σ- vs π-Binding: Lewis Acids Bind Lone Pairs, Not Double Bonds

The binding affinities for a series of Lewis acids (LAs) to acrolein and azabutadiene were investigated using density functional theory calculations at ZORA-BLYP-D3(BJ)/TZ2P. In all systems, LAs favor σ-binding to the lone pair of nitrogen or oxygen, rather than π-binding to the C=C double bond. Activation strain and energy decomposition analyses reveal that σ-binding is preferred over π-binding due to reduced steric (Pauli) repulsion between the LA and the substrate. Furthermore, LAs σ-bind more strongly to the nitrogen of azabutadiene than to the oxygen of acrolein due to enhanced electrostatic and orbital interactions. In addition, the role of solvation in the σ-binding versus π-binding preference was also examined: the preference for σ-binding clearly remains and even increases for strong LAs, but for weak LAs such as I2, the preference for σ-binding diminishes in solvent, though σ-binding remains dominant. Lastly, analysis of I2 catalysis of a model Diels-Alder reaction indicates that the preferred transition state features σ-binding. Overall, π-binding appears to be more of a (theoretical) curiosity and is unlikely to play a dominant role in catalysis.