A density functional theory study for the Diels–Alder reaction between N-acyl-1-aza-1,3-butadienes and vinylamines. Lewis acid catalyst and solvent effects

The molecular mechanism for the Diels–Alder reaction of N-acyl-1-aza-1,3-butadiene with dimethylvinylamine has been studied using density functional theory methods. This cycloaddition is the nucleophilic attack of the vinylamine to the conjugate position of the unsaturated acyl imine with concomitant ring-closure. The presence of a Lewis acid catalyst coordinated to the acyl oxygen atom decreases markedly the activation energy associated to the nucleophilic attack. This results from an increasing of the electrophilicity of the 1-aza-1,3-butadiene that shift the mechanism from a highly asynchronous concerted process to a polar stepwise one.