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.