  The mechanochemistry of ring-opening reactions of cyclopropane derivatives
turns out to be unexpectedly rich and puzzling. After showing that a rare
so-called uphill bifurcation in the case of trans-gem-difluorocyclopropane
turns into a downhill bifurcation upon substitution of fluorine by chlorine,
bromine and iodine in the thermal activation limit, the dichloro derivative is
studied systematically in the realm of mechanochemical activation. Detailed
exploration of the force-transformed potential energy surface of
trans-gem-dichlorocyclopropane in terms of Dijkstra path analysis unveils a
hitherto unknown topological catastrophe where the global shape of the energy
landscape is fundamentally changed. From thermal activation up to moderately
large forces, it is an uphill bifurcation that decides about dis-versus
conrotatory ring-opening followed by separate transition states along both
pathways. Above a critical force, the two distinct transition states merge to
yield a single transition state such that the decision about the dis-versus
conrotatory ring-opening process is taken at a newly established downhill
bifurcation. The discovery of a force-induced qualitative change of the
topology of a reaction network vastly transcends the previous understanding of
the ring-opening reaction of this species. It would be astonishing to not
discover a wealth of such catastrophes for mechanochemically activated
reactions which will greatly extend the known opportunities to manipulate
chemical reaction networks.
