Polyhalides 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). 

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... 

The order of ease of reductive dehalogenation of organic halides in the same type of structural environment is I > Br > Cl F. This order is parallel with the dissociation energy of carbon-halogen bonds (HsC—I 234 kJ mol- H3C—Br 293 kJ mol" H3C—Cl 351 kJ mol- H3C—F 452 kJ moL ) and is generally observed in the reduction of alkyl halides. Consequently, selective reduction of di- or polyhalides containing different halogen atoms is possible. Fluorides are often removed only with difficulty and examples of such reductions are comparatively limited. 

Dehalogenation of vicinal polyhalides with nickel was then investigated to establish that the second step in Equation 7.11 proceeds as shown. 1,2-Dibromo-l,2-diphenylethane (meso) reacted with metallic nickel at room temperature to yield stilbene trans) in 89% yield and a trace amount of cw-stilbene. Similarly, ethyl 2,3-dibromo-3-(4-nitrophenyl)propanoate (erythro) underwent debromination to give ethyl 4-nitrocinnamate (tram) in 76% yield. Dechlorination of l,2-diphenyl-l,l,2,2-tetrachloroethane at 85 C afforded 1,2-dichloro-l,2-diphenylethene (97%, cis trans = 70/30). 

These results show that the second step of Equation 7.11 proceeds smoothly with a variety of vicinal dihalides. Thus, it may be concluded that the reaction of benzylic polyhalides with nickel proceeds via a step-by-step dehalogenation to give ethene derivatives. 

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