Electron transfer oxidized ions

In the presence of bromide ion the slow one-electron transfer oxidation of the ArCH3 substrate is replaced by the rapid one-electron oxidation of bromide ion by cobalt(III) to afford a bromine atom. The latter, or rather its adduct with bromide ion, Br2 acts as the chain transfer agent in the reaction with the ArCH3 substrate (Fig. 10). 

Electrodes may be classified into the following two categories as shown in Fig. 4-3 one is the electronic electrode at which the transfer of electrons takes place, and the other is the ionic electrode at which the transfer of ions takes place. The electronic electrode corresponds, for instance, to the case in which the transfer of redox electrons in reduction-oxidation reactions, such as Fe = Fe + e,occurs and the ionic electrode corresponds to the case in which the transfer of ions, such as Fe , , = Fe, occiirs across the electrode interface. Usually, the former is found with insoluble electrodes such as platinum electrodes in aqueous solution containing redox particles and the latter is found with soluble metal electrodes such as iron and nickel. In practice, both electron transfer and ion transfer can take place simultaneously across the electrode interface. 

The feasibility of electron transfer oxidation is dictated by the thermodynamic potential , of the substrate RH and requires an anode potential or an oxidant to match the value of El. It is essential to choose an oxidant with an one-electron reduction potential sufficient for the desired oxidation and a two-electron reduction potential insufficient for further oxidation of the radical cation. The suitable oxidant may be a metal ion, a stable radical cation, or a typical PET-acceptor in its excited state. The advantage of electrochemically performed oxidation is obvious. 

Eq. 2). The free radical intermediates obtained frequently undergo second single-electron transfer oxidation to yield carbenium ions. 

Transition metals (iron, copper, nickel and cobalt) catalyse oxidation by shortening the induction period, and by promoting free radical formation [60]. Hong et al. [61] reported on the oxidation of a substimted a-hydroxyamine in an intravenous formulation. The kinetic investigations showed that the molecule underwent a one-electron transfer oxidative mechanism, which was catalysed by transition metals. This yielded two oxidative degradants 4-hydroxybenzalde-hyde and 4-hydroxy-4-phenylpiperidine. It has been previously shown that a-hydroxyamines are good metal ion chelators [62], and that this can induce oxidative attack on the a-hydroxy functionality. 

Photocatalytic oxidation of 2,4-dichlorophenoxyacetic acid (2,4-D) was investigated (Sun and Pignatello, 1995). In addition to the dominant hydroxyl radical mechanism, Sun and Pignatello found evidence that direct hole oxidation may be the mechanism for the photocatalytic degradation of some organic compounds. The assumed mechanism for this oxidation is H+ acting as an electron-transfer oxidant, while O behaves like a free OH and abstracts H or adds to C=C multiple bonds. Hole oxidation has been used to explain the oxidation of oxalate and trichloroacetate ions, which lack abstractable hydrogens or unsaturated C-C bonds. Whether the reaction... 

Ma J, Lin W, Wang W, Flan Z, Yao S, Lin N (2000) Triplet state mechanism for electron transfer oxidation of DNA. J Photochem Photobiol B Biol 57 76-81 Maeda M, Nushi K, Kawazoe Y (1974) Studies on chemical alterations of nucleic acids and their components VII. C-alkylation of purine bases through free radical process catalyzed by ferrous ion. Tetrahedron 30 2677-2682... 

Electron-transfer oxidation can be considered to consist of a series of equilibria as shown in Eq. (9), with formation of an electron donor-acceptor precursor complex, which leads to the contact ion-pair constrained by the solvent cage. Intermolecular reactions of RH-, which lead to oxidation products, take place after escaping from the cage.14... 

The rate-enhancing effect of bromide ion is explained by a scheme involving the formation of bromine atoms via electron transfer oxidation of bromide ion ... 

In an inert atmosphere, alkyl radicals are converted to alkanes by hydrogen transfer with solvent. Radicals can also undergo electron transfer oxidation by the metal oxidant and afford products (alkene, ester, etc.) ascribable to car-bonium ion intermediates,237 249, 288, 333 namely,... 

The inefficient trapping of thiyl radicals by dodecene-1 was attributed to the effective interception of the radicals by Mn(III), resulting in electron transfer oxidation to the thioxonium ion. By contrast, thiyl radicals formed in the oxidation of thiols by the weaker oxidant, ferric octanoate, were scavenged by dodecene-1. Disulfide was formed by dimerization of thiyl radicals.352 Thus, the mechanism for disulfide formation is dependent on the nature of the metal oxidant. 

SbClj has been claimed to act as an electron-transfer oxidant toward a number of reagents, such as N,N,N, N -tetramethylphenylenediamine, triphenylamine, 2,4,6-tri-t-butylphenolate ion, ferrocene, and N-vinylcarbazole. Among these, triphenylamine is the most difficult to oxidize, and was therefore chosen as a model compound in entry no. 18. By matching the lowest possible experimental rate constant, 103 s , with rate constants calculated as a function of E° for the SbClj/SbCl - couple, the latter was estimated to be 0.54 V. This is an entirely reasonable value (see Cowell et al., 1970). 

We have already discussed several cases of fast Fe(III) oxidations which occur by a non-bonded electron-transfer mechanism (Tables 13 and 14). One case of a relatively slow reaction, involving the substitution-inert hexacyanoferrate(III) ion, is shown in Table 14 (entry no. 17) and clearly demonstrates the electron-transfer oxidizing properties of this species with respect to easily oxidized aliphatic amines. Whether the same mechanism holds for compounds more resistant to oxidation, such as methylnaphthalenes (Andrulis et al., 1966) remains to be seen (the estimated rate constant at 25°C is ca. 10-7 M l s-1). Generally, hexacyanoferrate(III) seems to be a good non-bonded electron-transfer reagent (for a review, see Rotermund, 1975). 

GENERIC BEHAVIOR OF RADICAL IONS AS REACTIVE INTERMEDIATES IN ELECTRON-TRANSFER OXIDATION... 

It is important to emphasize the anodic, chemical and actinic activations of electron-transfer oxidation to be complementary methods that all commonly involve the reactive intermediates like those presented in equations (la) and (2). As such, cognizance must always be taken of the subtle differences of concentration, temperature, solvent polarity, etc. that affect the behavior of the transient radicals and ion radi-... 

When dte oxidation-reduction equilibria in equation (6a) ate included, the thermal activation of elec-tron-transfn oxidation in equation (3b) follows a course that is akin to charge-transfn activation in equation (5). In both, the A cotiq>lex [RH,A] is the important precursor which is directly converted into the critical contact ion pair [RH, A ]. Such an involvonent of reactive intermediates in common does widen the scqte of electron-transfer oxidations to include both thermal and photochemical pro-... 

Such electron transfers between ions and electrodes result in chemical changes (changes in the valence or oxidation state of the ions), i.e., in electrodic reactions. When ions receive electrons from the electrode, they are said to be electronated, or to undergo reduction when ions donate electrons to the electrodes, they are said to be deelectronated, or to undergo oxidation. 

Because of facile oxidation, there is no possibility for electrophilic substitution see Electrophilic Reaction) at cobaltocene. On the other hand, owing to its positive charge, the cobaltocenium ion is deactivated towards electrophilic substitution. Thus, apart from simple electron-transfer oxidation, one of the most important reactions undergone by cobaltocene is a two-step oxidative addition, as depicted in equations (57) and (58). 

Electron-transfer oxidation in equation (3b) can be considered to consist of a series of preequilibria, in the limit where the radical cation of the organic donor and radical anion of the acceptor are both persistent species (equation 6a). The fu-st set of brackets encloses the electron donor-acceptor or EDA precursor complex, and the second set the contact ion pair or CIP successor complex that is constrained by the solvent cage. Intermolecular reactions of that lead to the oxidation products largely occur subsequent to cage escape (ki). 

The fate of the contact ion pair [RH, A ] is critical to electron-transfer oxidation. Oxidative efficiency is the highest with those organic donors that yield unstable radical cations, such as hexamethyI(Dewar benzene), which undergoes spontaneous rearrangement (equation 7). 4i... 

---