Controlling Reactivity by Geometry in Retro-Diels-Alder Reactions under Tension.

Mechanical force, with its ability to distort, bend, and stretch chemical bonds, is unique in the way it activates chemical reactions. In polymer mechanochemistry, the force is transduced in a directional fashion, and the efficiency of activation depends on how well the force is transduced from the polymer to the scissile bond in the mechanophore (i.e., mechanochemical coupling). We have investigated the effects of regio- and stereochemistry on the rate of force-accelerated retro-Diels-Alder reactions of furan/maleimide adducts. Four adducts, presenting an endo or exo configuration and proximal or distal geometry, were activated in solution by ultrasound-generated elongational forces. A combination of structural (1H NMR) and computational (CoGEF) analyses allowed us to interrogate the mechanochemical activation of these adducts. We found that, unlike its thermal counterpart where the reactivity is dictated by the stereochemistry, the mechanical reactivity is mainly dependent on the regiochemistry. Remarkably, the thermally active distal-exo adduct becomes inert under tension due to poor mechanochemical coupling.