Two-electron oxidation mechanisms

Oxidation of guanine and 8-oxo-7,8-dihydroguanine with a Mn(IV)=0 species, a two-electron oxidant and riboflavin, a known photosensitizer and a one-electron oxidant, was studied. A quantification of the ratio between one- and two-electron oxidation mechanisms of guanine oxidation by electron transfer led to the conclusion that one-electron oxidation predominates and the two-electron oxidation process is a minor pathway.288... 

Another important aspect of peroxidase reactions is the relation between the substrate one-electron redox potential and the redox potential of compound I and compound II, since this restricts the number of possible redox partners (see Chap. 4 for a detailed description). Table 6.1 reports the redox potentials of some selected peroxidases as it can be seen, the values span an interval ranging from 1.35 V for reduction of myeloperoxidase (MPO) compound I to 1.0 V for reduction of HRP compound I [13-15]. But the selection of the preferred enzyme for a given radical reaction must consider not only the complementarities in the redox potentials but also the mechanism preferred by the enzyme, since some peroxidases, such as CPO and MPO, and also LPO in some cases, react through a two-electron oxidation mechanism. 

Normally, in cydohexene epoxidation with HP, Ti-silicates produce products that are typical of both two-electron oxidation mechanisms (cydohexene epoxide and... 

The Mn =0 entity of manganese porphyrin is a powerful chemical oxidant capable of abstracting electrons from guanine. However, the products of the reaction clearly result from a two-electron oxidation mechanism. 

Hence, our calculations emphasized a step-wise mechanism over most oxides H-abstraction leads to a surface hydroxyl, in conjunction with the alkyl radical formation, which rapidly rebounds to a nearby oxygen to form a surface alkoxy. This step-wise one-electron oxidation mechanism not only offers an energetically more favorable route than the one-step two-electron oxidation mechanism such as (5+2), but also provides a plausible solution to the puzzle as contrasting the EPR results [50] to the IR results [49] (cf. Fig. 6). We propose that EPR with higher time resolution (10 -10 s) detected the radical formation, whose stability was enhanced by the higher acidity of the tungstated zirconia catalyst which retarded the rebound process. On the contrary, IR with lower time resolution (10 -10 s) was unable to capture the active alkyl intermediates, showing the peaks related to the existence of both surface hydroxy and alkoxy after rebound. 

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). 

Tl(III) < Pb(IV), and this conclusion has been confirmed recently with reference to the oxythallation of olefins 124) and the cleavage of cyclopropanes 127). It is also predictable that oxidations of unsaturated systems by Tl(III) will exhibit characteristics commonly associated with analogous oxidations by Hg(II) and Pb(IV). There is, however, one important difference between Pb(IV) and Tl(III) redox reactions, namely that in the latter case reduction of the metal ion is believed to proceed only by a direct two-electron transfer mechanism (70). Thallium(II) has been detected by y-irradiation 10), pulse radiolysis 17, 107), and flash photolysis 144a) studies, butis completely unstable with respect to Tl(III) and T1(I) the rate constant for the process 2T1(II) Tl(III) + T1(I), 2.3 x 10 liter mole sec , is in fact close to diffusion control of the reaction 17). 

A relatively low potential, one-electron oxidation is observed (Equation (72)), followed above pH 2.2 by a two-electron oxidation, two-proton step (Equation (73)) and a one-electron oxidation (Equation (74)). In more acidic solutions a direct three-electron oxidation occurs leading also to the [Ruv O Ruv]4+ species. In various studies the Rulv O Rulv, RuIV-0 Ruv, and Ruv O Ruv species have been considered as the catalytically active form. Although these species have been characterized by resonance Raman and EPR spectroscopies,475,476,480 no definitive conclusion about the mechanism involved in the catalysis can be drawn and the question remains largely open. 

Further insight into the mechanism of this reaction was obtained with the help of MO theory and quantum mechanical calculations." The following orbital diagram (Scheme 35)100>101 describes the interaction of two sulfide moieties, which results in dication formation after a two-electron oxidation (cases A, B and C correspond to progressive increase in orbital perturbation and interaction between the sulfur atoms). 

In accord with this mechanism, a single two-electron oxidation of the enzyme into Compound I by hydrogen peroxide (Reaction (8)) is followed by two one-electron steps Reaction (9), in which substrate RH is oxidized to a radical R and Compound I is reduced to Compound II and Reaction (10), in which Compound II is reduced to native MPO, completing the catalytic... 

Several compounds can be oxidized by peroxidases by a free radical mechanism. Among various substrates of peroxidases, L-tyrosine attracts a great interest as an important phenolic compound containing at 100 200 pmol 1 1 in plasma and cells, which can be involved in lipid and protein oxidation. In 1980, Ralston and Dunford [187] have shown that HRP Compound II oxidizes L-tyrosine and 3,5-diiodo-L-tyrosine with pH-dependent reaction rates. Ohtaki et al. [188] measured the rate constants for the reactions of hog thyroid peroxidase Compounds I and II with L-tyrosine (Table 22.1) and showed that Compound I was reduced directly to ferric enzyme. Thus, in this case the reaction of Compound I with L-tyrosine proceeds by two-electron mechanism. In subsequent work these authors have shown [189] that at physiological pH TPO catalyzed the two-electron oxidation not only L-tyrosine but also D-tyrosine, A -acetyltyrosinamide, and monoiodotyrosine, whereas diiodotyrosine was oxidized by a one-electron mechanism. 

It has been pointed out earlier that peroxidases oxidize hydrogen peroxide by two-electron transfer mechanism to form Compound I. Thus for MPO, we have ... 

However, in addition to two-electron oxidation by native peroxidase, Compound I can oxidize hydrogen peroxide by one-electron mechanism ... 

Further reaction of Ph3P+ and Hg contributes the second ET for the overall two-electron oxidation. This was studied in detail for oxidations of tetraphenyllead, tetraethyllead and tetramethyllead at mercury electrodes in dichloromethane. The rationale of the mechanisms proposed above is based on the following observations122. 

One electron oxidation of monocarbanions leads to carbon radicals and two electron oxidation gives carbocations. In most of these oxidations, the mechanism is not known, though progress is being made on some mechanisms. But there appears to be a parallelism between base strength and ease of oxidation of carbanions. 

As a typical example, we consider the behaviour of the chloro-hydride complex [RuHC1(PP3)], which displays a rather simple conversion mechanism. In fact, Figure 16a shows that it undergoes an irreversible two-electron oxidation that generates, on the reverse scan, a voltam-metric profile identical to that observed for the chloro monocation [RuC1(PP3)] +, Figure 16b. 

In order to illustrate the application of LSV in mechanistic analysis we can look at the redox behavior of the formazan-tetrazolium salt system which we studied some years ago [17], 1,3,5-Triphenyl formazane was oxidized at controlled potential in CH3CN-Et4NC104 solution to 2,3,5-triphenyl tetrazolium perchlorate which was then isolated in quantitative yield. Coulometry showed that the overall electrode reaction was a two-electron oxidation. It has been shown that the rate of variation of Ep with log v was 30 mV per decade of sweep rate and that there was no variation of the peak potential with the concentration of 1,3,5-triphenylformazan. According to Saveant s diagnostic criteria (Table 1), four mechanistic schemes were possible e-C-e-p-p, e-C-d-p-p, e-c-P-e-p and e-c-P-d-p. If cyclization is the rate-determining step, then the resulting e-C-e-p-p and e-C-d-p-p mechanisms would not imply variation of Ep with the concentration of base. However, we have observed the 35 mV shift of Ep cathodically in the presence of 4-cyanopyridine as a b e. These observations ruled out the first two mechanisms. The remaining possibilities were then e-c-P-e and e-c-P-d, as shown in Scheme 3. 

Anodic oxidation of 45 in dry acetonitrile at 60 °C and at low current density provided a quantitative yield of 46, while oxidation of 45 in aqueous acetonitrile at 0 °C provided a high yield of 47. It has been shown that quinoneimine 47 can be transformed to 46 in 93% yield, through BF3Et20 catalyzed cyclization [75]. The reaction pathways leading to the formation of 46 or 47 are summarized in Scheme 25. Two-electron oxidation of 45 leads to the cation 45a through an ECE or e-p-e mechanism. It seems that the cyclization of 45a is the ratedetermining step in the overall intramolecular cyclization of 45 to 46. The high... 

The mechanism which could explain the formation of these products is described in Scheme 27. In an EC mechanism, the intermediate radical cation 48a could undergo a follow-up reaction with water as a nucleophile to form radical 48b which could than dimerize through S-N or S-S bond formation or react with 48a to yield 50 and 51 as the fianl one-electron oxidation products. In an ECE mechanism, intermediate 48b is further oxidized to 48c which reacts with acetonitrile as a solvent to give 49 as the final two-electron oxidation product. The cation intermediate 48c can react with the parent molecule 48 through [2 -f 3]-cycloaddition to give the final products 50 and 51. The [2 -f 3]-... 

Kinetics and mechanisms of oxidation of amines by Ru porphyrin complexes (particularly TMP species) have been reviewed [42]. rranx-Ru(0)2(TMP)/02/ CgHg/50°C/24h oxidised primary and secondary amines in the oxidation of ben-zylamine frani-Ru(NHj)jCHjPh)2(TMP) was isolated and characterised crystallo-graphically. A mechanism involving a two-electron oxidation of benzylamine to A-benzylideneamine by tra i-Ru(0)2(TMP) was proposed with concomitant reduction of the latter to Ru (0)(TMP). This disproportionates to tranx-Ru "(0)2(TMP) and Ru"(TMP) the latter regenerates Ru" (0)(TMP) with O, while the second two-electron oxidation of the imine to the aldehyde is effected by tranx-Ru(0)2(TMP) [597], (Table 5.1) [598]. 

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