Acyclic cucurbit[n]urils capped with alkylene linkers: synthesis and molecular recognition properties

Abstract

ABSTRACTWe report the synthesis and characterization of three new acyclic cucurbit[n]uril-type receptors that feature a covalent capping group in the form of an alkylene linker (2a – 2c) that we hypothesized would have higher binding affinity toward alkylammonium ions in water. Hosts 2a – 2c have lower aqueous solubility (≤2 mM) than the prototypical acyclic CB[n]-type host 1 (346 mM). Hosts 2a – 2c do not undergo significant intermolecular self-association over the experimentally accessible concentration range. In contrast, the results of 1H NMR experiments shows that the alkylene linkers of 2b and 2c undergo self-inclusion and that this process is reversed upon addition of ammonium ions 6–10 as guests. The host•guest Ka values were determined for hosts 1 and 2a – 2c toward guests 6–10 by isothermal titration calorimetry. We find that 2a – 2c bind less strongly toward guests 6–10 than host 1 due to the energetic penalty associated with expulsion of the alkylene linker from its own cavity.