Dehalogenation, with zinc

Dichloro- and l,2-dibromo-l,2-dihydro-3-benzoxepins are dehalogenated with zinc in methanol to give 3-benzoxepin (6a) in excellent yield.93 A methoxycarbonyl group in the 2-position, however, diminishes the yield of 6b considerably.91... 

Vinyl halides are so inert that none has been converted to a fluoride by halogen exchange. Vinyl fluorides have been synthesized from saturated polyhalides by dehalogenation with zinc and by dehydrohalo- genation with alcoholic alkali, and from acetylene by addition of one molecule of hydrogen fluoride.12 18... 

Reduction — A multistep reaction leads to desoxysaccharides. It involves 1-halopentaacetylated saccharide, which is dehalogenated with zinc into acetylated glucal (5.35). Hydrolyzed glucal accepts sulfuric acid, the S03H residue of which is readily hydrolyzed into deoxysugar (5.41). 

Hexafluoropropylene (CF3CF=CF2, CAS number 116-15-4) was first prepared by Downing et al. by pyrolysis. The full synthesis and identification of HFP was conducted by Henne and Woalkes. A six-step reaction scheme starting with the fluorination of 1,2,3-trichloropropane led to 1,2-dichlorohexa-fluoropropane, which was dehalogenated with zinc in boiling ethanol to yield hexafluoropropylene. 

Ketenes can be prepared by dehalogenation of a-halo acyl halides with zinc or with... 

Tetranitrodispiro[3.1.3.1 ]decane (24) was obtained as a mixture of isomers by treating the oxime (22) with chlorine in methylene chloride, followed by oxidation with hypochlorite and rednctive dehalogenation of the resnlting gm-chloronitro intermediate (23) with zinc... 

In addition to the procedures summarized in Table 6, modifications of the zinc dehalogenation have appeared. For example, copper-activated zinc in the presence of ammonium chloride with methanol as solvent apparently gives better yields and, in some cases, is the method of choice.90,91 124 An example of this procedure is the reductive dechlorination of 8,8-dichloro- l-phenylbicyclo[4.2.0]octan-7-one, with zinc in acetic acid, to give l-phenylbicyclo[4.2.0]octan-7-one (7) in 30% yield. The use of zinc and ammonium chloride in methanol gave 7 in 70% yield.124 However, no general trend can be seen. Another variation is to replace ammonium chloride with hydroxylamine hydrochloride.116... 

Another limitation is seen when extra strain is included in the compound to be reduced. Dehalogenation of 3,3-dichlorobicyclo[2.2.0]hexan-2-one with zinc/ammonium chloride in methanol gave, at best, a 25% yield of 3-chlorobicyclo[2.2.0]hexan-2-one (14) together with cyclohexenone and 6-chlorohex-5-enoic acid.128 The best results were achieved with the zinc/ acetic acid system, while addition of water, silver-promoted zinc reduction in methanol, tri-butyltin hydride reduction or hydrogenolysis with palladium in methanol did not result in formation of 14, but various other ring-opened products. 

Ketenes can be prepared by dehalogenation of ot-halo acyl halides with zinc or with tri-phenylphosphine.348 The reaction generally gives good results when the two R groups are aryl or alkyl, but not when either one is hydrogen.34,1 OS IV, 348 68, 41. 

Further improvements in dehalogenation selectivity and yields can be achieved by using dipolar aprotic solvents. Dimethylformamide has mostly been used for this purpose,18,56,84 97 although dimethyl sulfoxide,98,99 especially when combined with sonication at room temperature (vide infra), deserves attention in particular cases.100,101 Other polar and dipolar aprotic solvents have also been used, namely, acetone,4 butan-2-one,4 acetonitrile,102 acetic anhydride,103104 ethyl acetate,61 tetrahydrothiophene 1,1-dioxide (sulfolane)105 and hexamethyl-phosphoric triamide,106 but no details were reported on their advantages over dimelhylform-amide or dimethyl sulfoxide. Better performance of dipolar aprotic solvents, such as dimethyl-formamide, over other solvents has been demonstrated in the recent comparison of the dehalogenation of 4,5-dichloro-4,5,5-trifluoropentan-l-ol (4) with zinc in various solvents.90... 

Fluorine is simultaneously eliminated with other halogens if it is the only halogen vicinal to iodine, bromine or chlorine. The elimination tendency decreases in the series IF > BrF > C1F. The group CF2CHFX is relatively stable towards elimination and requires stronger reaction conditions. The dehalogenation of saturated fluorohalo compounds with zinc dust or zinc/ copper and zinc/silver couples is one of the most useful methods for the preparation of fluoroalkenes (see Table 8). Zinc/zinc(II) chloride has also been used instead of zinc alone. 

Dehalogenation to Allene (3). The chloride (2) is reductively dehalogenated to the corresponding C15-allene, 3,7,ll-trimethyldodeca-l,2-diene, by dissolving and stirring the chloride in a mixture of glacial acetic acid mixed with zinc... 

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