1,2-reduction to alkenes

Triple bonds in side chains of aromatics can be reduced to double bonds or completely saturated. The outcome of such reductions depends on the structure of the acetylene and on the method of reduction. If the triple bond is not conjugated with the benzene ring it can be handled in the same way as in aliphatic acetylenes. In addition, electrochemical reduction in a solution of lithium chloride in methylamine has been used for partial reduction to alkenes trans isomers, where applicable) in 40-51% yields (with 2,5-dihydroaromatic alkenes as by-products) [379]. Aromatic acetylenes with triple bonds conjugated with benzene rings can be hydrogenated over Raney nickel to cis olefins [356], or to alkyl aromatics over rhenium sulfide catalyst [54]. Electroreduction in methylamine containing lithium chloride gives 80% yields of alkyl aromatics [379]. 

The reduction of vinyl halides most often is carried out via a metallation-protonation reaction. For instance, sodium in ethanol was employed by Barton et a/. for the reduction of a steroidal vinyl iodide (Scheme 44). A number of conditions for vinyl halide reduction to alkenes have been reported, representative examples of which include vinyl chloride and bromide reduction in acyclic systems with... 

As was pointed out in the discussion of hydrostannation, allylic halides undergo reduction to alkenes rather than addition to the double bond 48,... 

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