Electrochemical, EPR, and quantum chemical study of reductive cleavage of cone-Calix[4]arene nosylates – New electrosynthetic approach

The mechanism of electrochemical reduction of a series of six cone-calix[4]arene-bis-nosylates (4-nitrophenylsulfonate aryl esters) was investigated on mercury electrodes using DC-polarography and cyclic voltammetry (CV) combined with in situ electron paramagnetic resonance (EPR)-spectroelectrochemistry in aprotic dimethylformamide. Model compounds – expected fragments and products - were studied for comparison. The experimental results are supported by quantum chemical calculations. All calix[4]arene-bis-nosylates are reduced in a first reversible step to bis-(radical anion) by two simultaneous one-electron transfers. Each of the two electrons is unpaired and separately localized on two nosylate groups.

In the second reduction step next 2×2 electrons are transferred and both sulfonate ester groups are cleaved to two 4-nitro-benzenesulfinate ions and a calixarene bis-phenolate (95%). This electroreductive generation of arylsulfinate anions is a significant finding from the electrosynthetic point of view. Activated arylsulfinates, the synthesis of which is generally difficult, can be easily prepared by electrochemical reduction of the nosyl esters.