Rotaxane or pseudorotaxane? Effects of small structural variations on the deslipping kinetics of rotaxanes with stopper groups of intermediate size

Three series of rotaxanes have been synthesized variously by slipping synth esis, in which axis and wheel are melted in admixture, by recognition of am ide groups inside the macrocyclic wheel, or by an anionic template method, in which the stoppering phenolates are hydrogen bonded to the wheel and the n 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 condi tions. The deslipping activation parameters for all rotaxanes are derived f rom H-1 NMR kinetic measurements and have been evaluated from the Arrhenius equation as well as according to Eyring theory. Small structural variation s give rise to surprising effects on the activation parameters. Firstly, in some examples, the axle length affects the deslipping barrier, although th e size complementarity of stopper and wheel remain unchanged. Secondly, sto pper flexibility has an important 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 pronounced desli pping rate enhancement. Fourthly, intramolecular hydrogen bonding within th e wheel decelerates deslipping by a factor of more than 10(4).