Debromination. reductive

It is not possible to use zinc for reductive debromination in the presence of (x-halo ketones and for transformations involving these intermediates, sodium iodide has been used. ° In some instances, e.g. 5,6-dihalo-3-ketones, iodide does not always give a completely halogen-free product, and zinc does not give clean debromination. The use of chromous chloride has proved advantageous in such cases and is the reagent of choice for vicinal dichlorides, which are inert to iodide ... 

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

Bromo-6,7,8,9-tetrahydro-l//-3-benzazepin-2-amine(6) with thiocyanate ion undergoes substitution of bromide to give the thiocyanatotetrahydro-l//-3-benzazepine 7.105 Attempts to replace bromide by azide ion failed, as did diazotization of the amine group with sodium nitrite in 6 M sulfuric acid. Oddly, treatment of the aminobromo compound with sodium borohydride in methanol results not in reduction, but in methoxy-debromination to give the 2-methoxy derivative which, on the basis of HNMR spectral data, is best represented as the 2-imino tautomer 8. 

Analogously, for preparation of racemic carba-a-glucopyranose 49 from 52, esterification of (—)-52 furnished the ester 95, which was transformed into compound 96 by debromination with zinc dust and acetic acid. Stereoselective hydroxylation of 96 with osmium tetraoxide and hydrogen peroxide, followed by acetylation, gave compound 97. Lithium aluminum hydride reduction of 97, and acetylation of the product, gave pentaacetate 98, which was converted into 99 by hydrolysis. ... 

Reduction of 1-benzyl-3,4-dibromophospholan oxide (125) with tri-chlorosilane, followed by debromination, gave 1-benzylphosphole. Determination of the molecular structure by X-ray analysis showed slight puckering of the ring with retention of pyramidal configuration at phosphorus. ... 

Morris PJ, JF Quensen III, JM Tiedje, SA Boyd (1992) Reductive debromination of the commercial polybro-minated biphenyl mixture Firemaster BP6 by anaerobic microorganisms from sediments. Appl Environ Microbiol 58 3249-3256. 

Selective reductive debromination of polyhalogenated methanes (Castro et al. 1985 Li and Wackett 1993). 

FIGURE 7.68 Reductive debromination by methanogenic bacteria. (From Neilson, A.H. and Allard, A.-S., The Handbook of Environmental Chemistry, Vol. 3R, pp. 1-74, Springer, 2002. With permission.)... 

He J, KR Robrock, L Alvarez-Cohen (2006) Microbial reductive debromination of polybrominated diphenyl ethers (PBDEs). Environ Sci Technol 40 4429-4434. 

Morris PJ, JF Quensen, JM Tiedje, SA Boyd (1993) An assessment of the reductive debromination of polybro-minated biphenyls in the Pine River reservoir. Environ Sci Technol 27 1580-1586. 

A combination of cat. Ybt and A1 is effective for the photo-induced catalytic hydrogenative debromination of alkyl bromide (Scheme 28) [69]. The ytterbium catalyst forms a reversible redox cycle in the presence of Al. In both vanadium- and ytterbium-catalyzed reactions, the multi-component redox systems are achieved by an appropriate combination of a catalyst and a co-reductant as described in the pinacol coupling, which is mostly dependent on their redox potentials. 

C gives a mixture of dibromoketones 166a and 166b, and when these are treated with zinc-copper in methanol saturated with ammonium chloride at room temperature, the debromination product 167 is obtained in quantitative yield. Reduction of 167 with an excess of DIBAH in THF at — 78°C and then at room temperature leads to a mixture of the 6,7-dehydro-3-hydroxytropanes 168 and 169 (93 7 ratio) (Scheme 10). 

In the case of deca-BDE, its suggested degradation process in mammals consists of a first reductive debromination where one, two or three bromine atoms can be replaced by hydrogen atoms followed by an oxidation to form hydroxylated metabolites, which are presumably formed from an intermediate epoxy [52]. This study detected traces of three nona-BDEs, which may be an indication of reductive debromination as a first step of degradation, and thirteen hydroxylated metabolites. Otherwise, the possibility of a deca-BDE oxidation as a first step to form the epoxy without an intermediate reductive debromination is also suggested [52]. The study conducted by Morck et al. [53] detected several hydroxylated products, from methoxy-hydroxy-pentabrominated to methoxy-hydroxy-heptabrominated compounds, which coincide with part of the metabolites obtained by Sandholm et al. [52]. In addition, both authors found that methoxy and hydroxy substituents are always on the same aromatic ring when both are present. Moreover,... 

Morck et al. [53] found traces of less brominated PBDEs than deca-BDE, indicating that the reductive debromination would be the first step in its degradation. In both articles, it is suggested the involvement of Cyp P450 in the oxidation reactions to form hydroxylated metabolites, but not in the reductive debromination proposed as the initial step. 

Regarding the reductive debromination as the first step of deca-BDE degradation in mammals, Huwe and Smith [54] detected the formation of different PBDEs (three nona-BDEs, four octa-BDEs and one hepta-BDE) from deca-BDE degradation in rats, which also suggests the existence of a reductive debromination process as the first step in deca-BDE degradation in mammals. In this case, it was not identified whether the specific enzymatic system responsible for the reductive debromination and the corresponding analyses to detect the formation of hydroxylated metabolites were not carried out. 

In the case of deca-BDE mixture, the formation of less brominated compounds was not detected for any of the samples (time of degradation corresponding to 24, 36, 60, 84 and 182 h). Regarding octa-BDE and penta-BDE mixtures, the analysis of samples at time 168 h did not detect an increase in concentration for any of the components present in each of the mixtures or the formation of any PBDE not present in the original composition of both mixtures. Therefore, the results obtained for the three mixtures demonstrated that the PBDEs degradation by the fungus does not follow the reductive debromination pathway, which is logical because this... 

One-electron reduction of organic halides is convenient for generating radicals. gem-Dibromocyclopropancs are reduced by such a system [77]. Using an excess of MoH2Cp2, stereoselective debromination is successful, possibly because of... 

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