Dehalogenation of vicinal

The stereospeciflcity of dehalogenation of vicinal dibromides to olefins was examined for reducing agents including Cr(II), iodide, and Fe° (Totten et al. 2001). For dibromostilbene, the ( )-stilbene represented >70% of the total olefin that was produced, and for threo-dibromopentane reduction by Cr(Il) produced ca. 70% of ( )-pent-2-ene, whereas values for iodide and Fe° were <5% of this. 

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. 

Dehalogenation of vicinal dibromides by electrogenerated polysulfide ions in DMA provides an indirect method to accomplish the same goal. A series of vicinal dibromides has been examined. These include methyl erj /2ro-2,3-dibromo-3-phenylpropanoate (152),... 

Fig. 15 Dehalogenation of vicinal-dibromides and dichlorides with 1,5-ditelluraoctane and 1,5-diselenaoctane.

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. 

An acetylenic bond may, however, be introduced also by dehalogenation of vicinal dihaloalkenes, occasionally by cleavage of nitrogenous compounds, or from enol esters by elimination reactions.178... 

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. 

Dehalogenation of vicinal dibromides by sodium iodide in acetone or alcoholic solvents, viz. 

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). 

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