Radical ions electron-transfer oxidation

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. 

Conversion of toluenes to the benzoic acid is also accomplished by anodic oxidation in acetic acid containing some nitric acid. It is not clear if this reaction involves the aromatic radical-cation or if the oxidising agents are nitrogen oxide radicals generated by electron transfer from nitrate ions [66, 67]. Oxidation of 4-fluorotoluene at a lead dioxide anode in dilute sulphuric acid gives 4-fluorobenzoic acid in a reaction which involves loss of a proton from the aromatic radical-cation and them in further oxidation of the benzyl radical formed [68]. 

The basic forms of phenols (phenolate anions) are easily oxidized to semiquinone radicals through electron transfer. These radicals can then react with another radical to form an adduct through radical coupling or, in the case of o-diphenols, undergo a second oxidation step yielding o-quinones that are electrophiles as well as oxidants. Oxidation reactions are very slow in wine, due to the low proportion of phenolate ions at wine pH values, but take place extremely rapidly when oxidative enzymes are involved (see Section 5.5.2.2). 

If the assumption of this reaction sequence is correct, the photolysis of tetraphenylphosphonium chloride must then only lead to biphenyl, diphenylphosphine, ethyl diphenyl-phosphinate and triphenylphosphine and its oxidation products. After 2 h of irradiation, biphenyl, diphenylphosphine and its oxidation products, triphenylphosphine and triphenylphosphine oxide, in a ratio of 3 1 5, along with raw material, are obtained. Ethyl diphenylphosphinate was detected in trace amounts7. These results support the postulate of the reversibility of phosphoranyl radical formation in such systems and indicate one-electron transfer processes15 in the formation and decomposition of the tetraarylphosphonium cation. This reaction is comparable to the observation of an electron transfer from halide ions to hydroxyl radicals or hydrogen atoms in aqueous solutions16,... 

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... 

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. 

In the late 70s, the interest of many groups was aroused by cyanoaromatic compounds (Sens) because they felt that, by proper choice of sensitizer redox potential, the cyanoaromatic radical anions (SensT) should be re-oxidized by molecular oxygen to produce, by a second exergonic electron-transfer process, superoxide ion OJ [Eq. (6)] [84]. 

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-... 

Photodissociation of dimer coupled to current measurement of electrochemical oxidation of the pyridinyl radical to the pyridinium ion has been described in Section 3.1.3. Oxidation of the l-methyl-3-carbamidopyridinyl and NAD (nicotinamide adenine dinucleotide radical) after dissociation of the dimers has been reported the agents being either oxygen or OH radical. Reasonable mechanisms for the latter are either electron transfer or radical combination, followed by dissociation to Py+ and OH. 

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... 

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