An electrocatalytic hydrogenation of cyanoarenes, nitroarenes, quinolines, and pyridines using a proton-exchange membrane (PEM) reactor was developed. Cyanoarenes were then reduced to the corresponding benzylamines at room temperature in the presence of ethyl phosphate. The reduction of nitroarenes proceeded at room temperature, and a variety of anilines were obtained. The quinoline reduction was efficiently promoted by adding a catalytic amount of p-toluenesulfonic acid (PTSA) or pyridinium p-toluenesulfonate (PPTS). Pyridine was also reduced to piperidine in the presence of PTSA.
Proton-exchange membrane (PEM) reactors, including a membrane electrode assembly (MEA) consisting of a proton exchange membrane and an electro-catalyst supported on carbon, has attracted attention as an useful device for electrocatalytic hydrogenation [1,2]. With this reactor, a continuous flow system can be constructed without the need for a supporting electrolyte, making hydrogenation reactions very ubiquitous. Although there have been several reports on the reductive transformations using PEM reactors, its application for nitrogen-containing organic compounds is still limited. We herein report the reduction of nitrogen-containing compounds having a cyano or nitro group by using a PEM reactor. We will also report on the reduction of quinoline and other nitrogen-containing aromatic compounds. [1] Takano, K.; Tateno, H.; Matsumura, Y.; Fukazawa, A.; Kashiwagi, T.; Nakabayashi, K.; Nagasawa, K.; Mitsushima, S.; Atobe, M. Bull. Chem. Soc. Jpn. 2016, 89, 1178–1183. [2] Mitsudo, K.; Inoue, H.; Niki, Y.; Sato, E.; Suga, S. Beilstein J. Org. Chem. 2022, 18, 1055–1061.
Electrochemical hydrogenation of enones using a proton-exchange membrane reactor is described. The reduction of enones proceeded smoothly under mild conditions to afford ketones or alcohols. The reaction occurred chemoselectively with the use of different cathode catalysts (Pd/C or Ir/C).
The first electrochemical dehydrogenative C-S bond formation leading to thienoacene derivatives is described. Several thienoacene derivatives were synthesized by dehydrogenative C-H/S-H coupling. The addition of n Bu4 NBr, which catalytically promoted the reaction as a halogen mediator, was essential.
