Electron transfer and subsequent reactions during electrochemical oxidation of aryl- and alkylthio derivatives of mucochloric acid

Abstract

The electrochemical oxidation of aryl and alkylthio derivatives of mucochloric acid (3,4 dichloro 5 hydroxyfuran 2(5H) one) in MeCN-Bu 4 NBF 4 (0.1 mol L -1 ) was investi gated. It was shown that all sulfides are electrochemically active, from one to five oxidation steps of sulfur containing groups were observed for them. The ease and direction of oxida tion of the thio group depend on its nature and position in the furanone ring. 3 Substituted 2(5H) furanones possess the lowest oxidation potential. 4 Substituted 2(5H) furanones are predominantly oxidized to sulfoxides, 5 aryl and alkylthio derivatives undergo fragmentation to give mucochloric acid, and 3 arylthio derivative gives complex unidentified mixture of products. In the case of 3,4 bis(4 methylphenylthio) derivative, the oxidation product of the arylthio group at the 3 position to the corresponding sulfoxide was isolated. Based on the data from cyclic voltammetry with different concentrations of a substrate and water added, the results of preparative electrolysis and quantum chemical calculations, possible mechanisms of electrochemical oxidation of mucochloric acid derived sulfides are discussed. The initial common step is a reversible single electron transfer from the substrate molecule to form highly reactive radical cation.Aromatic and aliphatic sulfides have been well studied by electrochemical methods. Both reduction and oxida tion processes of these compounds were investigated in detail. 1,2 At the same time, the current interest in this class of compounds does not wane due to the synthesis of new sulfides and, therefrom, sulfoxides and sulfones with practically useful properties. It appears that the structures combining a heterocyclic fragment and a sulfinyl or sulfonyl group will be particularly promising. Also, of great inter est are optically active sulfoxides widely used in the enantioselective syntheses. 3,4In recent years, we have synthesized a variety of sul fides based on mucochloric acid, 3,4 dichloro 5 hydroxy furan 2(5Н) one (1a). 5-7 This stimulated our search for the selective oxidation reactions of the obtained sulfides by chemical and electrochemical methods. In the present communication, the results of the experimental and theoretical studies on the processes of electrochemical oxidation (ECO) of sulfides 2-23 in MeCN by cyclic voltammetry, preparative electrolysis and quantum chemical simulation are presented. The pathways of for mation of their oxidation products including sulfoxides 24, 25 are discussed. ExperimentalCyclic voltammograms (CV curves) were recorded using a PI 50 1 potentiostat connected to an N 307/2 XY recorder. The working electrode was a glassy carbon disk electrode (d = 2 mm) pressed fitted into a fluoroplastic casing. Prior to each measurement, the electrode was mechanically polished. Potentials were measured relative to the standard potential of the redox system ferrocene/ferrocenium cation (Fc/Fc + ) (internal standard) using a silver reference electrode Ag/AgNO 3 (0.01 mol L -1 ) in MeCN. Dissolved oxygen w...