Upper-rim Schiff-base calix[4]arenes

Abstract

This study focuses on the synthesis of various Schiff-base calix[4]arenes that may be used as supramolecular building blocks. Two different routes to Schiff-base calix[4]arenes have been examined. One explores the reaction of different aminopyridines and p-formylcalix[4]arenes, while the other utilises the reaction between different p-amino-calix[4]arenes with various 2-hydroxybenzaldehydes. In addition, the amino-pyridines employed in the first route have also been used in a reductive amination procedure with p-formylcalix[4]arene as an alternative synthetic route due Schiff-base product instability. With respect to the 2-hydroxybenzaldehydes chosen for the second route, different functionality was introduced around the benzene ring in order to obtain a range of ligands that may influence the formation of supramolecular assemblies. This influence could be by hydrogen bonding, ionic interactions, halogen-halogen or π- staking interactions for example. Furthermore, this reaction was carried out on two different p-aminocalix[4]arenes. One was flexible (with respect to the movement around the annulus of the calixarene) while the other was locked in the cone conformation by the introduction of lower-rim crown-ether linkages. Several series of pro-ligands have been synthesised and fully characterised where possible. No supramolecular assemblies have been synthesised as yet due to time restrictions.