An experimental and theoretical reinvestigation of the Michael addition of primary and secondary amines to dimethyl acetylenedicarboxylate

Theoretical calculations of the Michael addition of diethylamine, pyrrolidine, and benzylamine to DMAD at the DFT (B3LYP/6-31+G∗) level indicate that the reaction follows a stepwise mechanism via a zwitterionic intermediate. The reactions have low activation barriers, 13-15 kcal mol−1 and are exothermic, ΔH° = −29 to −44 kcal mol−1. The detailed investigation of the reaction of benzylamine with DMAD reveals participation of the reactant-, transition structure-, and the product-complexes and that the 1,3-prototropic shift occurs through the benzylamine molecule. It also predicts formation of dimethyl 2-(N-benzylimino)butane-1,4-dicarboxylate as one of the products, which has been duly isolated and characterized experimentally.