Dehalogenation dechlorination

Formate is an excellent hydride source for the hydrogenolysis of aryl halides[682]. Ammonium or triethylammonium formate[683] and sodium formate are mostly used[684,685]. Dechlorination of the chloroarene 806 is carried out with ammonium formate using Pd charcoal as a catalyst[686]. By the treatment of 2,4,6-trichloroamline with formate, the chlorine atom at the /iiara-position is preferentially removed[687]. The dehalogenation of 2,4-diha-loestrogene is achieved with formic acid, KI, and ascorbic acid[688]. 

Aryl halides can be dehalogenated by Friedel-Crafts catalysts. Iodine is the most easily cleaved. Dechlorination is seldom performed and defluorination apparently never. The reaction is most successful when a reducing agent, say, Br or 1 is present to combine with the I" or Br coming off." Except for deiodination, the reaction is seldom used for preparative purposes. Migration of halogen is also found," both intramolecular and intermolecular." The mechanism is probably the reverse of that of 11-11." ... 

Tetrahalobenzynes, however, react with a variety of aromatic compounds to afford tetrahalobenzobarrelene derivatives in good yields, frequently in the range of 55 to 75%. The dehalogenation of a variety of alkenyl chlorides with alkali metals in tetrahydrofu-ran containing tert-butyl alcohol suggested this approach to the dechlorination of tetrachlorobenzobarrelenes. 

There has been considerable interest in the abiotic dechlorination of chlorinated ethenes at contaminated sites. Reductive dehalogenation has therefore been examined using a range of reductants, many of them involving reduced complexes of porphyrins or corrins. 

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... 

Reductive dechlorination in combination with the elimination of chloride has been demonstrated in a strain of Clostridium rectum (Ohisa et al. 1982) y-hexachlorocyclohexene formed 1,2,4-trichlorobenzene and y-l,3,4,5,6-pentachlorocyclohexene formed 1,4-dichlorobenzene (Figure 7.69). It was suggested that this reductive dechlorination is coupled to the synthesis of ATP, and this possibility has been clearly demonstrated during the dehalogenation of 3-chlorobenzoate coupled to the oxidation of formate in Desulfomonile tiedjei (Mohn and Tiedje 1991). Combined reduction and elimination has also been demonstrated in methanogenic cultures that transform 1,2-dibromoethane to ethene and 1,2-dibromoethene to ethyne (Belay and Daniels 1987). 

May HD, LA Cutter, GS Miller, CE Milliken, JEM Watts, KR Sowers (2006) Stimulatory and inhibitory effects of organohalides on the dehalogenating activities of PCB-dechlorinating bacterium o-17. Environ Sci Technol 40 5704-5709. 

Wu Q, DL Bedard, J Wiegel (1999) 2,6-dibromobiphenyl primes extensive dechlorination of Arochlor 1260 in contaminated sediment at 8-30°C by stimulating growth of PCB-dehalogenating microorganisms. 

The persistence of halogenated phenols and anilines in anaerobic environments is determined by the activity of anaerobic dehalogenating bacteria. Extensive effort has therefore been devoted to isolating the relevant organisms and an increasing number of strains have been obtained in pure culture. They display different specificities for the position of the halogen several bring about dechlorination specifically at positions ortho to the chlorine, while some of them can use the aromatic substrate as... 

Uotila JS, VH Kitunen, T Saastamoinen, T Coote, MM Hagblom, M Salkinoja-Salonen (1992) Characterization of aromatic dehalogenases of Mycobacterium fortuitum CG-2. J Bacteriol 174 5669-5675. van der Tweel WJJ, JB Kok, JAM de Bont (1987) Reductive dechlorination of 2,4-dichlorohenzoate to 4-chlorobenzoate and hydrolytic dehalogenation of 4-chloro-, 4-hromo-, and 4-iodohenzoate hyAcalig-enes denitrificans NTB-1. App/ Environ Microbiol 53 810-815. 

Tetrachoroethylene (perchloroethylene, PCE) is the only chlorinated ethene that resists aerobic biodegradation. This compound can be dechlorinated to less- or nonchlorinated ethenes only under anaerobic conditions. This process, known as reductive dehalogenation, was initially thought to be a co-metabolic activity. Recently, however, it was shown that some bacteria species can use PCE as terminal electron acceptor in their basic metabolism i.e., they couple their growth with the reductive dechlorination of PCE.35 Reductive dehalogenation is a promising method for the remediation of PCE-contaminated sites, provided that the process is well controlled to prevent the buildup of even more toxic intermediates, such as the vinyl chloride, a proven carcinogen. 

The novel, low-melting-point salt [N-pentylpyridinium][cJoso-CB11H12] has been used as solvent in several dehalogenations of mono- and polychlorides and -bromides, catalyzed by several Pd phosphine complexes [69]. The debromina-tion of hexabromo- and 1,2,4,5-tetrabromobenzene was accomplished quite rapidly, whereas the dechlorination of 1,2,4-trichlorobenzene proceeded more slowly, but with excellent selectivity to 1,2-dichlorobenzene. The system could be recycled at least seven times without noticeable decrease of activity. 

Reductive amination, of carbonyl compounds, 16 573 Reductive bleaching, 21 440 Reductive dechlorination defined, 3 757t Reductive dehalogenation defined, 3 757t... 

The metabolism of carbon tetrachloride proceeds via cytochrome P-450-dependent dehalogenation (Sipes et al. 1977). The first step involves cleavage of one carbon-chlorine bond to yield Cl- and a trichloromethyl free radical, which is then oxidized to the unstable intermediate trichloromethanol, the precursor of phosgene. Hydrolytic dechlorination of phosgene yields C02 and HC1 (Shah et al. 1979). Although there are similarities in the metabolism of chloroform and carbon tetrachloride, metabolic activation of chloroform produces primarily phosgene, whereas the level of phosgene production from... 

Oxidative dehalogenation of aromatic halogens should not occur because there is no hydrogen atom on the carbon involved however, it often does occur. One mechanism likely involves ipso addition as will be discussed later and as proposed for the dechlorination of pentachlorophenol (Fig. 4.65) (131). 

Tabaei SM H, Pittman CU Jr., Mead KT. 1991. Dehalogenation of organic compounds-2 the metal catalyzed sodium borohydride or sodium alkoxyborohydride/tetraethylene glycol/KOH dechlorination of mirex. Tetrahedron Lett 32(24) 2727-2730. 

Several studies have investigated the dechlorination of single PCB congeners under anaerobic conditions [298,457-460],but these have all focused on PCBs with six or fewer chlorine substituents. The dehalogenation of decachlorobi-phenyl over time has also been reported [228] however, the products were only tentatively identified and not quantified. 

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