Effect of Solvents on Electrogenerated Base-Driven Transfer Hydrogenation Reactions

Transfer hydrogenation is a crucial technology for synthesizing fine chemicals and pharmaceuticals, offering improved safety and convenience over traditional hydrogen methods, although it typically requires external bases. While isopropanol is commonly used as a hydrogen source, methanol is superior but faces challenges due to its high dehydrogenation energy barrier, limiting its use under mild conditions. This study focuses on investigating the differences in the electrogenerated base-driven transfer hydrogenation of aromatic ketones in isopropanol and methanol solvents, using Mn(CO)₅Br and cyclohexanediamine derivatives as the catalyst. The research demonstrates that high enantiomeric excess (ee) values were obtained in isopropanol in the presence of chiral Mn-based catalysts, while only racemic products were observed in methanol. The results indicate a strong dependence of the catalytic pathway on the choice solvent: in isopropanol, the catalyst operates via a metal-ligand cooperative transfer hydrogenation, resulting in high ee values, whereas in methanol, transfer hydrogenation occurs through metal hydride transfer with no stereoselectivity.