Examining the Ways To Bend and Break Reaction Pathways Using Mechanochemistry

Abstract

Mechanical forces can lead to qualitative changes in reaction mechanism, which depend on the specific mode of force induction and inherent chemistries of the mechanophore. To demonstrate these effects at an atomistic level, three challenging mechanochemical transformations of recent interest are herein studied: spiropyran ring opening, flex-activated small molecule release, and cyclopropane ring opening. These examples show that mechanical load can eliminate intermediates and transition states along a reaction path, preactivate reactions where the force is not parallel to the reaction path, and modulate force conveyance to the mechanophore in a polymerdependent fashion. Two different merocyanin products were identi-fiedcis and trans isomersfrom the spiropyran ring-opening reaction, and tuning the magnitude of the force allows for selection of one over the other. The reaction involving oxanorbornadiene where bonds perpendicular to the force are flexed through angular distortions has the interesting property that while the central bond contracts, the overall molecule is lengthened to effect activation. Cyclopropane ring openings demonstrate that the larger the extension of the mechanophore, determined by the rigidity of the polymer backbone under applied force, the larger the change in activation barrier. In this case, the application of a force represents a preactivation mechanism where the initial structure rises in energy, while the transition state stays relatively constant. For these three motifs, the reaction paths are readily uncovered using the growing string method, showing it to be a widely useful tool for studying mechanochemistry.

Validation of the CoGEF Method as a Predictive Tool for Polymer Mechanochemistry

“…Other computational studies have suggested that this reaction manifold is less sensitive to external mechanical perturbation and that a significant thermal component is still required for activation under relatively large forces. 20 Poor orientational alignment between the scissile bonds and the direction of applied force along the reaction coordinate results in weak mechanochemical coupling in these systems, and mechanical force alone is insufficient for activation. The formal retro-Diels−Alder reaction of phenyltriazolinedione−anthracene adduct 34 was also investigated experimentally in cross-linked elastomers under tension where mechanical activation is expected to proceed via force-induced planarization.…”

Polymer mechanochemistry-enabled pericyclic reactions

“…Roessler and Zimmerman used the growing string method (GSM) without using the θ value as a constraint coordinate and showed that the reaction mechanism for the flex activation depended on the applied force. 95 The activation energy decreased mostly linearly with increasing force and was approximately 24 kcal mol −1 if 4 nN of force were applied relative to the ground state reaction. Transition states calculated with different applied forces showed unique θ values, indicating that the reaction followed a multiparameter pathway and force influenced only one reaction coordinate.…”

“…Roessler and Zimmerman previously used GSM computations to locate reaction paths and products for SP1. 95 In their study, they identified two merocyanine products, the Z-and E-isomer (Scheme 34). At forces <2 nN the Z-product was found, but at higher forces the ring-open structure isomerized to the E-structure without an energy barrier.…”

Mechanochemically Gated Photoswitching: Expanding the Scope of Polymer Mechanochromism

“…26,27 Computational studies have established that the unique reactivity of mechanophores originates from a distortion of the potential energy surface under large forces, which fundamentally changes the reaction landscape and leads to different reaction pathways. [28][29][30]…”