Electrochemical reduction of some o-bis(phenylsulphonyl)benzene derivatives. Effect of the substrate structure and of the addition of bases on the product distribution

Abstract

A study of the electrochemical behaviour of the o-bis(phenylsulphonyl)benzene derivatives (1a–e) in dimethyl sulphoxide containing 0.1_M-tetrabutylammonium tetrafluoroborate has been undertaken. The results from cyclic voltammetry, controlled-potential electrolysis, and coulometry strongly argue in support of a mechanism involving initial formation of the radical anion (1)^–;˙ which fragments into the σ radical (2) and PhSO₂^–. Competing pathways for (2) are: (a) intramolecular homolytic arylation eventually leading to dibenzothiophene (4) together with dihydrodibenzothiophene (5) derivatives and (b) hydrogen-atom transfer leading to monosulphones (6). The fact that compounds (1a, b) undergo mainly cyclization, whereas the hydrogen-atom transfer predominates in the case of compounds (1c, d), indicates that the structure of the starting substrate is a major governing factor for the above competition. An explanation, based on a concomitance of steric effects of the methyl groups ortho to the phenylsulphonyl substituents, is given. Experiments carried out in the presence of different bases show that the intramolecular arylation leading to the cyclized product can occur also through an unprecedented chain mechanism whose efficiency, which increases as the strength and the concentration of the base is increased, is found in turn to be dependent on the substrate structure. Finally, when arenethiolates are used as bases, a third pathway (the nucleophile-radical coupling step of the S_RN1 process) is found to compete for the intermediate σ rdical (2) eventually leading to sulphides resulting from the overall substitution of an arylthio for a phenylsulphonyl moiety in (1). When the intramolecular cyclization does not compete efficiently almost quantitative yields of sulphides are obtained via an S_RN1 route.