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Cis/Trans Hydrogenation of Alkynes

Trans Hydrogenation of Alkynes

The catalytic hydrogenation of an alkyne with a Lindlar catalyst yields only the cis-hydrogenated product. A trans addition of elemental hydrogen is not known.

However, a trans addition of one equivalent hydrogen (H2) to an alkyne is possible if another hydrogenation reagent is applied. To this end, the alkyne is added to a solution of elemental sodium in liquid ammonia. An alkali metal is dissolved in liquid ammonia (at -33 °C) by forming the metal cation and a solvated electron. In contrast to a solution of sodium amide in liquid ammonia, which is a strong base, the deep blue solution of sodium in liquid ammonia is a very strong reducing agent.

Fig.1
Solvated electrons

The reducing power is strong enough so that an electron is added to the alkyne. This one-electron transfer yields a radical anion in which the additional unpaired electron occupies an antibonding p orbital because all bonding orbitals of the alkyne are already occupied. Therefore, the alkynyl radical anion is at a high-energy intermediate stage. It is stabilized by protonation by an ammonia molecule forming a neutral alkenyl radical because it is a very strong base. In this reaction step, the trans alkenyl radical is mainly formed, minimizing the steric hindrance between the initial alkyne substituents and therefore maximizing the alkenyl radical stability.

Fig.2
Reaction between an alkyne and a solvated electron and subsequent protonation by ammonia

Subsequently, the alkenyl radical is also reduced by an one-electron transfer and the formed alkenyl anion is protonated by ammonia, stereoselectively yielding the trans alkene. The alkene is the end product of the reaction because under the reaction conditions it is inactive towards an additional one-electron transfer.

Fig.3
Reaction between an alkenyl radical and a solvated electron and subsequent protonation by ammonia
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