Electron transfer, oxides

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). 

Lu, C. Y. Lui, Y.Y. (2002). Electron transfer oxidation of tryptophan and tyrosine by triplet states and oxidized radicals of flavin sensitizers a laser flash photolysis study. Biochimica et Biophysica Acta (BBA) - General Subjects, Vol. 1571, No.l, (May 2002), pp. 71-76, ISSN 0304-4165... 

FIGURE 12.3 Schematic depicting the electron flow in an enzyme-catalyzed mediated electron transfer oxidation of substrate. The relative magnitudes of the standard reduction potentials of each element for efficient mediation are shown beneath the scheme. 

The mechanisms of superoxide-dismuting activity of SODs are well established. Dismutation of superoxide occurs at copper, manganese, or iron centers of SOD isoenzymes CuZnSOD, MnSOD, or FeSOD. These isoenzymes were isolated from a variety of sources, including humans, animals, microbes, etc. In the case of CuZnSOD, dismutation process consists of two stages the one-electron transfer oxidation of superoxide by cupric form (Reaction (1)) and the one-electron reduction of superoxide by cuprous form (Reaction (2)). 

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... 

Diacyl peroxides are, however, also electron transfer oxidants, which according to a theoretical analysis should possess standard potentials, °[(ArCOO)2/RCOO RCOO ) of around 0.6 V in water, provided that the electron transfer process is of the dissociative type (50) (Eberson, 1982c). Such a value brings thermal ET steps involving DBPO within reach for redox-active organic molecules, as for example suggested by the so-called CIEEL mechanism of chemiluminescence (Schuster, 1982). 

Table 12 shows redox properties of some redox systems of biochemical nature. Generally, the redox potentials are modest, cytochrome P450 possibly being an exception. If cytochrome P450 functions as an electron transfer oxidant towards xenobiotic molecules, it is necessary to postulate a considerably higher potential (1.3-1.8V) from considerations of the Marcus theory (Eberson, 1990). 

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. 

The Sc -promoted photoinduced electron transfer can be generally applied for formation of the radical cations of a variety of fullerene derivatives, which would otherwise be difficult to oxidize [135]. It has been shown that the electron-transfer oxidation reactivities of the triplet excited states of fullerenes are largely determined by the HOMO (highest occupied molecular orbital) energies of the fullerenes, whereas the triplet energies remain virtually the same among the fullerenes [135]. 

The stability of [Fe4S4] core was utilized for electron-transfer oxidation catalysis. With 1,4-benzoquinone as oxidant, benzoin was catalytically oxidized by... 

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. 

Actually, the earliest derivative of a vinylcyclopropane radical cation was a serendipitous discovery. It was formed by an unusual hydrogen shift upon photo-induced electron transfer oxidation of tricyclo[4.1.0.0 ]heptane (26). This result has been questioned on the grounds that the same rearrangement was not observed in a Freon matrix. However, there is no basis for the assumption that radical cation reactions in frozen matrices at cryogenic temperatures should follow the same course as those at room temperature in fluid solution and in the presence of a radical anion, which is potentially a strong base. In several cases, matrix reactions have taken a decidedly different course from those in solution. For example, radiolysis of 8 in a Freon matrix generated the bicyclo[3.2.0]hepta-2,6-diene radical cation (27 ), or caused retro-Diels-Alder cleavage yet, the... 

The examples summarized above demonstrate that organometalhc derivatives of early transition metals can and will form dioxygen complexes, even though the stability of these adducts varies widely. The availability of some d-electrons is required i.e., d°-complexes do not show this mode of reactivity, presumably because binding of O2 requires some degree of electron transfer (oxidation of the metal). 

The oxidation of Co(NH3)gI+2 by double electron transfer oxidants, like H2O2, proceeds according to Mechanism 11 where the formation of IOH at an intermediate stage has been demonstrated (47). 

The process is induced photochemically and involves the single-electron transfer oxidation of cubane then completed with a backward electron transfer to the transient radical cations. A Li+ salt with a weakly coordinating anion is able to induce pericyclic transformations, including the rearrangement of cubane to cuneane, quadricyclane to norbomadiene, and basketene to Nenitzescu s hydrocarbon 392... 

Solid-supported FeCl3 is shown to act as an electron-transfer oxidant, providing a convenient method for coupling aromatic rings (80JOC749). The mechanism for the formation... 

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... 

Spontaneous oxidation of amines by one-electron transfer has been reported as a key process in polar solvents (35). It is not easy to distinguish the spontaneous and initiated mechanisms, because these pathways have a common intermediate (XI, Fig. 9). Thus, potassium hexacyanoferrate (III), a one-electron oxidant, gives electron transfer oxidation of amines (56) yielding the classical radical autoxidation products. 

Electron-transfer oxidation of organic compounds involves multiple steps with transient radicals as key reactive intermediates.14 The electron-transfer oxidation of a neutral, diamagnetic organic donor (RH), having an even number of electrons, produces a radical cation, as shown in Eq. (7). 

The energetic basis for the electron-transfer oxidation includes the thermodynamic potential of oxidation (E°ox) for the electron transfer from RH in Eq. (7). Such an electron detachment is commonly effected at an electrode, by an oxidant, or with light. The oxidation is driven electrochemically by the anodic electrode potential, which matches the E°m value. Likewise, the driving force in the chemical oxidation of RH is provided by the redox potential (fi°ed) of the electron acceptor or oxidant (A) according to Eq. (5). 

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