Electrochemical reduction of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan) in dimethylformamide

Abstract

Cyclic voltammetry and controlled-potential (bulk) electrolysis have been used to investigate the direct reduction of 5-chloro-2-(2,4-dichlorophenoxy)phenol (triclosan, 1) in dimethylformamide (DMF) containing tetra-n-butylammonium tetrafluoroborate (TBABF4). Cyclic voltammograms for reduction of 1 at glassy carbon electrodes exhibit three irreversible peaks attributed to successive reductive cleavage of the three aryl carbon–chlorine bonds. Bulk electrolyses of 1 at reticulated vitreous carbon cathodes held at a potential between the first and second cathodic peaks afford only 5-chloro-2-(4-chlorophenoxy)phenol; however, at a more negative potential, a mixture of 5-chloro-2-(4-chlorophenoxy)phenol, 5-chloro-2-phenoxyphenol, and 2-phenoxyphenol is obtained. A scheme consisting of electron-transfer steps and accompanying chemical reactions is proposed that follows the classic mechanism for the reduction of aryl halides. To provide support for this mechanism, theoretical calculations based on density functional theory have been performed to model the electronic structures of 1 and the likely intermediates formed via its electroreduction. Catalytic reduction of 1 by nickel(I) salen and nickel(I) diethylsalen, each mediator electrogenerated at a glassy carbon electrode in DMF – 0.10 M TBABF4, has been explored with the aid of cyclic voltammetry and controlled-potential electrolysis.