9.5 Reduction of Alkynes

Reduction of Alkynes

Reduction of alkynes lets you selectively build alkenes or alkanes by adding hydrogen across the triple bond. The key idea: your choice of reagent controls both how much hydrogen gets added and which stereoisomer you get. This makes alkyne reduction a powerful tool in organic synthesis.

Process of Alkyne Reduction

There are two main approaches, and they give you different products.

Catalytic hydrogenation uses a metal catalyst and $\ce{H2}$ gas:

• Pd/C (palladium on carbon) adds two equivalents of $\ce{H2}$, fully reducing the alkyne all the way to an alkane. The reaction doesn't stop at the alkene stage because Pd/C is too active.
• Lindlar's catalyst adds only one equivalent of $\ce{H2}$, partially reducing the alkyne to a cis-alkene. Lindlar's catalyst is Pd deposited on $\ce{CaCO3}$ and treated with lead acetate and quinoline. These additives "poison" (deactivate) the catalyst just enough to prevent over-reduction past the alkene.

Dissolving metal reduction uses an alkali metal as the electron source:

• $\ce{Na}$ or $\ce{Li}$ dissolved in liquid $\ce{NH3}$ reduces an alkyne to a trans-alkene. The ammonia serves as the proton source in this reaction.

Stereochemistry of Reduction Methods

Why do these methods give different stereochemistry?

Lindlar's catalyst delivers both hydrogen atoms from the catalyst surface to the same face of the triple bond. This is called syn addition, and it produces the cis-alkene.

Dissolving metal reduction (Na or Li in $\ce{NH3}$) proceeds through a stepwise radical anion mechanism (see below). The intermediates adopt geometries that minimize steric strain, so hydrogen ends up on opposite faces of the double bond. This is anti addition, producing the trans-alkene.

Being able to choose between cis and trans products from the same starting alkyne is what makes these reactions so useful in synthesis planning.

Mechanism of Dissolving Metal Reduction

This mechanism has four steps, alternating between electron transfers and protonations:

1. The alkali metal donates one electron to the alkyne, forming a radical anion (a species with both an unpaired electron and a negative charge).
2. Ammonia protonates the radical anion, producing a vinyl radical.
3. The alkali metal donates a second electron to the vinyl radical, forming a vinyl anion.
4. Ammonia protonates the vinyl anion, giving the final trans-alkene product.

The trans selectivity arises at the vinyl anion stage. The anion adopts a geometry where the two substituents (and the lone pair) are positioned to minimize electron-electron repulsion. This places the larger groups on opposite sides, so the trans-alkene is the favored product.

Applying Alkyne Reduction in Synthesis

When you're planning a synthesis, think of internal alkynes as versatile precursors to stereodefined alkenes:

• Need a cis-alkene? Reduce the alkyne with Lindlar's catalyst and $\ce{H2}$.
• Need a trans-alkene? Use Na (or Li) in liquid $\ce{NH3}$.
• Need an alkane? Use Pd/C with excess $\ce{H2}$.

This selectivity is especially valuable because directly controlling alkene geometry through other methods can be difficult. Starting from an alkyne and choosing your reduction conditions gives you reliable stereochemical control.