Two-electron mechanisms

The overall two-electron mechanism has been generally reported to obey an ECE (Electrochemical-Chemical-Electrochemical) process where the chemical reaction is the generally fast chemical scission of the C—S bond of the sulphone anion radical. 

The results of work [ 135] are of specific interest. The work surveyed the influence of the nature and structure of adsorbed layers upon the mechanism of deactivation of He(2 S) atoms. It has been shown that on a surface of pure Ni(lll) coated with absorbed bridge-positioned molecules of CO or NO, the deactivation of metastable atoms proceeds by the mechanism of resonance ionization with subsequent Auger-neutralization. With large adsorbent coverages, when the adsorbed molecules are in a position normal to the surface, deactivation proceeds by the one-electron Auger-mechanism. The adsorbed layers of C2H4 and H2O on Ni(lll) de-excite atoms of He(2 S) by the two-electron mechanism solely. In case of NH3 adsorption, both mechanisms of deactivation are simultaneously realized. Based on the given data, the authors infer that the nature of metastable atoms deactivation on an adsorbate coated metal surface is determined by the distance the electron density of adsorbate valance electrons is removed from the metal lattice. 

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. 

Manno et al. [43] observed the formation of superoxide during the oxidation of arylamines by rat liver microsomes. Noda et al. [44] demonstrated that microsomes are able to oxidize hydrazine into a free radical. In contrast, hepatic cytochrome P-450 apparently oxidizes paracetamol (4 -hydroxyacetanilide) to A-acetyl-p-benzoquinone imine by a two-electron mechanism [45]. Younes [46] proposed that superoxide mediated the microsomal S -oxidation of thiobenzamide. 

Microsomes are capable of oxidizing not only organic substrates but also inorganic ones. An interesting example is the metabolism of bisulfite (aqueous sulfur dioxide) in microsomes. Although mitochondrial sulfite oxidase is responsible for the in vivo oxidation of bisulfite by a two-electron mechanism, cytochrome P-450 is also able to reduce bisulfite to the sulfur dioxide radical anion [56] ... 

Chemical catalysts for transfer hydrogenation have been known for many decades [2e]. The most commonly used are heterogeneous catalysts such as Pd/C, or Raney Ni, which are able to mediate for example the reduction of alkenes by dehydrogenation of an alkane present in high concentration. Cyclohexene, cyclo-hexadiene and dihydronaphthalene are commonly used as hydrogen donors since the byproducts are aromatic and therefore more difficult to reduce. The heterogeneous reaction is useful for simple non-chiral reductions, but attempts at the enantioselective reaction have failed because the mechanism seems to occur via a radical (two-proton and two-electron) mechanism that makes it unsuitable for enantioselective reactions [2 c]. 

We also found that in the presence of ultrasound that not only was there a drop in overall cell voltage from 8.3 V to 7.3 V but the reaction approached completion in a shorter timescale despite the apparent switch to the two-electron process. Overall, ultrasound appeared to favour the two-electron mechanism, and the greatest effect of sonication upon product distribution was the substantial enhancement of alkene formation. 

Electrocatalytic Reduction of Dioxygen The electrocatalytic reduction of oxygen is another multi-electron transfer reaction (four electrons are involved) with several steps and intermediate species [16]. A four-electron mechanism, leading to water, is in competition with a two-electron mechanism, giving hydrogen peroxide. The four-electron mechanism on a Pt electrode can be written as follows ... 

Fig. 7 Proposed one- and two-electron mechanisms for electrocatalytic reduction of CO2 by c-Re(bpy) (CO)3Cl X is an oxide ion acceptor (adapted from Ref 55).

For the one-electron reduction of nitrite to NO by nitrate reductase, an O atom transfer mechanism is unlikely, inasmuch as O atom transfer is inherently a two-electron mechanism. 

Similar to peroxidases (Chapter 22) and LOXs (see above), cyclooxygenases are capable of catalyzing the oxidation of substrates during the reduction of PGG2 to PGH2. Potter and Hinson [96] proposed that prostaglandin H synthase catalyzed the oxidation of acetaminophen by both one-electron and two-electron mechanisms. Formosa et al. [91] showed that... 

Optical activity arises from the coupling of given electric-allowed transitions with a chiral orientation (coupled oscillator mechanism or two-electron mechanism) or from the electric or magnetic moments of a transition being pertubed by a chiral static field (asymmetrically perturbed field mechanism or one-electron mechanism) in the given one molecule. A similar mechanism of the optical activity can be expected for molecular assemblies which are composed of chiral and achiral ones. This type of optical activity is called induced optical activity and depends on types of inter-molecular interaction modes. 

It is known that, because of spin forbidding, molecular oxygen is weakly active in the normal state. However, when possible, it enters oxidation reactions much more easily by the one-electron rather than by the two-electron mechanism [71]. In this connection, it is believed that not so much molecular oxygen is toxic as reactive intermediates 02, H02 and OH, occurring in the course of its reduction to water. The radicals O and H02 are equivalent therefore, in the redox medium 02 possess the same unique properties, i.e. can be the oxidant and the reducer simultaneously. Thus, in principle, molecular oxygen is incapable of two-electron oxidation (because of spin forbidding), whereas H202 has this ability. This is the principal difference in the oxidation mechanisms of these radicals. 

Fig. 12. Two-electron mechanisms proposed by TAMREAC for the allyl-alkyl cross-coupling. The two numbers below each complex correspond to the complex number and to the valence electron number, respectively. Numbers in parentheses by the arrows correspond to the reaction numbers (Tables II, III, and IV). R = alkyl, X = halide, L = solvent molecule.

An additional catalase-type cycle may occur by a two electron mechanism [Mn]3+ + H202 - [Mn=0]5+ + H20... 

Chloro-l,6-heptadienes underwent alkylation/5-exo cyclization reactions in the presence of alkylaluminum reagents catalyzed by 5 mol% FeCl3 in the presence of PPI13 or DPEphos affording 2-alkylmethylenecyclopentanes in 30-80% yield. A polar two-electron mechanism was formulated for the reaction, but radicals may be involved [88]. 

In a more recent study Co(dppe)I2 was used as a catalyst for reductive additions of primary, secondary, and tertiary alkyl bromides or iodides 249 to alkyl acrylates, acrylonitrile, methyl vinyl ketone, or vinylsulfone 248 in an acetonitrile/water mixture using zinc as a stoichiometric reducing agent [305]. The yields of the resulting esters 252 were mostly good. The authors tested radical probes, such as cyclopropylmethyl bromide or 6-bromo-1-hexene (cf. Part 1, Fig. 8). However, the latter did not cyclize, but isomerized during addition, while the former afforded complicated mixtures. On this basis the authors proposed a traditional two-electron mechanism to be operative the results do not, however, exclude a radical-based Co(I) catalytic cycle convincingly (Fig. 61). 

Formylation reactions providing aldehydes succeeded under similar conditions in 51-86% yield. The process does not occur according to a p-hydride elimination/hydroformylation sequence. The authors favored, however, a two-electron mechanism for these reactions. A similar carbonylation/arylation method was also reported [243]. 

Another possible two-electron mechanism involves the direct transport of two electrons from a mononuclear transition metal complex to a substrate (S). Such a transport alters sharply the electrostatic states of the systems and obviously requires a substantial rearrangement of the nuclear configuration of ligands and polar solvent molecules. For instance, the estimation of the synchronization factor (asyn) for an octahedral complex, with Eq. 2.44 shows a very low value of asyn = 10 7to 10 8 and, therefore, a very low rate of reaction. The probability of two-electron processes, however, increases sharply if they take place in the coordination sphere of a transition metal, where the reverse compensating electronic shift from the substrate to metal occurs. Involvement of bi- and, especially, polynuclear transition metal complexes and clusters and synchronous proton transfer in the redox processes may essentially decrease the environment reorganization, and, therefore, provide a high rate for the two- electron reactions. 

Two-Electron Mechanisms of DNA Damage Triggered by Excess Electrons... 

Figure 21-29. Proposed two-electron mechanism of the DNA strand break induced by excess electrons (Figure 1 of ref. [36]. Reprinted with kind permission of Springer Science and Business Media.)...

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