The concept is logical and easy to follow--if reactions can be triggered by light, heat, pressure, or electrical potential, then why can't mechanical force also be harnessed to distort molecules in a way that promotes reaction? Actually, using molecules appropriately termed mechanophores, the Moore group has succeeded in employing the mechanical forces generated from ultrasound to promote and influence chemical reaction pathways. Although ultrasound generally has no effect on small molecules, the collapse of cavitation bubbles produced during sonication can agitate polymers in solution, generating friction, otherwise known as mechanical force. By incorporating small molecule mechanophores (either trans- or cis-1,2- dimethoxybenzocyclobutenes, BCBs) into a larger polymer, researchers were able to take advantage of this force and promote ring opening. According to the Woodward-Hoffmann rules, in a reaction promoted through light energy, both cis- and trans- BCBs undergo a disrotatory ring opening, while thermal activation produces conrotatory products. On the other hand, computational studies indicated that under mechanical influences, the cis-BCB would produce the disrotatory product, but the trans-BSB would generate the conrotatory product.
In order to test this hypothesis, the BCB-containing polymer was sonicated at 6-9 degrees C with an excess of a pyrene functionalized maleimide, which was meant to function as a a dienophile trap. Indeed, the mechanophore did behave as predicted; by distorting bond lengths and angles sonication produced the trans-BCB through a conrotatory process, while the mechanical force worked to reduce the energy barrier for a disrotatory reaction pathway for the cis-BCB. As the article title indicates, reactions can certainly be biased through the use of mechanical force.
So what will the next mechanophore be?