Lithium vicinal dihalides

Some instances of incomplete debromination of 5,6-dibromo compounds may be due to the presence of 5j5,6a-isomer of wrong stereochemistry for anti-coplanar elimination. The higher temperature afforded by replacing acetone with refluxing cyclohexanone has proved advantageous in some cases. There is evidence that both the zinc and lithium aluminum hydride reductions of vicinal dihalides also proceed faster with diaxial isomers (ref. 266, cf. ref. 215, p. 136, ref. 265). The chromous reduction of vicinal dihalides appears to involve free radical intermediates produced by one electron transfer, and is not stereospecific but favors tra 5-elimination in the case of vic-di-bromides. Chromous ion complexed with ethylene diamine is more reactive than the uncomplexed ion in reduction of -substituted halides and epoxides to olefins. ... 

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... 

Elimination from vicinal dihalides has also been accomplished with the reducing agent, lithium aluminium hydride and the nucleophilic thiophenox-ide but mechanistic details other than anti stereospecificity are lacking. 

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 . ... 

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