Towards assessing the aromaticity of the Diels-Alder transition state

The symmetry of the Diels-Alder transition state depends on the interplay of orbital interactions, aromaticity, which favors a Cs structure, and strain, which tends to break symmetry. Twenty Diels-Alder reactions are examined at MP4SDTQ/6-312G(d)//B3LYP/6-31G(d). The substituents utilized are alkyl groups in order to minimize the effect of orbital interactions. The underlying assumptions are examined and appropriate caveats discussed. These computations provide, as a first approximation, an estimate of 10 kcal mol−1 for the aromatic stabilization energy in the Diels-Alder TS.