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 macrocyclic wheel, or by an anionic template method, in which the stoppering phenolates 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 deslipping activation parameters for all rotaxanes are derived from 1H NMR kinetic measurements and have been evaluated from the Arrhenius equation as well as according to Eyring theory. Small structural variations give rise to surprising effects on the activation parameters. Firstly, in some examples, the axle length affects the deslipping barrier, although the size complementarity of stopper and wheel remain unchanged. Secondly, stopper flexibility has an important influence on the deslipping rate. Thirdly, exchange of a carbonamide for a sulfonamide in the wheel significantly reduces the entropic costs of the deslipping, resulting in a pronounced deslipping rate enhancement. Fourthly, intramolecular hydrogen bonding within the wheel decelerates deslipping by a factor of more than 104.

New [2]rotaxanes were prepared by the threading and the slipping procedure, the latter having the advantage of not needing templating interactions. As a consequence, the first [2]rotaxane consisting of a tetraamide macrocycle and a pure hydrocarbon thread was synthesized (see 12a in Scheme 2). Sterically matching wheels and axles being the basic requirement of a successful slipping approach to rotaxanes, mono‐ and bishomologous wheels 5b,c with larger diameters than the parent 5a were synthesized and mechanically connected to amide axles 10a – c which were stoppered with blocking groups of different spatial demand (Scheme 1). The deslipping kinetics of the resulting rotaxanes 8a – c and 9a,b were measured and compared; it emerges that even slight increases in the wheel size require larger stoppers to stabilize the mechanical bond. Moreover, when the deslipping rate of 8a (amide wheel and amide axle) was determined in either DMF or THF, a strong dependence on the solvent polarity, which is caused by a differing extent of intramolecular H‐bonds between the wheel and the axle, was observed. As expected, no such dependence was detected for rotaxane 12a (amide wheel and hydrocarbon axle) whose components cannot interact via H‐bonds. The comparison of the sterically matching pairs of macrocycles and blocking groups, found by a systematic fitting based on the results of slipping and deslipping experiments, with other rotaxane types bearing similar stoppers allows conclusions concerning the relative cavity size of wheels of various structure.

Abstract

organic chemistry, review organic chemistry, review Z 0200 52 -278 Catenanes, Rotaxanes, and Pretzelanes-Template Synthesis and Chirality -[13 refs.]. -(VOGTLE, F.; SAFAROWSKY, O.; HEIM, C.; AFFELD, A.; BRAUN, O.; MOHRY, A.; Pure Appl. Chem. 71 (1999) 2, 247-251; Kekule-Inst. Org. Chem. Biochem., Rhein.

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