Dehalogenation removal

DDT p,p -dichloro-diphenyl-trichloroethane. deacetylation removal of acetyl group, dealkylation removal of alkyl group, deaminate removal of amine group, dechlorination removal of chlorine group, de-ethylation removal of ethyl group, dehalogenation removal of halogen atom(s). 

Hydrolysis of esters and ethers, hydrolytic cleavage of C—N single bonds, hydrolytic cleavage of nonaromatic heterocycles, hydration and dehydration at m ultiple bonds, new atomic linkages resulting from dehydration reactions, hydrolytic dehalogenation removal of hydrogen halide molecules, various reactions. 

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]. 

However, treatment of cortisone 3,20-bissemicarbazone with acetic anhydride and pyridine removes the 20-semicarbazone group preferentially. Selective removal of a protecting group can be also achieved by a selective reaction to give a new intermediate which can be converted into the desired product ketone. Thus progesterone 20-monoenol acetate (42) is prepared from the 3,20-bisenol acetate (40) via selective electrophilic attack of iodine at C-6 followed by reductive dehalogenation of (41). ... 

Consequently, haloketenes can be readily generated in situ by two most widely used methods (a) the triethylamine dehydrohalogenation of an acyl halide (Eq. (3))50) (b) the dehalogenation of an a-haloacyl halide with activated zin (Eq. (4))51). Since the halogen substituents on the cyclobutanone can be reductively removed by usual procedures, the synthesis of a halocyclobutanone constitutes a formal preparation of the cyclobutanone, the synthetic utility of which is convincingly demonstrated by the following examples. 

Phenylthioacetylene has been prepared by elimination of thiophenol and dehydrobromination of cis-1,2-bis(phenylthio)ethylene5 and cis-1-bromo-2-phenylthioethylene,2 7 respectively. The latter was obtained by addition of thiophenol to propiolic acid in ethanol and subsequent one-pot bromine addition, decarboxylative dehalogenation, and careful distillation to remove the trans isomer.2.7 on the other hand, cis-1,2-bis(phenylthio)ethylene was prepared by double addition of thiophenol to cis-1,2-dichloroethylene.5a d Although these procedures can provide useful amounts of phenylthioacetylene, they were found to be somewhat less satisfactory in our hands as far as operation and/or overall yields are concerned. Furthermore, we have encountered problems with regard to the reproducibility of one-pot dehydrobrominations of phenylthio-1,2-dibromoethane.6 However, the stepwise execution of the double dehydrobromination, as described in the modified procedure reported here, provides preparatively useful quantities of phenylthioacetylene in a practical manner. 

Kinetic studies performed on model compounds were aimed at understanding the effect of different parameters on the selectivity. They showed that selectivity was achieved only when A336 was present. In fact, in the absence of A336 and of the base the hydrodehalogenation of p-chloroacetophenone proceeded aU the way to ethylcyclohexane in the biphasic aqueous-organic system. When A336 was added, selectivity was reversed—chloride was removed first—and the selective dehalogenated benzyl alcohol was obtained. ... 

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. 

Catalytic dehalogenation (mostly by H2/Pd) and desulfurization (by Raney-Ni) are important tools for structural analysis. This way chlorine can be removed from positions 2, 5, and 7 [64CPB204 66JCS(C)2031]. Zinc was used for dechlorination at C-7 (59YZ903). The 5,7-dichloro-TPs (61CPB801) and 6,7-dichloro-TPs (59CPB903) in the presence of H2/Pd first lose the more reactive chlorine from C-7 the remaining 6-chloro compound can be dechlorinated only by Raney-Ni. 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

Oxidative dehalogenation. Halogen atoms may be removed from xenobiotics in an oxidative reaction catalyzed by cytochromes P-450. For example, the anesthetic halothane is metabolized to trifluoroacetic acid via several steps, which involves the insertion of an oxygen atom and the loss of chlorine and bromine (Fig. 4.28). This is the major metabolic pathway in man and is believed to be involved in the hepatotoxicity of the drug. Trifluoroacetyl chloride is thought to be the reactive intermediate (see chap. 7). 

In the presence of a strong reductant, abiotic reductive dechlorination of CPs is catalyzed by the reduced form of Vitamin B12 (Gantzer Wackett, 1991Smith Woods, 1994). These abiotic dechlorinations favor removal of m- and /(-chlorines, which differs substantially from reductive dechlorinations by anaerobic microbial consortia or by the CP-dehalogenating bacteria isolated to date. This indicates that abiotic dechlorinations of CPs are not central reactions in environmental or engineered anaerobic systems. 

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