Stereochemical inversion of rim-differentiated pillar[5]arene molecular swings

Abstract

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 CC or CC bonds give rise to lower inversion barriers, presumably as a result of attractive π-πinteractions in the transition state. Finally, data on a rim-differentiated penta-methyl-penta-propargyl P[5] exhibited the complexity of the overall inversion dynamics.