Dehalogenation catalysis

Berardi, S., Carraro, M., Iglesias, M., etal. (2010).Polyoxometalate-Based A-HeterocyclicCar-bene (NHC) Complexes for Palladium-Mediated C-C Coupling and Chloroaryl Dehalogenation Catalysis, Chem. Em J., 16, pp. 10662-10666. 

The low reactivity of alkyl and/or phenyl substituted organosilanes in reduction processes can be ameliorated in the presence of a catalytic amount of alkanethiols. The reaction mechanism is reported in Scheme 5 and shows that alkyl radicals abstract hydrogen from thiols and the resulting thiyl radical abstracts hydrogen from the silane. This procedure, which was coined polarity-reversal catalysis, has been applied to dehalogenation, deoxygenation, and desulfurization reactions.For example, 1-bromoadamantane is quantitatively reduced with 2 equiv of triethylsilane in the presence of a catalytic amount of ferf-dodecanethiol. 

Li S, LP Wackett (1993) Reductive dehalogenation by cytochrome P450(,j jy[ substrate binding and catalysis. Biochemistry 32 9355-9361. 

Orotic acid readily forms dimers even when irradiated in liquid medium [582, 583]. 5-Bromouracil (5-BrU) in DNA is dehalogenated, rather than forming cyclobutane-type dimers. Such DNA derivatives are more sensitive to ultraviolet irradiation than normal DNAs [584-594], Irradiation of 5-bromo-uracil and derivatives in aqueous medium produces 5,5 -diuracil [590, 591]. However, derivatives such as 3-sbutyl-5-bromo-6-methyluracil have been reported to yield cyclobutane dimers either by irradiation of frozen aqueous solutions, or by catalysis with free radical initiators, such as aluminium chloride, ferric chloride, peroxides or azonitriles [595]. 5-Hydroxymethyluracil is reported to dimerize very slowly in frozen water at 2537 A [596]. The fundamental research in the photochemistry of the nucleic acids, the monomeric bases, and their analogues has stimulated new experiments in certain micro-organisms and approaches in such diverse fields as template coding and genetic recombination [597-616]. 

Consequently, degradation of halogenated compounds in gaseous effluents, especially in diluted effluents, is often problematic, dehalogenation being the limiting step. In order to overcome this problem, a new process, based on solid/gas catalysis with whole dehydrated cells as the catalyst, has been proposed [14]. The aim is to convert halogenated compounds into alcohols. 

Zhang and Rusling [66] employed a stable, conductive, bicontinuous microemulsion of surfactant/oil/water as a medium for catalytic dechlorination of PCBs at about 1 mA cm-2 on Pb cathodes. The major products were biphenyl and its reduced alkylbenzene derivatives, which are much less toxic than PCBs. Zinc phthalocyanine provided better catalysis than nickel phthalocyanine tetrasulfonate. The current efficiency was about 20% for 4,4 -DCB and about 40% for the most heavily chlorinated PCB mixture. A nearly complete dechlorination of 100 mg of Aroclor 1260 with 60% Cl was achieved in 18 hr. Electrochemical dehalogenation was thus shown to be feasible in water-based surfactant media, providing a lower-cost, safer alternative to toxic organic solvents. 

In dehalogenating enzymes of anaerobic microorganisms, corrinoid cofactors have a newly discovered further role in the redox catalysis of the energy conserving dehalogenation of chloro(hydro)carbon compounds ( dehalorespiration ), and the specific redox properties of the protein-bound unusual corrinoids are of particular current interest. 

While the dehalogenation of chlorinated species is often referred to as zero-valent metal catalysis, it is not strictly a catalytic but rather an electrochemical corrosion process. The metal is consumed by reaction, leading to the formation of metal salts and dechlorinated by-products. 

Atom Variations E2 Heteroatom Variants, Dehalogenation, Fragmentation Vinylogous Variations Sn2 and E2. 1,4 additions Extent of Proton Transfer Variations General Acid and General Base Catalysis of Additions and Eliminations, Summary by Media, Push-Pull Catalysis of Enolization... 

Heterogeneous dehalogenations of vicinal dihalides can be achieved smoothly with zinc in protic solvents or magnesium, lithium or sodium in ethereal solution. In general the elimination occurs at the surface of the metal and the reactivity of this surface is maintained by the presence of a solvent which redissolves any produced olefin and any metallic salt. However, in one case, catalysis by the zinc dihalide deposited on the metal surface during the elimination has been reported . ... 

Dravis BC, LeJeune KE, Hetro AD et al. (2000) Enzymatic dehalogenation of gas phase substrates with haloalkane dehalogenase. Biotechnol Bioeng 69(3) 235—241 D Souza S (1999) Immobdized enzymes in bioprocess. Curr Sci 77 69—78 Durand J, Teuma E, Gomez M (2007) Ionic liquids as a medoium for enantioselective catalysis. C R Chimie 10 152-177... 

Dehalogenation of a-bromoacetophenones and -bromopropiophenones with or without general acid catalysis can be effected by triphenylphosphine (24) but not by trialkylphosphites. The a-chloroketones fail to undergo a similar reaction. The catalyzed reaction conforms to Saville s rule. 

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