Steering the Mechanism of the Furan-Maleimide Retro-Diels-Alder Reaction to a Sequential Pathway with an External Mechanical Force

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.