The Tension Activated Carbon–Carbon Bond

Abstract

Over the last century, chemists have mastered the ability to precisely connect pairs of carbon atoms for the synthesis of complex structures ranging from pharmaceuticals to polymeric materials. Less attention has been given to precision C–C bond disconnection. In the past two decades, mechanical force has emerged as a unique vectoral stimulus to drive selective and productive C–C bond activations, leading to distinct reaction trajectories, as well as unprecedented mechanoresponsive materials. However, the molecular details of force to chemical transduction are poorly captured by conventional chemical intuition, making it challenging to understand and predict structure-reactivity relationships under tension. Here, we utilize a physical organic model inspired by the classical Morse potential and its differential forms to identify the effective force constant (keff) and the force-free reaction energy (ΔE) as key molecular features that govern mechanochemical kinetics. Through a comprehensive experimental and computational investigation with four norborn-2-en-7-one (NEO) mechanophores, we establish the relationship between these features and the force-dependent energetic changes along the reaction pathways. We found a linear model accurately predicts the transition force (f*) required for C–C bond activation in over 30 mechanophores. These results demonstrate a general mechanistic framework for mechanochemical reactions under tensile force, and provide a highly accessible tool for the large-scale computational screening in the design of mechanophores.

Polymer mechanochemistry: from single molecule to bulk material

“…7b). 97 The EFEI method could yield a more exact distortion of the molecular structure, which is in agreement with experiment results.…”

Mechanochemistry of Pterodactylane

“…We calculated the spring constant of the central rung of pterodactylane to be 11.0 nN/Å (Figure S25) and found it to be among the lowest for cyclobutane-based mechanophores studied in developing the TMBA model, which ranges from 7.0 to 44.3 nN/Å. Only a cyclobutane with flanking phenyl groups has a lower value of 7.0 nN/ Å. , We also calculated the change in force-free potential energy upon conversion of 5 to 6 to be 26.1 kcal/mol. This was also among the lowest for homolytic C–C bond scission events evaluated by the TMBA model, which ranged from 17.2 to 67.8 kcal/mol.…”

“…An intriguing comparison is between ladderane being pulled at the central or end rung and another strained compound with multiple fused cyclobutanes, cubane. The end rung of [4]-ladderane has a calculated potential energy of 55.3 kcal/mol for bond cleavage and a spring constant of 20.9 nN/ Å, both of which are significantly greater than those for the central rung, reflecting the diminished steric crowding around the end rung and decreased radical stability upon its cleavage. The threshold force for mechanoactivation of 1,2-cubane was recently reported to be 1.55 nN under the same SMFS conditions, still significantly higher than that for pterodactylane .…”

“…Mechanophores with low activation threshold forces are highly desired for the development of materials with a greater force response . To rationalize the low force required for the reactivity of pterodactylane within concepts of the physical organic canon, we turned to the recently proposed tension model of bond activation (TMBA) . This model deploys two key parameters that together are highly predictive of threshold force: the Hookean spring constant of the first broken bond and the potential energy difference between the reactant and the diradical intermediate.…”

Solvent Polarity Effects on the Mechanochemistry of Spiropyran Ring Opening

“…The connection highlights continuing oppor-tunities to connect covalent polymer mechanochemistry and classic physical organic principles of reactivity. 74…”