Bond breaking of furan-maleimide adducts via a diradical sequential mechanism under an external mechanical force

Substituted furan-maleimide Diels-Alder adducts are bound by dynamical covalent bonds that make them particularly attractive mechanophores. Thermally activated [4+2] retro Diels-Alder (DA) reactions predominantly proceed via a concerted mechanism on the ground electronic state. We show that an asymmetric stretching direction along the anchoring bonds in both the endo and exo isomers of proximal dimethyl furan-maleimide adducts favors a sequential pathway. The switching from a concerted to a sequential mechanism occurs at external forces ≈ 1nN. The first bond rupture occurs for a projection of the pulling force on the scissile bond ≈ 4 nN for the exo adduct and ≈ 4.5 nN for the endo one. The reaction is inhibited for external forces up to ≈3.1 nN for the endo adduct and 3.6 nN the exo one after which it is activated. In the activated region, at 4 nN, the rupture rate of the first bond for the endo adduct is computed to be ≈ 2 orders of magnitude larger than for exo one in qualitative agreement with recent sonication experiments [ Z. Wang, S. L. Craig, Chemical Communications 2019, 55, 12263-12266.] In the intermediate region of the path between the rupture of the first and the second bond the lowest singlet state exhibits a diradical character for both adducts and is close in energy to a diradical triplet state. The computed values of spin-orbit coupling along the path are too small for inducing intersystem crossings. These findings open the way for the rational design of DA mechanophores.