Two-Electron Oxidation Processes

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... 

Anodic oxidation has been employed for water-soluble triphenyl-methane dyes. It has been shown that the formation of dye is an irreversible two-electron oxidation process.21-23 This method has been used for the oxidation of diamino triphenylmethane leuco compounds containing two to four sulfonic acid groups to obtain food-grade colored materials.24... 

For compound (Scheme 1 and Table 1) the oxidation pattern is quite different the differential pulse voltammetry exhibits two peaks of equal height, both corresponding to a two-electron oxidation process (Figure 11). The first oxidation occurs at nearly the same potential as the four-electron process of compound 6F. This shows that, as expected, the two Os(bpy)2( i-2,3-dpp) units are the first to be oxidised (Table 2). The second process concerns the oxidation of the two Ru(bpy)2(p-2,5-dpp) units. Since such units lie far away from the previously oxidized Os-containing units, their oxidation occurs at a potential (Table 2) close to that of the equivalent peripheral units of 6D. As in the case of the compounds 6A-F the oxidation of the two inner units are displaced outside the accessible potential window. 

In contrast to the active site of galactose oxidase, to pre-catalyst 13, and to the system reported by Stack et al., the proposed catalytic species 15 does not imdergo reduction to Cu intermediates, as the oxidation equivalents needed for the catalysis are provided for solely by the phenoxyl radical Hgands. Since the conversion of alcohols into aldehydes is a two-electron oxidation process, only a dinuclear Cu species with two phenoxyl ligands is thought to be active. Furthermore, concentrated H2O2 is formed as byproduct in the reaction instead of H2O, as in the system described by Marko et al. [159]. 

The reaction follows the consensus mechanism for aliphatic —H activation by oxyl-ferryl compounds (35) in which the first step is H-atom abstraction via TS1 to give a hydroxo-Fe(III) complex with a C-centered alkyl radical, labeled IN. This is followed by a rebound step via TS2 to give the final product, ethanol and the ferrous active site. Overall, this is a two-electron oxidation process where the bonding orbital serves as the electron donor and the H-atom abstraction is rate limiting. 

Table 11 Half-wave Potentials for One and Two Electron Oxidation Processes ...

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... 

Mechanisms of sulfoxidation catalyzed by compound I of heme enzymes were investigated by direct observation of sulfide-induced reduction of three different compound I species including HRP, His 64 Ser Mb, and 0=FeIVTMP+ (Fig. 9) (60). The reaction of thioanisole and compound I of HRP gave the resting state of HRP with accumulation of compound II (0=FeIV species) as an intermediate. The yield of sulfoxide by a stoichiometric reaction of HRP compound I with thioanisole was only 25%. On the other hand, the same sulfoxidation by 0=FeIVTMP+ and compound I of His 64 Ser Mb exclusively exhibited a two-electron oxidation process, resulting in the quantitative formation... 

The bioactivity of the frans-2-phenylcyclopropylamine 10 (tranylcypromine) which is also a potent inhibitor of MAO and an efficient, albeit dangerous tranquillizing drug, would result of a net two-electron oxidation process leading to the cyclopropyliminium ion 11, which then undergoes nucleophilic substitution by a thiol group of cysteine yielding 12, Eq. (5) [14]. 

Although there is no proof that the two-electron oxidation process leads to a product that is still dimeric, the fact that the cyclic voltam-mogram does not reveal the formation of new species even during repeated cycling is consistent with either a dimeric product or mononuclear complexes that rapidly re form the starting dimer (85). 

Particular cyclic enaminoesters derived from proline underwent a two-electron oxidation process, with loss of carbon dioxide103 (Scheme 76). 

Fig, 15. A schematic diagram illustrating the different two-electron oxidation processes undergone by Fe(III) hemes in horseradish peroxidase [Fe(IV)(por )], yeast cytochrome c peroxidase [Fe(IV(por)(R )], and diheme cytochrome c peroxidase. The porphyrin ring is represented by the square. Histidine is the proximal ligand in all cases. R represents an amino acid side chain. 

Electrochemical studies of the tetranuclear ruthenium complex of ligand (62) indicate two reversible two-electron redox waves in the anodic region attributed to oxidation of the Ru11 metal centers. This complex is said to act as two binuclear subunits with little or no electronic interaction between subunits. The first two-electron oxidation process results in a mixed valence complex with each subunit having a Ru11 and Ru111 metal center. 

SECM SG/TC experiments were carried out to prove that the product of the initial two-electron oxidation process diffused into the solution, where it would react homogeneously and irreversibly. For these measurements, a 10 /xm diameter Au tip UME was stationed 1 /xm above a 100 /xm diameter Au substrate electrode. With the tip held at a potential of —1.3 V versus saturated mercurous sulfate electrode (SMSE), to collect substrategenerated species by reduction, the substrate electrode was scanned through the range of potentials to effect the oxidation of borohydride. The substrate and tip electrode responses for this experiment are shown in Figure 16. The fact that a cathodic current flowed at the tip, when the substrate was at a potential where borohydride oxidation occurred, proved that the intermediate formed in the initial two-electron transfer process (presumed to be mono-borane), diffused into the solution. An upper limit of 500 s 1 was estimated for the rate constant describing the reaction of this species (with water or OH ), based on the diffusion time in the experimental configuration. This was consistent with the results of the cyclic voltammetry experiments (11). 

The materials were of low to medium molecular weight = 4,000-13,500) and showed very interesting electrochemical behavior. Studies of model compounds show that on reduction or oxidation the Fe-Fe bond is cleaved. The polymers, on the other hand, show reversible two-electron oxidation processes in THF/[ Bu4N][BF4] at fast scan rates, as the Fe-Fe bonds reform after cleavage due to the fact that Fe atoms are held in close proximity by the bridging organosiloxane units. 

A more versatile approach is to use a transformation reaction in which one type of active terminal species is converted into a second type. Two general reactions have been identified (1) a terminal unit anion-cation transformation by a two-electron oxidation process and (2) carbanion to free-radical conversion, which is a one-electron oxidation step. 

By far, one on the most common pathways of reactive metabolite formation is via an enzymatic two-electron oxidation process on aromatic rings containing... 

Like the Kolbe dimerization, the success of the Hofer-Moest reaction depends upon the judicious control of a number of experimental variables. These include among others (1) the use of low current density, (2) carbon electrodes, and (3) the presence of additives. The low current density ensures that the concentration of radical species on the surface of the electrode is kept low so as to favor the abstraction of the second electron. Carbon electrodes proved particularly adapted to such two-electron oxidation processes. The addition of additives leads to competitive adsorption of substrate and additives on the surface of the electrodes, resulting in a decrease of the local concentration of the radical species. 

The two-electron oxidation process can be diverted to provide an interesting route to variously substituted alkenes. The presence of a function (COOH, SiMe3, SR, NR2) beta to the carboxylic acid is useful in controlling the regiochemistry of the olefination reaction. However, it is not a prerequisite (Scheme 11). 

Cyclic voltammetry and controlled potential coulometry investigations on a MeCN solution, made 0.1 M in BU4NCIO4, have indicated that cis-[Ptn(FcPy)2Cl2] imdergoes a two-electron oxidation process at the platinum electrode, according to the two consecutive one-electron steps (4) and (5) ... 

Cyclic voltammetry investigation on a DMSO solution of the supercomplex, abbreviated as [ Ni (Lpy))2Pt Cl2], disclosed just one reversible wave, with Ei/2 = 0.023 V vs Fc /Fc. This potential is very close to that observed for the oxidation of the reference system [Ni (7)](C104)2, under the same conditions 0.032 V vs Fc" /Fc. Moreover, the coulometry experiment on a solution of [ Ni (Lpy) 2Pt kI l2] at a potential 200 mV more positive than Ey2 showed the consumption of 2 electrons. This indicates that the [ Ni (Lpy) 2Pt Cl2] supercomplex undergoes a two-electron oxidation process according to the following two one-electron reversible steps ... 

As a matter of fact, the electrochemiced investigation (cyclic voltammetry and coulometry) of a MeCN solution 0.1 M in BU4NCIO4 and lO M in DRu (bipy)2 Ni cyclamCOpy))2] showed first a one-electron oxidation process, to be ascribed to the Ru /Ruin change, followed, at a more anodic potential, by a two-electron oxidation process, to be assigned to the statistically controlled oxidation of the two metallocyclam subunits. The corresponding potential values are reported in the unidimensional diagram in Figure 2. 

The controlled-potential oxidation of selenium and tellurium has been studied extensively by Lingane and Niedrach (222). Both selenium and tellurium (—II) undergo well defined two-electron oxidation processes at all pH values. For selenium (—II) the primary anodic reaction seems to involve oxidation of the mercury electrode followed by formation of HgSe while tellurium (—II) seems to be directly oxidized to tellurium metal. In strongly alkaline solution tellurium (—II) appears to undergo an additional incomplete oxidation to TeOs-... 

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