Understanding the high reactivity of triazolinediones in Diels-Alder reactions. A DFT study.

The participation of 4-substituted-1,2,4-triazoline-3,5-diones (TADs) in Diels-Alder reactions toward a series of dienes is studied at the M05-2X/6-31+G(d,p) level. These reactions show very low activation energies and complete endo selectivity, in agreement with the experimental data. For a dienic steroid model, the reaction presents an α-facial selectivity. Analysis of reactivity indices explains the superelectrophilic character of TADs, and low activation energy. The substituent and solvent effects are also evaluated.