Acceleration of Diels-Alder reactions by mechanical distortion

Challenges in quantifying how force affects bond formation have hindered the widespread adoption of mechanochemistry. We used parallel tip-based methods to determine reaction rates, activation energies, and activation volumes of force-accelerated [4+2] Diels-Alder cycloadditions between surface-immobilized anthracene and four dienophiles that differ in electronic and steric demand. The rate dependences on pressure were unexpectedly strong, and substantial differences were observed between the dienophiles. Multiscale modeling demonstrated that in proximity to a surface, mechanochemical trajectories ensued that were distinct from those observed solvothermally or under hydrostatic pressure. These results provide a framework for anticipating how experimental geometry, molecular confinement, and directed force contribute to mechanochemical kinetics. Description Editor’s summary Using ball mills to conduct chemical reactions in solid state has the potential to eliminate vast quantities of solvent waste. However, the energetics of these reactions are much harder to discern and optimize than reactions in the solution phase. Zholdassov et al. used a modified atomic force microscope to study in a more precise, controlled fashion how the forces squeezing molecules together in a ball mill might influence the Diels-Alder cycloaddition reaction (see the Perspective by Weiss). The authors observed much greater acceleration through mechanical distortion than hydrostatic pressure would deliver. —Jake Yeston A modified atomic force microscope is used to probe activation effects that may accelerate reactions in a ball mill.