Vicinal dihalide dehalogenation

On the other hand, benzylic polyhalides were converted to the corresponding olefins via vicinal dihalide intermediates. Metallic nickel was also shown to be useful for the dehalogenation of vicinal dihalides(36,43). 

Preparation of alkenes Dehalogenation of vicinal-dihalides with Nal in acetone produces alkene via E2 reactions. 

Kray W. C., Jr. and Castro C. E. (1964) The cleavage of bonds by low-valent transition metal ions the homogeneous dehalogenation of vicinal dihalides by chromous sulfate. J. Am. Chem. Soc. 86, 4603-4608. 

Removal of halogen from a vicinal dihalide with zinc can be used to form olefins, although it is not a common method for alkene synthesis. Similarly, dehalogenation of >SiX-SiX< with alkali metals appears to be successful in a limited sense. When R2Si(X)Si(X)R2 (R = t-Bu X = C1, Br) or RR Si(Cl)Si(Cl)RR (R = t-Bu R = c-C6H,) are reacted with lithium (sodium or potassium), the radical anion of the disilene is formed which is particularly long-lived in the latter case181. However, several intermediate... 

Based on the limited number of vicinal dihalides tested thus far, it appears that negative groups on both carbons bearing halogen atoms are necessary for dehalogenation. Thus, methyl 2,3-dichloro- (2,160) and dibromopropionate (2), 2,3-dichloro- and dibromopropionitrile (2), and... 

Describe dehalogenation of vicinal dihalides to obtain an alkene. 

Dehalogenation of vicinal dihalides is severely limited by the fact that these dihalides are themselves generally prepared from the alkenes. However, it is sometimes useful to convert an alkene to a dihalide while some operation on another part of the molecule is being performed, and then regenerate the alkene by treatment with zinc. 

This reaction is called the dehalogenation of vicinal dihalides, i.e. it is the abstraction of two halogens from adjacent carbon atoms to yield the alkene. The metal Zn acts as a donor of electrons. 

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 . ... 

OTHER 1,2-ELIMINATION REACTIONS Dehalogenation of Vicinal Dihalides... 

In each of the examples of 1,2-elimination reactions discussed above, one of the groups eliminated was a proton. Another iype of 1,2-elimination reaction is the dehalogenation of vicinal dihalides. Iodide ion can be used as the dehalogenating agent in both protic and aprotic solvents (equation 10.40). 

General Preparations.—Alkenes. Dehalogenation of vicinal dihalides is easily carried out using sodium in liquid ammonia, e.g. 1,2-dichlorocyclo-octane gave ds-cyclo-octene in 95% yield.The scope of the synthesis of ap-unsaturated carbonyl compounds by syn-elimination from 2-phenylselenoxides has been studied only low yields of the desired ketone were obtained from 2-phenylseleno-cycloheptanone and cyclo-octanone because of the competing Pummerer rearrangements, but improved yields could be obtained by prior conversion of the ketones into ketals. The reaction worked well for 2-methoxycarbonyl cyclic ketones, e.g. 2-methoxycarbonylcyclo-oct-2-en-l-one was prepared in 93% yield. ... 

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