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 (Delta G(double dagger) = 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 pi-pi interactions in the transition state. Finally, data on a rim-differentiated pentamethyl-penta-propargyl P[5] exhibited the complexity of the overall inversion dynamics.