Anodic synthesis of benzaldehydes—II. Optimization of the direct anodic oxidation of toluenes in methanol and ethanol

Abstract

The direct anodic oxidation of 3,4,5-trimethoxytoluene, p-methoxytoluene, toluene and p-chlorotoluene had been investigated by preparative electrolyses in dry methanol and ethanol solution with NaClO 4, NaBF 4 and Et 4N · BF 4 as a supporting electrolyte at graphite anodes in batch experiments. 3,4,5-trimethoxytoluene oxidation yields almost no benzaldehyde dimethylacetal but anodic oligo- and polymers. If oxidized, however, in the presence of 0.1 molar sodium methoxylate, the yield and selectivity of the benzaldehyde synthesis approaches 100%. With the three other toluenes, benzylmethyl (or ethyl) ethers and benzaldehyde dimethyl (or diethyl) acetals are obtained due to consecutive reaction steps. The acetals are further oxidized to a number of compounds among which benzoic acid ortho-esters and esters had been identified. The direct anodic oxidation of the three toluenes is almost always accompanied by anodic polymerization, so that material yields of less than 100% and selectivities between 0 and, at best, 70% are obtained for the anodic synthesis of benzaldehyde dialkylacetals at close to 100% conversion of the toluenes. The amount of benzylmethyl ethers produced as by-product is always low and approaches zero if the toluene conversion is almost complete. The observed polymerization reaction which in unfavourable cases consumes almost all converted toluene is contrary to predictions and model calculations which are based on voltametric data. Modelling of the anodic conversion of toluenes by using voltammetric data leads only to a qualitative agreement between experiment and model-predictions and optimization has to be performed by trial and error. However, optimal selectivities do not surmont 70 to 75% for p-methoxytoluene and p-chlorotoluene and only 30% for toluene.