How Alkali Cations Catalyze Aromatic Diels‐Alder Reactions

We have quantum chemically studied alkali cation‐catalyzed aromatic Diels‐Alder reactions between benzene and acetylene forming barrelene using relativistic, dispersion‐corrected density functional theory. The alkali cation‐catalyzed aromatic Diels‐Alder reactions are accelerated by up to 5 orders of magnitude relative to the uncatalyzed reaction and the reaction barrier increases along the series Li(+) < Na(+) < K(+) < Rb(+) < Cs(+) < none. Our detailed activation strain and molecular‐orbital bonding analyses reveal that the alkali cations lower the aromatic Diels‐Alder reaction barrier by reducing the Pauli repulsion between the closed‐shell filled orbitals of the dienophile and the aromatic diene. We argue that such Pauli mechanism behind Lewis‐acid catalysis is a more general phenomenon. Also, our results may be of direct importance for a more complete understanding of the network of competing mechanisms towards the formation of polycyclic aromatic hydrocarbons (PAHs) in an astrochemical context.