Q4(e). But-2-yne to cis- and trans-but-2-ene

Introduction. A reaction is stereospecific when stereoisomeric starting materials give stereoisomerically different products (the mechanism fixes the result, e.g. cis- and trans-but-2-ene with Br₂). It is stereoselective when one substrate can give two or more stereoisomers and one is formed preferentially.

1. (i) cis-But-2-ene: H₂, Lindlar catalyst (Pd on CaCO₃ poisoned with Pb(OAc)₂ and quinoline); or H₂/Ni₂B (P-2). Both H atoms are delivered from the metal surface to the same face: syn addition. The poison stops reduction at the alkene.
2. (ii) trans-But-2-ene: Na (or Li) in liquid NH₃ at -33°C (dissolving-metal reduction). Electron transfer gives a radical anion; protonation gives a vinyl radical, which takes the trans geometry (alkyl groups far apart); a second electron and protonation give the E-alkene: net anti addition.
3. Classification. But-2-yne has no stereoisomers, so neither reaction can be stereospecific in the strict sense. Each can in principle give cis or trans but-2-ene and gives mainly one: both are stereoselective (highly; syn for Lindlar, anti for Na/NH₃).

Conclusion. (i) H₂, Lindlar catalyst (syn) → cis-but-2-ene; (ii) Na, liquid NH₃ (anti) → trans-but-2-ene. Both are stereoselective reactions.

Application. Lindlar and dissolving-metal reductions give the Z and E double bonds of insect pheromones (for example bombykol) from a single alkyne precursor.