Oxidation potentials of carbanions

The oxidation potential of carbanions, ox> or the reduction potential of carbocations, red> could be a practical scale of stability as defined by (3). These potentials can be measured by voltammetry, although the scale is subject to assumptions regarding elimination of the diffusional potential and solvation effects. 

TABLE 7. pKa (DMSO) of sulfones and standard oxidation potentials of the corresponding carbanions in DMSO... 

Advantage has been taken of the ready accessibility of eleven para-substituted trityl and 9-phenylxanthyl cations, radicals, and carbanions in a study of the quantitative relationship between their stabilities under similar conditions.2 Hammett-type correlations have also been demonstrated for each series. Heats and free energies of deprotonation and the first and second oxidation potentials of the resulting carbanions were compared. The first and second reduction potentials and the p/CR values of the cations in aqueous sulfuric acid were compared, as were calorimetric heats of hydride transfer from cyanoborohydride ion. For radicals, consistent results were obtained for bond dissociation energies derived, alternatively, from the carbocation and its reduction potential or from the carbanion and its oxidation potential. 

The possibility of a thermally activated electron transfer from an anion to an acceptor is not always excluded. As explained by Bordwell [106], this will of course depend on the oxidation potential of the anion or on its related basicity and this author has shown that the electron transfer reactivity of carbanions rapidly decreases with a decrease of basicity. It is thus possible that among the dark SRN1 reactions some of them are activated by an initial ground state electron transfer from the anion to the accepting substrate. 

The 9-phenylxanthyl radical is a resonance-stabilized triphenylmethyl analog. The corresponding carbonium ion and carbanion are also stabilized and can be prepared in sulfolane, so that A//het can be directly measured.The data for benzyl and t-butyl are obtained by measuring the reduction and oxidation potentials of the radicals in acetonitrile. The results show that C6H5CH2 and (CH3)3C are much harder than the 9-phenylxanthyl radical (the latter is just one of several studied with similar properties ). The solution hardnesses are then responsible for the difficulty in forming the ions in the benzyl and t-butyl cases, and the stability of the ions in the resonance-stabilized cases. The effect of the small hardness in the latter cases also is evident in the small bond energy for homolytic dissociation. 

We have noted that both 5 2 and SnAr reactions may occur through SET processes. There is good evidence that the SnAc reaction may involve such a pathway also. Figure 8.55 shows species identified by Bacaloglu and coworkers in a fast kinetic spectroscopy study of the reaction of hydroxide ion with l-chloro-2,4,6-trinitrobenzene (picryl chloride, 71). D ending on reaction conditions, these workers could see transients ascribed to the n complex (72), an intermediate produced by single electron transfer (73), and one or more cr complexes (74, 75). In addition, evidence was obtained for the reversible formation of a phenyl carbanion (76) and a dianion (77) that probably do not lead directly to the substitution product (78). Further support for the role of SET processes in SNAr reactions comes from the detection of radical intermediates by EPR spectrometry and by correlations of reactivity with the oxidation potentials of the nucleophiles in some studies. ... 

Kuhn s carbanion analogues, [44 ], [45 ] and [46 ], have recently been synthesized, and the precursor hydrocarbons [44]-H, [45]-H and [46J-H dissociate into the respective anions in DMSO to show deep blue colours without any added base (Kinoshita et al., 1994). A fullerene anion, Bu Qb [47 ], has also been obtained as a stable carbanion (Fagan et al., 1992) its lithium salt has been isolated in the form Li [47 ]-4MeCN or Li [47"]-3-4THF. Several stable all-hydrocarbon anions of precursor hydrocarbons with low pKa values ( 7) are listed in Table 2, along with their oxidation potentials, ox-... 

A typical cyclic voltammetric trace for the anodic oxidation of the fluorenyl anion 2 at platinum is shown in Figure 1. The oxidation potential for this and several other resonance stabilized carbanions lies conveniently within the band gap of n-type Ti02 in the non-aqueous solvents, and hence in a range susceptible to photoinduced charge transfer. Furthermore, dimeric products (e. g., bifluorenyl) can be isolated in good yield (55-80%) after a one Faraday/mole controlled potential (+1.0 eV vs Ag quasireference) oxidation at platinum. 

As the oxidation potential clearly shows, carbanions may easily be oxidized by the anodic method. The most typical process of the anodic oxidation of carbanions is the formation of radical species (equation 52). 

The reactions described above all involve the displacement of a carbanion from the cobalt atom of methylcobalamin. These reactions occur under aerobic conditions with rate constants in the order of milliseconds. It is apparent that metals which react by electrophilic attack on the Co-C bond (SE2 mechanism) occur with the more oxidized state of the metal, i.e., Pb, Tl, Hg, Pd, which have standard reduction potentials greater than 0.8 volts. Because of the "base on - base off" equilibrium these reactions are pH dependent. 

Reductlon/Oxidatlon of Organic Radicals Inner sphere electron transfer is generally considered the predominant redox process that occurs in ATRP. However, outer sphere electron transfer (OSET) may also occur between organic radicals and transition metal complexes whereby growing radicals are oxidized to carbocations by Cu or reduced to carbanions by Cu. The extent to which OSET occurs in ATRP is dictated by the relative redox potentials of the species involved. 

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