Oxidative electron transfer

The classical syntheses of phenanthrene and fluorenone fit well into the electron transfer scheme discussed in Section 8.6 and in this chapter. The aryl radical is formed by electron transfer from a Cu1 ion, iodide ion, pyridine, hypophosphorous acid, or by electrochemical transfer. The aryl radical attacks the neighboring phenyl ring, and the oxidized electron transfer reagent (e. g., Cu11) reduces the hexadienyl radical to the arenium ion, which is finally deprotonated by the solvent (Scheme 10-76). 

The redox potential diagram in eq. 1 illustrates that the effect of optical excitation is to create an excited state which has enhanced properties both as an oxidant and reductant, compared to the ground state. The results of a number of experiments have illustrated that it is possible for the excited state to undergo either oxidative or reductive electron transfer quenching (2). An example of oxidative electron transfer quenching is shown in eq. 2 where the oxidant is the alkyl pyridinium ion, paraquat (3). 

The NO/NO+ and NO/NO- self-exchange rates are quite slow (42). Therefore, the kinetics of nitric oxide electron transfer reactions are strongly affected by transition metal complexes, particularly by those that are labile and redox active which can serve to promote these reactions. Although iron is the most important metal target for nitric oxide in mammalian biology, other metal centers might also react with NO. For example, both cobalt (in the form of cobalamin) (43,44) and copper (in the form of different types of copper proteins) (45) have been identified as potential NO targets. In addition, a substantial fraction of the bacterial nitrite reductases (which catalyze reduction of NO2 to NO) are copper enzymes (46). The interactions of NO with such metal centers continue to be rich for further exploration. 

Let s look at the little strip cartoon in Figure 7.7, which shows the surface of a copper electrode. For clarity, we have drawn only one of the trillion or so atoms on its surface. When the cell of which it is a part is permitted to discharge spontaneously, the copper electrode acquires a negative charge in consequence of an oxidative electron-transfer reaction (the reverse of Equation (7.7)). During the oxidation, the surface-bound atom loses the two electrons needed to bond the atom to the electrode surface, becomes a cation and diffuses into the bulk of the solution. 

The initial steps of the Kolbe reaction, the oldest organic electrochemical reaction, constitute a good illustration of the loss of an acid moiety upon oxidative electron transfer (Scheme 2.24). The issue of the stepwise versus concerted character of the electron transfer/bond-breaking process in this reaction is discussed in Chapter 3. 

R acts as an electron donor and is therefore oxidised (oxidative electron transfer Figure 6.18) ... 

Figure 6.18 Molecular orbital representation of oxidative electron transfer...

We suggest that these results can be explained if the aggregation process in these solid TTF polymers proceeds by means of a two-step mechanism (Figure 7) in which the fast oxidative electron transfer step is followed by a slow process of ion clustering/reorganization which is favored by a low viscosity environment. This mechanism is consistent with the fact that the starting neutral homopolymer shows no spectroscopic evidence for site-site interaction between the pendant donors. The absorption spectrum of the polymer is... 

ASPECTS OF CARBOHYDRATE OXIDATION, ELECTRON TRANSFER, AND OXIDATIVE PHOSPHORYLATION... 

Scheme 16 Formation of radicals by oxidative electron transfer and the following reactions.

The collision between reacting atoms or molecules is an essential prerequisite for a chemical reaction to occur. If the same reaction is carried out electrochemically, however, the molecules of the reactants never meet. In the electrochemical process, the reactants collide with the electronically conductive electrodes rather than directly with each other. The overall electrochemical Redox reaction is effectively split into two half-cell reactions, an oxidation (electron transfer out of the anode) and a reduction (electron transfer into the cathode). 

Methyl viologen (/V, /V - d i m e t h I -4,4 - b i p r i d i n i u m dication, MV2+ ) can function as an electron acceptor.34 When MV2+ is linked to electron donor, photoinduced electron transfer would occur. For example, within molecule 24 the 3MLCT excited state of [Ru(bpy)3]2+ is quenched by MV2+ through oxidative electron transfer process. The excited state of [Ru(bpy)3]2 + can also be quenched by MV" + and MV°. The transient absorption spectroscopic investigations show that the quenching of the excited state of [Ru(bpy)3]2+ by MV + and MV° is due to the reductive electron transfer process. Thus, the direction of the photoinduced electron transfer within molecule 24 is dependent on the redox state of MV2 +, which can be switched by redox reactions induced chemically or electrochemically. This demonstrates the potential of molecule 24 as a redox switchable photodiode.35... 

Cyclobutane Fragmentations, irradiation of oxidative electron transfer sensitizers in the presence of aryl cyclobutanes causes ring fragmentation, in a formal retrocyclo-addition sense. For example, Mukai has shown, eq. 33 (98),... 

CONTENTS Acknowledgments, Margery G. Ord and Lloyd A. Stocken. Introduction. Biochemistry Before 1900. Early Metabolic Studies Energy Needs and the Composition of the Diet. Carbohydrate Utilization Glycolysis and Related Activities. Aspects of Carbohydrate Oxidation, Electron Transfer, and Oxidative Phosphorylation. Amino Acid Catabolism in Animals. The Utilization of Fatty Acids. The Impact of Isotopes 1925-1965. Biochemistry and the Cell. Concepts of protein Structure and Function. Chronological Summary of Main Events Up to ca. 1960. Principal Metabolic Pathways. Index. 

We suggest that electron transfer and electrophilic substitutions are, in general, competing processes in arene oxidations. Whether the product is formed from the radical cation (electron transfer) or from the aryl-metal species (electrophilic substitution) is dependent on the nature of both the metal oxidant and the aromatic substrate. With hard metal ions, such as Co(III), Mn(III), and Ce(IV),289 reaction via electron transfer is preferred because of the low stability of the arylmetal bond. With soft metal ions, such as Pb(IV) and Tl(III), and Pd(II) (see later), reaction via an arylmetal intermediate is predominant (more stable arylmetal bond). For the latter group of oxidants, electron transfer becomes important only with electron-rich arenes that form radical cations more readily. In accordance with this postulate, the oxidation of several electron-rich arenes by lead(IV)281 289 and thallium(III)287 in TFA involve radical cation formation via electron transfer. Indeed, electrophilic aromatic substitutions, in general, may involve initial charge transfer, and the role of radical cations as discrete intermediates may depend on how fast any subsequent steps involving bond formation takes place. 

Catalyst coking may involve carbonaceous species such as partially hydrogenated fragments (QHy) and may be initiated on metal or than acidic-oxide sites [1]. Three types of carbonaceous deposits may be formed on say Pt [2], which may be differentiated by temperature-programmed oxidation. SnO,-promoted Pt catalysts are important in reforming of alkanes [3] and low temperature CO oxidation [4]. Of course Sn02 is an n-type semiconductor and certainly in photoelectrolysis one expects metal-oxide electron transfers across the junction [51, but the nature of the Pt-SnOt interaction in catalytic systems remains unclear. 

One cannot distinguish between the analogous copper intermediates involved in oxidative electron-transfer and ligand-transfer reactions. In each the ionization of the ligand to copper(II) has an important role in the formation of carbonium ion intermediates. A reaction analogous to the copper-catalyzed decomposition of peroxides is the copper-promoted decomposition of diazonium salts (178). The diazonium ion and copper(I) afford aryl radicals which can undergo ligand-transfer oxidation with copper(II) halides (Sandmeyer reaction) or add to olefins (Meerwein reaction). 

Radical ions from arenes Birch reduction and arene oxidation Electron transfer in aliphatic substitution 38 Electron transfer in aromatic substitution 38 Electrochemical electron transfer 39... 

The voltammogram for a simple oxidative electron transfer process (19) ... 

In spite of the fact that numerous oxidation reactions are known, that lead to a-functionalization of ketones [159,160], in most cases enol radical cations are not involved in these transformations, and rigorous evidence for their formation through selective oxidation of the enol tautomer (Fig. 2, path 2) has only been obtained in a few cases. For example, it could be inferred from kinetic studies that in many cases enols are not intermediates in aqueous oxidation reactions with V(V), Co(III), Ce(IV) and Mn(III) [161-163], whereas in acetic acid Mn(III) was postulated to attack the enol form of ketones [164,165], but not by electron transfer [166]. On the other hand, oxidants as Cr(VI), Tl(III), Hg(II) and Mn(VII) [167] as well as Pb(IV) [168] definitely react with the enol form, but since with these inner-sphere oxidants electron transfer is assumed to occur in a bonded fashion, radical cation intermediates are most likely not implicated. 

Floreancig [42] has described a new solid-phase linker system cleaved by oxidative electron transfer. The cleavage process is based on the oxidative fragmentation of homobenzylic ethers. Acetal 230, immobilized on a soluble oligonorbornene scaffold prepared by ROMP polymerization, was efficiently... 

Scheme 58 Linker cleavage and cyclization by oxidative electron transfer by Floreancig [42]...

Finally, divalent and tetravalent selenium can be involved in reductive and oxidative electron transfer processes. 

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