Dibromoalkene, elimination

Alkenylstannanes react with 1,1-dibromoalkenes to give enynes.204 These reactions are thought to involve elimination of the elements of HBr prior to reductive elimination. 

Disubstituted alkynes and terminal alkynes form E-dibromoalkenes [4] when the tribromide is formed in situ in an essentially basic medium, an addition reaction followed by elimination of hydrogen bromide results in the conversion of terminal alkynes into the 1-bromoalkynes [5]. When the addition reaction is conducted in methanol, l,l-dibromo-2,2-dimethoxyalkanes are produced, in addition to the 1,2-dibromoalkenes [6], The dimethoxy compounds probably result from the initial intermediate formation of bromomethoxyalkenes. Under similar conditions, alkenes yield methoxy bromo compounds [7]. 

Analogous to the Wit tig olefmation, triphenylphosphine and carbon tetrabromide react to give an intermediate dibrominated phospho-nium ylide. Ylide 33 extends the chain of aldehyde 11 with elimination of triphenylphosphine oxide to give 1,1-dibromoalkene 12.17... 

Geminal substituted dibromoalkenes can be prepared by the alkylation of dibromomethyllithium with a-chloroalkyl methyl ethers. Deprotonation of the alkylation products results in the elimination of methanol and the formation of the corresponding 1,1-dibromoalkenes. Despite the lower acidity of 1 -bromo-1 -chloroalkyllithiums relative to the dibromo analogs they exhibit similar nucleophilic proper-ties. Alkyl dichloroacetates can also be deprotonated with lithium diethylamide and alkylated with a range of alkyl halides. ... 

Chain extension of aldehydes to 1,1-dibromoalkenes followed by elimination to alkynes by means of BuLi or RMgX. 

Reactions. - Elimination and Dehalogenation. Mild conditions for eliminating primary alkyl halides to form 1-alkenes have been reported in which a solution of the halide and DBU in THF is added to a solution prepared by the reaction of an equivalent of dichlorobis(triphenylphosphinato)nickel and two equivalents each of triphenylphosphine and butyl-lithium.1 Polyethylene glycol promoted basic dehydrohalogenation of 1,1-dibromoalkenes has been... 

The conversion of alcohols into ynol ethers has been achieved [188, 189]. This methodology requires the use of powerful bases thus, the potential for functional group incompatibility needs to be addressed prior to implementation of this approach Ynol ethers have been generated from gem-dibromoalkenes through initial formation of a brominated vinyl ether followed by elimination [193] Terminal aUcynes are convenient starting points for this chemistry and several excellent approaches are known [187, 191, 192]... 

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