A stereoselective and electrocatalytic coupling reaction of isoeugenol has been reported for the first time in a 1,1,1,3,3,3-hexafluoro-2-propanol (HFIP)/boron-doped diamond (BDD) electrode system. This particular C-C bond formation and diastereoselectivity is driven by a solvate interaction between the radical species and another isoeugenol molecule. Due to an electrocatalytic cycle, only understoichiometric amounts of charge are necessary. Since electric current is directly employed as the oxidant, the reaction is metal and reagent-free. In addition, the electrolysis can be conducted in a very simple undivided beaker-type cell under constant current conditions. Therefore, the protocol is easy to use, suitable for scale-up, and inherently safe.

An electrochemical pinacol coupling reaction of acetophenone using a boron‐doped diamond (BDD) electrode has been reported. This transformation is driven by the one‐electron reduction of acetophenone on the BDD cathode, followed by an intermolecular radical coupling. Owing to the BDD's outstanding electrochemical properties, the pinacol‐type compound has been obtained in good conversion yield. The roles of a supporting electrolyte and solvent were addressed; (1) lithium ions contribute to increase in the reactivity of the radical intermediate. (2) Addition of water promotes the electron transfer process at the BDD surface. The reaction is relatively tolerant to para‐substituted acetophenone derivatives. Since electric current is directly employed as reducing reagents to generate a radical intermediate, the reaction is metal‐free, sustainable, and inherently safe.

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.

Electrochemical homo-coupling reaction of brominated phenols was investigated. In the asymmetric synthesis, the structural motif of 2,2’-biphenyl provides chiral chelating ligand systems and therefore they are categorized as privileged chiral catalysts. Also, the 2,2’-biphenyl unit has been found as bioactive centers in several natural products. The 2,2’-biphenyl molecule with bromide functional group would be a useful synthetic tool to construct further C-C bond formation. However, synthetic methods to obtain brominated 2,2’-bipnenyls are less common, and direct coupling reaction of brominated phenols resulted with low to moderate yield. Recently we have found iodine mediated one step coupling reaction to construct brominated 2,2’-biphenyl from 4-brominated phenol. This reaction was applied to electrochemical reaction. The electrochemical reactions and results of the biological assay of the obtained compounds will be discussed.