Force‐Induced Catastrophes on Energy Landscapes: Mechanochemical Manipulation of Downhill and Uphill Bifurcations Explains the Ring‐Opening Selectivity of Cyclopropanes

Abstract

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.

Three concomitant C–C dissociation pathways during the mechanical activation of an N-heterocyclic carbene precursor

“…The product distribution is likely the result of a post-transition-state bifurcation on the force-modified potential energy surface. This phenomenon has been recently observed in other mechanochemical transformations, 33,34 and might be more common than anticipated. Interestingly, the shape of the force-modified PES can be altered to favour a particular dissociation pathway by tuning the electronic properties of the mechanophore.…”

“…6d). The emergence of a post-TS bifurcation in a force-modified PES has been recently observed in other mechanophores, 33,34 and could be a common phenomenon in mechanochemical transformations. Since the minor homolytic pathway is not observed when the importance of the heterolytic pathway decreases (Fig.…”

Mechanochemical generation of acid-degradable poly(enol ether)s

“…Interestingly, other studies of cyclobutane mechanochemical cycloreversion by Craig 28 and Wang 15 groups reported higher fractions of stereochemical inversion. The stereochemical distribution of the mechanochemically generated diene can be ascribed to the instability of the diradical transition state upon the first bond scission and dynamic effect, 29,30 and warrants comprehensive computational studies. 22% and 28% mechanophore activation per scission cycle was achieved for P1-H and P2-H , respectively (Fig.…”

Polymer mechanochemistry-enabled pericyclic reactions

“…They further showed the impact of halogen substitution on the thermally activated ring-opening process followed by halogen migration of (2S,3S)-1,1-dihalo-2,3-dimethylcyclopropanes in the case of F, Cl, Br and I distribution. 141 Surprisingly, the direction of halogen migration in the case of disrotatory ring-opening was qualitatively different for the difluorinated cyclopropane compared with the homologous Cl, Br and I species. For gDFC E-isomer, the decision was governed by an uphill bifurcation followed by two separate transition states for each migration direction, whereas the disrotatory transition state was first surmounted in the other three cases followed by a downhill bifurcation into the disrotatory products.…”