Dehalogenation reactions mediated

Dehalogenation Reactions Mediated by Iron Sulfide Minerals... 

As indicated in Chapter 8, the production of alkanes, as by-products, frequently accompanies the two-phase metal carbonyl promoted carbonylation of haloalkanes. In the case of the cobalt carbonyl mediated reactions, it has been assumed that both the reductive dehalogenation reactions and the carbonylation reactions proceed via a common initial nucleophilic substitution reaction and that a base-catalysed anionic (or radical) cleavage of the metal-alkyl bond is in competition with the carbonylation step [l]. Although such a mechanism is not entirely satisfactory, there is no evidence for any other intermediate metal carbonyl species. 

Reduction reactions mediated by microorganisms may include the reduction of nitro bonds, sulfoxide reduction, and reductive dehalogenation. Reduction of the nitro group to amine involves the intermediate formation of nitrase and hydroxy amino groups. Selected reductive reactions may involve the saturation of double bonds, reduction of aldehydes to alcohols or ketones to secondary alcohols, or of certain metals. The main reductive processes in the subsurface environment have been discussed earlier in this chapter. 

Scheme 18 Examples of the tris(trimethyIsilyl)silane 61-mediated deoxygenation and dehalogenation reactions conducted under continuous flow.

STEVEN E. ROKITA, PhD, is Professor in the Department of Chemistry and Biochemistry at the University of Maryland. His research interests lie in sequence and conformation specihc reachons of nucleic acids, enzyme-mediated activation of substrates and coenzymes, halogenation and dehalogenation reactions in biology, and aromatic substitution and quinone methide generation in bioorganic chemistry. 

Figure 9 Model representation of organic-mediated dehalogenation reactions in the presence (oval) and absence of...

Under anaerobic conditions, Gossett and Zinder showed that the reductive dehalogenation of the chlorinated ethenes occurs as a series of consecutive irreversible reactions mediated by the addition of 1 mole of hydrogen gas for every mole of chloride removed. Thus, the theoretical minimum hydrogen requirement for dechlorination can be calculated on a mass basis as shown below ... 

Dehalogenation of oi,a dihaloalkenes and Ketones. The preparation (eq 24) of the iron carbonyl complex of trimethylen-emethane (42) and some Fc2(CO)9-mediated rearrangements of dibromo ketones constitute the pioneering work in this area. Noyori has extended the dehalogenation reaction of a-halo ketones to a,a -dibromo ketones and o,a,a, a -tetrabromo ketones which can be dehalogenated with Fe(CO)5 or Fe2(CO)9. ... 

Considerable attention has been directed to dehalogenation mediated by corrinoids and porphyrins in the presence of a chemical reductant (references in Gantzer and Wackett 1991 Glod et al. 1997 Workman et al. 1997). Illustrations are provided by the dechlorination and elimination reactions carried out by titanium(III) citrate and hydroxocobala-min (Bosma et al. 1988 Glod et al. 1997). The involvement of corrinoids and porphyrins is consistent with the occurrence of analogous mechanisms for biological reactions that... 

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 ... 

Pletcher and associates [155, 159, 160] have studied the electrochemical reduction of alkyl bromides in the presence of a wide variety of macrocyclic Ni(II) complexes. Depending on the substrate, the mediator, and the reaction conditions, mixtures of the dimer and the disproportionation products of the alkyl radical intermediate were formed (cf. Section 18.4.1). The same group [161] reported that traces of metal ions (e.g., Cu2+) in the catholyte improved the current density and selectivity for several cathodic processes, and thus the conversion of trichloroacetic acid to chloroacetic acid. Electrochemical reductive coupling of organic halides was accompanied several times by hydrodehalogena-tion, especially when Ni complexes were used as mediators. In many of the reactions examined, dehalogenation of the substrate predominated over coupling [162-165]. 

Several factors govern the transport and fate of hydrophobic organic chemicals in sediment/water environments microbially mediated reactions and sorption are major processes affecting the fate of these compounds in aquatic systems [166,366-368]. Aryl halides have been shown to undergo microbially-mediated dehalogenation under anaerobic conditions [38, 52, 68, 105, 116,... 

Beurskens, J.E.M., Dekker, C.G.C., van der Heuvel, H., Swart. M., de Wolf, and Dolfing, J. Dehalogenation of chlorinated benzenes by an anaerobic microbial consortium that selectively mediates the thermodynamic and most favorable reactions,... 

Iron carbonyls also mediate the cycloaddition reaction of allyl equivalents and dienes. In the presence of nonacarbonyldiiron a,a -dihaloketones and 1,3-dienes provide cycloheptenes (Scheme 1.5) [14,15]. Two initial dehalogenation steps afford a reactive oxoallyliron complex which undergoes a thermally allowed concerted [4 + 31-cycloaddition with 1,3-dienes. The 1,3-diene system can be incorporated in cyclic or heterocyclic systems (furans, cyclopentadienes and, less frequently, pyrroles). Noyori and coworkers applied this strategy to natural product synthesis, e.g. a-thujaplicin and P-thujaplicin [14, 16]. 

In Chapter 4, we saw that Sml2 mediates the dehalogenation of a range of alkyl halides. These reactions proceed by reduction to alkyl radicals that are then... 

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