Size complementarity of macrocyclic cavities and stoppers in amide-rotaxanes

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). Steri cally 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 conne cted to amide axles 10a-c which were stoppered with blocking groups of diff erent spatial demand (Scheme 1). The deslipping kinetics of the resulting r otaxanes 8a-c and 9a,b were measured and compared; it emerges that even sli ght increases in the wheel size require larger stoppers to stabilize the me chanical bond. Moreover, when the deslipping rate of 8a (amide wheel and am ide axle) was determined in either DMF or THF, a strong dependence on the s olvent polarity, which is caused by a differing extent of intramolecular H- bonds between the wheel and the axle, was observed. As expected, no such de pendence was detected for rotaxane 12a (amide wheel and hydrocarbon axle) w hose components cannot interact via ii-bonds. The comparison of the sterica lly matching pairs of macrocycles and blocking groups, found by a systemati c fitting based on the results of slipping and deslipping experiments, with other rotaxane types bearing similar stoppers allows conclusions concernin g the relative cavity size of wheels of various structure.