TEMPO-Mediated Electro-Oxidation of 1,2-Diol on Boron-Doped Diamond Electrode

Abstract

Electroorganic synthesis is an attractive method, in which only electrons serve as reagent and therefore complies with a “green chemistry” condition. In addition, reactions of electroorganic synthesis can be regarded as a special heterogeneous catalytic one. Furthermore, products can be obtained with high selectivity and efficiency by optimizing a reaction condition, such as electrode materials, potential, and others. Recently, a boron-doped diamond (BDD) electrode attracts much attention in the field of electroorganic chemistry. This is particularly because the BDD electrode enables to generate active species such as a hydroxyl radical with high efficiency under an appropriate electrolysis condition. Here, we report on the electroorganic synthesis using BDD electrode, especially TEMPO-mediated oxidation of the 1,2-diol derivative. TEMPO (2,2,6,6,-tetramethylpyperidine-1-oxyl) has been widely used as a catalyst in organic synthesis, for converting primary alcohol to an aldehyde selectively even in the presence of a secondary alcohol. However, the conventional TEMPO oxidation reaction requires a co-oxidant such as sodium hypochlorite and a hypervalent iodine compound. First, we prepared an electrolyte solution of TEMPO (0.1 mmol) and LiClO4 (0.1 M in CH3CN) or n-Bu4N•PF6 (0.1 M in CH2Cl2). Cyclic voltammetry (CV) was performed to examine an electrochemical behavior of TEMPO. For CV measurementsusinan undivided cell, BDD, Pt wire, and Ag/AgCl electrodes were used as the working, counter, and reference electrode, respectively. Next, for an electrolysis experiment, a diol substrate, 3-phenyl-1,2-propanediol, was synthesized according to the previous report. The diol substrate (10 mmol) was added to the electrolyte solution (10 mL), and a constant current electrolysis (1 F/mol for the diol substrate) was conducted at room temperature. After electrolysis, a resulting compound containing in solution was evaluated by 1H NMR. For acetylation of a hydroxyl group in oxidized products, pyridine (20 mmol) and acetic anhydride (20 mmol) was added and stirred at room temperature for 6 h. The resulting acetylated products were analyzed by a thin layer chromatography (TLC). In the cyclic voltammogram of TEMPO solution, oxidation and reduction peaks of TEMPO were clearly observed at 0.8 V and 0.6 V (vs. Ag/AgCl), respectively. On the other hand, the reduction peak of TEMPO almost disappeared in the presence of 3-phenyl-1,2-propanediol. Based on the reaction mechanism of TEMPO oxidation, such a CV behavior would ascribed to oxidation of 3-phenyl-1,2-propanediol substrate by TEMPO catalyst. Next, we examined the solvent dependence of TEMPO-mediated electro-oxidation. When using CH3CN electrolyte solution, surface of the Pt cathode was covered with a black film and the current dropped immediately. On the other hand, in case of CH2Cl2 electrolyte solution, a couple of oxidized products were detected by a TLC analysis. Furthermore, in the 1H NMR spectrum, a signal at 10 ppm derived from an aldehyde group was detected. We investigated TEMPO-mediated selective electro-oxidation using a BDD electrode. First, we confirmed both oxidation and reduction of TEMPO on a BDD electrode. Next, TEMPO-mediated electro-oxidation of 3-phenyl-1,2-propanediol gave a couple of products containing an aldehyde group.