Halogens, reductive dehalogenation

The Ni(II) complexes 6 and 7 have been found by Stiles [60] to be soluble catalysts for reductive dehalogenation when combined with NaBH4 or hydrazine at 25-45 °C in protic solvents. Reactivity toward the reducing system increased with the halogen content of the substrate. Aryl bromides were converted much faster than chlorides, polychlorobenzenes, however, reacted readily with stepwise loss of chlorine. 

Halogenated aliphatics can be partially or completely degraded under anaerobic conditions through a transformation reaction called reductive de-halogenation. Often a co-metabolic degradation step, reductive dehalogenation... 

Fig. 24. Reductive dehalogenation of a chlorinated hydrocarbon in the presence of a metal forming an alkyl radical, showing (Pathway (I)) the alkyl radical scavenging a hydrogen atom, and (Pathway (II)) the alkyl radical losing a second halogen to form an alkene...

A broad spectrum of chemical reactions can be catalyzed by enzymes Hydrolysis, esterification, isomerization, addition and elimination, alkylation and dealkylation, halogenation and dehalogenation, and oxidation and reduction. The last reactions are catalyzed by redox enzymes, which are classified as oxidoreductases and divided into four categories according to the oxidant they utilize and the reactions they catalyze 1) dehydrogenases (reductases), 2) oxidases, 3) oxygenases (mono- and dioxygenases), and 4) peroxidases. The latter enzymes have received extensive attention in the last years as bio catalysts for synthetic applications. Peroxidases catalyze the oxidation of aromatic compounds, oxidation of heteroatom compounds, epoxidation, and the enantio-selective reduction of racemic hydroperoxides. In this article, a short overview... 

The reductive dehalogenation of polyfluoroarenes by zinc in aqueous ammonia gave products derived from the removal of one or two halogen atoms. A radical anion is suggested to form initially by direct electron transfer from the zinc to substrate which then fragments. Ceo undergoes single-electron reduction by the electron-rich. 

Baum and Archibald reported the synthesis of 2,2,6,6-tetranitrobicyclo[3.3.1]nonane (142). This synthesis starts from the dioxime (140), which on halogenation with chlorine, followed by oxidation with hypochlorite and reductive dehalogenation with hydrogen in the presence of palladium on carbon, yields 2,6-dinitrobicyclo[3.3.1]nonane (141). Oxidative nitration of (141) with sodium nitrite and silver nitrate under alkaline conditions yields 2,2,6,6-tetranitrobi-cyclo[3.3.1]nonane (142). The greater molecular freedom in 2,2,6,6-tetranitrobicyclo[3.3.1] nonane (crystal density-1.45 g/cm ) compared to the isomeric 2,2,6,6-tetranitroadamantane (crystal density-1.75 g/cm ) is reflected is their considerably different crystal densities. 

The typical reactions of halogenothiophenes can be classified under the following headings nucleophilic displacement, halogen-metal exchange, reductive dehalogenation and formation of thienyl radicals. 

The term dehalogenation generally denotes any reaction in which one or more halogen atoms leave a molecule, e.g. in reductive dehalogenation. In this section, dehalogenation is used in the sense of 1,2-didehalogenation leading to the formation of a multiple bond. 

The order of ease of reductive dehalogenation of carbon-halogen bonds of the same structural type is generally as follows ... 

Dehalogenation can occur by several reductive pathways. The simplest results in replacement of a C-bonded halogen atom with a hydrogen and is known as hydrogenolysis or reductive dehalogenation. The process is illustrated for trichloroethene, TCE,... 

In conclusion, reductive dehalogenation with Pd catalysts offers a number of advantages it can treat a wide variety of compounds, including mixtures it generally results in simple alkanes, with few halogenated intermediates, if any and it is extremely rapid, which allows small, in-well reactors. As more studies are conducted, the applicability to a broad range of conditions will be tested and will provide opportunities to better understand the process. This will facilitate optimization of catalyst parameters and column operation for the most effective remediation under a variety of field conditions. 

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