Indirect Electrooxidation (an Ex-Cell Method) of Alkylbenzenes by Recycle Use of (NH4)₂Ce(NO₃)₆ in Various Solvent Systems

Indirect electrooxidation (an ex-cell method) of the side chain of alkylbenzenes has been performed by recycle use of (NH₄)₂Ce(NO₃)₆ (CAN) in various solvent systems. The side-chain oxidation of p-methoxytoluene (1) with CAN in methanol proceeds smoothly, yielding anisaldehyde (3, 94%), while use of aqueous methanol or acetic acid resulted in a mixture of 3 and the dimeric product 4 (19–35%) along with complex products (28–47%). Electrooxidation of the recovered cerium(III) salts was performed smoothly in methanol by passing 1.1–1.2 F/mol of electricity to give a reddish methanol solution of CAN, which can be used for the next oxidation of 1. The repeated use of CAN over 10 times produced the desired 3 in over 90% yields. Similarly, the oxidation of alkylbenzenes, bearing a p-methoxy group, gave the corresponding side-chain-oxidized products. In contrast, p-tert-butyltoluene (9e) could be oxidized by heating with CAN in aqueous 50% acetic acid at 85–90 °C to give p-tert-butylbenzaldehyde in over 90% yield. The regeneration of CAN after the oxidation of 9e was also accomplished by electrolysis in the aqueous 50% acetic acid system. The reaction mechanisms in connection with the nature of CAN in various solvent systems are discussed.