One-Electron Transfer Scheme

The concept of the one-electron transfer scheme provides a common feature to the seemingly unrelated reductive transformation pathways presented in Section 3.B. Each of these processes occurs initially by transfer of a single electron in the ratedetermining step. In each case, a radical anion is formed that is more susceptible to reduction than the parent compound. 

Although it is not possible to predict the absolute rates for the reduction of organic chemicals in environmental systems, recent laboratory studies have established relationships between substrate properties and relative reactivities (Tratnyek et al., 1991). For example, Schwarzenbach et al. (1990) studied the quinone and iron porphyrin mediated reduction of a series of nitrobenzenes and nitrophenols (see Section 3.C.2 for further discussion of electron mediated reductions). From the experimentally determined rate law, it was determined that the transfer of the first electron from the monophenolate species of the hydroquinone, lawsone (HLAW ), to the nitroaromatic was rate determining (Equation 3.50). 

Hammett a constants also have been successfully correlated with reduction rate constants. For example, the reduction rate constants of a series of 4-substituted nitrobenzenes in anaerobic sediment-water systems and the Hammett a constants exhibited a positive correlation (Delgado and Wolfe, 1992). The slopes of the linear-free energy plots for a river sediment, pond sediment, and aquifer material were similar, suggesting that reduction was occurring through the same mechanism in each of these systems. 

The disappearance rate constants for a large number of halogenated hydrocarbons, which include halogenated alkanes and alkenes, in anoxic sediment-water systems were correlated with several molecular descriptors (Peijnenburg et al., 

Equation 3.51) and experimentally determined rate constants (k p) for the series of halogenated alkanes and alkenes. 

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A- Alky In i tropy razo I cs are also reduced in two stages the first stage corresponds to reversible one-electron transfer (Scheme 3.35). In comparison with nitropyrazoles not substituted on nitrogen atom, the first half-wave potentials of A-alkylnitro-pyrazoles are essentially moved in cathodic region. Using the ESR method the signals of primary radical anions are recorded. 

The polarogram of 1,4-dinitropyrazole is considerably more complex and has five waves (Table 3.36, Section 3.3). This compound is reduced more easily than all the investigated nitroazoles. The first wave corresponds to an irreversible one-electron transfer (Scheme 3.19). As with 1-nitropyrazole, at this stage an unstable anion radical is formed and then breaks up at bond N-N02. The N02 anion is reduced at potential -1.7 V. 4-Nitropyrazolyl radical further is dimerized with subsequent reduction [851],... 

Redox active dioxolene derivatives exist as different isomers of benzo-quinone, semiquinonate and catecholate by two-step one-electron transfer (Scheme 55). A linear chain structure with direct a Rh Rh bond was reported with 3,6-di-tert-butyl-l,2- benzosemiquinonate (3,6-DBSQ) radical anion in [Rh (C0)2(3,6-DBSQ)]. It was originally synthesized by the reaction of T1(3,6-DBSQ)... 

In 1970, a new reacdon, the displacement of a nitro group from ct-nitro esters, ct-nitro nitnles, ct-nitro ketones, iind ct,ct-dinitro compounds by nitroalkiine salts, was described. These displacements, which are exemplified by the reacdon presented in Eq. 7.1, take place at room temperanire iind give excellent yields of pure products. The reacdon proceeds via a radiciil chain mechanism involving one electron-transfer processes as shovmin Scheme 7.1 the details of the mechanism are described in a review. ... 

Meanwhile, it was found by Asai and colleagues [48] that tetraphenylphosphonium salts having such anions as Cl, Br , and Bp4 work as photoinitiators for radical polymerization. Based on the initiation effects of changing counteranions, they proposed that a one-electron transfer mechanism is reasonable in these initiation reactions. However, in the case of tetraphenylphosphonium tetrafluoroborate, it cannot be ruled out that direct homolysis of the p-phenyl bond gives the phenyl radical as the initiating species since BF4 is not an easily pho-tooxidizable anion [49]. Therefore, it was assumed that a similar photoexcitable moiety exists in both tetraphenyl phosphonium salts and triphenylphosphonium ylide, which can be written as the following resonance hybrid [17] (Scheme 21) ... 

This statement does not mean, however, that the mechanism of diazotization was completely elucidated with that breakthrough. More recently it was possible to test the hypothesis that, in the reaction between the nitrosyl ion and an aromatic amine, a radical cation and the nitric oxide radical (NO ) are first formed by a one-electron transfer from the amine to NO+. Stability considerations imply that such a primary step is feasible, because NO is a stable radical and an aromatic amine will form a radical cation relatively easily, especially if electron-donating substituents are present. As discussed briefly in Section 2.6, Morkovnik et al. (1988) found that the radical cations of 4-dimethylamino- and 4-7V-morpholinoaniline form the corresponding diazonium ions with the nitric oxide radical (Scheme 2-39). 

A particular case of a [3C+2S] cycloaddition is that described by Sierra et al. related to the tail-to-tail dimerisation of alkynylcarbenes by reaction of these complexes with C8K (potassium graphite) at low temperature and further acid hydrolysis [69] (Scheme 24). In fact, this process should be considered as a [3C+2C] cycloaddition as two molecules of the carbene complex are involved in the reaction. Remarkable features of this reaction are (i) the formation of radical anion complexes by one-electron transfer from the potassium to the carbene complex, (ii) the tail-to-tail dimerisation to form a biscarbene anion intermediate and finally (iii) the protonation with a strong acid to produce the... 

Molecules have some occupied and some unoccupied orbitals. There occur diverse interactions (Scheme 1) when molecules undergo reactions. According to the frontier orbital theory (Sect 3 in Chapter Elements of a Chemical Orbital Theory by Inagaki in this volume), the HOMO d) of an electron donor (D) and the LUMO (fl ) of an electron acceptor (A) play a predominant role in the chemical reactions (delocalization band in Scheme 2). The electron configuration D A where one electron transfers from dio a significantly mixes into the ground configuration DA where... 

It has been shown recently that the selective reductive homo-coupling polymerization of aromatic diisocyanates via one electron transfer promoted by samarium iodide in the presence of hexamethylphosphoramide [PO(NMe2)3] (HMPA) can produce poly(oxamide)s in nearly quantitative yield (Scheme 9). 

A number of heteroaromatic monothiocarboxylic acids are formed by Pseudomonas sp. From P. putida, there was isolated pyridine-2,6-di-(mon-othiocarboxylic acid) 46 (Scheme 16). Of interest is the fact that in P. stutzeri KC, a copper complex of 46 is the active agent for a one electron transfer in the bacterial biodegradation of CCI4. Methylation of P. putida extracts provides a number of related structures such as 47. In addition, a P. fluorescens sp. contains 8-hydroxy-4-methoxy-quinoline-2-monothiocarboxylic acid 48.98... 

The fact that the anodic oxidation of allylsilanes usually gives a mixture of two regioisomers suggests a mechanism involving the allyl cation intermediate (Scheme 3). The initial one-electron transfer from the allylsilane produces the cation radical intermediate [9], Although in the case of anodic oxidation of simple olefins the carbon-allylic hydrogen bond is cleaved [28], in this case the... 

Flavoprotein dehydrogenases usually accept electrons from reduced pyridine nucleotides and donate them to a suitable electron acceptor. The oxidation-reduction midpoint potential of the FAD of the oxidase has been determined by ESR spectroscopy and shown to be -280 mV. The NADP+/ NADPH redox potential is -320 mV and that of the cytochrome b is -245 mV hence, the flavin is thermodynamically capable of accepting electrons from NADPH and transferring them to cytochrome b. As two electrons are transferred from NADPH, although O2 reduction requires only one electron, the scheme of electron transfer shown in Figure 5.8 has been proposed by Cross and Jones (1991). 

The above three examples involved reactions where the electron transfer takes place from the metal to the organic substrate. The reverse scenario can also be used in radical reactions via oxidative generation of cationic radical species, which can undergo coupling reactions. Kurihara et al. have used chiral ox-ovanadium species as a one-electron transfer oxidant to silylenol ethers in a hetero-coupling process [165]. Treatment of 246 with a catalyst prepared in situ from VOCI3/chiral alcohol/MS 4 A followed by addition of 247 provided the coupling product 248 (Scheme 63). 8-Phenyl menthol 251 was found to be... 

Introduction of nitrobenzene sulfenates into the same mixture of trichlorosilane and tributylamine results in the evolution of hydrogen. As proven by Todres and Avagyan (1978), trichlorosilane with tributylamine yields the trichlorosilyl anion and tributylammonium cation. This stage starts the process involving one-electron transfer from the anion to a nitrobenzene sulfenate. At that time, nitrobenzene sulfenate produces the stable anion-radical with the tributylammonium counterion. The anion-radical gives off an unpaired electron to the proton from the counterion (see Scheme 1.14). 

If the snlfate anion-radical is bonnd to the snrface of a catalyst (sulfated zirconia), it is capable of generating the cation-radicals of benzene and tolnene (Timoshok et al. 1996). Conversion of benzene on snlfated zirconia was narrowly stndied in a batch reactor under mild conditions (100°C, 30 min contact) (Farcasiu et al. 1996, Ghencin and Farcasin 1996a, 1996b). The proven mechanism consists of a one-electron transfer from benzene to the catalyst, with the formation of the benzene cation-radical and the sulfate radical on the catalytic snrface. This ion-radical pair combines to give a snrface combination of sulfite phenyl ester with rednced snlfated zirconia. The ester eventually gives rise to phenol (Scheme 1.45). Coking is not essential for the reaction shown in Scheme 1.45. Oxidation completely resumes the activity of the worked-out catalyst. 

As an analogous example, the behavior of sulfonium salts can be mentioned. At mercury electrodes, sulfonium salts bearing trialkyl (Colichman and Love 1953) or triaryl (Matsuo 1958) fragments can be reduced, with the formation of sulfur-centered radicals. These radicals are adsorbed on the mercury surface. After this, carboradicals are eliminated. The carboradicals capture one more electron and transform into carbanions. This is the final stage of reduction. The mercury surface cooperates with both the successive one-electron steps (Scheme 2.23 Luettringhaus and Machatzke 1964). This scheme is important for the problem of hidden adsorption, but it cannot be generalized in terms of stepwise versus concerted mechanism of dissociative electron transfer. As shown, the reduction of some sulfonium salts does follow the stepwise mechanism, but others are reduced according to the concerted mechanism (Andrieux et al. 1994). 

If the nitro group is located at the ethylene fragment, one-electron transfer initiates dimerization of the developing anion-radicals. a-Nitrostilbene, w-methyl-co-nitrostyrene, and a-nitro-p-ferrocenylethylene give anion-radicals, which dimerize spontaneously. It is interesting to compare reactions of cyclooctatetraene dipotassium (C8HgK2) with a-nitro and a-cyano ferrocenylethylenes (Todres and Tsvetkova 1987, Todres and Ermekov 1989 Scheme 3.4). 

Irradiation accelerates the reactions of Scheme 4.1, and the substitution products are formed in 70-80% yields. Acceptors of radicals (e.g., di-tert-butylnitroxyl) or electrons (e.g., m-dinitro-benzene [DNB]) completely inhibit the snbstitution even if the acceptors are added to the reaction mixture in small amonnts. The mentioned snbstitution reactions do not take place when no cyano groups are present in the initial a-phenylsnlphonyl cumene. Hence, the cyano groups send the reaction via the ion-radical pathway. Like the nitro gronp, the cyano group promotes the formation of anion-radical, which originates on one-electron transfer from the thiophenolate or malonate ions to the substrate. 

To decrease the stationary concentration of complex (HetH- - - ArH) +, it will suffice to lower the concentration of the oxidizer, that is, substrate (HetH)+. This also decreases the equilibrium concentration of the cation-radical complex (HetH- - ArH)+. The rate of anisylation—the main process—drops sharply. The side process, one-electron transfer from anisole to the cation-radical of thianthrene, also decelerates, but not so markedly. So this side process (route b on Scheme 5.11) remains the only one. 

Once formed, the twisted cation-radical become surrounded by the chair-form neutral molecules. It does not transfer the hole to the neighboring chair-form neutral molecules until this twisted cation-radical acquires the chair configuration in a spontaneous manner. As a result, fast migration of the hole, that is, the one-electron transfer from the neutral solvent molecule to its cation-radical, is detained. Hummel and Luthjens (1986) considered the idea that conformation dynamics is involved in the electron transfer. Scheme 5.20 illustrates this dynamics. 

On one-electron rednction, aldehydes and ketones give anion-radicals. It is the carbonyl group that serves as a reservoir for the unpaired electron Ketones yield pinacols exclusively. Thus, acetophenone forms 2,3-diphenylbutan-2,3-diol as a result of electrolysis at the potential of the first one-electron transfer wave (nonaqueous acetonitrile as a solvent with tetraalkylammonium perchlorate as a supporting electrolyte) (van Tilborg and Smit 1977). In contrast, calculations have shown that the spin densities on the carbonyl group and in the para position of the benzene ring are equal (Mendkovich et al. 1991). This means that one should wait for the formation of three types of dimer products head-to-head, tail-to-tail, and head-to-tail (cf. Section 3.2.1). For the anion-radical of acetophenone, all of the three possible dimers are depicted in Scheme 5.21. 

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