Mechanochemical Retro-Diels-Alder-Heteroatom Radical Stabilization of a Two-Step Mechanism?

In organic molecules, heteroatom substitutions are a powerful molecular engineering strategy for modulating chemical reactivity. This is particularly interesting in polymer mechanochemistry, in which the reaction mechanism can change due to the force modification of the potential energy surface of mechanophores. Here, we investigate the effects of inserting a single heteroatom (N, O, and S) next to the bridging carbon of anthracene-maleimide (AnM) adduct mechanophores. It has been previously proposed that upon application of force, the reaction mechanism changes from concerted to a two-step mechanism based on homolytic bond scission, and therefore, such heteroatoms could affect this conversion. Indeed, the mechanochemistry experiments revealed a meaningful acceleration in force-induced retro-Diels-Alder reaction. Notably, sulfur atom insertion resulted in the highest enhancement in reactivity, with a ca. 2-fold increase compared to a carbon atom at the same position. Computational studies reveal that a simplistic look based on transition state theory does not explain such enhanced reactivity, and therefore, ab initio steered molecular dynamics are used, indicating that the radical lifetime after bond scission correlates well with the experimental results. Importantly, both experimental and computational data confirm that heteroatom insertion is an effective and practical approach to enhancing mechanophore reactivity.