Radical cations electron-transfer oxidation

On the basis of a quantum yield as great as 20, they suggests a chain radical cation electron-transfer oxidation, initiated by the reaction of the radical cation (DCMC+) with 02 with formation of the oxygenated radical cation (DCMC-O+). 

We suggest that electron transfer and electrophilic substitutions are, in general, competing processes in arene oxidations. Whether the product is formed from the radical cation (electron transfer) or from the aryl-metal species (electrophilic substitution) is dependent on the nature of both the metal oxidant and the aromatic substrate. With hard metal ions, such as Co(III), Mn(III), and Ce(IV),289 reaction via electron transfer is preferred because of the low stability of the arylmetal bond. With soft metal ions, such as Pb(IV) and Tl(III), and Pd(II) (see later), reaction via an arylmetal intermediate is predominant (more stable arylmetal bond). For the latter group of oxidants, electron transfer becomes important only with electron-rich arenes that form radical cations more readily. In accordance with this postulate, the oxidation of several electron-rich arenes by lead(IV)281 289 and thallium(III)287 in TFA involve radical cation formation via electron transfer. Indeed, electrophilic aromatic substitutions, in general, may involve initial charge transfer, and the role of radical cations as discrete intermediates may depend on how fast any subsequent steps involving bond formation takes place. 

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... 

Transfer of an electron from a a- or jr-bond leads to a radical cation (Eq. la), oxidation with a preceding (Eq. lb) or a succeeding deprotonation generates a radical, and oxidation of the radical forms a cation (Eq. Ic). The radical cations can react according to Scheme 1 [1, 2]. 

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. 

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]. 

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 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... 

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). 

These results were reconciled with a mechanism involving reversible formation of a radical cation by electron transfer oxidation of the hydrocarbon by Co(III)244 ... 

In summary, the oxidation of aromatic hydrocarbons carried out with high concentrations of cobalt catalysts involve two competing processes, namely, electron transfer oxidation of the hydrocarbon to the radical cation and electron transfer oxidation of the ligand to the corresponding radical ... 

Copper(II) is known61 63b>65 to be more effective than other metal oxidants for the electron transfer oxidation of radicals to the corresponding cations. 

Reactions of xenon difluoride with various carbanions have been extensively studied by Tselinski and coworkers25-28. They found that the solvent plays the most important role in the transformation reactions in dichloromethane give mixtures of up to four products, while acetonitrile is found to be the most convenient solvent (Scheme 6). Excellent results were established with various substrates, while cationic species do not play a very important role. The authors suggested a one-electron transfer oxidation of the carbanion and further formation of various radicals, which is especially evident in reactions in mixtures of acetonitrile and benzene. 

The features of initiation of free radical reactions in polymers by dimers of nitrogen dioxide are considered. The conversion of planar dimers into nitrosyl nitrate in the presence of amide groups of macromolecules has been revealed. Nitrosyl nitrate initiates radical reactions in oxidative primary process of electron transfer with formation of intermediate radical cations and nitric oxide. As a result of subsequent reactions, nitrogen-containing radicals are produced. The dimer conversion has been exhibited by estimation of the oxyaminoxyl radical yield in characteristic reaction of p-benzoquinone with nitrogen dioxide on addition of aromatic polyamide and polyvinylpyrrolidone to reacting system. The isomerisation of planar dimers is efficient in their complexes with amide groups, as confirmed by ab initio calculations. 

Facile ligand-transfer oxidation of alkyl radicals is accomplished by copper(II) halides or pseudohalides 143a). Two processes occur simultaneously (1) oxidative substitution via cationic intermediates and an alkylcopper species, as in electron-transfer oxidation processes and (2) homolytic atom transfer. The former is akin to the oxidative displacement... 

A combination of radical and electron transfer steps mediated by manganese triacetate has been used in the synthesis of 5-acetoxyfuranones 21 through carbox-ymethyl radical addition to mono- and disubstituted alkynes 20, followed by oxidative cyclization of the resulting vinyl radicals 22 (Scheme 2.4). The cyclic intermediate 24 is transformed into the furanone 21 through stepwise one-electron oxidation and capture of the resulting aUyl cation 26 by acetate. 

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 The first set of brackets encloses the electron donor-acceptor or EDA precur-... 

Owing to the central role of radical cations and radical anions, any general description of electron-transfer oxidation must rely on their individual behavior, as described in the next section. <"... 

By convention, the electrode potentials for redox are referred to only in terms of reduction (E n ). For electron-transfer oxidation, however, the values of are employed for RH donors here with the understanding that = -E i of the radical cation. 

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