Electron transfer like charge radical ions

Pairs of radical ions of like charge also react by electron transfer (i.e., they disproportionate). One classic example involves reduction of tetraphenylethylene and subsequent ET between two tetraphenylethylene anions. A more recent interesting example is that of cyclooctatetrene radical anion 148 . Alkali metals readily reduce the nonplanar cyclooctatetraene, generating a persistent planar radical anion... 

Under this type of catalysis we may classify those reactions in which reversible electron transfer takes place. The electron to be transferred may be carried by an atom, and in such a case we speak about hydrogen or chlorine transfer and the like. While in the previous case the reaction was characterized by the development of a positive charge at the reactive centre, and we could speak about carbonium ions, in the present case the reaction centre is characterized by the presence of an unpaired spin, and we may speak about free radical reactions, taking into account the same precautions as previously. 

In the context of mechanistic studies, the electrochemical behavior and reactions with nucleophiles of 4-chloro-2,6-diphenylpyrylium and 4-chloro(bromo)flavylium have been studied <1999CHE653>. The proposed mechanism for nucleophilic substitution in halogen-substituted pyrylium and flavylium salts passes through formation of a charge-transfer complex that is converted into an ion-radical pair by simple electron transfer. Heterocyclic cleavage of the C-halogen bond occurs at the stage of the radical or the adduct from the reaction of the pyrylium salt and the nucleophile. In this study, an amine nucleophile was used however, the data are likely relevant for other types of nucleophiles as well (Scheme 5). 

Photoreactions employing an electron transfer are discussed. Among these are recent examples of photochemical SRN1 reactions, photoalkylations of carbanions and photoreductions initiated by oxyanions and radical anions. Anions used in their ground state as electron donating quenchers are also considered. Intra ion pair electron transfers as well as the use of anion-like precursors in charge transfer complexes or charge transfer excited states are presented. 

It is found that, upon irradiation in toluene, a highly efficient triplet-triplet energy transfer governs the deactivation of the RuP, while electron transfer from the porphyrin to the fullerene (RuP +-C6o ) prevails in polar solvents [389]. Complexation of ZnP by the fullerene derivative is reversible and, following excitation of the ZnP, gives rise to very efficient charge separation. In fluid polar solvents like THE and benzonitrile radical ion pairs (ZnP +-C6o ) are generated both by intramolecular electron transfer in the complex and by intermolecular electron transfer in the uncomplexed form [386]. In the latter case radical pairs live about 10 ps in THE and about 50 ps in benzonitrile at room temperature [386]. Thus, com-... 

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