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...

Mechanical forces are known to control rates of chemical reactions and govern reaction pathways, possibly inducing a change of mechanism with respect to the zero force one. We report on a switching of mechanism of the retro Diels-Alder bond breaking from concerted at zero force to sequential under tension for four furan–maleimide adducts, mechanophores widely used in polymer mechanochemistry because they can undergo reversible breakage under tension. The four different adducts differ by their regio- and stereochemistry. The reaction paths on the force modified potential energy surfaces were characterized by isometric and isotensional approaches and determining stationary points (equilibrium geometries and transition states) as a function of the applied force, as well as by analyzing the redistribution of strain energy over the internal degrees of freedom. We evidence different bond breaking pathways and rate constants for the four isomers, the proximal configurations being favored over the distal ones. The switch from a concerted pathway at zero force to a sequential one occurs for a threshold force that is significantly higher (≈ 2.4 nN) for the distal-exo adduct than for the other three (≈ 1 nN), explaining its larger resistance to breaking and its almost inert character under tension. The switch is accompanied by the rupture of one of the two scissile bonds which leads to a twice smaller imaginary frequency of the transition state and an increase of the activation barrier, which then decreases for higher force strengths (> 3nN) to become barrierless at a critical force value.

Correction for ‘Bond breaking of furan–maleimide adducts via a diradical sequential mechanism under an external mechanical force’ by Manuel Cardosa-Gutierrez, et al., Chem. Sci., 2023, https://doi.org/10.1039/D2SC05051J.

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.