Mutual Induced Fit Transition Structure Stabilization of Corannulene's Bowl-to-Bowl Inversion in a Perylene Bisimide Cyclophane

Additional data to report https://doi.org/10.1039/D3SC05341E:

Corannulene is known to undergo a fast bowl-to-bowl inversion at r.t. via a planar transition structure (TS). Herein we present the catalysis of this process within a perylene bisimide (PBI) cyclophane composed of chirally twisted, non-planar chromophores, linked by para-xylylene spacers. Variable temperature NMR studies reveal that the bowl-to-bowl inversion is significantly accelerated within the cyclophane template despite the structural non-complementarity between the binding site of the host and the TS of the guest. The observed acceleration corresponds to a decrease in the bowl-to-bowl inversion barrier of 11.6 kJ mol−1 compared to the uncatalyzed process. Comparative binding studies for corannulene (20 π-electrons) and other planar polycyclic aromatic hydrocarbons (PAHs) with 14 to 24 π-electrons were applied to rationalize this barrier reduction. They revealed high binding constants that reach, in tetrachloromethane as a solvent, the picomolar range for the largest guest coronene. Computational models corroborate these experimental results and suggest that both TS stabilization and ground state destabilization contribute to the observed catalytic effect. Hereby, we find a “mutual induced fit” between host and guest in the TS complex, such that mutual geometric adaptation of the energetically favored planar TS and curved π-systems of the host results in an unprecedented non-planar TS of corannulene. Concomitant partial planarization of the PBI units optimizes noncovalent TS stabilization by π–π stacking interactions. This observation of a “mutual induced fit” in the TS of a host–guest complex was further validated experimentally by single crystal X-ray analysis of a host–guest complex with coronene as a qualitative transition state analogue.

补充数据详见：https://doi.org/10.1039/D3SC05341E：
已知碗烯（corannulene）在室温下可通过平面过渡态（transition structure, TS）发生快速的碗-碗翻转过程。本文报道了苝二酰亚胺（perylene bisimide, PBI）环蕃对该过程的催化作用；该环蕃由手性扭曲的非平面发色团构成，并通过对二甲苯间隔基连接。变温核磁共振（variable temperature NMR）研究表明，尽管主体的结合位点与客体过渡态的结构并不互补，但环蕃模板内部的碗-碗翻转过程被显著加速。与未催化的过程相比，观测到的加速效应对应碗-碗翻转能垒降低了11.6 kJ·mol⁻¹。我们通过对碗烯（含20个π电子）以及其他14至24个π电子的平面多环芳烃（polycyclic aromatic hydrocarbons, PAHs）开展对比结合研究，对该能垒降低现象进行了合理化阐释。研究结果显示结合常数极高，以四氯化碳为溶剂时，最大客体分子晕苯（coronene）的结合常数可达皮摩尔级。计算模型验证了上述实验结果，并表明过渡态稳定化与基态去稳定化共同促成了观测到的催化效应。据此，我们在过渡态复合物中发现了主体与客体之间的“相互诱导契合”现象：能量上更稳定的平面过渡态与主体的弯曲π体系发生相互几何适配，使得碗烯形成了前所未有的非平面过渡态。与此同时，苝二酰亚胺单元发生部分平面化，通过π-π堆积相互作用优化了对过渡态的非共价稳定作用。以晕苯作为定性过渡态类似物的主客体复合物单晶X射线衍射分析，进一步从实验上验证了主客体复合物过渡态中“相互诱导契合”的这一观测结果。