Formation of Arylsulfinates Using Electroreductive Cleavage of Cone-Calix[4]Arene Nosylates – EPR and Theoretical Study

Abstract

Nosylates are used as protecting groups. In a non-redox hydrolytic process, the leaving group is the corresponding sulfonate anion. Upon electrochemical reduction of nosylates (here six cone-calix[4]arene-bis-nosylates; i.e. 4-nitrophenylsulfonate aryl esters) using mercury electrodes (DC-polarography and cyclic voltammetry - CV) combined with in-situ EPR-spectroelectrochemistry in aprotic DMF, the mechanism is different. All studied calix[4]arene-bis-nosylates are reduced in a first reversible step to bis-nitroradical anion) which was proved by EPR. The stability of the multiple radical ions proves the absence of electron communication among the redox centers on the calixarene macrocycle which was observed previously in other systems1-3.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%) under splitting of the S-O bond.4 The mechanism of electroreductive generation of arylsulfinate anions is a significant finding from electrosynthetic point of view. Synthesis of activated arylsulfinates is generally difficult for their low stability. Nevertheless, they can be easily prepared by electrochemical reduction of calixarene nosyl esters. Acknowledgement This work was supported by the grant GAČR 21-23261S (Czech Science Foundation) with the institutional support RVO: 61388955. The authors are grateful to colleagues ing. Markéta Řezanková, prof. Jan Budka and prof. Pavel Lhoták (University of Chemistry and Technology, Prague) for synthesis and granting the substances.