Electron-Transfer Reduction of

Electron-Transfer Reduction of 02. Within aqueous solutions the most direct means to the electron-transfer reduction of dioxygen is by pulse radiolysis. Irradiation of an aqueous solution by an electron beam yields (almost instantly 10-12 s) solvated electrons [e (aq)], hydrogen atoms (H-), and hydroxyl radicals (HO-)- If the solution contains a large excess of sodium formate [Na+ 0(0)CH] and is saturated with 02, then the radiolytic electron flux efficiently and cleanly reduces 02 to superoxide ion (O ) 21-25 

This represents electrodeless electrochemistry, whereby the only process is electron transfer and the only product is the one-electron adduct of 02, the superoxide ion (02). Through the use of redox mediator dyes and spectrophotometry, the electron-transfer thermodynamic reduction potential for 02 has been evaluated  

This relation in combination with the 02/HOOH couple at pH 7.0 of Table 9.3 yields the one-electron reduction potential for Oj  

A direct electrochemical measurement of the reduction potential for the 02/Oj couple in aqueous solutions is complicated by the rapid proton-induced 

The aqueous electrochemical reduction of 02 at inert electrodes occurs via an initial reversible electron-transfer process [analogous to the homogeneous process of pulse radiolysis, Eq. (9.19)] 

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Fukuzumi, S., Ohkubo, K., Wenbo, E., Ou, Z., Shao, J., Kadish, K.M., Hutchison, J.A., Ghiggino, K.P., Sintic, P.J. and Crossley, M.J. (2003) Metal-centered photoinduced electron transfer reduction of a gold(III) porphyrin cation linked with a zinc porphyrin to produce a long-lived charge-separated state in nonpolar solvents. Journal of the American Chemical Society, 125, 14984-14985. 

Fig. 5 Marcus plot of electron transfer reductions of alkyl halides, as contrasted with archetypal SN2 substitution processes (Finkelstein reactions, circled see Eberson,...

The electron-transfer reduction of a large number of aromatic molecules involving an aryl carbon-heteroatom a-bond produces a frangible anion radical which decomposes to the corresponding aryl radical and an anion containing the heteroatom. The most widely investigated compounds in this... 

In the numerous cases where k jk = 0, the overall reaction amounts to a substitution of the halogen in the halide by the aromatic anion radical followed by the electron-transfer reduction of the ensuing radical by another aromatic anion radical, i.e. the sum of steps (92) and (94) might rather occur as in Scheme 6. 

At this point, it can be concluded that the direct and indirect electrochemical approach of the reaction in the case of aryl halides has provided a quantitative kinetic demonstration of the mechanism and the establishment of the nature of the side-reactions (termination steps in the chain process). In poor H-atom donor solvents, the latter involve electron-transfer reduction of the aryl radical. 

Electron transfer reduction of pyridines in both acid and alkaline solution generates the protonated radical-anion. This rapidly accepts a further electron and a proton to give a mixture of dihydropyridines. Enamine structures in these dihydro-pyridines can tautomerise to the imine, which is more readily reduced than the original pyridine molecule. Further reaction of the 1,4-dihydropyridine leads to piperidine while reduction of the t, 2-dihydropyridine leads to a tetrahydropyridine in which the alkene group cannot tautomerise to the imine and which is not therefore reduced to the piperidine stage. The reaction sequence is illustrated for 2,6-dimethyl-pyridine 18 which yields the thermodynamically favoured cis-2,6-dimethylpiperidine in which the two alkyl substituents occupy equatorial conformations. 

Reduction of a Stable System Resulting in Reduction of a Ligand. Ligands, as a group, are far better electron donors than acceptors. Thus, electron transfer reductions of ligands should be rather unlikely, and in fact, no such case has hitherto been reported. 

The nickel(IV) complex of ligand (4) has been used to oxidize thiourea and alkyl derivatives to the respective disulfides, NH2C(S)NHR—> NH=C(NHR)—S—S— C(NHR)=NH. Autocatalysis is observed around pH 4.5, whilst at pH > 6, a faster Ni(IV) >Ni III) reduction step is followed by slower Nifffl) —> Ni(II) reduction. In the intervening pH region (ca 5.5), behaviour indicative of a single step two-electron-transfer reduction of the Ni(TV) is observed.43... 

Potassium hydroxide when merely dissolved in methanol is not effective in the electron-transfer reduction of 9-diazofluorene and fluoren-9-ylides. Addition of DMSO to the system makes a drastic change, with the highest efficiency in pure DMSO (Handoo Kaul 1992 Handoo et al. 1983). 

This interesting methodology broadens the preceding electron transfer reduction of acetates and pivaloates, and seems very efficient for synthesizing protected 2 -deoxy uridine such as 85 in high yields (85 %) (Scheme 44). 

To utilize the strong reducing power of the 3(da po) excited states of the platinum and iridium dimers, the nonproductive back electron transfer reactions need to be inhibited. An effective way to accomplish this is to use acceptors that are thermally unstable after the initial electron transfer. Reduction of alkyl halides has been shown to lead to short-lived radical anions RXT, which rapidly decompose to give R- and X (k... 

A parabolic driving force dependence of logket is also observed for electron-transfer reduction of fullerenes in PhCN, as shown in Fig. 13.10 [18, 28-32]. 

J.C. Ruhl, D.H Evans, P. Hapiot, P. Neta, Fast Cleavage Reactions following Electron Transfer. Reduction of 1,1-Dinitrocyclohexane, J. Am. Chem. Soc., 113 (1991) 5188-5194. 

Likewise, the electron-transfer reduction of H20 (with uncharged hydrogen and oxygen atoms), which must overcome the stabilization of the strong O—H bonds, results in a — 1 charge for oxygen rather than hydronium ion reduction ... 

Because the electron-transfer reduction of 02 is a reversible one-electron process, the hope for an electron-transfer catalyst is futile. Atom-transfer catalysts [Eqs. (9.54) and (9.55)] can promote more extensive reduction and higher potentials (those that correspond to the reduction of the metal oxide). 

The proposition of a one-electron mechanism for the electron-transfer reduction of dioxygen and the associated conclusions present significant ramifications relative to the development of improved fuel cells and metal-air batteries. To date the practical forms of such systems have used strongly acidic or basic electrolytes. Such solution conditions normally cause atom transfer to be the dominant reduction process for molecular oxygen at metal electrodes. Hence, the search for effective catalytic materials should be in this context rather than in terms of a one-electron-transfer process. 

Carbon Dioxide (C02). Figures 11.12 and 11.13 illustrate the electron-transfer reduction of C02 in Me2SO at gold, platinum, and mercury electrodes.18,19 Whereas the reduction at a gold electrode is a one-electron per C02 process on a voltammetric time scale, at mercury it is a sequential two-electron process. In both cases the overall reduction is two electrons per C02. The products for anhydrous conditions are C03 and CO, and with H20 present H0C(0)0-and HC(0)0 ... 

The examples presented in this chapter illustrate that many molecules without metals undergo redox processes in which the voltammetric current is proportional to their concentration. Often these nonmetallic substrates give responses that are due to the facilitated electron-transfer reduction of H30+/H20 or oxidation of H0 /H20. Hence, any substrate that forms a strong bond with H- or HO1 (or has an HO—/ or an R—H group with weak bonds to yield H—OH) will facilitate these electron-transfer processes at less extreme potentials to give peak currents that are proportional to the substrate concentration. The next two chapters (on organic compounds and organometallic compounds) include many more examples of matrix-centered electron-transfer redox processes. 

The following are examples of other generation methods of the same kind of reactive sp2 carbon-centered radicals. Treatment of aromatic diazocarboxylate ester (11) at pH 7.2 forms the phenyl radical, through hydrolysis of the ester, decarboxylation to the phenyldiimide, and finally, reaction with molecular oxygen (eq. 11.9a). Electron transfer reduction of 1,4-diazonium (12) with Cu+ generates the corresponding /7-phenylene biradical (probably step-by-step formation) (eq. 11.9b). These simple sp2 carbon-centered radicals also destroy DNA plasmid at pH 7.6, under living-body conditions, like esperamicin [37-39]. 

In this enantiodifferentiating photoreduction, the chiral amine plays two roles, as a chiral inductor and as an electron donor. Irradiation of 25 (Scheme 10) in a hexane slurry of unmodified NaY zeolite gave only the intramolecular hydrogen abstraction product 26. However, photolysis of 25 coimmobilized with ephedrine, pseudoephedrine, or norephedrine in NaY supercages afforded the reduction product 27 along with 26. It is clear that the immobilized amine plays the decisive role in the photoinduced electron-transfer reduction of 25, since 27 was not formed in unmodified or (— )-diethyl tartrate-modified zeolites. Consequently, the ee of obtained 27 was independent of the loading level of the chiral inductor. 

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