Towards Controlling the Threading Direction of a Calix[6]arene Wheel by Using Nonsymmetric Axles

Traffic control: By exploiting the interplay of kinetic and thermodynamic effects, the direction of threading/dethreading in a nonsymmetric calixarene wheel can be selected by an appropriate choice of the head group incorporated in the molecular axle (see figure). The possibility of obtaining full control on the direction of axle threading in calix[6]arene wheel 1 either from its upper or lower rim was evaluated in solution. To this aim, we prepared nonsymmetric axles characterised by a 4,4′-bipyridinium recognition unit with two alkyl side chains, one of which terminates with a stopper, and the other with either ammonium (2), hydroxy (3) or methyl (4 and 5) head groups. When the axles were mixed with 1 in apolar solvents at room temperature, the formation of oriented pseudorotaxanes derived from the threading of the axles from the upper rim was observed. The stability constants of such complexes are in the order of 107 M−1 and are almost independent of the type of axle. A detailed thermodynamic and kinetic study revealed that stability constants and activation parameters for complex formation between 1 and axles 2 and 3 are of the same order of magnitude, suggesting a common threading process. However, upon heating a solution of 1 and 2 in benzene at 340 K, the formation of another supramolecular complex was observed, the structure of which is consistent with an oriented pseudorotaxane derived from the threading of axle2 from the lower rim of the calixarene wheel. By carrying out the threading–stoppering reaction sequence between 1 and 2 in the presence of an excess of diphenylacetyl chloride, the orientational rotaxane isomers R1 and R2, derived from lower- and upper-rim threading, respectively, were collected in about a ratio of 7:3 as the unique chromatographic fraction. Our results suggest that at room temperature the threading process is under kinetic control for all axles. On increasing the temperature only the threading behaviour of axle 2 is substantially modified, most likely because the process becomes thermodynamically controlled owing to the peculiar recognition properties of the ammonium head of this axle.