9.11: Reduction of Alkynes to cis-Alkenes: Catalytic Hydrogenation

Catalytic hydrogenation of alkenes involves the syn addition of one mole of hydrogen across the π bond in the presence of transition metal catalysts like platinum, palladium, or nickel to form alkanes.

Similarly, alkynes also undergo catalytic reduction to alkanes. However, since there are two π bonds, two moles of hydrogen are required to saturate the triple bond.

The addition of the first equivalent forms an alkene, which upon further reduction gives an alkane. Both steps follow syn stereochemistry, with the hydrogen adding to the same side of the π bond.

It is possible to stop the reaction at the cis-alkene stage using a poisoned transition metal catalyst synthesized by adding certain chemical compounds that deactivate the metal.

One such example is Lindlar's catalyst, a mixture of palladium deposited on calcium carbonate and deactivated by treatment with lead salts and quinoline. Another is a nickel–boron complex, known as the P-2 catalyst.

The modified catalyst lowers the activation energy and effectively reduces the first π bond of the alkyne. However, it is not reactive enough to reduce the second π bond. Thus, partial hydrogenation of alkynes using a Lindlar or P-2 catalyst forms cis-alkenes.

The mechanism begins with the adsorption of molecular hydrogen onto the surface of the metal catalyst, thereby breaking the H–H bond and forming new metal–H bonds.

Next, the alkyne binds to the catalytic surface, forming a π complex with the metal. A sequential transfer of two hydrogens from the metal surface to the same face of the alkyne leads to the formation of the cis-alkene product. 

Hydroboration-protonolysis is an alternative non-catalytic approach for converting internal alkynes to cis-alkenes.

The reaction proceeds via a syn-stereoselective addition of borane to an internal alkyne to form a trialkenylborane. Subsequent treatment of the trialkenyl intermediate with acetic acid replaces the boron with hydrogen to give a cis-alkene.