Electron transfer number pathway

The reaction between benzyne derivatives and selenium analogues of Barton s thiopyridone esters provided a convenient entry into complex, fused benzo[3]selenophenes <2004JHC13, 2004ARK51>. For example, the generation of the benzyne 86 in the presence of selenoester 85 provided benzo[. ]seleno[2,3-. ]pyridine 87, presumably via a single electron transfer (SET) pathway (Equation 11). This methodology was examined utilizing a number of benzyne precursors (anthranilic acids, iodium triflates, and trimethysilyl triflates) and provided access to an impressive number of fused benzo[. ]selenophenes. 

Figure 7. Proposed proton-coupled electron transfer (PCET) pathway between the R2 and R1 subunits of the E. coli ribonucleotide reductase (RNR) complex. The conserved amino acids are shown schematically E. coli numbering).

Good electron donors such as sulfides, phosphines, or arsines can react with N-fluoropyridinium cation by a single-electron transfer (SET) pathway. This conclusion was reached after finding products known to be derived from free-radical processes. For example, it is believed that the SET process is operative in the reaction of sulfides (74) to give pyridyl-substituted sulfides (78) through the intermediary of a radical 75 and a radical cation 76 (Scheme 7). In addition to 78 this reaction produces a dimer of a radical 77 derived from the radical cation (76) and a number of other products known to be formed from 76 or 77 (93JHC329). The... 

Due to the production of H2O2 or HO2 through a 2-electron-transfer pathway, the overall electron-transfer number of the ORR process is always less than 4. This electron-transfer number is normally called the apparent number of electrons transferred per O2 molecule. Actually, this apparent number of electron transfer can be measured by the RRDE technique, from which the percentage of H2O2 formation in the ORR can also be calculated. Generally to say, the apparent number of electron transfer and the percentage of peroxide produced in the ORR process are two important pieces of information in evaluating the ORR catalyst s catalytic activity. 

In this mechanism. Reaction (12.1) is the chemical reaction to form the adduct, which has a reaction rate constant of 1.0 x 10 cms, as determined by the RDE measurements in this work. Reaction (12.11) is the ORR RDS on the Ti407 electrode surface, whose rate constants are given in Table 12.1. Reaction (12.III) represents the reactions for peroxide formation. After HO2 formation, HO2 can react in one of two ways further 2-electron reduction to OH through Reaction (12.1V), or chemical desorption through Reaction (12.V) to form a free peroxide ion, which then enters into the bulk solution and can be detected by the ring electrode of the RRDE. The ORR on the Ti407 electrode has a mixed 2- and 4-electron transfer pathway and gives an overall electron transfer number of <4. The relative portion of Reaction (12.IV) can be expressed as x, and the portion of Reaction (12.V) can be expressed as (1-x). When x= 1, the mechanism will follow a totally 4-electron transfer pathway, and when x = 0, the mechanism will be a totally 2-electron pathway. If the x value is >0 and < 1, the ORR will have a mixed 2- and 4-electron transfer pathway. Note that this ORR mechanism is only hypothetical, to facilitate further discussion. More evidence is needed to validate the mechanism. 

High electron transfer number of 12 for complete oxidation (methanol is 6 and hydrogen is 2), resulting in reduced theoretical fuel requirement. However, this does also indicate a more complex molecule requiring a greater number of intermediate reaction steps and pathways and generally worse kinetics. 

The pathway model makes a number of key predictions, including (a) a substantial role for hydrogen bond mediation of tunnelling, (b) a difference in mediation characteristics as a function of secondary and tertiary stmcture, (c) an intrinsically nonexponential decay of rate witlr distance, and (d) patlrway specific Trot and cold spots for electron transfer. These predictions have been tested extensively. The most systematic and critical tests are provided witlr mtlrenium-modified proteins, where a syntlretic ET active group cair be attached to the protein aird tire rate of ET via a specific medium stmcture cair be probed (figure C3.2.5). 

A number of mechanistic pathways have been identified for the oxidation, such as O-atom transfer to sulfides, electrophilic attack on phenols, hydride transfer from alcohols, and proton-coupled electron transfer from hydroquinone. Some kinetic studies indicate that the rate-determining step involves preassociation of the substrate with the catalyst.507,508 The electrocatalytic properties of polypyridyl oxo-ruthenium complexes have been also applied with success to DNA cleavage509,5 and sugar oxidation.511... 

Analysis of the above data led to the conclusion that all of the redox reactions proceed with electron transfer through the [CuL2]2+/+ redox couple, and that the change in number of ligands occurs in the Cu(I) oxidation state. This interpretation is given as pathway I in Fig. 5. 

Ferredoxins (Fds) are widespread in the three domains of life and an abundance of sequence data and structural information are available for Fds isolated from several sources. In particular, the bacterial type Fds are small electron-transfer proteins that posses cubane xFe-yS clusters attached to the protein matrix by Fe ligation of Cys via a conserved consensus ligating sequence. The archaeal type ferredoxins are water-soluble electron acceptors for the acyl-coenzyme A forming 2-oxoacid/ferredoxin oxidoreductase, a key enzyme involved in the central archaeal metabolic pathways. Fds have been distinguished according to the number of iron and inorganic sulphur atoms, 2Fe-2S, 4Fe-4S/3Fe-4S (Fig. Ib-d) and Zn-containing Fds. 

Electron-transfer chains in plants differ in several striking aspects from their mammalian counterparts. Plant mitochondria are well known to contain alternative oxidase that couples oxidation of hydroquinones (e.g., ubiquinol) directly to reduction of oxygen. Semiquinones (anion-radicals) and superoxide ions are formed in such reactions. The alternative oxidase thus provides a bypass to the conventional cytochrome electron-transfer pathway and allows plants to respire in the presence of compounds such as cyanides and carbon monoxide. There are a number of studies on this problem (e.g., see Affourtit et al. 2000, references therein). 

Electrochemical reductions of CO2 at a number of metal electrodes have been reported [12, 65, 66]. CO has been identified as the principal product for Ag and Au electrodes in aqueous bicarbonate solutions at current densities of 5.5 mA cm [67]. Different mechanisms for the formation of CO on metal electrodes have been proposed. It has been demonstrated for Au electrodes that the rate of CO production is proportional to the partial pressure of CO2. This is similar to the results observed for the formation of CO2 adducts of homogeneous catalysts discussed earlier. There are also a number of spectroscopic studies of CO2 bound to metal surfaces [68-70], and the formation of strongly bound CO from CO2 on Pt electrodes [71]. These results are consistent with the mechanism proposed for the reduction of CO2 to CO by homogeneous complexes described earlier and shown in Sch. 2. Alternative mechanistic pathways for the formation of CO on metal electrodes have proposed the formation of M—COOH species by (1) insertion of CO2 into M—H bonds on the surface or (2) by outer-sphere electron transfer to CO2 followed by protonation to form a COOH radical and then adsorption of the neutral radical [12]. Certainly, protonation of adsorbed CO2 by a proton on the surface or in solution would be reasonable. However, insertion of CO2 into a surface hydride would seem unlikely based on precedents in homogeneous catalysis. CO2 insertion into transition metal hydrides complexes invariably leads to formation of formate complexes in which C—H bonds rather than O—H bonds have been formed, as discussed in the next section. 

In terms of the development of an understanding of the reactivity patterns of inorganic complexes, the two metals which have been pivotal are platinum and cobalt. This importance is to a large part a consequence of each metal having available one or more oxidation states which are kinetically inert. Platinum is a particularly useful element of this pair because it has two kinetically inert sets of complexes (divalent and tetravalent) in addition to the complexes of platinum(O), which is a kinetically labile center. The complexes of divalent and tetravalent platinum show significant differences. Divalent platinum forms four-coordinate planar complexes which have a coordinately unsaturated 16-electron d8 platinum center, whereas tetravalent platinum is an 18-electron d6 center which is coordinately saturated in its usual hexacoordination. In terms of mechanistic interpretation one must therefore consider both associative and dissociative substitution pathways, in addition to mechanisms involving electron transfer or inner-sphere atom transfer redox processes. A number of books and articles have been written about replacement reactions in platinum complexes, and a number of these are summarized in Table 13. 

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