Development of a Streaming Potential-Driven Electrolysis System: Reductive Hydroxylation of Aromatic Boronic Acids

Bipolar electrodes (BPEs), driven by an electric field in low-concentrated electrolytes, are paying more and more attention due to their unique features such as wireless nature and gradient potential distribution(Figure 1a) 1 . Recently, we achieved electrochemical fluorination using a bipolar electrode, reducing the amount of supporting electrolyte that becomes waste after the reaction 2,3 . However, the driving electrodes generating an electric field may induce side reactions during electrolysis. Using streaming potential is a key strategy to solve this problem (Figure 1b). Streaming potential is a potential difference generated by a laminar flow of low-concentrated electrolytes inside a narrow channel 4 . Indeed, an anodic dissolution of metallic silver 5 and electropolymerization of aromatic monomers 6 were achieved in bipolar electrode systems driven by the streaming potential (Figure 1c). In this work, we aimed to expand this electrolysis system to molecular transformations. The reductive hydroxylation of aromatic boronic acid was selected as a model reaction (Figure 1d) 7 . To amplify the streaming potential, we employed various porous fillers packed in narrow flow channels. Polymer resin fillers with continuous porous monolith structures showed high streaming potential. Then, the hydroxylation using streaming potential was carried out by pumping the solution containing 4-(methylthio)phenylboronic acid. As a result, the reaction current was observed between bipolar electrodes, and the desired phenol product was detected by 1 H NMR, suggesting that the proposed electrochemical hydroxylation proceeded without a power supply. Through the investigation of the concentration of substrates, electrode surface area, and additives, the yield and conversion yield were increased to 9.0% and 97.3%, respectively. The details of the streaming potential measurements and hydroxylation will be presented in the talk. References N. Shida, Y. Zhou, S. Inagi, Acc. Chem. Res. , 2019 , 52 , 2598–2608. K. Miyamoto, H. Nishiyama, I. Tomita, S. Inagi, ChemElectroChem , 2019 , 6 , 97–100. H. Sakagami, H. Takenaka, S. Iwai, N. Shida, E. Villani, A. Gotou, T. Isogai, A. Yamauchi, Y. Kishikawa, T. Fuchigami, I. Tomita, S. Inagi, ChemElectroChem , 2022 , 9 , e202200084. A. V. Delgado, F. González-Caballero, R. J. Hunter, L. K. Koopal, J. Lyklema, J. Colloid Interface Sci. , 2007 , 309 , 194–224. I. Dumitrescu, R. K. Anand, S. E. Fosdick, R. M. Crooks, J. Am. Chem. Soc. , 2011 , 13