Covalent Post-assembly Modification Triggers Structural Transformations of Borromean Rings

A series
of supramolecular transformation cascades are presented
here, employing ligand exchange reactions, concentration-dependent
Borromean ring (BR) linking and unlinking, and inverse electron-demand
Diels–Alder (IEDDA) reactions. The new family of template-free,
tetrazine-edged BRs are constructed by using ligand exchange reactions,
and their concentration-dependent linking and unlinking reactions
have been observed. Moreover, Borromean precursors have been demonstrated
to further undergo supramolecular structural transformations induced
by rapid and efficient IEDDA reactions to afford corresponding post-assembly
modified architectures. Remarkably, subtle steric increases of the
pyridazine fragments obtained by IEDDA reactions is regarded to induce
controlled topological transformations in the cascade, unlinking the
Borromean structures by using electron-rich dienophiles as triggering
signals.

本文报道了一系列超分子转化级联反应，所采用的核心策略包括配体交换反应、浓度依赖型博罗米恩环（Borromean Ring, BR）的连接与解连过程，以及反电子需求狄尔斯-阿尔德（inverse electron-demand Diels–Alder, IEDDA）反应。本研究通过配体交换反应构建了一类新型无模板、带有四嗪边缘修饰的博罗米恩环家族，且观测到其具备浓度依赖型的连接与解连行为。此外，实验证实博罗米恩环前驱体可进一步经快速高效的反电子需求狄尔斯-阿尔德反应诱导发生超分子结构转化，得到相应的组装后修饰架构。值得注意的是，经反电子需求狄尔斯-阿尔德反应生成的哒嗪片段所产生的微小空间位阻提升，被认为可诱导级联反应中的可控拓扑结构转变，以富电子亲双烯体作为触发信号实现博罗米恩结构的解连。