Rotaxane or Pseudorotaxane? Effects of Small Structural Variations on the Deslipping Kinetics of Rotaxanes with Stopper Groups of Intermediate Size

摘要

Three series of rotaxanes have been synthesized variously by slipping synthesis, in which axis and wheel are melted in admixture, by recognition of amide groups inside the macro-cyclic wheel, or by an anionic template method, in which the stoppering phenotates are hydrogen bonded to the wheel and then joined by reaction with a semi-axle. The 3,5-di-tert-butylphenyl stopper used for most of these rotaxanes is large enough to permit their isolation, but still allows the wheel to deslip from the axle under appropriate conditions. The de-slipping activation parameters for all rotaxanes are derived from ~1 H NMR kinetic measurements and have been evalu-ated from the Arrhenius equation as well as according to Eyr-ing theory. Small structural variations give rise to surprising effects on the activation parameters. Firstly, in some ex-amples, the axle length affects the deslipping barrier, al-though the size complementarity of stopper and wheel re-main unchanged. Secondly, stopper flexibility has an import-ant influence on the deslipping rate. Thirdly, exchange of a carbonamide for a sulfonamide in the wheel significantly re-duces the entropic costs of the deslipping, resulting in a pro-nounced deslipping rate enhancement. Fourthly, intramolec-ular hydrogen bonding within the wheel decelerates deslip-ping by a factor of more than 1O~4.