Insights into Tetrazine-Benzene Cycloadditions.

Arene-tetrazine cycloadditions involving benzene as the dienophile represent an exceptionally rare class of Diels-Alder reactions. In this study, we computationally investigate the origin of the intrinsic reactivity of simple aromatic systems toward tetrazines and identify the factors that enable successful cycloaddition. Our results show that only highly activated tetrazines can overcome the unfavorable orbital overlap associated with the compact π-system of benzene. Energy decomposition analysis reveals that reactivity is governed by a combination of frontier molecular orbital interactions and charge-control effects. Substituent effects strongly influence the activation barrier, particularly when they promote a shift toward a more asynchronous and polar transition state. These findings provide mechanistic insight into a rare mode of arene reactivity and clarify the electronic requirements for enabling tetrazine Diels-Alder reactions with aromatic dienophiles.