Kinetic versus thermodynamic control during the formation of rotaxanes by a dynamic template-directed clipping process
A template-directed dynamic clipping procedure has generated a library of nine rotaxanes that have been formed from three dialkylammonium salts - acting as the dumbbell-shaped components-and three dynamic, imino bond-containing, crown-8-like macrocycles - acting as the ring-shaped components - which are themselves assembled from three dialdehydes and one diamine. The rates of formation of these rotaxanes differ dramatically, from minutes to days depending on the choice of dialkylammonium ion and dialdehyde, as do their thermodynamic stabilities. Generally, rotaxanes formed by using 2,6-diformylpyridine as the dialdehyde component, or bis(3,5-bis(trifluoromethyl)benzyl)ammonium hexafluorophosphate as the dumbbell-shaped component, assembled the most rapidly. Those rotaxanes containing this particular electron-deficient dumbbell-shaped unit, or 2,5-diformylfuran units in the macroring, were the most stable thermodynamically. The relative thermodynamic stabili ties of all nine of the rotaxanes were determined by competition experiments that were monitored by 1H NMR spectroscopy.