www.elsevier.nl/locate/jelechem Journal of Electroanalytical Chemistry 481 (2000) 24 – 33 Catalytic reduction of ethyl chloroacetate by cobalt(I) salen electrogenerated at vitreous carbon cathodes Lee J. Klein, Kent S. Alleman, Dennis G. Peters *, Jonathan A. Karty, James P. Reilly Department of Chemistry, Indiana Uniersity, Bloomington, IN 47405, USA Received 14 May 1999; received in revised form 19 November 1999; accepted 22 November 1999 Abstract Cyclic voltammetry, controlled-potential electrolysis, and spectroelectrochemistry have been employed to investigate and characterize the catalytic reduction of ethyl chloroacetate by cobalt(I) salen, electrogenerated at a carbon cathode in dimethylform- amide containing tetraalkylammonium salts as supporting electrolytes. A cyclic voltammogram for the reduction of cobalt(II) salen in the presence of excess ethyl chloroacetate exhibits a prewave, which is attributed to the formation of an ethoxycarbonyl- methylcobalt(III) salen complex. This prewave is followed by a large catalytic wave which involves the formation of both ethoxycarbonylmethyl and ethoxide anions; the first anion then undergoes a series of chemical reactions with the remaining starting material (ethyl chloroacetate) to yield ethyl acetate and 1,2,3-cyclopropane tricarboxylic acid triethyl ester, whereas ethoxide is protonated to afford ethanol. From the results obtained by means of cyclic voltammetry and controlled-potential electrolysis, a mechanism is proposed to explain our findings, and the validity of this mechanism has been probed with the aid of experiments involving the liquid chromatographic separation and mass spectrometric identification of the ethoxycarbonyl- methylcobalt(III) salen complex. © 2000 Elsevier Science S.A. All rights reserved. Keywords: Cobalt(I) salen; Ethyl chloroacetate; Ethoxycarbonylmethylcobalt(III) salen complex; Catalytic reduction; Glassy carbon electrodes 1. Introduction Cobalt-containing complexes have been studied ex- tensively as catalysts for the reduction and synthesis of organic compounds [1,2]. In recent years, considerable attention has been focused on the reactions of electro- generated cobalt(I) salen with halogenated organic species, including benzal chloride [3 – 5], benzotrichlo- ride [6], bromobenzene [7], bromoethane [7], tert -butyl bromide and chloride [8], 1,2-dibromocyclohexane [9,10], 1-bromobutane [9,11,12], 1-iodobutane [12], 1,2- dibromobutane [12], benzyl bromide [13], benzyl chlo- ride [14], and 3-chloro-2,4-pentanedione [15]. Recent papers, with additional pertinent references, from our laboratory describe the catalytic reduction of iodoethane [16] and 1,8-diiodooctane [17] by electro- generated cobalt(I) salen. However, there appears to have been no previous report of the catalytic reduction of an -haloester by a low-valent cobalt complex. On the other hand, the direct electrochemical reduc- tion of halogenated esters has been the subject of a modest number of publications, beginning with polar- ographic studies of the behavior of some ethyl bro- moalkanoates by Elving et al. [18] and by Lambert [19]. In an investigation of the electrochemistry of several halogenated ethyl acetates, Baizer and Chruma [20] discovered that ethyl bromoacetate undergoes di- rect two-electron reduction at mercury to form an ethoxycarbonylmethyl anion and that subsequent non- electrochemical reactions lead to the following prod- ucts (with yields in parentheses): ethyl acetate (51%); diethyl succinate (20%); 1,2,3-cyclopropane tricar- boxylic acid triethyl ester, 1 (7%); 1,2,3-propane tricar- boxylic acid triethyl ester, 2 (6%); and diethyl fumarate (4%). * Corresponding author. Tel.: +1-812-8559671; fax: +1-812- 8558300. E-mail address: peters@indiana.edu (D.G. Peters) 0022-0728/00/$ - see front matter © 2000 Elsevier Science S.A. All rights reserved. PII:S0022-0728(99)00473-8