The Role of AlCl ₃ in Catalysis and Regioselectivity in Diels–Alder Reactions Between Polycyclic Aromatic Hydrocarbons and Maleimides

The reaction between anthracenes and N‐substituted maleimides was the first example for which the catalytic effect of AlCl 3 on Diels–Alder reactions was shown. The current article presents a combined experimental and theoretical (DFT) study that extends the scope of accessible cycloadducts and also quantifies the influence of AlCl 3 on reaction barrier and regioselectivity for a broad scope of reagents and different catalyst ratios. DFT calculations indicate that each aluminum trichloride bound to maleimide lowers the activation energy by ≈15–30 kJ mol −1 in reactions with anthracenes or naphthalenes. Moreover, calculations point to steric factors and not reaction barrier heights as the most relevant to the formation of anti‐ 1,4 isomers in cycloaddition reactions to 9,10‐dihaloanthracenes. Additionally, the inertness of 9‐cyanoanthracene can be ascribed to its high binding energy to AlCl 3 .