Stereochemical Inversion of Rim-Differentiated Pillar[5]arene Molecular Swings

To
investigate the dynamic stereochemical inversion behavior of
pillar[5]­arenes (P[5]­s) in more detail, we synthesized a series of
novel rim-differentiated P[5]­s with various substituents and examined
their rapid rotations by variable-temperature NMR (203–298
K). These studies revealed for the first time the barrier of “methyl-through-the-annulus”
rotation (ΔG‡ = 47.4 kJ·mol–1 in acetone) and indicated that for rim-differentiated
P[5]­s with two types of alkyl substituents, the smaller rim typically
determines the rate of rotation. However, substituents with terminal
CC or CC bonds give rise to lower inversion barriers,
presumably as a result of attractive π–π interactions
in the transition state. Finally, data on a rim-differentiated penta-methyl-penta-propargyl
P[5] exhibited the complexity of the overall inversion dynamics.

为更细致地探究柱[5]芳烃（pillar[5]arenes，P[5]s）的动态立体化学翻转行为，我们合成了一系列带有多样取代基的新型边缘差异化柱[5]芳烃，并通过变温核磁共振（variable-temperature NMR，203~298 K）对其快速旋转过程进行了考察。本研究首次揭示了“甲基穿环”旋转的活化能垒（丙酮溶剂中ΔG‡ = 47.4 kJ·mol–1），同时指出：对于带有两类烷基取代基的边缘差异化柱[5]芳烃，体积更小的边缘通常决定旋转速率。而带有末端碳碳双键（C=C）或碳碳三键（C≡C）的取代基则会降低翻转能垒，这一现象大概率源于过渡态中存在的吸引力π-π相互作用。最后，针对一例边缘五甲基-五炔丙基差异化柱[5]芳烃的研究数据，展现了整体翻转动力学的复杂特性。