Single-electron donor

A comprehensive description of the mechanism of molybdenum nitrogenase has been provided by the Lowe-Thorneley scheme 102) (Figs. 8 and 9). In this scheme the Fe protein (with MgATP) functions as a single electron donor to the MoFe protein in the Fe protein cycle (Fig. 8), which is broken down into four discrete steps, each of which may be a composite of several reactions ... 

Coming back to thermal homogeneous dissociative electron transfer reactions, the question arises whether the electron-donor molecule reacts as a single electron donor or as a nucleophile in an Sn2 reaction. We will review this long-debated question in Section 7, including the most recent developments. 

The reduction of arylalkyl halides of the triphenylmethyl type by electro-chemically generated outer sphere single electron donors offers an example of a sequencing of the bond-breaking and electron-transfer steps different from what has been described before. The cleavage of the halide ion then precedes electron transfer which thus involves the carbocation, a mechanism reminiscent of 8, 1 reactions (Andrieux et ai, 1984b) as shown in (102). The... 

ScoOiOPorph/RCoooporph-1 The principle of the method is illustrated in Fig. 18 with the example of reaction (142). The rate constants obtained with the investigated nucleophiles (or with single electron donors—that is the question ) are compared to those of the reaction of a series of anion radicals with the same alkyl halide in the same medium. As discussed on p. 59, aromatic anion radicals behave in this reaction as outer sphere electron donors and the alkyl halide undergoes a dissociative electron transfer. For... 

Lund s organic nucleophiles, which are all members of perfectly reversible redox couples and might therefore have been deemed to behave as outer sphere single electron donors, in fact react in an S 2 fashion in the absence of too severe steric constraints. 

The surface basicity of a solid catalyst can be defined in a way analogous to that applied to conventional bases. Thus, a surface Lewis base site is one that is able to donate an electron pair to an adsorbed molecule. If we take the definition of surface basicity in a more general way, it could be said that the active surface corresponds to sites with relatively high local electron densities. This general definition will include not only Lewis basicity but also single electron donor sites. We emphasize that the literature of heterogeneous catalysis often reports that both single-electron and electron-pair donor sites exist on basic catalysts. 

The preceding results and Ashby s observation (81JOC2429) that LDA acts as the single-electron donor towards aromatic hydrocarbons, stimulated Newkome (82JOC599) to study the reaction of pyridine with LDA (HMPA at 0°C) (Scheme 149). The products of this reaction, 2,4 - (77) and 4,4 - (494) bipyridines, obtained in the ratio 1 8, were rationalized as being formed from a pyridine radical-anion 493. In support of this proposal, complex ESR signals corresponding to both pyridine radical-anion and the diisopropylamino radical were observed under these conditions. 

Figure 6.3 Catalase redox transformation diagram. Compounds II, III and IV represent complexes of the enzyme with H202 and iron valence states, Fe5+, Fe4+ and Fe6+, respectively HXOH is a two-electron donor (reducer) X=0, NH, C=0, H(CH2) CH, where n = 1,2, 3 AH is a single-electron donor (reducer) ROOH is hydroperoxide (R is alkyl or acyl radical) and ROH is alcohol.

Note that if for complex VI reduction, catalase applies substrates representing two-electron donors, then peroxidase applies single-electron donors with complex II reduction in two stages through complex III formation (refer to Figure 6.3). 

Free radicals mentioned in this chapter are not useful for organic synthesis. However, it is important to know the radical character of metal hydride reagents, since metal hydride reagents sometimes behave not only as a polar (ionic) hydride donor species, but also a single electron donor species, depending on the substrates and reaction conditions. 

The study of photoinduced ET in covalently linked donor-acceptor assemblies began with comparatively simple dyad systems which contain a transition metal center covalently linked to a single electron donor or acceptor unit [26]. However, work in this area has naturally progressed and in recent years complex supramolecular assemblies comprised of one or more metal complexes that are covalently linked to one or more organic electron donors or acceptors have been synthesized and studied [27-36]. Furthermore, several groups have utilized the useful photoredox properties of transition metal complexes to probe electron and energy transfer across spacers comprised of biological macromolecules such as peptides [37,38], proteins [39,40], and polynucleic acids [41]. 

Organomagnesium compounds sometimes behave as single-electron donors. Such behaviour is most likely to be observed in highly polar, cation-solvating media and/or when steric hindrance inhibits alternative pathways. Ate complexes of magnesium can also act as single electron donors (see p. 97). It should be noted that the presence of traces of transition metals may lead to electron transfer processes [2]. 

As stated above, CO2 reduction proceeds by the OER species of the rhenium complex serving two roles. That is, (1) serving as a single-electron donor and (2) ligand elimination to generate a... 

Electron Transfer Chemistry Involving C-C Multiple Bonds as Single-electron Donors... 

Single electron donors such as sodium metal, sodium iodide, or sodium naph-thalenide (the last in an amount as low as 5 % molar) convert triazolinediones to deaza dimers 77 via a reaction that has been formulated as a radieal anion chain... 

The mobile phases with two polar modihers (acetone-methanol, ethylmethylketone-methanol) assure relatively high values of the a factor and the separation in the area of less polar taxoids is better in comparison with the separation obtained by single electron-donor mobile phases. 

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