An energetic analysis of the Diels-Alder endo:exo selectivity reaction by using composite methods

The G3(MP2)-CEP and G3(MP2) theories are applied to analyze the energetic tendency of six Diels-Alder reactions. Different energetic data were analyzed in this paper calculated by both levels of theory: energy differences, Gibbs energies, and distortion/interaction energies. The quantitative and qualitative results obtained by both theories tend to agree with the experimental data in most cases regarding the relative barrier heights, major product tendency and distortion/interaction effects, although in some cases the sufficiently small energy difference is lower than the error of the method itself. The examples studied showed that the endo rule cannot be followed in some of the reactions discussed.