Retrosynthesis of Alkenes Elimination

If an elimination reaction is planned, the retrosynthesis of an alkene involves an FGI that adds either HX or water to the alkene to give an alkyl halide or an alcohol starting material, respectively. The stability of the alkene TM will help determine whether an E2 or El elimination would be more suitable. For example, an unstable, terminal alkene cannot be prepared via dehydration because a rearrangement of the carbocation intermediate would be expected. Care must be taken such that the alkyl halide or alcohol starting material would give only the desired TM as the major product. 

Alkynes can be reduced to either a cis or trans alkene. Catalytic hydrogenation of an alkyne using a poisoned catalyst (H2, Lindlar catalyst) results in the syn addition of one equivalent of H2 to give a cis alkene product. Dissolving metal reduction (Li, NH3) of an alkyne produces the corresponding trans alkene. This strategy is well suited for synthesizing monosubstituted alkenes and disubstituted alkenes with a specific stereochemistry. 

2-disubstituted or monosubstituted alkenes can be prepared from alkynes 

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