one- or two-electron oxidative additions,... 

Carboxylic acids with an electron donating substituent in the a-position decarboxylate in a two-electron oxidation to carbocations (see chap. 7). These can react with the solvent (alcohol, acetic acid, water) or the unreacted carboxylate to ethers, esters, or alcohols (Eq, 14). In some cases the carbon skeleton rearranges, which is a clear indication of the cationic pathway. 

Addition of two oxygen atoms and two-electron oxidation (carboxylic acid formation)... 

During oxidation of the MoFe protein the P clusters are the first to be oxidized at about -340 mV. This redox potential was first measured (40) using Mossbauer spectroscopy and exhibited a Nemst curve consistent with a two-electron oxidation process. It is possibly low enough for this redox process to be involved in enzyme turnover (see Section V). No additional EPR signal was observed from this oxidized form at this time. However, later a weak signal near g = 12 was detected and was finally confirmed, using parallel mode EPR... 

A relationship between the redox state of an iron—sulfur center and the conformation of the host protein was furthermore established in an X-ray crystal study on center P in Azotobacter vinelandii nitroge-nase (270). In this enzyme, the two-electron oxidation of center P was found to be accompanied by a significant displacement of about 1 A of two iron atoms. In both cases, this displacement was associated with an additional ligation provided by a serine residue and the amide nitrogen of a cysteine residue, respectively. Since these two residues are protonable, it has been suggested that this structural change might help to synchronize the transfer of electrons and protons to the Fe-Mo cofactor of the enzyme (270). 

Of these steps, the last three can be discounted (10.6) on the grounds that there is no significant V(II) dependence, (10.7) is considered unimportant since Tl(II) is present only in minute concentrations, (10.8) is slow by comparison with the other steps in the set (k 0.13 1.mole . sec in 1 M HCIO4 at 0 °C). Both rate and stoichiometric data infer that the reaction between Tl(III) and V(n) occurs essentially by a two-electron oxidation (step (10.1)). In the presence of chloride, less V(IV) is produced. It is interesting to note that oxidation of V(II) by molecular oxygen or hydrogen peroxide generates V(IV). However, the oxidation of V(III) by Tl(III) does not occur as a two-electron step (see p. 231). 

Le Mest Y, L Her M. 1995. Electrochemical generation of a new type of dioxygen carrier complex. Reversible fixation of dioxygen by the highly electron-deficient two-electron oxidized derivative of a dicobalt face-to-face diporphyrin. J Chem Soc Chem Commun 1441. 

The enzymatic reactions of peroxidases and oxygenases involve a two-electron oxidation of iron(III) and the formation of highly reactive [Fe O] " species with a formal oxidation state of +V. Direct (spectroscopic) evidence of the formation of a genuine iron(V) compound is elusive because of the short life times of the reactive intermediates [173, 174]. These species have been safely inferred from enzymatic considerations as the active oxidants for several oxidation reactions catalyzed by nonheme iron centers with innocent, that is, redox-inactive, ligands [175]. This conclusion is different from those known for heme peroxidases and oxygenases... 

Taking into account the results obtained by polarography as well as controlled potential electrolysis, the reaction which proceeded in Range A and gave the polarographic wave was estimated to be composed of two-electron oxidation of NADH and one-electron reduction of CQ at the W/DCE interface. The oxidation of NADH is accompanied by the dissociation of one H in W. 

The reaction in Range B did not give any polarographic currents but produced NAD and CQH2. Hence, the reaction was estimated to be two-electron oxidation of NADH by CQ accompanied by two-H transfer at the W/DCE interface,... 

The reaction can occur by a concerted fragmentation process initiated by a two-electron oxidation. 

Chromanoxylium cation 4 preferably adds nucleophiles in 8a-position producing 8a-substituted tocopherones 6, similar in structure to those obtained by radical recombination between C-8a of chromanoxyl 2 and coreacting radicals (Fig. 6.4). Addition of a hydroxyl ion to 4, for instance, results in a 8a-hydroxy-tocopherone, which in a subsequent step gives the /zara-tocopherylquinone (7), the main (and in most cases, the only) product of two-electron oxidation of tocopherol in aqueous media. A second interesting reaction of chromanoxylium cation 4 is the loss of aproton at C-5a, producing the o-QM 3. This reaction is mostly carried out starting from tocopherones 6 or /zora-tocopherylquinone (7) under acidic catalysis, so that chromanoxylium 4 is produced in the first step, followed by proton elimination from C-5a. In the overall reaction of a tocopherone 6, a [ 1,4] -elimination has occurred. The central species in the oxidation chemistry of a-tocopherol is the o-QM 3, which is discussed in detail subsequently. 

The third primary intermediate in the oxidation chemistry of a-tocopherol, and the central species in this chapter, is the orr/zo-quinone methide 3. In contrast to the other two primary intermediates 2 and 4, it can be formed by quite different ways (Fig. 6.4), which already might be taken as an indication of the importance of this intermediate in vitamin E chemistry. o-QM 3 is formed, as mentioned above, from chromanoxylium cation 4 by proton loss at C-5a, or by a further single-electron oxidation step from radical 2 with concomitant proton loss from C-5a. Its most prominent and most frequently employed formation way is the direct generation from a-tocopherol by two-electron oxidation in inert media. Although in aqueous or protic media, initial... 

The third fact that seemed to argue in favor of the occurrence of radicals 10 was the observation that reactions of a-tocopherol under typical radical conditions, that is, at the presence of radical initiators in inert solvents or under irradiation, provided also large amounts of two-electron oxidation products such as o-QM 3 and its spiro dimerization product 9 (Fig. 6.8).16,25,26 This was taken as support of a disproportionation reaction involving a-tocopheroxyl radical 2 and its hypothetical tautomeric chromanol methide radical 10, affording one molecule of o-QM 3 (oxidation) and regenerating one molecule of 1 (reduction). The term disproportionation was used here to describe a one-electron redox process with concomitant transfer of a proton, that is, basically a H-atom transfer from hypothetical 10 to radical 2. 

FIGURE 6.8 Hypothetical disproportionation of two a-tocopherol-derived radicals 2 and 10 in the absence of other coreactants to account for the formation of typical two-electron oxidation products (o-QM 3, a-tocopherol spiro dimer 9). 

SCHEME 10.2 Common pathways of QM formation in biological systems, (a) Stepwise two-electron oxidation by cytochrome P450 or a peroxidase, (b) Enzymatic oxidation of a catechol followed by spontaneous isomerization of the resulting n-quinone. (c) Enzymatic hydrolysis of a phosphate ester followed by base-catalyzed elimination of a leaving group from the benzylic position. 

A relatively stable QM is produced by initial P450-catalyzed aromatic hydroxylation of the SERM tamoxifen to yield 4-hydroxytamoxifen, followed by a cytochrome P450-catalyzed direct two-electron oxidation (Scheme 10.9).7 58 This QM is extremely long lived at physiological pH and temperature (tl/2 3 h, Table 10.2),59 most likely... 

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