Orientational isomers of α-cyclodextrin [2]semi-rotaxanes with asymmetric dicationic threads

Abstract

Two series of novel dicationic threading molecules [Quin(CH₂)₁₀R]²⁺ and [3,5-Lut(CH₂)₁₀R]²⁺, where Quin⁺ = quinuclidinium, 3,5-Lut⁺ = 3,5-lutidinium, and R⁺ = N(CH₃)₃⁺ and N(CH₃)₂CH₂CH₃⁺, form [2]semi-rotaxanes with α-cyclodextrin (α-CD) in aqueous solution. The quinuclidinium and 3,5-lutidinium are sufficiently bulky to prevent threading while the R⁺ groups allow for slow threading by α-CD at 25 °C. The resulting [2]semi-rotaxanes exist in two orientational isomers owing to the asymmetry of both the α-CD cavity and the threading molecules. Two-dimensional ¹H NMR spectroscopy and kinetics experiments reveal that the isomer in which the narrower rim (primary OHs) is positioned near the R⁺ group is the kinetically preferred isomer, while the other isomer is the thermodynamically preferred product. The kinetics and mechanism of the formation, dissociation, and interconversion of the two isomers have been determined at 25 °C.