Reductive dehalogenation electrochemical

Magdesieva, T.V., Graczyk, M., Vallat, A., Nikitin, O.M., Demyanov, P.I., Butin, K.P. and Vorotyntsev, M.A. (2006) Electrochemically reduced titanocene dichloride as a catalyst of reductive dehalogenation of organic halides. Electrochim. Acta 52, 1265-1280. 

Rondinini, S., Mussini, P.R., Specchia, M. and Vertova, A. (2001a) The electrocatalytic performance of silver in the reductive dehalogenation of bromophenols. J. Electrochem Soc. 148, D102-D107. 

The melt crystallization yields a mother liquor that is usually rejected the alternative is reductive dehalogenation by electrochemistry (Scheme 6). The electrochemical step is shown in Fig. 12. 

Reductive Dehalogenations Microemulsions are usually more useful than micelles for electrochemical synthetic applications because larger amounts of polar and nonpolar reactants can be solubilized. Electrochemical catalysis has been used in microemulsions for the electrolytic conversion of organohalide pollutants to hydrocarbons [53] using mediators such as metal phthalocyanines and cobalt complexes. Microemulsions were used for the complete electrochemical catalytic... 

Highly reactive metal complexes serve as electrocatalysts for a number of detoxification reactions. An example is the use of Co(II) N, N -bis(salicylidene)ethylene (CoSalen) to facilitate the electrochemical reductive elimination of chlorine from hexachlorobenzene (HCB) [15]. Even though more environmentally friendly conditions have yet to be developed to scale up this process, the CV voltammogram in Fig. 6 shows separate dehalogenation electrochemical steps from the sequence below. 

He J, Ela WP, Bettertrar EA, Arnold RG, Saez AE (2004) Reductive dehalogenation of aqueous-phase chlruinated hydrocarborts in an electrochemical reactor, htd Eng Chem Res 43 7965—7974... 

Electrochemical enzyme catalysis can often be identified by cyclic voltammetry. Mb, Hb and cyt P450 enzymes in various films catalyze the reductive dehalogenation of organohalides [11,17,33,34,48,62,63,72,84]. Figure 21 shows CVs of films of Mb and dimyristoylphosphatidylcholine (DMPC). In the absence of reactant trichloroacetic acid (TCA), the voltammogram features reversible reduction-oxidation peaks for the Fe /Fe couple of Mb. In a solution 2 mM in... 

Dehalogenating reduction Salt elimination Disproportionation Electrochemical formation Dehydrogenative catalytic coupling... 

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]. 

Coordinatively unsaturated complexes and those giving easily such species by ligand dissociation favor pathways related to that described in Eqs. (10) and (13). Coordinatively saturated complexes reduce halocarbons via outer-sphere ET [193, 194]. In cases of electrochemical dehalogenations, the species formed by one-electron reduction of the mediators on the cathode often react in this way [156, 157, 198], For example (Eq. (14)) [157, 166] ... 

In outer-sphere SET reductions (e.g., in electrochemical dehalogenations), hydrogen abstraction by R leads to the product RH (i.e., no step related to (ii) is required to occur). Process (ii) follows generally the activation of the substrate in the proposed hydrodehalogenation cycles, but we know also of opposite examples [77, 82, 106, 112],... 

Toxic compounds are frequently rendered less toxic by electrochemical treatment, for instance dehalogenation of chlorinated derivatives such as PCBs or AOX (performed in a divided cell or in the absence of Cl ions in a monocell) odors are eliminated or greatly reduced, i.e. reduction of nitrotoluene which can be a serious odor nuisance biodegradability is improved. Elimination of colloids and surfactants is possible. 

Organochalcogen(II) compounds, 100-102 Organochalcogen(IV) compounds, 100-102 Organoselenium compounds dehalogenation reactions, 96 electrochemical reduction, 113-117 haloperoxidase-like activity, 108-113 with odd number of ligands, 100-102 one-electron oxidation, 117-118 oxidation of thiols, 102-106 redox reactions, 79-80 thioperoxidase-like activity, 108-113 Organotellurium compounds... 

In several examples the reductive halide-hydrogen exchange has been studied on a preparative scale. For example, the indirect electroreduction of 2-chloropyridine in DMF using anthracene as mediator gives pyridine in 83-86 % yield 2 . For the dehalogenation of 1-chlorohexane (80% yield), naphthalene is applied as redox catalyst. Similarly, 6-chloro-hexene yields 1-hexene (60%) and methylcyclopentane (25%), which is the product of the radical cyclization . The indirect electrochemical reduction of p- and y-bromocarboxylic esters forms coupling and elimination products besides the dehalogenated products... 

Selective dehalogenation of halopyridines is an important industrial process for the same reason that reduction of carboxylic acids, esters, amides, and nitriles are also important. There is a dearth of selective oxidation technologies whether by conventional or electrochemical methods. Therefore, many intermediate oxidation stage products are made by overoxidation, i.e., overhalogenation, followed by selective reduction. 

Electrochemical reduction processes of CFCs leading to partially or completely dehalogenated compounds for synthetic purposes have been... 

Polyhaloacetic acids and their partially hydrodehalogenated products represent a second important family of herbicide-/pesticide-derived substrates. In their review on the environmental applications of industrial electrochemistry, Juttner and co-authors (Juttner et al. 2000) documented the electroreductive dechlorination of dichloroacetic acid (a by-product of monochloroacetic acid), a way to recover the valuable compound and avoid wastes. The electrochemical reduction of polychloro- and polybromo-derivatives was performed by Korshin and Jensen (2001) on Cu and Au cathodes. Complete dehalogenation was obtained for all substrates, but for monochloroacetic acid. To overcome the intrinsic poor reactivity of the monochloro-derivative the photoelectrochemical properties of a p-doped SiC electrode were investigated (Schnabel et al. 2001) however, the dehalogenation stopped at monochloroacetic acid. 

An electrochemical reduction procedure using a chromium(II) salt as the catalyst is effective for the dehalogenation of P-hydroxy halides. This procedure is a convenient entry to deoxynucleosides, as shown in Scheme 8. 

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