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Trans-Hydrogenation of alkynes
The catalytic hydrogenation of alkynes in the presence of a Lindlar catalyst only provides the productcis- permanent hydrogen. One trans-Addition cannot be achieved by hydrogenolysis.
But there is a way out for them trans-Addition of one equivalent of hydrogen to alkynes, namely if one goes through one-electron transfer reactions. An alkyne dissolved in a solution of sodium metal in liquid ammonia gives a trans-Alken. In contrast to sodium amide dissolved in ammonia, which functions as a strong base, sodium dissolved in liquid ammonia acts as a strong electron donor, i.e. as a reducing agent. Alkali metals dissolve in liquid ammonia (-33 ° C) to form a deep blue solution that contains the metal cation and solvated electrons.
When an alkyne is added, a solvated electron adds to the triple bond to form a radical anion. The additional unpaired electron in the radical anion must be in an antibonding p orbital, since the binding ones are all fully occupied. While this destabilizes this intermediate stage, there are still enough occupied binding orbitals that hold the system together. Because of its excess electrons, the radical anion is a strong Lewis base and easily withdraws a proton from the ammonia, forming a vinyl radical and the amide. There are two stereochemical possibilities for the vinyl radical: cis- and the trans-Form, of which the transArrangement is more stable for steric reasons.
This vinyl radical takes up a solvated electron again and the resulting vinyl anion is again replaced by ammonia with the formation of the trans-Alkenes protonated.