Exploration of the Differences between Amine and Thiolate Addition to Acetylenedicarboxylates

Nucleophilic addition of thiolates to diethyl acetylenedicarboxylate in chloroform at room temperature affords solely the meso dithioaddition product, whereas the addition of amines in ethanol gives only the corresponding (Z)-enamine, as confirmed by X-ray crystal analysis. The monoaddition product of thiolate addition, prepared and isolated at lower temperatures, also exhibited (Z)-stereochemistry. The accompanying computational study on simplified model systems explains the reasons for the observed stereochemistry and for acetylenedicarboxylate readily undergoing two addition reactions with thiolate nucleophiles and the (Z)-enamine being much less reactive toward addition of thiolate or amine nucleophiles.