How Lewis Acids Catalyze Ene Reactions
Eveline H. Tiekink, Pascal Vermeeren, F. Matthias Bickelhaupt, Trevor A. Hamlin
Abstract
Abstract The catalytic effect of various Lewis acids (LAs) on the ene reaction between propene (ene) and but‐3‐en‐2‐one (enophile) was studied quantum chemically using density functional theory and with coupled‐cluster theory. The studied LAs efficiently accelerate the ene reaction by lowering the reaction barrier up to 12 kcal mol −1 compared to the uncatalyzed reaction. Our detailed activation strain and Kohn‐Sham molecular orbital analyses reveal that coordination of a LA catalyst to the enophile decreases the reaction barrier of the ene reaction by inducing an asymmetry in the π‐electronic system, which increases the asynchronicity and hence relieves the otherwise highly destabilizing activation strain and Pauli repulsion between the closed‐shell filled π ‐ orbitals of the ene and enophile. In all, these findings further demonstrate the generality of the Pauli‐lowering catalysis concept.