Bicyclo[2.2.0]hexene (BCH): A Multimodal Mechanophore with Force-Governed Chemoselectivity

The identity of reactive sites in a molecule and the selectivity between them in chemical transformations depends on applied reagents and external drives. Polymer mechanochemistry has been an enabling tool in accessing chemical reactivity and reaction pathways that are distinctive from the thermal counterparts. However, mechanochemical selectivity, i.e., activation of multiple unique reaction pathways from the same mechanophore, remains elusive. Here, we report the design of bicyclo[2.2.0]hexene (BCH) as a multimodal mechanophore to leverage its structural simplicity and relatively low molecular symmetry to demonstrate the idea of force-governed chemoselectivity. Upon changing the attachment points of pendant polymer chains, different C–C bonds in bicyclo[2.2.0]hexene are specifically activated via sonication by externally applied force. Extensive experimental characterization confirms that each mode of activation results in a unique reaction, entailing retro-[2+2] cycloreversion, 1,3-allylic migration, and 4π-electrocyclic ring-opening reactions, respectively. Control experiments with small-molecule analogues reveal that observed chemoselectivity of BCH regioisomers is only possible with mechanical force. Theoretical studies further elucidate that the changes of substitution have minimal impact on the potential energy surface of parent BCH. The mechanochemical bond-specific activation is a result of selective and effective coupling of force to the targeted C–C bonds in each mode.