Calix[4]pyrroles with Long Alkyl Chains: Synthesis, Characterization, and Anion Binding Studies

Abstract

Calix[4]pyrrole derivatives bearing long n-alkyl ester chains have been synthesized from calix[4]pyrroles containing carboxylic acid functional groups. These systems, which contain ester groups on either the meso- carbon atoms of the calixpyrrole ring or the β-positions of individual pyrrole rings, were prepared from the corresponding carboxylic acids. Esterification was effected using dicyclohexylcarbodiimide/4-dimethylaminopyridine (DCC/DMAP) to obtain long alkyl chain substituted calix[4]pyrroles. In the context of this work, several brominated calixpyrrole derivatives were prepared using N-bromosuccinimide (NBS) as the brominating agent. Anion binding studies carried out by isothermal titration calorimetry (ITC) in 1,2-dichloroethane with Cl⁻  and in the form of their respective tetrabutylammonium salts, revealed that the functionalized ester derivatives have anion binding affinities similar to those of the unsubstituted “parent” systems, octamethylcalix[4]pyrrole (1) and β-octabromocalix[4]pyrrole (2). The new alkylated systems proved soluble in nonpolar solvents, such as hexanes. Structure identification studies, carried out by single crystal X-ray diffraction analyses, revealed that the control ester system 4 contains two unique crystal structures per asymmetric unit. These asymmetric units interact via intermolecular hydrogen bonds in the solid state to produce a continuous intermolecular structure. Such interactions are not present in the case of the corresponding brominated ester 5.

Keywords:

• Calix[4]pyrrole
• Anions
• Hydrogen bonding
• Receptors

Acknowledgements

This research was supported by grants of National Institute of Health (Grant GM 58907 to J.L.S.).