Understanding the catalysis by bis-selenonium cations as bidentate chalcogen bond donors

The mode of activation and catalysis of bidentate bis-selenonium cations have been computationally explored in detail by means of Density Functional Theory calculations. To this end, the parent Diels–Alder cycloaddition reaction between cyclohexadiene and methyl vinyl ketone is compared to the analogous reaction mediated by selenonium cations. It is found that, although the catalyst induces a significant stabilization of the LUMO of the dienophile, the observed acceleration of the process is not due to an enhancement of the orbital interactions between the reactants but to a remarkable reduction of both the steric (Pauli) repulsion between them and the destabilizing strain energy. Issues such as the origin of the endo:exo selectivity of the transformation and the influence of the nature of the tether connecting both selenium centers on the activity of the catalysts are also analyzed in detail.