One-electron oxidation/reduction

Transition metal catalysts arc characterized by their redox ehemistry (catalysts can be considered as one electron oxidants/reductants). They may also be categorized by their halogen affinity. While in the initial reports on ATRP (and in most subsequent work) copper266,267 or ruthenium complexes267 were used, a wide range of transition metal complexes have been used as catalysts in ATRP. 

The overall reaction catalyzed by PS I is one electron oxidation-reduction reaction. 

Figure 8. Electrochemically controlled movements of the ring components upon one-electron oxidation/reduction in a catenane containing a non-symmetric ring.

The mechanism of the reaction depicted in Scheme 4.6 differs from the Sf.,1 or Sf.,2 mechanism in that it involves the stage of one-electron oxidation-reduction. The impetus of this stage may be the easy detachment of the bromine anion followed by the formation of fluorenyl radical. The latter is unsaturated at position 9 near three benzene rings that stabilize the radical center. The radical formed is intercepted by the phenylthiolate ion. This leads to the anion-radical of the substitution product. Further electron exchange produces the substrate anion-radical and final product in its neutral state. The reaction consists of radical (R)-nucleophilic (N) monomolecular (1) substitution (S), with the combined symbol Sj j l. Reactions of Sj j l type can have both branch-chain and nonchain characters. 

This problem has two aspects—consumption of spin traps in one-electron oxidation/reduction either of a free radical or an initial ion-radical. An electron exchange between a trap and radical depends on a relative rate of the exchange as compared to rates of the addition reactions considered. An electron exchange between a trap and an ion-radical is represented by the following sequence (Nu is a nucleophile) ... 

Sano, M., and Matsubara, T. (1988). Structural change in the one-electron oxidation-reduction at the copper site in nitrite reductase. Evidence from EXAFS. Inorg. Chem. Acta Bioinorg. Chem 152, 53-54. 

Since the initial observation of flavin radical species by Michaelis and coworkers the involvement of flavins in one-electron oxidation-reduction processes in biological systems has occupied the attention of workers in the field of redox enzymology up to the present time. Flavin coenzymes occupy a unique role in biological oxidations in that they are capable of functioning in either one-electron or two-electron transfer reactions. Due to this amphibolic reactivity, they have been termed in a recent review to be at the crossroads of biological redox processes. 

NADH dehydrogenase and succinate dehydrogenase also contain Fe atoms that are bound by the S atoms of cysteine residues of the protein, in association with additional, inorganic sulfide atoms. Structures of these complexes are shown in figure 10.19. Succinate dehydrogenase has three iron-sulfur centers, one with a [2Fe-2S] cluster, one with [4Fe-4S], and one with a cluster containing 3 Fe atoms and 3 (or possibly 4) sulfides. Iron-sulfur centers undergo one-electron oxidation-reduction reactions. 

Complex IV Cytochrome Oxidase. Cytochrome oxidase contains two atoms of Cu in addition to the hemes of cytochromes a and a3. The Cu atoms undergo one-electron oxidation-reduction reactions between the cuprous (Cu+) and cupric (Cu2+) states. One of the Cu atoms (CuB) is close to the Fe of cytochrome a3 (fig. 14.12). The other (CuA) is associated with cytochrome a, but not so intimately. Oxidation of cytochrome c takes place on the side of the membrane facing the intermembrane space, whereas the reduction of 02 by cytochrome a3 and CuB occurs on the matrix side. 

Homolytic oxidations involve free radical intermediates and are catalyzed by first-row transition metals characterized by one-electron oxidation-reduction steps, eg. Com/Con, Mnln/Mnn. The hydrocarbon substrate is generally not coordinated to the metal and is oxidized outside the coordination sphere. Consequently, the oxidation is not very selective and does not often preserve the stereochemical configuration of the substrate. 

Kumar and Endicott673 have examined the one-electron oxidation-reduction of [Co([14]aneN4) (H20)(02)]2+ in aqueous solution. Reduction competes successfully with dimerization in this system and even mildly reducing metal ions such as Fe2+ react via an inner-sphere process. Presumably this pathway is preferred owing to the large free-energy changes associated with formation of the p-peroxo adducts. With Fe2+ the adduct is observable as a transient CoOOFe species, which decays to Fe3+ and unspecified cobalt products. 

Unusually persistent remarkable open-shell structures were discovered upon one-electron oxidation/reduction of C4 and N4 fragments embedded into rigid carbon skeletons. The thus generated radical ions reveal electron deficient bonding. ... 

This is the electrode at which the reaction of interest takes place, e.g. the simple one-electron oxidation-reduction processes given in equations (1) and (2). 

In doped polymers, hole or electron transport occurs by the transfer of charge from states associated with the donor or acceptor molecules, respectively. This can be described as a one-electron oxidation-reduction or donor-acceptor process between molecules in their neutral charged states (Pfister, 1977 Mort and Pfister, 1979 Pai et al., 1983 Facci and Stolka, 1986). For hole transport, some dopant molecules are initially positively charged (cation radicals). Under an applied field, neutral molecules will transfer electrons to the cation radicals. This results in the motion of positive charge. For this to occur, the dopant molecules must be donor-like in their neutral state. For electron transport, electrons are displaced from the anion radicals to neutral molecules, which requires that the dopant molecule be acceptor-like in its neutral state. It is generally accepted tliat these processes occur by hopping. 

The potentials of reversible one-electron oxidations (reductions) of alternant aromatic hydrocarbons have been found to give good straight lines when plotted against the calculated energies of the highest occupied molecular orbitals (HOMOs lowest unoccupied molecular orbitals = LUMOs) [Heilbronner (91)]. The results indicate two important trends ... 

Iron-sulfur proteins are found in a variety of organisms, bacteria, plants, and animals, and serve as electron transfer agent.s via one-electron oxidation-reduction step [redox potential --0.43V in chloroplasts to 1-0.35 V in... 

In the salt [K (diglyme)][Ce(CgHg) ] the anion has staggered 0, symmetry. The cerium system is unusual for a lanthanide complex in showing two one-electron oxidation-reduction steps. 

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