Radical anions electrochemical generation

Figure 2. UV-Vis spectra for a 0.5 mil thick Kapton film after a 5 sec immersion in a solution containing 0.0475 M benzil/0.0025 M benzil radical-anion electrochemically generated in 0.1 M TBAFB/ACN solution.

In order to measure the absorption spectra, the radical anions were generated electrochemically in the optical path of a spectrophotometer. The absorption spectrum of 3,5-dinitroanisole radical anion (Figure 11, curve c) is very similar to that of the 550-570 nm species produced photochemically. So we believe this species to be the radical anion formed by electron transfer from the nucleophile to the excited 3,5-dinitroanisole and decaying by interaction with its surroundings including the nucleophile radical cation. The behaviour described seems to be rather general for aromatic nitro-compounds since it is observed with a series of these compounds with various nucleophilic reagents. 

A photoexcited anion also plays a role in the photoelectrocatalytic reduction of 4-chlorobiphenyl using the anion radicals of anthracene and 9,10-diphenylanthracene409. The anion radicals were electrochemically generated and excited by means of visible light. Formation of the aryl anion radical then takes place either by direct electron transfer or by... 

Radical-anions are generated by electrochemical reduction of fluorinated pyridines (Scheme 31348) and other heterocycles.349 Radical-anions have also been produced from polyfluoropyridines by X-ray irradiation and studied by ESR using matrix isolation techniques. It was concluded that crossover of [Pg.64]

The formation of o-complex seems to be a crucial step of the reaction between a nucleophile and an electron-deficient nitroarene. At the same time, there might be another strategy. By changing the electron character of substrate through the formation of radical-anions we generate electrrMi rich species, which are able to react with neutral nucleophiles (electron poor). Thus, the choice of appropriate reactants for electrochemical reactions appears to be a crucial point. Indeed, 1,3,5-trinitrobenzene and N-methylformamide (nucleophile/solvent) proved to be suitable reactants [29]. 

An electron transfer alone to the nitro compound is not sufficient to promote oxygen transfer, since the nitroarene radical anion, independently generated by electrochemical techniques in the presence of phosphine, did not afford any phosphine oxide. An ion pair is a reasonable representation of the rate-limiting transition state and neither the N-0 bond breaking nor the P-0 bond making are determining factors in the kinetics of the oxygen-transfer process. The actual transfer must be facile and the collapse of the ion pair via a cyclic intermediate like ... 

One aspect that reflects the electronic configuration of fullerenes relates to the electrochemically induced reduction and oxidation processes in solution. In good agreement with the tlireefold degenerate LUMO, the redox chemistry of [60]fullerene, investigated primarily with cyclic voltammetry and Osteryoung square wave voltammetry, unravels six reversible, one-electron reduction steps with potentials that are equally separated from each other. The separation between any two successive reduction steps is -450 50 mV. The low reduction potential (only -0.44 V versus SCE) of the process, that corresponds to the generation of the rt-radical anion 131,109,110,111 and 1121, deserves special attention. 

Fig. 3.6 EPR spectmtn of the radical anion [PhC(NSN)2CPh] generated by electrochemical reduction." ...

The electrophilic character of sulfur dioxide does not only enable addition to reactive nucleophiles, but also to electrons forming sulfur dioxide radical anions which possess the requirements of a captodative" stabilization (equation 83). This electron transfer occurs electrochemically or chemically under Leuckart-Wallach conditions (formic acid/tertiary amine - , by reduction of sulfur dioxide with l-benzyl-1,4-dihydronicotinamide or with Rongalite The radical anion behaves as an efficient nucleophile and affords the generation of sulfones with alkyl halides " and Michael-acceptor olefins (equations 84 and 85). 

Further experimental studies involved the determination of the rate constant of the reaction of several alkyl halides with a series of electrochemically generated anion radicals so as to construct activation driving force plots.39,40,179 Such plots were later used to test the theory of dissociative electron transfer (Section 2),22,49 assuming, in view of the stereochemical data,178 that the Sn2 pathway may be neglected before the ET pathway in their competition for controlling the kinetics of the reaction. 

The radical anions of 309, 311 (Me derivative of enol), and 308 were generated electrochemically ESR spectroscopy indicated that the unpaired electron was delocalized in the triazine moiety (81MI3). 

Simple 1,2,3-thiadiazoles show three absorption bands in the ultraviolet (UV) 211-217 (emax 4380-5300), 249-253 (1460-2100), 290-294 (195-245) nm <1996CHEC-II(4)289>. The ESR spectrum for the radical anion generated by the electrochemical reduction of the 1,2,3-thiadiazolium ion 8 has been reported. A number of 5-substituted derivatives were also examined and the splitting constants in the ESR spectrum were analyzed <1998MRC8>. 

ESR spectra of the radical anions of 3-phenoxy-, 3,4-diphenoxy-, and 3,4-dichloro-l,2,5-thiadiazole and of the radical cations of various 3-aryloxy-4-morpholino-l,2,5-thiadiazoles have been obtained by the electrochemical generation of the ions in 3 x 10-3M solutions of the thiadiazoles in the system MeCN/Et4NC104 (ca. 0.1 M) on a platinum helix electrode directly in an ESR resonator at first wave potentials <2003RJC806>. 

Energetic electron transfer reactions between electrochemically generated, shortlived, radical cations and anions of polyaromatic hydrocarbons are often accompanied by the emission of light, due to the formation of excited species. Such ECL reactions are carried out in organic solvents such as dimethylformamide or acetonitrile, with typically a tetrabutylammonium salt as a supporting electrolyte. The general mechanism proposed for these reactions is as follows. 

FIGURE 2.27. Redox catalysis of the reductive cleavage of 1-chloronaphthalene by the electrochemically generated anion radical of 4-methoxybenzophenone in DMSO at 0.05 V/s for excess factor of 0.5. Fitting of the experimental data points. 

The direct electrochemical reduction of carbon dioxide requires very negative potentials, more negative than —2V vs. SCE. Redox catalysis, which implies the intermediacy of C02 (E° = —2.2 V vs. SCE), is accordingly rather inefficient.3 With aromatic anion radicals, catalysis is hampered in most cases by a two-electron carboxylation of the aromatic ring. Spectacular chemical catalysis is obtained with electrochemically generated iron(0) porphyrins, but the help of a synergistic effect of Bronsted and Lewis acids is required.4... 

For the reduction of CCI4 by electrochemically generated aromatic anion radicals in DMF (Eberson et al., 1989), the kinetics were treated according to a reaction scheme in which CCl, " appears as an intermediate, but no strong evidence was offered that they would not fit a concerted pathway as well. As discussed on pp. 32, 33, it is indeed not possible to distinguish between the two pathways in such types of experiment when, in the stepwise pathway, the cleavage reaction is so fast that the electron-transfer step would be rate determining, a situation likely to be met in the present case. 

The reduction of aryl-substituted vinyl halides by electrochemically generated aromatic anion radicals has also been investigated in DMF (Gatti et al., 1987). Counter-diffusion behaviour at low driving forces (pp. 34, 35) does not appear as clearly as in the case of aryl halides (Fig. 11). However, analysis of the log k vs E° plot according to a quadratic activation-driving force relationship gave standard potential and intrinsic barrier values that... 

Fig. 14 Reduction of PhjCSPh by electrochemically generated aromatic anion radicals (in DMF at 25°C). Variation of the rate-determining step rate constant, A , with the standard potential of the aromatic anion radical, p,g (from left to right azobenzene, benzo[c]cinnoline, 4-dimethylaminoazobenzene, terephthalonitrile, naphthacene, phlhalonitrile, perylene, fluoranthene, 9,10-diphenylanthracene). The dotted lines are the theoretical limiting behaviours corresponding to the concerted (right) and stepwise (left) pathways. (Adapted from Severin et al 1988.)...

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