One-electron reductions

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. 

The electrochemical features of the next higher fullerene, namely, [70]fullerene, resemble the prediction of a doubly degenerate LUMO and a LUMO + 1 which are separated by a small energy gap. Specifically, six reversible one-electron reduction steps are noticed with, however, a larger splitting between the fourth and fifth reduction waves. It is important to note that the first reduction potential is less negative than that of [60]fullerene [31]. 

The one-electron reduction of thiazole in aqueous solution has been studied by the technique of pulse radiolysis and kinetic absorption spectrophotometry (514). The acetone ketyl radical (CH ljCOH and the solvated electron e were used as one-electron reducing agents. The reaction rate constant of with thiazole determined at pH 8.0 is fe = 2.1 X 10 mole sec in agreement with 2.5 x 10 mole sec" , the value given by the National Bureau of Standards (513). It is considerably higher than that for thiophene (6.5 x 10" mole" sec" ) (513) and pyrrole (6.0 X10 mole sec ) (513). The reaction rate constant of acetone ketyl radical with thiazolium ion determined at pH 0.8 is lc = 6.2=10 mole sec" . Relatively strong transient absorption spectra are observed from these one-electron reactions they show (nm) and e... 

The Ei/2 values for one-electron reductions in this series have been measured and compared with theory 78MI21502). 

Apart from the nuclear bromination observed (Section 2.15.13.1) in the attempted radical bromination of a side-chain methyl group leading to (396), which may or may not have involved radical intermediates, the only other reaction of interest in this section is a light-induced reduction of certain hydroxypyrido[3,4-f)]pyrazines or their 0x0 tautomers analogous to that well-known in the pteridine field (63JCS5156). Related one-electron reduction products of laser photolysis experiments with 1 -deazaflavins have been described (79MI21502). 

The polarographic half-wave reduction potential of 4-nitroisothiazole is -0.45 V (pH 7, vs. saturated calomel electrode). This potential is related to the electron affinity of the molecule and it provides a measure of the energy of the LUMO. Pulse radiolysis and ESR studies have been carried out on the radical anions arising from one-electron reduction of 4-nitroisothiazole and other nitro-heterocycles (76MI41704). 

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 ... 

Alkyl hydroperoxides give alkoxy radicals and the hydroxyl radical. r-Butyl hydroperoxide is often used as a radical source. Detailed studies on the mechanism of the decomposition indicate that it is a more complicated process than simple unimolecular decomposition. The alkyl hydroperoxides are also sometimes used in conjunction with a transition-metal salt. Under these conditions, an alkoxy radical is produced, but the hydroxyl portion appears as hydroxide ion as the result of one-electron reduction by the metal ion. ... 

Cyclooctatetraene provides a significant contrast to the preference of aromatic hydrocarbons for one-electron reduction. It is converted to a diamagnetic dianion by addition of two electrons. It is easy to understand the ease with which the cyclooctatetraene radical accepts a second electron because of the aromaticity of the 10-7t-electron aromatic system which results (Section 9.3). 

Two classes of charged radicals derived from ketones have been well studied. Ketyls are radical anions formed by one-electron reduction of carbonyl compounds. The formation of the benzophenone radical anion by reduction with sodium metal is an example. This radical anion is deep blue in color and is veiy reactive toward both oxygen and protons. Many detailed studies on the structure and spectral properties of this and related radical anions have been carried out. A common chemical reaction of the ketyl radicals is coupling to form a diamagnetic dianion. This occurs reversibly for simple aromatic ketyls. The dimerization is promoted by protonation of one or both of the ketyls because the electrostatic repulsion is then removed. The coupling process leads to reductive dimerization of carbonyl compounds, a reaction that will be discussed in detail in Section 5.5.3 of Part B. 

One-electron reduction of a-dicarbonyl compounds gives radical anions known as setnidiones. Closely related are the products of one-electron reduction of aromatic quinones, the semiquinones. Both semidiones and semiquinones can be protonated to give neutral radicals which are relatively stable. 

Generated by one-electron reduction of the pyridinium salt. Stable, distillable, and only moderately reaetive to oxygen. 

Tile ESR spectra of the radical anions, generated by one-electron reduction of the a-oxothioketone 173 and the dithiete 172, were determined, and spin densities were calculated from the coupling constants and, especially, from the anisotropic values (87CB575). 

Scheme 10.27 Catalytic cycle of HppE. Dashed arrows indicate electron transport. In this scheme HPP binds to iron1". After a one-electron reduction, dioxygen binds and reoxidizes the iron center. The peroxide radical is capable of stereospecifically abstracting the (pro-R) hydrogen. Another one-electron reduction is required to reduce one peroxide oxygen to water. Epoxide formation is mediated by the resulting ironlv-oxo species.

Two-electron reduction of the quinone provides the hydroquinone 10, which may also be formed by two sequential one-electron reductions. It has also been... 

One electron reduction to Fe gives stable complexes for peralkylatedarenes... 

Of some relevance in this connection is a study216 on the structure of the anion radicals formed when diaryl sulphones react with n-butyllithium in hexane-HMPA solution under an argon atmosphere. Apparently, a dehydrogenative cyclization and a further one-electron reduction occurs to produce the anion radicals of substituted dibenzothiophene-S, S-dioxides. These anion radicals were studied by ESR spectroscopy. 

More recently it has become apparent that proton equilibria and hence pH can be equally important in aprotic and other non-aqueous solvents. For example, the addition of a proton donor, such as phenol or water, to dimethylformamide has a marked effect on the i-E curve for the reduction of a polynuclear aromatic hydrocarbon (Peover, 1967). In the absence of a proton donor the curve shows two one-electron reduction waves. The first electron addition is reversible and leads to the formation of the anion radical while the second wave is irreversible owing to rapid abstraction of protons from the solvent by the dicarbanion. 

The Eo values for 2-substituted 1,4-benzoquinones (sets 45-4 through 45-7, 45-10) show an average value of pr of 59. Thus the resonance effect predominates. For most of these sets, the Op constants are not the best parameters for correlation. By contrast, the electron reduction potentials (set 45-8) show a Pr value of 39, which indicates predominance of the localized effect. The 2,5-disubstituted 1,4-benzoquinones differ distinctly in their behavior from the 2-substituted 1,4-benzoquinones in that they show an average Pr value of 53. The one-electron reduction potentials of these compounds show about the same composition of the electrical effect, with a value of Pr of 50. The only set of Eq values available for the 2,6-disubstituted 1,4-benzoquinones pve a Pr value of 51, comparable to the values observed for the 2,5-disubsti-tuted 1,4-benzoquinones. The 2,3,5,6-tetrasubstituted 1,4-benzoquinones have... 

One-electron reduction to the [FesSJ state results in partial bleaching of the visible absorption. As illustrated by P. furiosus 3Fe Fd in Fig. 12, the resulting absorption spectrum is relatively featureless, gradually increasing with increasing energy, except for a pronounced shoulder at 430 nm and a weak shoulder at 660 nm. The... 

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