Mechanical Manipulation of Chemical Reactions: Reactivity Switching of Bergman Cyclizations.

Photoswitches incorporated into molecular frameworks have been used since a long time to trigger chemical processes on demand. Here, it is shown how mechanophores can be used as switches in order to drastically change the reactivity of a neighboring functional group as a function of external stress. The reactivities of cyclic enediynes, which are highly toxic agents when undergoing Bergman cyclization, roughly correlate with the distance between the bond-forming carbons in many cases. It is demonstrated how this distance, and thus enediyne reactivity, can be tuned upon applying mechanical stress. Depending on suitable substitution patterns, chemically inert species can be turned into highly reactive ones and vice versa, thus extending the concept of photoswitching to mechanoswitching. Moreover, depending on the derivative, it is found that C1-C5 cyclization becomes energetically preferred over the Bergman (C1-C6) pathway at nano-Newton forces, thus leading to a force-induced switch in selectivity in such cases.