Mechanochemistry of Cyclopropane Ring-Opening Reactions

Since the end of the 1960th, the Woodward–Hoffmann rules have been a well-grounded and powerful tool to understand and predict pericyclic reactions. Recently, astonishing results on such reactions subject to mechanochemical activation by external forces have revealed reaction pathways at sufficiently large forces which are not expected from the Woodward–Hoffmann rules. These findings have started a controversy whether the “Woodward–Hoffmann rules are broken in mechanochemistry”. Our study of ring opening of cyclopropane shows 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. It is rather the mechanical work that lowers the activation barrier and therefore promotes reaction pathways to products not expected from the Woodward–Hoffmann rules. A front cover and an additional cover profile article have been devoted to these findings (Wollenhaupt et al., Chem Phys Chem 16:1593–1597, 2015)