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

Abstract

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.

Consensus Model of Mechanophore Sensors for Biological Force Measurement

“…To estimate the forces involved in generating a fluorescent signal from the mechanophore, we first sought to evaluate the two stages of the process, namely: (1) force-induced release of the furanmaleimide adduct, and (2) fragmentation and decarboxylation of the resulting 2-furyl carbonate to uncage the resorufin fluorophore (Scheme 1B). We considered that the retro-DA reaction in the first stage could proceed through either a concerted or a stepwise carbon-carbon bond breaking mechanism 24 . The energetics of these two possible pathways are presented in Fig.…”