Alkenes alkynes from

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. 

Conjugation between the triple bond and the carbonyl function lowers the reduction potential considerably whereas alkyl substitution makes reduction more difficult (entries 1-5). A comparison between the half-wave potentials for reduction of PhC=CPh (1-69 V, vs. Hg pool) and // <7/t -PhCH=CHPh (1-65 V) substantiates the fact that, at least for this case, a likely product of reduction is more vulnerable to electroreduction than the starting material. In practice electrolyses in protic media aimed at producing alkene from alkyne usually proceed to give alkane. 

Z)-Alkenes. The method for generating (Z)-alkenes from alkynes can be extended to conjugated and methylene-skipped enynes. On quenching the titanacyclopropene intermediates with D O, dideuterio products are obtained. 

The most obvious method for the formation of alkenes from alkynes is by partial reduction. This reaction can be effected in high yield with a palladium-calcium carbonate catalyst that has been partially deactivated by addition of lead(II) acetate or quinoline (Lindlar s catalyst). It is aided by the fact that the more electrophilic alkynes are adsorbed on the electron-rich catalyst surface more strongly than the corresponding alkenes. An important feature of these reductions is their high stereoselectivity. In most cases the product consists very largely of the thermodynamically... 

The Lindlar catalyst is a deactivated hydrogenation catalyst that stops alkyne hydrogenation at the alkene stage, allowing conversion of allies to ds-alkenes. Hydroboration followed by an acid workup (instead of basic peroxide) also gives cfs-alkenes from alkynes. Hydroboration-protonolysis is an alternative way to prepare a ds-alkene from an alkyne. 

Dissolving metal reduction gives irans-alkenes from alkynes. 

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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