Unidirectional threading of triphenylureidocalix[6]arene-based wheels: Oriented pseudorotaxane synthesis

Triphenylureidocalix[6]arenes 5 a,b are heteroditopic receptors having a pinched cone structure able to interact with both the cation and the anion of ion pairs. They are able to act as wheels and form complexes of the pseudorotaxane type with axles derived from dialkylviologen salts. An investigation into the possibility of exploiting the different structural and chemical information present on the two distinct rims of the calixarene wheel as control elements to pivot the direction of the axle threading processes and give access to oriented pseudorotaxanes is reported. It was verified that, in C6D6, an asymmetric dicationic axle derived from 4,4′-bipyridil bearing two alkyl chains, one of which has a stopper, and triphenylureidocalix[6]arenes 5 a or 5 b form 1:1 supramolecular complexes belonging to the class of pseudorotaxanes. The structure of these complexes has been inferred through 1H NMR techniques. The data show that the axle accesses the calixarene cavity only through the wider rim. To further verify this issue, the new rotaxane 8, obtained by stoppering the pseudorotaxane derived from 5 b and the symmetrical axle 7 with diphenylacetyl chloride, was synthesised. In the 1H NMR spectrum of 8, the aliphatic protons of the axle portion that resides at the wide rim of the wheel show chemical shifts that are almost identical to those observed in pseudorotaxanes 6. On the other hand, those that stick out of the narrow rim of 8 experience chemical shifts that could not be found in the oriented pseudorotaxanes 6.