Reduction by electron transfer

Figure 12. CID of precursor ion Cu(DMSO)3+, which shows that this ion undergoes charge reduction by electron transfer (see equation 22 which leads to Cu(DMSO)2 and DMSO+. From Blades, A. T. Jayaweera, P. Ikonomou, M. G. Kebarle, P. J. Chem. Phys. 1990, 92, 5900, with permission.

Reduction by electron transfer yields superoxide ion (02 -)> which has its negative charge and electronic spin density delocalized between the two oxygens. As such, it has limited radical character (H-OO bond energy AGbf, 72kcalmoU ) and is a weak Bronsted base in water... 

Shine and co-workers have carried out product studies of the reactions of fV-substituted phenothiazine cation-radicals with nucleophilic re-agents. As with the cation-radicals of thianthrene (178) and phenoxathiin (198), addition of certain nucleophiles at S may occur to give, ultimately, such adducts as 270-272 (see Sections III,C,4,b V,B,1). Attack by other nucleophiles may result in reduction by electron transfer or substitution at position 3. Shine has reviewed much of this work. The mechanisms of such reactions have been controversial (see Section... 

Reduction by electron transfer, the third type of competitive reaction, is more prevalent for main group chemistry relative to the previously discussed organic chemistry, but it is not as significant relative to transition metal chemistry. An example of a reduction reaction is shown in Eq. (40) [89],... 

Electron transfer is also common at electrodes. An example of such a process is the laser irradiation at 406.7 nm of methylviologen (20) as a thin film on a roughened silver electrode at liquid nitrogen temperatures which results in its reduction by electron transfer to its radical cation (21). Again, electron transfer from the metal to the dication of 20 is considered to be a reasonable mechanism for the process63. Apparently, the radical... 

Interestingly, the reaction of the (reducing) a-hydroxyalkyl radicals with thiols [reaction (9)] is relatively fast while the (oxidizing) formylmethyl radical does not react with thiols at an appreciable rate [36]. In contrast, in basic solution where the thiolate form predominates, a rapid reduction, by electron transfer, of the formylmethyl radical is observed [reaction (10)] [36]. 

Carbonyl compounds undergo similar reductions by electron transfer, the formation of pinacols (7.95) and the acyloin condensation (7.96) being typical examples ... 

Electron carriers which contain iron (and in the case of cytochrome oxidase also copper) they undergo oxidation and reduction by electron transfer alone. These are the cytochromes, in which the iron is present in a haem molecule, and nonhaem iron proteins, sometimes called iron—sulphur proteins, because the iron is bound to the protein through the sulphur of the amino acid cysteine. Figure 3.19 shows the arrangement of the iron in non-haem iron proteins and the three different types of haem that occur in cytochromes ... 

Hydroperoxides are decomposed readily by multivalent metal ions, ie, Cu, Co, Fe, V, Mn, Sn, Pb, etc, by an oxidation-reduction or electron-transfer process. Depending on the metal and its valence state, metallic cations either donate or accept electrons when reacting with hydroperoxides (45). Either one... 

Reductants such as zinc or sodium dithionite generate the semidione from diketones. Electrolytic reduction can also be used. Enolates can reduce diones to semidiones by electron transfer. 

Thus, the reaction by which a metal dissolves in an acid is conveniently discussed in terms of oxidation and reduction involving electron transfer. The reaction can be divided into half-reactions to show the electron gain (by H+ ions) and the electron loss (by metal atoms). 

Novi and coworkers124 have shown that the reaction of 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene with sodium benzenethiolate in dimethyl sulfoxide yields a mixture of substitution, cyclization and reduction products when subjected at room temperature to photostimulation by a sunlamp. These authors proposed a double chain mechanism (Scheme 17) to explain the observed products. This mechanism is supported by a set of carefully designed experiments125. The addition of PhSH, a good hydrogen atom donor, increases the percent of reduction products. When the substitution process can effectively compete with the two other processes, the increase in the relative yield of substitution (e.g., with five molar equivalents of benzenethiolate) parallels the decrease in those of both cyclization and reduction products. This suggests a common intermediate leading to the three different products. This intermediate could either be the radical anion formed by electron transfer to 2,3-bis(phenylsulfonyl)-l,4-dimethylbenzene or the a radical formed... 

This process involves a series of reactions, including dissolution, hydrogen reactions and chlorine withdrawal [20], The second type of reactions include reduction of protons at the catalyst by electron transfer yielding hydrogen radicals that are consumed by reaction or give elemental hydrogen otherwise. 

Lastly, we have shown that transport of ions across the double layer is facilitated by water via proton transfer and that the barrier for the reduction of O2 is controlled by electron transfer that occurs as the proton moves close to the adsorbed O2 to form a reactive center. Electron transfer appears to occur before the actual formation of the 00 H bond. 

The benzyl group can serve as a hydroxy-protecting group if acidic conditions for ether cleavage cannot be tolerated. The benzyl C—O bond is cleaved by catalytic hydrogenolysis,176 or by electron-transfer reduction using sodium in liquid ammonia or... 

The reduction of tetrazolium salts by NADH is greatly accelerated by electron transfer agents (ETAs) such as phenazine methosulfate (PMS 233) or its derivatives.451-454 Other classes of ETAs such as quinones.455,456 ferricinium,457 phenothiazine,458 the viologens,459 acridiniums,460 and phe-nazinium or quinoxalinium salts461 as well as the enzyme diaphorase462 have been used. 

Tetranuclear iron-sulfur clusters of the type [Fe4S4(SR)4]2, where R = CH2C6H5 and C6H5, were found138 to catalyze the reduction of C02 in DMF solutions. Controlled-potential electrolyses were carried out in a C02-saturated 0.1 M tetrabutylammonium tetrafluoroborate (TBAT)-DMF solution at a mercury pool cathode. In the absence of a catalyst, C02 was substantially reduced only at potentials more negative than -2.4 V versus SCE, while in the presence of a cluster, the reduction took place at around -1.7 V thus, potential shift of ca. 0.7 V was achieved. The products were analyzed by means of gas chromatography and isotachophoresis. Without a catalyst, oxalate was the main product, and addition of small amounts of water to the DMF solution favored formate production, whereas in the presence of the catalyst, formate was produced predominantly even in a dry DMF solution. This result was interpreted in terms of indirect reduction of C02, proceeding by electron transfer from the reduced cluster to C02 in the bulk... 

Extensive studies have established that the catalytic cycle for the reduction of hydroperoxides by horseradish peroxidase is the one depicted in Figure 6 (38). The resting enzyme interacts with the peroxide to form an enzyme-substrate complex that decomposes to alcohol and an iron-oxo complex that is two oxidizing equivalents above the resting state of the enzyme. For catalytic turnover to occur the iron-oxo complex must be reduced. The two electrons are furnished by reducing substrates either by electron transfer from substrate to enzyme or by oxygen transfer from enzyme to substrate. Substrate oxidation by the iron-oxo complex supports continuous hydroperoxide reduction. When either reducing substrate or hydroperoxide is exhausted, the catalytic cycle stops. 

PS1 The PS 1-prep, introduced in this communication is the first reported with a polyhistidine tag fused to the N-terminus of the PsaF subunit. This construct was possible due to the fact that cyanobacterial PsaF-deletion mutants show no impact on photoautotrophic growth - in contrast to Chlamydomonas reinhardtii, where inactivation of PsaF results in a severe reduction of electron transfer from plastocyanin to PS 1 [Hippier et al. 1997], Also, the N-terminus of the F-subunit which was decorated by the tag is located towards the lumen side which enables an attachment of the isolated PS1 with the lumen-exposed /donor-side to the electrode surface in our hydrogen-producing device. 

Electron transfer of the glucose oxidase/polypyrrole on the electrode surface was confirmed by differential pulse voltammetiy and cyclic voltammetry. The glucose oxidase clearly exhibited both reductive and oxidative current peaks in the absence of dissolved oxygen in these voltammograms. These results indicate that electron transfer takes place from the electrode to the oxidized form of glucose oxidase and the reduced form is oxidized by electron transfer to the electrode through polypyrrole. It may be concluded that polypyrrole works as a molecular wire between the adsorbed glucose oxidase and the platinum electrode. 

According to Taube, the inner sphere mechanism can takes place when both oxidizing and reducing agents are substitution inert and when ligand transfer from oxidant to reductant is accompanied by electron transfer. The inner sphere electron transfer mechanism may be represented by the scheme... 

Although thermodynamically favorable, reductive dissolution of Fe(III)(hydr)oxides by some metastable ligands (even those, such as oxalate, that can form surface complexes) does not occur in the absence of light. The photochemical pathway is depicted in Fig. 9.3e. In the presence of light, surface complex formation is followed by electron transfer via an excited state (indicated by ) either of the iron oxide bulk phase or of the surface complex. (Light-induced reactions will be discussed in Chapter 10.)... 

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