Conservation of orbital symmetry can be circumvented in mechanically induced reactions

In first-principles molecular dynamics simulations of the mechanically induced ring-opening of substituted benzocyclobutene we observe both con- and disrotatory ring-opening reactions. We show that this finding does not contradict the fundamental principle that the orbitals develop continuously in time. However, it constitutes an exception from the principle of the conservation of orbital symmetry and thus is indeed an exception from the Woodward-Hoffmann rules. In contrast, the ring-opening of unsubstituted cyclobutene proceeds in a conrotatory fashion. This shows that the breaking of the Woodward-Hoffmann rules is significantly facilitated by the substituents.