Electrocarboxylation and related radical coupling processes of aryl and benzyl halides in microemulsion

Abstract

Electrocarboxylation of aryl halides and benzyl halides in cetyltrimethylammonium bromide (CTAB) based bicontinuous microemulsion is reported. Benzyl halides exhibited higher conversion of coupling products between the benzyl radical and the phenyl acetate radical. Benzylphenylacetate is found to be the predominant product in galvanostatic electrolysis experiments. Electrocarboxylation of aryl halides is conventionally carried out in dimethylformamide (DMF) using nickel cathode and magnesium or aluminium anodes [1, 2]. Further improvements in the electrocarboxylation processes employing redox mediators [3–5] and silver cathodes [6, 7] have been reported. Poly halogenated aromatic compounds [8], iodo aromatic compounds [9] and aliphatic halides [10, 11] also undergo facile electrocarboxylation. Al ion generated at the anode appears to have a catalytic influence on the electrocarboxylation reactions [12]. Highly pure and moisture free DMF still remains the medium of choice for electrocarboxylation reactions [13]. Quite recently both cyclo addition of carbon dioxide to epoxides [14] and electrocarboxylation of cinnamate ester [15] were achieved in moisture free ionic liquid. In contrast to dimethylformamide and ionic liquids, bicontinuous microemulsions contain significant amount of water and are indeed much greener media for chemical and electrochemical processes [16]. Rusling et al. have reported free radical generation and coupling processes in bicontinuous microemulsion [17–19]. Similar electro generated free radical reactions have also been reported [20, 21]. Electrochemical coupling of benzyl free radicals generated from benzyl bromide with active methylene compounds under galvanostatic experimental conditions in cationic microemulsions was reported from this laboratory [21]. The objective of this communication is to explore the possibility of another important coupling reaction, namely electrocarboxylation, involving the carbon dioxide and the electro generated aryl and benzyl free radicals in a bicontinuous microemulsion.