Two-electron transfer pathway

Both phenols and Cr(VI) are chemicals of high environmental concern. In this redox reaction, one- or two-electron transfer pathways could take place, and a proposed mechanism is shown in Scheme 4.10 [20]. In aqueous acidic media (pH < 5), it involves fast equilibrium... 

Scheme 4.10 Proposed one-electron and two-electron transfer pathways for the oxidation of phenols by Cr(VI) [20],...

Accordingly, an overall second-order rate law is expected at constant pH (first order in phenol and in Cr(VT)), and there will be a complex dependence on acidity varying between kinetic order 0 and 2. In addition, the k2 (two-electron transfer) pathway should be the main route at higher acidities, with predominance of the products derived from the phenoxy cation, X2. 

Ruthenium catalysts found many applications in C-C bond formation reactions (selected reviews [157-161]). Ruthenium occurs mostly in oxidation states +2 and +3, but lower as well as higher oxidation states can easily be reached. Thus ruthenium compounds are frequently used in oxidative transformations proceeding by either single or two electron transfer pathways (selected reviews [162-164]). It has long been known that ruthenium complexes can be used for the photoactivation of organic molecules (selected reviews [165, 166]). Ruthenium complexes are applied as catalysts in controlled or living radical polymerizations [167-169]. 

The relatively high quantum yield for the final charge separated state in 15 may be attributed in part to the fact that two electron transfer pathways (steps 3 and 4 in Figure 11) compete with charge recombination of the initial C-P" -Qa "-Qb species (step 7), rather than just one, as in triad-type systems. 

Kaplan and DeGrado [9] to catalyze the oxidation of aminophenol by a two-electron transfer pathway, Scheme 5.4. The catalyst has been extensively characterized with regards to structure and the designed protein provided not only an active site cavity but also a channel through which the active site could be reached. The reaction mechanism and reactive site residues were determined. 

Some studies have shown that certain modification procedures can be used to transform two-electron reduction metalloN4-macrocyclic complexes into hybrid materials with the capability to reduce oxygen to water, either via the direct four-electron transfer pathway or in the series two-electron transfer pathway. Carbon nanomaterials, carbon nanotubes in particular [58-65], have been reported to significantly increase the catalytic oxygen reduction current, with a substantial reduction of the overpotential for ORR reported in some cases, as shown by the examples in Table 7.4. 

Vanadium(v) is also produced in the oxidation of V" by Cla. Initial rate studies, however, show that the proportion of resulting from a two-electron transfer pathway is small compared with production of At its simplest, the... 

Vanadium(v).—The reaction of vanadium(v) with thiourea has been investigated over the range [H+] = 0.2—1.40 mol h. Intermediate complexes have been identified within the time of mixing. Both one- and two-electron transfer pathways are postulated involving 1 1 and 1 2 vanadium thiourea species. At [H+]<0.40mol 1, the reaction order with respect to reductant is two and the complex [V(OH)2L2] + may act as a template for formation of the disulphide in a two-electron step similar to that already postulated for Cr. ... 

Allen et al., 1987 Chang et al., 1986). In contrast to this structural symmetry, electron transfer in this system has been shown to be very asymmetric, with detectable electron transfer only occurring down one of the two possible chains of electron acceptors (Kirmaier et al., 1985 Parson, 1986). Site-specific mutagenesis has been used to address this issue (e.g., Bylina et al., 1988), but so far there has been no case in which changing a single amino acid has been shown to substantially alter the balance between the two electron transfer pathways. 

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