Oxidation-reduction mechanisms hydrogen atom transfer

Classification exclusively in terms of a few basic mechanisms is the ideal approach, but in a comprehensive review of this kind, one is presented with all reactions, and not merely the well-documented (and well-behaved) ones which are readily denoted as inner- or outer-sphere electron transfer, hydrogen atom transfer from coordinated solvent, ligand transfer, concerted electron transfer, etc. Such an approach has been made on a more limited scale. Turney has considered reactions in terms of the charges and complexing of oxidant and reductant but this approach leaves a large number to be coped with under further categories. 

This approach has been taken for the reaction of chlorinated ethenes with Zn° [125,165] and Fe° [88,166], resulting in separate rate constants for all the reactions shown in Fig. 3. Care must be taken in using these parameters in predictive modeling, however, as it is not yet known how sensitive the relative values of these rate constants are to pH, thickness and composition of the oxide film, etc. The same caution applies where the approach represented by Eq. (25) is used to describe parallel mechanisms of transformation. For example, it has recently been reported that several experimental factors influence the relative contributions of dissociative electron transfer, hydrogen atom transfer, and reductive elimination to the dechlorination of carbon tetrachloride and TCE by Fe° [177],... 

A review of the phototochemistry of A-oxides has appeared and this includes a section on the photo-induced deoxygenation of heterocyclic A-oxides. A second review deals with the mechanistic aspects of the photochemistry of bicyclic azoalkanes and considers their photo-reduction by hydrogen donors, a process for which hydrogen-atom transfer and CT mechanisms have been suggested. 

Figure 11-4. Mechanism of oxidation and reduction of nicotinamide coenzymes. There is stereospecificity about position 4 of nicotinamide when it is reduced by a substrate AHj. One of the hydrogen atoms is removed from the substrate as a hydrogen nucleus with two electrons (hydride ion, H ) and is transferred to the 4 position, where it may be attached in either the A or the B position according to the specificity determined by the particular dehydrogenase catalyzing the reaction. The remaining hydrogen of the hydrogen pair removed from the substrate remains free as a hydrogen ion.

Although the oxidation (hydroxylation) of hydrocarbons is usually believed to occur via hydrogen atom abstraction [51], the one-electron transfer mechanism of cytochrome P-450 catalyzed oxidation has also been proposed for the oxidation of Ar, Ar-dialkylaniIines [52]. This mechanism (Figure 24.4) is generally preferred for the substrates with low reduction... 

An extremely interesting feature of these mechanisms is the fact that superoxide and the alkene radical cation are both formed in the reduction (Fig. 20) and also in the Frei oxidation (Fig. 19). In the Frei photo-oxidation, however, they are formed concurrently in a tight ion pair and collapse to product more rapidly than their diffusive separation. In the reduction (Fig. 20), the formation of the radical cation and superoxide occur in independent spatially separated events allowing the unimpeded diffusion of superoxide which precludes back-electron transfer (BET) and formation of oxidized products. The nongeminate formation of these two reactive species provides the time necessary for the radical cation to abstract a hydrogen atom from the solvent on its way to the reduced product. 

The chemistry of electrochemical reaction mechanisms is the most hampered and therefore most in need of catalytic acceleration. Therefore, we understand that electrochemical catalysis does not, in principle, differ much fundamentally and mechanistically from chemical catalysis. In addition, apart from the fact that charge-transfer rates and electrosorption equilibria do depend exponentially on electrode potential—a fact that has no comparable counterpart in chemical heterogeneous catalysis—in many cases electrocatalysis and catalysis of electrochemical and chemical oxidation or reduction processes follow very similar if not the same pathways. For instance as electrochemical hydrogen oxidation and generation is coupled to the chemical splitting of the H2 molecule or its formation from adsorbed hydrogen atoms, respectively, electrocatalysts for cathodic hydrogen evolution—... 

The chemistry of the reduction of NAD+ has been solved most elegantly (Chapter 8, section Bi).2 Oxidation of the alcohol involves the removal of two hydrogen atoms. One is transferred directly to the 4 position of the nicotinamide ring of the NAD+, and the other is released as a proton (equation 16.1).3,4 It is generally thought that the hydrogen is transferred as a hydride ion H , but a radical intermediate cannot be ruled out. For convenience, we shall assume that the mechanism is the hydride transfer. 

Reduction of 02 by Atom Transfer. In contrast to the preceding electron-transfer mechanism with inert electrodes, the reduction of dioxygen at freshly preoxidized metal electrodes is pH-dependent, yields negligible amounts of hydrogen peroxide, and occurs at the same potential as that for the metal oxide of the electrode material. A plausible explanation for these observations is represented by the reaction sequence... 

Cytochrome c, a small heme protein (mol wt 12,400) is an important member of the mitochondrial respiratory chain. In this chain it assists in the transport of electrons from organic substrates to oxygen. In the course of this electron transport the iron atom of the cytochrome is alternately oxidized and reduced. Oxidation-reduction reactions are thus intimately related to the function of cytochrome c, and its electron transfer reactions have therefore been extensively studied. The reagents used to probe its redox activity range from hydrated electrons (I, 2, 3) and hydrogen atoms (4) to the complicated oxidase (5, 6, 7, 8) and reductase (9, 10, 11) systems. This chapter is concerned with the reactions of cytochrome c with transition metal complexes and metalloproteins and with the electron transfer mechanisms implicated by these studies. 

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