1,2-dihalides, dehalogenation

Deuteration of arenes, 206 Deuterium, role in nmr, 243 Dextrorotatory, 70 Diastereomers, 69 conformational, 80 Diastereoselective reactions, 91 Diazine, 458, 460 Diazomethane, 67, 353 Diazonium ions, 409 Diazonium salts, 416 Dicarboxylic acids, 342 Diels-Alder reaction, 154 Dienes, polymerization, 153j summary of chemistry, 154 Dienophile, 154 Dihalides, dehalogenation, 91 Dimethyl sulfoxide (DMSO), 305 Dioxane, 285 Dioxin, 447... 

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). 

Cleavage Gem-dihalides and monohalides have been dehalogenated to chiral monohalides in the presence of alkaloids [397, 398]. l,l-Diphenyl-2-bromo-2-carboxyl (bromo or methyl carboxylate) cyclopropanes are cathodically debromi-nated in the presence of alkaloid cations with enantioselectivities up to 45% ee. A mechanism is proposed whereby the alkaloid is adsorbed at the Hg cathode, which protonates face selectively the carbanion generated by 2e reduction from the bromide [399]. 

Problem 7.35 Dehalogenation of Wc-dihalides with active metals (Mg or Zn) is also an anti elimination. Predict the products from (a) meso- and (b) either enantiomer of 2,3-bromobutane. ... 

The reactant cannot be a v/c-dihalide, because it would undergo dehalogenation ... 

Alkenes are obtained by the transformation of various functional groups, e.g. dehydration of alcohols (see Section 5.4.3), dehalogenation of alkyl halides (see Section 5.4.5) and dehalogenation or reduction of alkyl dihalides (see Section 5.4.5). These reactions are known as elimination reactions. An elimination reaction results when a proton and a leaving group are removed from adjacent carbon atoms, giving rise to a tt bond between the two carbon atoms. 

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

The use of zinc in ethanol to dehalogenate haloallyl halides to allenes was reported earlier by Gustavson and Demjanoff [17a, b]. This method is still a useful one and has been applied when the starting dihalides are available. Recently chromous sulfate has also been shown to be an effective reagent at room temperature [17c] (Eq. 10). 

Dehalogenation of vic-dihalides 7-31 Elimination of a halo and a hetero group (Boord)... 

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