Solvent cage electron-transfer oxidation

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... 

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). 

Each ion-radical reaction involves steps of electron transfer and further conversion of ion-radicals. Ion-radicals may either be consnmed within the solvent cage or pass into the solvent pool. If they pass into the solvent pool, the method of inhibitors will determine whether the ion-radicals are prodnced on the main pathway of the reaction, that is, whether these ion-radicals are necessary to obtain the hnal prodnct. Depending on its nature, the inhibitor may oxidize the anion-radical or reduce the cation-radical. Thns, quinones are such oxidizers whereas hydroquinones are reducers. Because both anion and cation-radicals are often formed at the first steps of many ion-radical reactions, qninohydrones— mixtures of quinones and hydroquinones—turn out to be very effective inhibitors. Linares and Nudehnan (2003) successfully used these inhibitors in studies on the mechanism of reactions between carbon monoxide and lithiated aromatic heterocycles. 

One important discrepancy should be noted between photochemical and chemical ion radical reactions. In the photochemical mode, an oxidized donor and a reduced acceptor remain in the same cage of a solvent and can interact instantly. In the chemical mode, these initial products of electron transfer can come apart and react separately in the bulk solvent. For example, one-electron oxidation of phenylbenzyl sulfide results in formation of the cation radical both in the photoinduced reaction with nitromethane and during treatment with ammoniumyl species. Sulfide cation radicals undergo fragmentation in the chemical process, but they form phenylbenzyl sulfoxide molecules in the photochemical reaction. The sulfoxide is formed at the expense of the oxygen atom donor. The latter comes from the nitromethane anion radical and is directly present in the solvent cage. As for the am-... 

The reactant state is converted to the product state by the transfer of one electron. The participants in the reactant state may be individual molecules held transiently in proximity by a solvent cage or they can be distinct parts of a supramolecular unit. Several types of chemical species can make up the reactant state it may contain only ground-state, spin-paired entities, or electronically excited entities (singlet or other multiplicity), or reactive entities (free radicals, metal complexes in unusual oxidation states, etc.) Many combinations are possible, and a large variety of reactant states can be prepared from some precursor state by photon absorption. The chapters in this series of volumes contain an abundance of examples. In every case, however, no matter what the identity of the entities participating in the process, the... 

Few examples of this mechanism have been clearly demonstrated because of tfie difficulty in establishing that this path occurs from experimental data. The most well-established examples are reactions of nickel complexes with aryl halides Studied by Tsou and Kochi. The rate of the reaction of Ni(PEt3)jWith aryl halides was shown to be first order in nickel and in ArX and retarded by added PEtj, Ortho-methyl substituents had little effect on the rate. Because of the lack of steric effect, electron transfer was proposed to occur after formation of a TT-complex between Ni(PEt3)j and ArX, rather than by direct insertion of the metal into the carbon-halogen bond by a three-centered mechanism. Moreover, the products of the reaction included the Ni(I) species L3NiX and arene. Tliese products are likely to result from the pathway in Scheme 7.4, involving electron transfer from Ni(0) to the aryl halide and escape of the aryl radical from the solvent cage. Other studies of oxidative additions of aryl halides and sulfonates to Ni(0) complexes have been reported. " ... 

Once electron transfer has occurred, coulombic attractions keep the oppositely charged ions in the solvent cage. If there is an accessible excited state of one of the partners, then the back transfer of the electron will occur at an immeasurably fast rate. This makes direct examination of this crucial part of the mechanism extremely difficult, if not impossible. Nevertheless, there is good indirect evidence for this sequence of events. The remaining essential feature of the mechanism is that in order to allow the formation of an energetic excited state within the second law of thermodynamics, the second electron transfer must occur from the now reduced oxidant so as to leave a stable molecule. Certain structural types are thus required. Examples of these can be seen in the reactions depicted on pages 47 and 35. 

The parallels between the reactions of firefly luciferin, as presently understood, and those of the acridan are striking, as the comparison below shows. Note that the oxidation to form the peroxide does not take place in one step. Investigation [37] has shown that electron transfer and recombination within the solvent cage, very similar to other carbanion oxidations, gives the appearance of a one step reaction. 

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