Redox transformations

Homoenolate Protonation The p-protonation of homoenolates has been observed by Scheidt and co-workers, resulting in a redox transformation of enals to afford saturated esters 48. This process is catalysed by the NHC derived from imidazolium salt 46 and utilises phenol as a proton source [14]. A range of primary and secondary alcohols, and phenol itself, are competent nucleophiles with which to trap the acylazolium intermediate 47 generated by protonation (Scheme 12.8). 

Related redox transformations allow the conversion of ynals to a,P-nnsaturated esters [33], as well as the ring expansion of formyl P-lactams [34], oxacycloalkane-2-carboxaldehydes [35], and 2-acyl-1-formyIcyclopropanes [36], Farther developments allow the synthesis of amides from a range of a-fnnctionalised compounds, but require an additive (imidazole, HO At or HOBt) for efficient amidation [37],... 

Rovis and co-workers have extended the application of redox transformations to generate chiral acylazolium species from a-haloaldehydes 245 and the NHC derived from pre-catalyst 247, allowing the desymmetrisation of mei o-hydrobenzoin 246 to give ester 248 in good yield and enantioselectivity (Scheme 12.54) [30]. 

Many methods including photometric, fluorimetric, chromatographic, and electrochemical methods have been used to detect the antioxidants so far. Recently, electrochemical methods have intensively been used for antioxidant detection. Among the electrochemical methods, the detection of antioxidant based on the direct redox transformation of cyt c has been studied over the decade. Since cyt c can act as an oxidant of superoxide, the superoxide level in solution can be detected as an oxidation current at the sensor electrode due to electron transfer from the radical via cyt c to the electrode. 

Photoreduced (C.) species undergo a number of dark redox transformations (including disproportionation 2WV —> WVI + W ) leading to their original oxidized form (A) with the release of hydrogen. In a simplified form, hydrogen production can be written as follows ... 

Su C., Puls R.W., Arsenate and arsenite removal by zero-valent iron kinetics, redox transformation, and implications for in situ groundwater remediation, Environ. Sci. Technol. 35,2001, 1487-1992. 

Fig. 9.13 (from Davison, 1985) shows the redox transformations schematically for the water column. The important items are ... 

Similarly the redox transformations Mn(III,IV) oxides / Mn2+ causes rapid electron cycling at suitable redox intensities. Two differences of Mn and Fe in their redox chemistries are relevant ... 

It undergoes either two separated reductions or one oxidation, all having features of chemical reversibility in the cyclic voltammetric time scale. Since no detailed studies have been carried out on the underlying electrode mechanism, it is noted that these redox transformations could formally correspond to the sequence V(III)/V(II)/V(I), but, as far as the reduction processes are concerned, it is also likely that they are centred on the polypyridine ligand. 

REDOX TRANSFORMATIONS FOLLOWING IRREVERSIBLE ELECTRON-TRANSFER PATHWAYS... 

On the basis of these data it is generally assumed that, as shown in Scheme 3, the 2-/3- redox transformation involves a structural reorganisation from a compressed to a elongated geometry. 

The most important redox transformation of this complex (134 valence electrons) is the one-electron oxidation to the corresponding monoanion, Figure 51. 

Selenium (masses 74, 76, 77, 78, 80, and 82 Table 1) and chromium (masses 50, 52, 53 54 Table 1) are treated together in this chapter because of their geochemical similarities and similar isotope systematics. Both of these elements are important contaminants in surface and ground water. They are redox-active and their mobility and environmental impact depend strongly on valence state and redox transformations. Isotope ratio shifts occur primarily during oxyanion reduction reactions, and the isotope ratios should serve as indicators of those reactions. In addition to environmental applications, we expect that there will be geological applications for Se and Cr isotope measurements. The redox properties of Se and Cr make them promising candidates as recorders of marine chemistry and paleoredox conditions. 

Biological action is very important in Se redox transformations. Rates of abiotic selenium redox reactions tend to be slow, and in soils and sediments, Se(VI), Se(IV), Se(0) and organically bormd Se often coexist (Tokrmaga et al. 1991 Zhang and Moore 1996 Zawislanski and McGratii 1998). Bacteria use Se(VI) and Se(IV) as eleclron acceptors (Blum et al. 1998 Dungan and Frankenberger 1998 Oremland et al. 1989), or oxidize elemental Se (Dowdle and Oremland 1998), and it is likely that most of the important redox transformations are microbially mediated. 

Figure 3. Isotopic effects for redox transformations of Se. Modified from Johnson (in press)...

In the theory of SWV, two different types of surface EE mechanisms have been treated [91,92], O Dea et al. [91] considered a mechanism in which the first redox step was chemically reversible, whereas the second one was a totally irreversible process. In the succeeding study [91], a more general case has been treated consisting of two quasireversible redox transformations, as indicated by (2.129) ... 

In DET, the enzymatic and electrode reactions are coupled by direct (mediatorless) electron transfer. In this case, the electron is transferred directly from the electrode to the substrate molecule (or vice versa) via the active site of the enzyme. In such a system, the coupled overall process is the redox transformation of the substrate(s), which can be considered as an enzyme-catalyzed electrode process. According to this mechanism, the electrode surface acts as the enzyme cosubstrate, and the enzymatic and electrode reactions cannot be considered as separate, but as formal stages of the bioelectrocatalytic reaction mechanism. The catalytic effect of the enzyme is the... 

Being compared to ionic reactions, inorganic redox transformations usually proceed slowly. Therefore, sometimes, a more active couple needs to be selected. For instance, the one-electron reduction of aromatic diazocompounds by FF (FF /FF couple) takes place sluggishly, whereas Tiiii (Ti /Ti couple) reacts with the same substrates rapidly (Heinrich et al. 2006). Two-electron redox systems are represented by couples TFVTF, Pb""/PF Pd /Pd°, Mg /Mg°, Hg"/Hg°, Au VAu, Pt /PF, and Pt /Pt°. 

An example of a redox transformation in natural water (oxidation) and sediments (reduction) of an organochlorinated contaminant (aldicarb insecticide) is given in Fig. 13.6. 

Fig. 13.6 Redox transformation pathways of aldicarb. (Macalady et al. 1986 Wolfe et al. 1986)...

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