Semihydrogenasjon

Semihydrogenasjon, or partial hydrogenation, is the catalytic hydrogenation of unsaturated substrates in which only part of the unsaturation is reduced. In organic synthesis it is most often used to convert alkynes to alkenes, with the aim of stopping at the alkene stage rather than going all the way to alkanes. This approach is particularly valued for producing stereochemically defined alkenes, especially cis-alkenes, from internal alkynes.

The process relies on poisoned or deactivated hydrogenation catalysts that limit activity enough to prevent full saturation. The classical example is the Lindlar catalyst, palladium on calcium carbonate poisoned with lead(II) acetate and often supplemented with quinoline or sulfur compounds. Poisoning promotes selective partial reduction and typically proceeds with syn addition of hydrogen, favoring cis products for many internal alkynes. Other milder or differently poisoned palladium or nickel systems can achieve similar partial hydrogenation, and alternative catalyst formulations such as nickel boride-based systems are also used in some contexts.

Typical conditions involve hydrogen gas at low to moderate pressure and modest temperatures, with alcohol or ether solvents common. The presence of the poisoning agent is crucial to minimize over-reduction to alkanes.

Applications include the preparation of cis-alkenes from alkynes in pharmaceutical, fine chemical, and materials synthesis. A classic demonstration is the conversion of diphenylacetylene to cis-stilbene under Lindlar-type conditions. Semihydrogenation is distinguished from complete hydrogenation by its selectivity and is often chosen when the geometry of the resulting alkene is important. Limitations include sensitivity to functional groups that coordinate to the catalyst and the need for careful control of conditions to avoid over-reduction.