Radical anion disproportionation

The NMR spectrum indicates a planar aromatic structure. It has been demonstrated that the dianion is more stable than the radical anion formed by one-electron reduction, since the radical anion disproportionates to cyclooctatetraene and the dianion ... 

The cleavage of alkyl aryl sulfones by sodium amalgam and alcohols65 probably proceeds also through the intermediacy of a radical anion, followed by splitting to the arylsulfinate anion and an alkyl radical. Both the sulfinate anion and the disproportionation products of the radical have been observed. 

Under physiological conditions, the disproportionation of superoxide radical is very fast and involves reaction of the radical anion 02 with a molecule of the neutral radical, H02 (Eq. 2, Scheme 8.36). Disproportionation yields one molecule of molecular oxygen and one molecule of hydrogen peroxide. 

The electrochemical reduction of cycloheptatriene (CHT) in liquid ammonia takes place at about —2.5 V vs SCE and forms the radical anion of CHT. The radical anion is stable in ammonia on the voltammetric time scale but decays slowly by disproportionation and coupling reaction pathways to give respectively 1,3- and 1,4-cycloheptadienes (total yield 34-39%) and C14H18 (in yields of 55-58%) isomers which incorporate the bitropyl carbon skeleta20. 

The redox properties elicited for Rh(bpy)3 + and its congeners are thus entirely consistent with the description of these species as bound-ligand radicals. On the other hand, the disproportionation reactions eq 2-6 are not known to be characteristic of ligand-centered radicals, but are consistent with behavior expected for rhodium(II). Furthermore the substitution lability deduced for Rh(bpy)3 + and Rh(bpy)2 +> while consistent with that expected for Rh(II), is orders of magnitude too great for Rh(lII). Finally the spectrum observed for the intermediate Rh(bpy)3 + is not that expected for [RhIII(bpy)2(bpy")]2+. The spectrum measured has an absorption maximum at 350 nm with e 4 x 10 M 1 cm l and a broad maximum at 500 nm with e = 1 x 1()3 M 1 cm l. The spectra of free and bound bpy radical anions are quite distinctive (23.35-38) very intense absorption maxima (e 1 x 10 to 4 x 10 M - cm l) are found at 350-390 nm and are accompanied by less intense maxima (e 5 x 10 cm ) at 400 to 600 nm. While the Rh(bpy)3 +... 

Disproportionation mechanisms have been proposed for protonahon reachons and intramolecular rearrangements (see Sects. 4.3.2 and 4.3.3) [54]. The prominent feature is that follow-up processes at the level of the dianion can already take place at potentials corresponding to radical anion formation. In order to evaluate data for disproportionation reactions it is necessary to know the value of the disproporhonahon equilibrium constant. 

Drastic changes in the disproportionation constants occur when alkaK cations are used instead of tetraalkylammonium ions. Typically, the potentials of the radical anion formation are less affected than that of the dianion formation. In the presence of alkali cations, AE shifts may reach values of more than 600 mV, which correspond to an increase in the K constant of more than 10 orders of magnitude [52, 53]. 

Azobenzenes, (29), and analogous heteroaromatic azo compounds, (30), are in aprotic solvents reduced in two sequential one-electron steps to the radical anion and the dianion [61-66]. Disproportionation of the radical anion to the dianion is favored by the presence of Li+ [67]. The dianion is considerably more basic than the radical anion, and the dianion is only stable in very dry nonacidic solvents [64, 65, 67, 68]. Both the dianion and the radical anion derived from (29) have been used as EGBs. The anion resulting from protonation of the dianion is less basic (by several pK units) than the dianion but more basic than... 

Esters of ethenetetracarboxylic acid, (37), and disubstituted (fluoren-9-ylidene)me-thane derivatives, (38), are reduced sequentially to radical anions and dianions [2, 68, 84, 85]. Only the dianions are sufficiently basic to be useful as EGBs [53,86]. For (37), the two reduction potentials are separated by 0.2 V [68], and even with a working potential allowing formation only of the radical anion, the dianion can be formed by disproportionation. The protonated form of the dianionic EGBs, (37H) and (38H) , will normally be stable in solution since the pK values of the dihydro products are expected to be in the range 12 to 16. 

Stilbene derivatives are reduced in dimethylformamide to the dihydrocompound and in a number of cases [6,18] the mechanism is known to involve accumulation of the radical-anion in solution. Ihis disproportionates to the starting material and the dianion, which is protonated. 

The formation of butanes by reduction of arylethenes may arise by radical-radical coupling of two radical-anions giving a dianion, which is then protonated. An alternative route is by nucleophilic addition onto one neutral molecule of the radical-anion, followed by further reduction and protonation. In support of this alternative, cyclobutanes have been isolated from electrochemical reduction of phenylvinylsulphones [21] and vinylpyridines [22], A mechanism for the latter process is illustrated for the case of 2-vinylpyridine 7. Nucleophilic attack of a radical-anion on the substrate gives an intermediate and this disproportionates to form the cyclobutane and a 1,4-diary Ibutane. Cyclobutanes are themselves reduced with ring opening to form the 1,4-diarylbulane. 

The mechanism of the electron transfer from electron donor such as MV to producing molecular hydrogen and catalyzed by Au [187] or Pt clusters [188], and that of the disproportionation reaction of the superoxide radical anion O2 by Pt clusters in solution or supported on colloidal Ti02 particles [189], have been studied by pulse radiolysis. 

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... 

It may not always be clear from the conditions for electrochemical generation which species is the effective EGB. In some cases a possible complication is fast disproportionation of radical-anion to dianion (Scheme 12). This can mean that for electrogeneration at, say, the first reduction potential E Jl) it is possible for either the radical-anion or the dianion to act as base, depending on the relative rates of protonation by acid HA (k and kp, the value of the disproportionation constant (Kj), and the rate at which equilibrium between radical-anion and dianion is attained. In principle, of course, it is also possible that electrogeneration at E p2) could lead to a situation where radical-anion was the effective base as a consequence of rapid reproportionation causing it to be present in high concentration, thus offsetting its probably much lower kinetic basicity. These points are discussed in more detail on p. 157. 

Derivatives of the general formula (7) in Table 6 have been successfully used as probases and their properties in this context are being further explored. In common with the azobenzenes and ethenetetracarboxylate esters, the fluoren-9-ylidene derivatives usually display two reversible one-electron peaks in cyclic voltammetric experiments. Although disproportionation is possible (cf. Scheme 12) it is the dianions which are the effective bases. It was shown early on that the radical-anions of such derivatives are long-lived in relatively acidic conditions (e.g. in DMF solution the first reduction peak of Ph C -.QCN) remains reversible in the presence of a 570-fold molar excess of acetic acid, at 0.1 V s ). Even the dianions are relatively weak bases, useful mainly for ylid formation from phosphonium and sulphonium salts (pKj s 11-15) they are not sufficiently basic to effect the Wittig-Homer reaction which involves deprotonation of phosphonate esters... 

Cathodic reduction of oxygen is the most convenient method of production of the superoxide radical-anion,. The properties of this important species have been well reviewed and key references to the extensive work on the electrochemistry of oxygen are contained therein. Of immediate significance is the large cathodic shift in E° for the 0 /0 couple which accompanies a change from aqueous to aprotic solvent (e.g. DMF, DMSO, and MeCN) this is interpreted in terms of relatively weak solvation in aprotic media which enhances the nucleophilicity of the superoxide anion. However, in the presence of acids the chemistry of superoxide is dominated by the disproportionation shown in equation 1. 

This could be explained in terms of disproportionation of the radical-anion to dianion with subsequent protonation. However, a much more complete explanation followed the realisation that, in most cases, the radical-anion acts not only as a base but also as a single electron-transfer agent (the so-called DISP mechanisms). In particular a comparison of observed cyclic voltammetric behaviour of substituted azobenzenes in the presence of weak acids with that predicted using digital simulation based on various mechanistic possibilities has established the DISPl route given in Eq. (3) (reactions 1-4). 

On the other hand, for anthraquinone and benzophenone, the ESR results showed that the concentration of the radical anion was almost time-independent, although, in polarography, the height of the first wave increased, consuming the height of the second wave as in Fig. 8.12. This inconsistent result was explained as the establishment of the following disproportionation equilibrium in the solution ... 

Unlike the tub-shaped parent azocines which are antiaromatic, their 1 Orr-electron dianions (e.g. 30) are planar and aromatic in nature (7iJAi6i). The dianiones are formed by two-electron reduction of azocines. An intermediate radical-anion (29) was obtained from 3,8-dimethyl-2-methoxyazocine (28) (83JA6078) which has a strong tendency to disproportionate into dianion (30) and neutral azocine. 

The most common electrochemical effects exerted in bulk solution are related to association (solvation, ion-pairing, complex formation, etc.) with the electroactive substance or electrochemically generated intermediates [4,19]. The importance of solvation can be gauged by comparing calculated and measured values of the parameter AE1/2 (defined as the difference, in volts between the half-wave potentials of the first and second polarographic waves) exhibited by polycyclic aromatic hydrocarbons (PAH) in dipolar aprotic solvents [46,47], It can be shown that AE1/2 is related to the equilibrium constant for disproportionation of the aromatic radical anion into neutral species and dianion, that is,... 

The structure and energy of a series of ions generated from penta-cyclo[3.3.1.13,7.01 3.05 7]decane (7) has been explored by using HF, MP2 and DFT methods to estimate enthalpy changes of isodesmic disproportionation reactions and by considering the reorganization of frontier orbitals as a consequence of addition or removal of electrons from the neutral molecule.8 The dication (72+), which is considered to be Three-dimensionally homoaromatic , is stable relative to a localized structure with similar features but is highly unstable compared to the radical cation (7+i)- hi contrast, the dianion (72 ) is unstable relative to the radical anion (T) and shows no evidence of electron delocalization. 

Haynes and Sawyer observed C02 reduction in DM SO at Au electrodes. In this case, CO was produced, with the mechanism proceeding through a disproportionation of two C02 radical anions to form CO and C032" [85],... 

Other examples concern the interaction between iron carbonyles and potassium alkylthiolates that is accompanied by disproportionation. The anion radicals Fe2(CO)g, Fe3(CO)fi, Fe4(CO)i3, and Fe(CO)2 are formed (Belousov et al. 1987). The interaction of iron carbonyls Fe(CO)5, Fe2(CO)9, and Fe3(CO)i2 with (CH3)3NO occurs according to a one-electron redox-disproportionation scheme, giving rise to iron carbonyl anion radicals Fe2(CO)s Fe3(CO)n, l e3(CO)i2, and Fe4(CO)ii (Belousov Belousova 1999). 

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