Should the Woodward-Hoffmann Rules be Applied to Mechanochemical Reactions?

Since decades, pericyclic reactions have been well-understood by means of the Woodward-Hoffmann rules and their classification as thermally or photochemically allowed or forbidden. Recently, stunning results on such reactions subject to mechanochemical activation by external forces instead of heat or light have revealed reaction pathways at sufficiently large forces, which are not expected from the Woodward-Hoffmann rules. This led to the much reiterated idea that the Woodward-Hoffmann rules are broken in mechanochemistry. Here, by studying ring-opening of cyclopropane, we show that the electronic structure underlying the dis- and conrotatory pathways, which are greatly distorted upon applying forces to an extent that eventually the thermally forbidden process becomes mechanochemically allowed, does not change along both pathways. It is rather the mechanical work that lowers the activation barrier of the thermally forbidden conrotatory process relative to the disrotatory one at large forces.