Mechanical forces can be exploited to control the rates of chemical reactions and to govern reaction pathways. Furan–maleimide adducts are widely used as mechanophores in polymer mechanochemistry for their ability to undergo reversible breakage under tension via a retro Diels-Alder reaction (rDA). We show that an asymmetric mechanical force along the anchoring bonds in both the endo and exo isomers of proximal dimethyl furan-maleimide adducts favors a sequential pathway involving a single-bonded diradical intermediate. The switching from a concerted to a sequential mechanism occurs at external forces of ≈ 1nN. The reaction is inhibited for external forces up to ≈3.4 nN for the endo adduct and 3.6 nN for 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 ≈ 3 orders of magnitude larger than for exo one in qualitative agreement with recent sonication experiments. An anti and a gauche-out conformational diradicalar diastereomers were identified: in the case of proximal-exo they can interconvert via a rotation, with a 3 kcal/mol barrier, about the bond that is not broken. The open-shell singlet and triplet states are quasi degenerate in the vicinity of the intermediates, where minimum energy crossings points are localized. The reaction paths on the force-modified potential energy surfaces were characterized by the isometric approach CoGEF, and the diradical character of the intermediate was elucidated from the analysis of the determinantal compositions of the CASSCF wavefunction, natural molecular orbital occupations, and electronic localization. These findings open the way for the rational design of DA mechanophores.

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

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