Reductive dehalogenation, reaction pathways

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). 

Primarily, reductive dechlorination occurs on the surface of ZVl. According to Matheson and Tratnyek (1994), the reaction mechanisms and pathways of reductive dehalogenation of chlorinated compounds by iron metal can be summarized as follows First, chlorinated compound(s) (designated RCl) transport to the surface of ZVl via mass transfer. Owing to the oxidation of ZVl, electrons are released to the system. By accepting the electrons, dechlorination of RCl takes place. During this reaction process, Fe is generated as a result of Fe° oxidation. Meanwhile, CL forms due to dechlorination. Second, further oxidation of Fe to Fe releases... 

TEDs include hydrogen, natural organic carbon, humic material, and nonhalo-genated fuel compounds. For reductive dehalogenation to occur, there must be both a TEA and a TED. For the biotic pathway, organisms must derive energy from the reaction. 

Two competitive pathways have been discovered for the reaction of phosphites with haloacetonitriles (335). The addition of phosphites to the nitrile bond afforded N-unprotected iminophosphonates (336), while reductive dehalogenation of perfluoro(chloro) acetonitriles yielded dihaloacetonitriles (337) and corresponding halophosphates (338) (Scheme 95). The direction and chemoselectivity of the reactions were controlled by the nature and quantity of halogen atoms in the starting nitrile (335) and by the nature of the phosphites. ... 

The suppression of the SET pathway can be affected by addition of a radical scavenger, e.g., tetraphenylhydrazine, to the polymerization (14). Addition of 0.01 mole% of tetraphenylhydrazine to the polymerization of 1,3-bis(p-chlorobenzoyl)benzene and hydroquinone resulted in high polymer with an inherent viscosity equivalent to those polymers prepared with the difluoro monomer. In addition, the reductive dehalogenation side reaction can be avoided by the use of diphenylsulfone rather than NMP or DM AC, due to the absence of labile hydrogen atoms towards hydrogen abstraction (16). 

Chlorobenzoates may be formed during the initial steps in the aerobic degradation of PCBs, and their further metabolism illustrates a number of pathways. There are several reactions that carry out dehalogenation including dioxygenation, hydrolysis, and reduction. 

Both Ni and Pd reactions are proposed to proceed via the general catalytic pathway shown in Scheme 8.1. Following the oxidative addition of a carbon-halogen bond to a coordinatively unsaturated zero valent metal centre (invariably formed in situ), displacement of the halide ligand by alkoxide and subsequent P-hydride elimination affords a Ni(II)/Pd(ll) aryl-hydride complex, which reductively eliminates the dehalogenated product and regenerates M(0)(NHC). ... 

The reduction potentials for various alkyl halides range from +0.5 to +1.5 V therefore, when Fe° serves as an electron donor, the reaction is thermodynamically favorable. Because three reductants are present in the treatment system (Fe°, H2, and Fe2+), three possible pathways exist. Equation (13.9) represents the oxidation of Fe° by reduction of a halogenated compound. In the second pathway, the ferrous iron behaves as a reductant, as represented in Equation (13.10). This reaction is relatively slow because the ability to reduce a pollutant by ferrous iron is dependent on the speciation ferrous ions, which is determined by the ligands present in the system. The third possible pathway, Equation (13.11), is dehalogenation by hydrogen. This reaction does not occur easily without a catalyst. In addition, if hydrogen levels become too high, corrosion is inhibited (Matheson and Tratnyek, 1994) ... 

The key butenolide needed by Buszek, for his synthesis of (—)-octalactin A, had already been prepared by Godefroi and Chittenden and coworkers some years earlier (Scheme 13.4).9 Their pathway to 10 provides it in excellent overall yield, in three straightforward steps from l-ascorbic acid. The first step entails stereospecific hydrogenation of the double bond to obtain L-gulono-1,4-lactone 13. Reduction occurs exclusively from the sterically less-encumbered ot face of the alkene in this reaction. Tetraol 13 was then converted to the 2,6-dibromide 14 with HBr and acetic anhydride in acetic acid. Selective dehalogenation of 14 with sodium bisulfite finally procured 10. It is likely that the electron-withdrawing effect of the carbonyl in 14 preferentially weakens the adjacent C—Br bond, making this halide more susceptible to reductive elimination under these reaction conditions. 

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... 

Figure 23.2.2. Anaerobic degradation of carbon tetrachloride. An example of anaerobic dehalogenation, using carbon tetrachloride as the model compound. In many cases, these reactions occur under cometabolic conditions meaning that an alternative growth substrate must be present to serve as an electron donor to drive the reduction reactions whereby carbon tetrachloride is used as the electron acceptor. Three known pathways for microbial degradation of carbon tetrachloride have been identified [U.E. Krone, R.K. Thauer, H.P. Hogenkamp, and K. Steinbach, Biochemistry, 3d 0), 2713 (1991) C.H. Lee, T.A. Lewis, A. Paszczynski, andR.L. Crawford Biochem Biophys Res Commun, 261(3), 562 (1999)]. These pathways are not enzymatically driven but rely on corrinoid and corrinoid-like molecules to catalyze these reactions.

Reduction of NHC-borane adducts has proven useful for purposes other than the synthesis of boron-boron multiple bonds. Bissinger et al. [124] successfully employed this technique to prepare a trapped form of borylene, BH. NHC-stabi-lized dichloroborane (IMe-BHCl2) was prepared from BHCl2 SMe2 and then subsequentiy dehalogenated with 2 equivalents of sodium naphthalenide (NaN), yielding the naphthalene-trapped IMe BH adduct (Scheme 15.10). The authors propose that this reaction occurs via a [2+1] cycloaddition pathway and provide quantum chemical calculations to support this notion. 

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