Mechanochemistry of Cubane
Liqi Wang, Xujun Zheng, Tatiana B. Kouznetsova, Tiffany Yen, Tetsu Ouchi, Cameron L. Brown, Stephen L. Craig
Abstract
We report the mechanochemical reactivity of the highly strained pentacyclic hydrocarbon cubane. The mechanical reactivity of cubane is explored for three regioisomers with 1,2-, 1,3-, and 1,4-substituted pulling attachments. Whereas all compounds can be activated thermally, mechanical activation is observed via pulsed ultrasonication of cubane-containing polymers only when force is applied via 1,2-attachment. The single observed product of the force-coupled reaction is a thermally inaccessible syn-tricyclooctadiene, in contrast to cyclooctatetraene (observed thermally) or a pair of cyclobutadienes that would result from sequential cyclobutane scission. We further quantify the mechanochemical reactivity of cubane by single molecule force spectroscopy, and force-coupled rate constants for ring opening reach ∼33 s–1 at a force of ∼1.55 nN, lower than forces of 1.8–2.0 nN that are typical of conventional cyclobutanes.