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Stereochemical Inversion of Rim-Differentiated Pillar[5]arene Molecular Swings
Ke Du1, Paul Demay-Drouhard1,2, Kushal Samanta1,2
1Institute for Molecular Design and Synthesis, School of Pharmaceutical Science & Technology, Tianjin University, 92 Weijin Road, Nankai District, Tianjin 300072, P. R. China.
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 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.
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