Base‐Promoted 1,4‐Elimination Reactions: On the Origin of an Eventual syn ‐Stereoselectivity

A “conveyor” or a concerted, E1cb‐like mechanism is involved in the 1,4‐elimination of HCl from E‐(1) and Z‐(1). The particular mechanism depends on the solvent: In polar solvents the concerted process is effective, in non‐polar solvents the “conveyor” process. Due to the larger distance between the reaction centers in polar solvents alkyne‐forming 1,4‐eliminations are less stereoselective than analogous 1,2‐elimininations.