One-electron transfer pathway

In summary, product analysis and kinetic analysis support the proposed mechanism comprising three reaction channels - two of them involving a one-electron transfer pathway... 

Gersmann et al.74 suggested another mode of initiation, which proceeds by deprotonation of RH to a carbanion that transfers one electron to an oxygen molecule capture of oxygen produces a peroxy radical that oxidizes another molecule of the initial carbanion, the last two steps constituting a chain-propagation process. In hydrocarbon oxidations, Russell73 also showed that reactions of carbanions with 02 proceed via a two-step one-electron transfer pathway [Eqs. (29)-(32)]. 

Paquette has reported an intramolecular oxidative coupling using ferric chloride to prepare the intermediate 30 for the synthesis of cerorubenic acid-III. Addition of the dienolate of 28 to FeCls in dmf at —78°C produced the cyclopropane intermediate 29 in 54% yield (equation 16). Although the mechanism of this oxidative cyclization is not discussed in the paper, it is likely that a one-electron transfer pathway is involved. Copper(n) salts have also been utilized for intramolecular enolate coupling, but they proved to be somewhat less effective in the present context. 

Miscellaneous. Evidence has been presented for a one-electron transfer pathway for the oxidation of V " by chlorine in aqueous solution. The reaction rate shows an inverse dependence on hydrogen-ion concentration, suggesting that the hydroxo-vanadium(iii) ion may be the reactive species. In the first stage of the reaction, V is oxidized to and chlorine reduced to the dichloride ion. 

For convenience, this reaction can be written in terms of two different one electron transfer pathways ... 

At present, new developments challenge previous ideas concerning the role of nitric oxide in oxidative processes. The capacity of nitric oxide to oxidize substrates by a one-electron transfer mechanism was supported by the suggestion that its reduction potential is positive and relatively high. However, recent determinations based on the combination of quantum mechanical calculations, cyclic voltammetry, and chemical experiments suggest that °(NO/ NO-) = —0.8 0.2 V [56]. This new value of the NO reduction potential apparently denies the possibility for NO to react as a one-electron oxidant with biomolecules. However, it should be noted that such reactions are described in several studies. Thus, Sharpe and Cooper [57] showed that nitric oxide oxidized ferrocytochrome c to ferricytochrome c to form nitroxyl anion. These authors also proposed that the nitroxyl anion formed subsequently reacted with dioxygen, yielding peroxynitrite. If it is true, then Reactions (24) and (25) may represent a new pathway of peroxynitrite formation in mitochondria without the participation of superoxide. 

As assumed, the small and positive valne of H/D kinetic isotope effect may be used as a criterion for an electron-transfer pathway. For example, anion-radicals of a-benzoyl-co-haloalkanes can react in two routes (Kimura and Takamnkn 1994). The first ronte is the common one—an electron is transferred from the oxygen anion of the carbonyl gronp to a terminal halogen. The transfer provokes fission of the carbon-halogen bond. The second ronte is the S 2 reaction, leading to a cyclic product as shown in Scheme 2.37. 

One should be aware, however, that none of the Grignard reactions of benzophenone proceeds through a completely free coupling process of benzophenone anion-radicals with alkyl radicals. For example, the portion of electron-transfer pathway in the Grignard reactions of benzophenone with isomeric C4H5MgCl was estimated to be 65, 61, and 26% for (CH3)3C—, CH3CH2CH(CH3)—, and CH3CH2CH2CH2-, respectively (Lund et al. 1999). 

Irradiation accelerates the reactions of Scheme 4.1, and the substitution products are formed in 70-80% yields. Acceptors of radicals (e.g., di-tert-butylnitroxyl) or electrons (e.g., m-dinitro-benzene [DNB]) completely inhibit the snbstitution even if the acceptors are added to the reaction mixture in small amonnts. The mentioned snbstitution reactions do not take place when no cyano groups are present in the initial a-phenylsnlphonyl cumene. Hence, the cyano groups send the reaction via the ion-radical pathway. Like the nitro gronp, the cyano group promotes the formation of anion-radical, which originates on one-electron transfer from the thiophenolate or malonate ions to the substrate. 

It is shown first that hydride transfer is not a likely reaction pathway. The rate-limiting step is established to be the initial one-electron transfer by using rate constants obtained by independent... 

It is now clear that in addition to their widespread involvement in electron transfer pathways, iron-sulfur clusters function as catalytic centers in a wide variety of enzymes. The first example of such an enzyme is aconitase. It was at first thought that the role of the iron-sulfur group was regulatory, but it is now clear that in this enzyme the iron-sulfur group is part of the catalytic site. One of the iron atoms can coordinate water or hydroxyl and plays a key role in the isomerization catalyzed by the enzyme (Emptage et al., 1983). 

Ceruloplasmin is involved in copper storage and transport as well as in iron mobilisation and oxidation. Among the blue oxidases it is unique since it contains, in addition to the usual motif of a type 1 combined with the trinuclear cluster, two other type 1 coppers. Electron transfer occurs, however, only between five of the six copper ions since one of the type 1 centres is not catalytically relevant due to its too high redox potential. The redox potentials of the centres were determined and possible electron transfer pathways among the copper sites were discussed.101... 

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