Electrochemical oxidation of calixarenes: stabilisation by intramolecular π–π interactions of electrogenerated phenoxylium cations

5,11,17,23-Tetra-tert-butyl-25,27-bis(p-toluenesulfonyl)-26,28-bis(hydroxy)calix[4]arene (3) has been prepared from p-tert-butylcalix[4]arene and p-toluenesulfonyl chloride. The oxidative electrochemistry of 3 was studied in dichloromethane solution at a platinum electrode. As shown by detailed voltammetric, coulometric and spectroelectrochemical experiments the hydroxyl groups of 3 behave as non interacting sites, each hydroxyl group undergoing an irreversible two-electron transfer governed by an ECE mechanism to form the corresponding phenoxylium cation. The resulting di-(phenoxylium cation) [3-2H]2+ 4 was found to be stable for at least 2 h in dichloromethane. Nucleophilic attack of residual water on 4 ultimately led to the corresponding calix[4]diquinone 5.