Aza-Diels–Alder addition of cyclopentadiene to propynyliminoglyoxylates

The aza-Diels–Alder reaction of cyclopentadiene with protonated propynylimine of methyl (Me) and tert-butyl (t-Bu) carboxylate was studied, using density functional theory (DFT) at the X3LYP/6-31G (d) level, in order to elucidate the role of the ester group of the dienophile. Four independent reaction pathways were found for each diene/dienophile pair, all of them proceeding through a concerted, highly asynchronous, mechanism. Both systems exhibit a clear tendency for the formation of exo adducts and show the same overall behavior, despite steric hindrances. The exo cycloadducts are always favored relative to the endo analogs, both by kinetic and thermodynamic reasons. Gas-phase results showed that the exo/endo rate would be mostly controlled by the relative thermodynamic stability of the final products. However, the application of the polarizable continuum model (PCM) preferentially lowered the activation enthalpy of the reaction exo channels, stressing the importance of the kinetic over the thermodynamic aspects of the proposed mechanism. This resulted in a reaction model that satisfies the experimental evidence.