Electron-Mediated Reductions

Numerous studies have demonstrated that the facile reduction of organic pollutants such as nitroaromatics, azo compounds, and polyhalogenated hydrocarbons can occur in anaerobic environments with exceedingly fast reaction rates. The most abundant electron donors in these systems include reduced iron and sulfur species. Laboratory studies, however, indicate that the rates of reduction by these bulk reductants are much slower than those observed in natural reducing systems. The addition of electron carriers or mediators to the laboratory systems is found to greatly accelerate the measured reduction rates. An electron shuttle system has been postulated to account for the enhanced reactivity observed in these laboratory studies and presumably in reducing natural systems (Dunnivant et al., 1992). 

In this scenario, the bulk electron donor or reductant rapidly reduces an electron carrier or mediator, which in turn transfers electrons to the pollutant of interest. The oxidized electron mediator is then rapidly reduced again by the bulk reductant, which enables the redox cycle to continue. 

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C.2 for further discussion of electron-mediated reductions) (Schwarzenbach, et al., 1990 Tratnyek and Macalady, 1989). Quinoid-type compounds are thought to be constituents of natural organic matter (Thurman, 1985 see Chapter l.B.3c). It has been hypothesized that some free radicals in humic substances are quinone-hydroquinone redox couples (Tollin et al., 1963 Steelink and Tollin, 1967). 

Although it is not possible to predict the absolute rates for the reduction of organic chemicals in environmental systems, recent laboratory studies have established relationships between substrate properties and relative reactivities (Tratnyek et al., 1991). For example, Schwarzenbach et al. (1990) studied the quinone and iron porphyrin mediated reduction of a series of nitrobenzenes and nitrophenols (see Section 3.C.2 for further discussion of electron mediated reductions). From the experimentally determined rate law, it was determined that the transfer of the first electron from the monophenolate species of the hydroquinone, lawsone (HLAW ), to the nitroaromatic was rate determining (Equation 3.50). 

Recent laboratory studies suggest that electron-mediated reductions may not be limited to environmental systems containing NOM. Mineral systems found in groundwaters, such as iron-bearing phyllosilicates, iron sulfides, and sulfide miner-... 

Humic acid and the corresponding fulvic acid are complex polymers whose structures are incompletely resolved. It is accepted that the structure of humic acid contains oxygenated structures, including quinones that can function as electron acceptors, while reduced humic acid may carry out reductions. These have been observed both in the presence of bacteria that provide the electron mediator and in the absence of bacteria in abiotic reactions, for example, reductive dehalogenation of hexachloroethane and tetrachloromethane by anthrahydroquininone-2,6-disulfonate (Curtis and Reinhard 1994). Reductions using sulfide as electron donor have been noted in Chapter 1. Some experimental aspects are worth noting ... 

Anaerobic bio-reduction of azo dye is a nonspecific and presumably extracellular process and comprises of three different mechanisms by researchers (Fig. 1), including the direct enzymatic reduction, indirect/mediated reduction, and chemical reduction. A direct enzymatic reaction or a mediated/indirect reaction is catalyzed by biologically regenerated enzyme cofactors or other electron carriers. Moreover, azo dye chemical reduction can result from purely chemical reactions with biogenic bulk reductants like sulfide. These azo dye reduction mechanisms have been shown to be greatly accelerated by the addition of many redox-mediating compounds, such as anthraquinone-sulfonate (AQS) and anthraquinone-disulfonate (AQDS) [13-15],... 

The dimethylpyrrolidinium-Sn mediated reduction of aryl bromides, bearing electron-withdrawing and electron-donating groups to give the corresponding debrominated products, has been described [531]. 

The pyridinium salt NAD 19a and its reduced form NADH 20a are important co-factors for many enzymes, fhe reduced form is involved in enzyme mediated reductions where it is converted to NAD. In natural systems, NAD is converted back to NADH by another enzyme-controlled process. Attempts to effect the direct electrochemical conversion of NAD to NADH are not very successful. Reduction on a mercury cathode at -1.1 V see on the first one-electron reduction wave leads to the radical-zwitterion, which reacts further to give dimers. Three stereoisomers of the 4,4 -dimer account for 90 % of the mixture and three 4,6 -dimers form the remainder [78]. Reduction at -1.8 V on the second reduction wave produces only 50 % of enzymatically active 1,4-NADH. The NAD analogue 19b shows related behaviour and one-electron reduction affords two diastereoisomers... 

A Rh-catalyzed Reformatsky reaction of chiral imine (24) led to the stereoselective preparation of the a,a-difluoro-jS-amino acid (25). 25 was converted to difluor-oalkene (26), and subsequently L-Val-i/r[(Z)CF=CH]Gly derivative (23) in greater than 82% for both steps. The samarium diiodide-mediated reductive transformation of the y,y-difluoro-a, S-enoates proceeded via successive two-electron transfers to form a dienolate species which upon kinetically controlled trapping with fert-BuOH formed 23 (Scheme 6). 

More complex reductions of CO2 by enzyme cascades have also been achieved. A combination of an electron mediator and two enzymes, formate dehydrogenase and methanol dehydrogenase, was used to reduce CO2 to methanol. This system operates with current efficiencies as high as 90% and low overpotentials (approximately —0.8 V vs. SCE at pH 7) [125]. The high selectivity and energy efficiency of this system indicate the potential of enzyme cascades. There are also drawbacks to these systems. In general, enzymes are... 

A model proposed for photochemical conversion of solar energy 11,14) is shown in Fig. 3. The system is made of a photoreaction couple, two kinds of electron mediator, and reduction as well as oxidation catalysts. It is designed to share the necessary functions among the various compounds because it would be difficult for one single compound to bear all the functions. A single component carrying out the total conversion would of course be the best system. 

Pi and P2 are the photochemical reaction center serving also as light-harvesting unit. They can be two kinds of compounds or a single compound (P) such as a metal complex. The photoreaction center must have a strong absorption in the visible region. Tt and T2 are the electron mediators which take out photochemically separated charges rapidly to prevent back reactions. C and C2 are the reduction and oxidation... 

Fig. 3. A model system for photochemical conversion of solar energy. R2 Reducing agent, R, Oxidizing agent C2 Oxidation catalyst Cj Reduction catalyst T2, T, Electron mediators P2-Pj = P Photoreaction center...

A third prominent set of biologically mediated reactions used for the initial transformations of xenobiotic compounds are reductions. As discussed in Chapter 14, reduction reactions entail transferring electrons to the organic compound of interest. Microbially mediated reductive transformations involve the same structural moieties that are susceptible to abiotic reductions (Table 17.5). The common characteristic for the structures at the point of reduction is that electron-withdrawing... 

The reduction of bipyridyls was discussed in Part I (see also Refs. 223-226). Paraquat dimer molecules (155) held together by a chain of methylene groups are reduced through the cation-radical and further to a diradical the splitting of the waves depends on the length of the methylene bridge.227 They have been considered as two-electron mediators in indirect electrode... 

The corrinoid-mediated reduction of polyhaloethenes has been the subject of a recent study, which reports reaction via homolytic C-halogen bond fission. The elimination of a further halogen radical affords haloalkynes, which lead to acetylene itself.56 The electron transfer-induced reductive cleavage of alkyl phenyl ethers with lithium naphthalenide has been re-examined in a study which showed that it is possible to reverse regioselectivity of the cleavage (i.e. ArOR to ArH or ArOH) by introduction of a positive charge adjacent to the alkyl ether bond.57 A radical intermediate has been detected by ESR spectroscopy in the reduction of imines to amines with formic acid58 which infers reacts takes place via Lukasiewicz s mechanism.59... 

In aqueous solution, [Ru(bpy)3]2+ was used as the photosensitizer and methyl viologen as the electron mediator, with TEOA or EDTA as the sacrificial reductant... 

First, C02 reduction at metal electrodes in both aqueous and nonaqueous media, as well as in systems coupled with electron-mediating complexes are detailed. The faradaic efficiency of such a system can be used as a measure of efficiency and selectivity. For a specific, electrochemically generated product, the faradaic efficiency is the ratio of the actual and theoretical amounts of product formed within the same time interval, based on charge passed. An efficient and selective system will lead to a 100% faradaic yield for a single product in other words, all of the charge passed in the system has gone into the production of that product. 

The rhenium complexes described in Section 11.2 have also been studied as electron mediators for C02 reduction at metal electrodes. Hawecker et al. used the complex Re(bpy)(CO)3Cl in DMF/water (9 1) at glassy carbon electrodes at a potential of-1.44V (versus SCE) to produce CO with 98% faradaic efficiency [15, 87]. Likewise, Sullivan et al. reported the production of CO with similar efficiency at a platinum electrode at -1.5 V (versus SCE) by using the complex fac-Re(bpy) (CO)3Cl [88]. Ruthenium complexes that have been used in photochemical... 

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