Polymer mechanochemistry has been established as an enabling tool in accessing chemical reactivity and reaction pathways that are distinctive from their thermal counterparts. However, eliciting diversified reaction pathways by activating different constituent chemical bonds from the same mechanophore structure remains challenging. Here, we report the design of a bicyclo[2.2.0]hexene (BCH) mechanophore to leverage its structural simplicity and relatively low molecular symmetry to demonstrate this idea of multimodal activation. Upon changing the attachment points of pendant polymer chains, three different C–C bonds in bicyclo[2.2.0]hexene are specifically activated via externally applied force by sonication. Experimental characterization confirms that in different scenarios of polymer attachment, the regioisomers of BCH undergo different activation reactions, entailing retro-[2+2] cycloreversion, 1,3-allylic migration, and retro-4π ring-opening reactions, respectively. Control experiments with small-molecule analogues reveal that the observed diversified reactivity of BCH regioisomers is possible only with mechanical force. Theoretical studies further elucidate that the differences in the positions of substitution between regioisomers have a minimal impact on the potential energy surface of the parent BCH scaffold. The mechanochemical selectivity between different C–C bonds in each constitutional isomer is a result of selective and effective coupling of force to the aligned C–C bond in each case.

中文翻译：

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双环[2.2.0]己烯：一种多环机械载体，其反应性因外力而多样化

聚合物机械化学已被确立为一种获得化学反应性和反应途径的有利工具，这些化学反应性和反应途径与热反应途径不同。然而，通过激活同一机械载体结构中不同的化学键来引发多样化的反应途径仍然具有挑战性。在这里，我们报告了双环[2.2.0]己烯（BCH）机械载体的设计，利用其结构简单性和相对较低的分子对称性来证明这种多模式激活的想法。当改变聚合物侧链的连接点时，双环[2.2.0]己烯中的三个不同的C-C键通过超声处理的外力被特异性激活。实验表征证实，在不同的聚合物连接情况下，BCH的区域异构体经历不同的活化反应，分别需要逆[2+2]环化反应、1,3-烯丙基迁移和逆4π开环反应。小分子类似物的对照实验表明，观察到的 BCH 区域异构体的多样化反应性只有通过机械力才有可能实现。理论研究进一步阐明，区域异构体之间取代位置的差异对母体 BCH 支架的势能表面影响最小。每种结构异构体中不同 C-C 键之间的机械化学选择性是在每种情况下将力选择性且有效地耦合到对齐的 C-C 键的结果。