Cis-alkenes, from alkynes

Although catalytic hydrogenation is a convenient method for preparing cis alkenes from alkynes, it cannot be used to prepare trans alkenes. With a dissolving metal reduction (such as Na in NH3), however, the elements of H2 are added in an anti fashion to the triple bond, thus forming a trans alkene. For example, 2-butyne reacts with Na in NH3 to form rram-2-butene. 

Hydrogenation is a stepwise process that may be stopped at the intermediate alkene stage by the use of modified catalysts, such as the Lindlar catalyst. This catalyst is palladium that has been precipitated on calcium carbonate and treated with lead acetate and quinoline. The surface of the metal rearranges to a less active configuration than that of palladium on carbon so that only the first rr bond of the alkyne is hydrogenated. As with catalytic hydrogenation of alkenes (Section 12-2), the addition of H2 is a syn process (see margin). As a result, this method affords a stereoselective synthesis of cis alkenes from alkynes. 

Lindlar s catalyst A heterogeneous catalyst containing palladium that forms cis alkenes from alkynes in a hydrogenation reaction. 

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See also in sourсe #XX -- [ ]

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