Subtle “supramolecular buttressing effects” in Cucurbituril/guest assemblies†
Biphenyl derivatives bearing a dimethylsulfonium group at position 3 and three different substituents at position 4 (H, F and CH3) have been prepared as probes to test the validity of the “supramolecular buttressing” concept. We define the latter as the alteration, by a neighboring unit, of a substituent effect on intermolecular recognition. In this case, the 4-substituents exert some pressure on the 3-dimethylsulfonium groups and control the ratio of their syn and anti conformations. As free species, biphenyls bearing 4-H and 4-F substituents are present as approximately equimolar mixtures of syn and anti-conformers, while the biphenyl scaffold with a 4-CH3 group adopts the anti-conformation exclusively. The 3-dimethylsulfonium substituents then interact with one of the carbonylated portals of Cucurbituril (CB), and their conformations affect the position of the guests inside the cavity of the macrocycle, thereby validating our “supramolecular buttressing” model. Surprisingly however, binding affinities towards CB are barely affected by the nature of the 4-substituents and the conformations of the neighboring sulfonium groups, despite very different electronic densities presented to the CB portal in their syn or anti conformations. Solvation was found to dramatically smoothen host–guest Columbic interactions, although the latter remain important in the recognition process. Replacing the positively charged 3-dimethylsulfonium unit with an isopropyl substituent decreases the affinity of the biphenyl guest by 1000-fold.
- This article is part of the themed collection: In Celebration of Andrew D. Hamilton’s Career in Chemistry