Cucurbit[n]uril Analogues

Abstract

Molecular recognition and self-assembly in aqueous solution have experienced rapid growth in recent years. The use of alpha-, beta-, and gamma-cyclodextrins, calixarenes, and cyclophanes have served as the platforms for molecular recognition in aqueous solution. Recently, the investigation of an alternative platform based on cucurbituril has become the focus of several research groups. The rigid structure and capability of forming complexes with molecules and ions through hydrophobic, ion-dipole and hydrogen-bonding interactions make cucurbituril an attractive candidate as a synthetic receptor as well as a building block for the construction of supramolecular architectures.

However, before cucurbituril can be used to supplant the more common platforms as the molecule of choice for molecular recognition and self-assembly in aqueous solution, several advances must be made: 1) cucurbiturils must become available in a variety of sizes, 2) their solubility must be improved, and 3) synthetic procedures must be advanced to include the ability to selectively generate specific cucurbituril homologues, derivatives, and analogues.

Herein, the synthesis of cucurbit[n]uril analogues is presented with control over their size, shape, and solubility. These CB[5], CB[6], and CB[7] analogues all contain bis(phthalhydrazide) walls which are incorporated into the macrocycle. The tailor-made synthesis of these CB[n] analogues proceeds by the condensation of the appropriate bis(electrophile) with bis(phthalhydrazide) which delivers the CB[6] and CB[7] analogues in good yield whereas the CB[5] analogue is formed in low yield. To help rationalize the high yields obtained in these macrocyclization reactions, we performed mechanistic studies of model methylene bridged glycoluril dimers.

The molecular recognition properties of a water soluble cucurbit[6]uril analogue in aqueous buffer toward a variety of guests including alkanediamines, aromatics, amino acids, and nucleobases were studied by fluorescence spectroscopy. For the alkanediamines studied, as the length of the alkane is increased between the amines, the association constants also increase. The CB[6] analogue is capable of forming strong complexes with guests containing aromatic rings with association constants (Ka) ranging from 10e2 to 10e6 /M due to the favorable pi-pi interactions that occur between the host and the aromatic portion of the guest while encapsulated in its hydrophobic cavity.