Novel Electrocatalytic Hydrogenation Using a Solid Polymer Electrolyte (SPE) Reactor

Organic electrosynthetic reactions, which are driven by electricity, can generally be carried out under mild conditions (room temperature and ambient pressure). In addition, they do not require any hazardous reagents and produce less waste than other conventional chemical syntheses. Therefore, electrosynthesis is known to be a mild and clean method for organic synthesis and there has recently been renewed interest in its development. However, electrosynthesis also has some disadvantages. Ordinary chemical reactions are homogeneous, while the reaction field of electrolysis is a heterogeneous interface, therefore electrosynthesis has a productive drawback. In addition, a large amount of supporting electrolyte is usually required. Therefore, the availability of solvents is limited by the necessity for dissolution of a supporting electrolyte. The presence of the supporting electrolyte might also cause separation problems in the reaction workup. Moreover, in an ordinary electrolysis system, solution resistance is generated between the working and counter electrodes, resulting in a decrease in energy efficiency. To overcome these problems, we focused on a solid polymer electrolyte (SPE) reactor which was originally developed for fuel cell technologies and demonstrated various electrocatalytic hydrogenations in a SPE reactor (Figure 1) [1-5]. As shown in Figure 1, a SPE membrane such as a proton exchange membrane (PEM) is integrated into the central part of the reactor, and is sandwiched between a pair of catalyst layers on the anode and cathode sides. For this reason, the substrate solution does not require a supporting electrolyte and the cell resistance can be minimized. The anode and cathode have highly porous 3D structures in order to conduct an efficient electrochemical reaction. Moreover, the reaction can be carried out in a flow operation. Thus, the SPE reactor system possesses many characteristics designed to overcome the disadvantages of conventional electrosynthetic processes. In a series of studies we have conducted, in this presentation, we will discuss two topics such as diastereoselective reduction of cyclic ketones and electrocatalytic hydrogenation of pyridines. The authors gratefully acknowledged support by JST CREST Grant No. 18070940, Japan. [1] A. Fukazawa, M. Atobe, et al. ACS Susutain. Chem. Eng. 2019, 7, 11050. [2] S. Nogami, M. Atobe, et al. J. Electrochem. Soc. 2020, 167, 155506. [3] A. Fukazawa, M. Atobe, et al. Org. Biomol. Chem. 2021, 19,7363. [4] S. Nogami, M. Atobe, et al. ACS Catalysis, 2022, 12, 5430. [5] Y. Shimizu, M. Atobe, et al. ACS Energy Lett., 2023, 8, 1010. Figure 1