Reductive Transformation Pathways

Reductive transformations are most conveniently categorized according to the type of functional group that is reduced. General schemes illustrating the reductive transformations that are known to occur in natural reducing environments are summarized in Table 3.1. 

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The concept of the one-electron transfer scheme provides a common feature to the seemingly unrelated reductive transformation pathways presented in Section 3.B. Each of these processes occurs initially by transfer of a single electron in the ratedetermining step. In each case, a radical anion is formed that is more susceptible to reduction than the parent compound. 

One of these alternate models, postulated by Gunter Wachtershanser, involves an archaic version of the TCA cycle running in the reverse (reductive) direction. Reversal of the TCA cycle results in assimilation of CO9 and fixation of carbon as shown. For each turn of the reversed cycle, two carbons are fixed in the formation of isocitrate and two more are fixed in the reductive transformation of acetyl-CoA to oxaloacetate. Thus, for every succinate that enters the reversed cycle, two succinates are returned, making the cycle highly antocatalytic. Because TCA cycle intermediates are involved in many biosynthetic pathways (see Section 20.13), a reversed TCA cycle would be a bountiful and broad source of metabolic substrates. 

The other major dehalogenation pathway involves elimination of two halogens, leaving behind a pair of electrons that usually goes to form a carbon-carbon double bond. Where the pathway involves halogens on adjacent carbons, it is known as vicinal dehalogenation or reductive -elimination. The major pathway for reductive transformation of lindane involves vicinal dehalogenation, which can proceed by steps all the way to benzene (28). Recently, data has shown that this pathway not only can convert alkanes to alkenes, but can produce alkynes from dihaloalkenes (29). 

Thus sorption, followed by intra-particle diffusion of the dyes, causes rapid initial loss followed by slow long term loss. Transformation pathways probably involve azo reduction. Diffusion may limit the dyes transformation rates because of the large size of the molecules. 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

Estimation methods for reductive transformations (e.g., dehalogenation or nitro reduction reactions) are limited because it is not yet possible to predict the rates of reductive transformations quantitatively. The choice of appropriate descriptors is complicated by the variability in rate-limiting steps with contaminant structure and environmental conditions. Most QSARs for reduction reactions have been developed as diagnostic tools to determine reduction mechanisms and pathways. So far, only a few of these QSARs provide sufficiently precise predictions and are sufficiently general in scope that they might be useful to predict environmental fate (Tratnyek et al. 2003). They mostly use LFER-type correlations or quantum-chemically derived parameters (e.g., Peijnenburg et al., 1991 Rorije et al., 1995 Scherer et al., 1998 Tratnyek and Macalady, 2000) and many of them are compiled in a recent review by Tratnyek et al. (2003). 

Superoxide radical, Fe, and the hydroperoxide anion (H02 ) are potential reducing agents and may facilitate reductive transformations of the contaminants. Therefore, in a treatment system generally perceived to be oxidative, contaminant transformation by reductive pathways may also occur. Indeed, reductive transformation of chloroaliphatic compounds has been reported.However, the specific mechanisms and the environmental conditions in which they are facilitated have not been identified or optimized yet for contaminant transformation. 

Other reductive transformations are sometimes involved in nitroaromatic degradation pathways. Pseudomonase pseudoalcalignes ]S S is able to grow on nitrobenzene as sole carbon source. Nitrobenzene is reduced, first to nitrosobenzene, and then to the hydroxylamine, which is isomerized via a mutase enzyme to 2-aminophenol, as shown in Figure 8. The remaining pathway then follows an oxidative meta-cleavage route. 

Because the reduction of aromatic nitro groups is such a facile process, reductive transformation of chemicals containing this moiety is often the dominant pathway for their transformation in the environment. For example, the reduction of methyl parathion, which is representative of the nitro-containing organophosphorus insecticides, to amino methyl parathion has been observed in anaerobic sediments (Wolfe et al., 1986) and flooded soils (Wahid and Sethunathan, 1979 Wahid et al., 1980 Adhya et al., 1981a and 1981b Gambrell et al., 1984), with half-lives on the order of minutes to hours (Equation 3.22). 

Pentachloronitrobenzene (PCNB) is another example of a nitroaromatic agrochemical that is known to undergo facile reduction in anaerobic systems. PCNB has been identified as a pollutant in river water and groundwater (Fushiwaki et al., 1990). Reduction of PCNB results in the formation of pentachloroaniline, which is fairly resistant to further transformation pathways (Wang and Broadbent, 1973 Kuhn and Suflita, 1989a) (Equation 3.23). 

Figure 3.5. Proposed pathway for the reductive transformation of Disperse Blue 79 in anaerobic sediments.

Dealkylation appears to be a common transformation pathway for agrochemicals with the requisite functionality however, the reaction mechanisms for this process have not been thoroughly investigated. The lack of reactivity often observed in sterile systems has led to the general conclusion that reductive dealkylation requires the presence of viable microorganisms. Examples of N-dealkylation include the dealkylation of carbofuran (Equation 3.45) (Hassall, 1982) and atrazine, which may... 

Organolanthanide complexes differ from late d-block transition metal complexes in several aspects. They are electrophilic, kinetically labile and lack conventional oxidative addition/reductive elimination pathways in their reactions. They have alternative mechanisms to perform catalytic transformations and are being increasingly used in homogeneous catalysis. The hydrophosphination reaction was proposed to proceed through the cycle depicted in... 

While the mechanism of many of the transformations described herein has not been investigated in detail, there are several examples where the reductive elimination pathway has been thoroughly evaluated via experiment and/or theory. Ultimately, a molecular-level mechanistic imderstanding of these processes should provide insights into the factors that control the relative and absolute rates of C-X bond-formation (and other fundamental organometallic transformations) at Pd. This, in turn, will serve as critical data to inform the rational design of new catalytic transformations via the Pd manifold. 

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