Reductive dechlorination, halogenated

Gerritse J, BJ van der Woude, JC Gottschal (1992) Specific removal of chlorine from the ortho-position of halogenated benzoic acids by reductive dechlorination in anaerobic enrichment cultures. FEMS Microbiol Lett 100 273-280. 

Biodegradation. Halogenated solvents are degraded under aerobic and anaerobic conditions. The anaerobic process is typically a reductive dechlorination that progressively removes one halide at a time,... 

This reaction is the dominant dehalogenation pathway in reduction of halogenated methanes [84] and haloacetic acids [85]. In Fig. 4, this reaction is illustrated for perchloroethene (PCE), where complete dechlorination by this pathway requires multiple hydrogenolysis steps. The relative rates of these steps are a critical concern because they determine whether... 

By similar reasoning, we can further conclude that oxidation reactions will be most favorable for the lesser-halogenated isomers. This concept is illustrated in Figure 5(b) where relative rates of oxidation and reduction are plotted as a function of level of chlorination. In practice, the literature has revealed that there may be a (chemical-dependent) zone where oxidation and reduction reactions are equally likely. This zone captures aryl halides with four to six chlorines, and alkyl halides with one to three chlorines per molecule. Figure 5(c) shows a trend between the Gibbs free energy for reductive dechlorination and the HOMO-LUMO gap (HOMO = highest occupied... 

In general, highly halogenated aliphatic compounds have high reduction potentials, so much so that hexachioroethane is a better electron acceptor than molecular oxygen The likelihood that such compounds will be reduced in anaerobic soils is high. There is evidence that polychlorinated biphenyls buried in anaerobic river sediments very slowly undergo reductive dechlorination, but this is believed to be a microbial process (Quensen et al, 1988). 

Nucleophilic displacement of halogen in pyrazines and quinoxalines is the most common method for the synthesis of otherwise inaccessible heterocycles, in which, over the past 12 years, the most innovative methodology has been the organometallic catalyzed cross-coupling reaction (Scheme 10). The first example in the pyrazine series was the reductive dechlorination of chloropyrazines... 

Lithium or sodium dissolving in tetrahydrofuran-t-butanol can accomplish efficient reductive dechlorination of chlorine compounds. This synthetic method has been important in obtaining hydrocarbons via halogenated intermediates. Important examples are conversion of dichlorocyclopropanes (prepared by dichlorocarbene... 

A pH-dependent equilibrium between the acid and conjugate base forms of FeS surface species, discussed above to explain the influence of pH on rate constants for FeS-mediated reductive dechlorination of HCA, might also explain the pH-dependence of the TCE product distribution (22). Other factors proposed to explain the product distributions of TCE and other halogenated compounds in related systems include the reduction potential of reactive surface species (d2), electron availability at the mineral surface 63, 64), and differences in enthalpies for dichloroelimination versus hydrogenolysis 46). 

Empirical linear correlation analysis has also produced results that may be used in predicting the rates of FeS-mediated pollutant transformation based on pollutant thermodynamic or molecular properties. One study (77) performed linear correlation analysis of log kohs values for 8 halogenated aliphatic molecules with five molecular properties one-electron reduction potentials, lowest unoccupied molecular orbital (LUMO) energies, fi ee energies of formation of aqueous phase radicals formed upon one-electron reduction, gas-phase homo-[ytic bond dissociation enthalpies, and aqueous solubilities. Of these parameters, homolytic bond dissociation enthalpies (7)r.x values) were best correlated with log obs values for FeS reductive dechlorination (R =0.82). The correlation between log obs and 7)r x is illustrated in Figure 4. Another parameter shown... 

The anaerobic degradation of halogenated alkanoic acids has, however, been much less exhaustively examined. Geobacter (Trichlorobacter) thiogenes was able to transform trichloroacetate to dichloroacetate by coupling the oxidation of acetate to CO2 with the reduction of sulfur to sulfide that carries out the dechlorination (De Wever et al. 2000). 

Atom Transfer Atom Transfer (AT) takes place typically in the case of d7 complexes, which abstract the halogen atom from RX. The radical formed combines then with a second metal [193, 194]. A classical example of this mechanism is the hydrodehalogenation with cyanocobaltates(II) (see Section 18.2.1) [8, 9], but an analogous pathway was suggested recently for the Co(II) corrin-catalyzed dechlorination of CC14 in the presence of S2 /cysteine as reductant (Eqs. (11)—(12))... 

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