Arene—onium cations—V. Preparative investigation concerning yields and selectivities for the anodic formation of some selected trisarene sulphonium cations

Abstract

Based on kinetic data for the anodic oxidation of bisarenesulphonium cations[1] the preparative anodic oxidation of bisanisolesulphide, bisbenzenesulphide and bistoluenesulphide was performed in acetonitrile in presence of different arenes as coupling partners aiming to show that higher selectivities for the formation of desired trisarene sulphonium cations can be obtained provided appropriate process parameters are chosen. It is decisive to establish a concentration ratio {c(arene)/c(bisarenesulphide)} in the reaction layer which is high enough to keep the rate of anodic self-coupling vs the coupling with the desired arene low enough. For the formation of trisanisole sulphonium cations, where not self-coupling but coupling with water (yielding bisanisole oxide) is the most important competing reaction, a selectivity of 80% is already obtained if the ratio of the bulk concentrations of the competing coupling partners water and anisole is unity. In the case of the formation of tristoluenesulphide further advantage for obtaining a higher selectivity may be drawn from heterogeneous catalysis of the coupling reaction at carbon anodes. The yield for tristoluene sylphonium cations can be raised to nearly 90% at carbon anodes whereas oxidizing 0.1 M bistoluenesulphide in the presence of 0.1 M toluene at platinum anodes yields only undesired self-coupling products. From bisbenzenesulphide no trisbenzene sulphonium cations could be obtained; however, coupling with mesitylene, xylene and toluene was achieved.