How Ionization Catalyzes Diels‐Alder Reactions

The catalytic effect of ionization on the Diels‐Alder reaction between 1,3‐butadiene and acrylaldehyde has been studied using relativistic density functional theory (DFT). Removal of an electron from the dienophile, acrylaldehyde, significantly accelerates the Diels‐Alder reaction and shifts the reaction mechanism from concerted asynchronous for the neutral Diels‐Alder reaction to stepwise for the radical‐cation Diels‐Alder reaction. Our detailed activation strain and Kohn‐Sham molecular orbital analyses reveal how ionization of the dienophile enhances the Diels‐Alder reactivity via two mechanisms: (i) by amplifying the asymmetry in the dienophile's occupied π‐orbitals to such an extent that the reaction goes from concerted asynchronous to stepwise and thus with substantially less steric (Pauli) repulsion per reaction step; (ii) by enhancing the stabilizing orbital interactions that result from the ability of the singly occupied molecular orbital of the radical‐cation dienophile to engage in an additional three‐electron bonding interaction with the highest occupied molecular orbital of the diene.