Nickel polyhalides

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). 

Although 1, -elimination of o xylene polyhalides with sodium iodide, zinc, copper, or iron metal is a fundamental method for the formation of o-xylylene intermediates, it is difficult to carry out the reaction under mild conditions such as at room temperature. o Trimethylsilylmethyl)-benzyltrimethylammonium halides were devised for this purpose and were shown to generate the o-xylylene at room temperature. However, we have successfully generated o-xylylene at room temperature by the reaction of a, 0,-dibromo o-xylene with metallic nickel(51). The Diels-Alder reaction of... 

Halogen oxidation of planar complexes, particularly of the nickel triads that contain bidentate glyoximes or macrocyclic ligands, such as, phthalocyanines, substituted porphyrins, or tetraazaannulenes, leads to highly conducting substances that may possess chains of metal atoms as well as chains of polyhalides ... 

Partial replacement. BrFj added to a vigorously stirred soln. or startg. polyhalide in Freon 113 at 20-5" in the presence of a little SbClj, and worked up after ca. 1.5 h product. Y 90%. A Teflon reactor, fitted with a nickel stirrer and quartz dropping funnel, was used. The method is useful for halogen exchange in polychloroalkanes of low reactivity. F.e.s. A.V. Kartashov et al., Zh. Org. Khim. 24, 2522-5 (1988). 

There are two likely mechanistic pathways for the reductive coupling of benzylic polyhalides, either via a carbene or carbene-nickel complex intermediate... 

Table 7.14 Reaction of benzylic mono- and polyhalides with metallic nickel."...

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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