Versatile (Pentamethylcyclopentadienyl)rhodium‐2,2′‐Bipyridine (Cp*Rh‐bpy) Catalyst for Transfer Hydrogenation of N‐Heterocycles in Water

Abstract An investigation employing the catalytic system consisting of (pentamethylcyclopentadienyl)rhodium dichloride dimer [Cp*RhCl 2 ] 2 and 2,2′‐bipyridine (bpy) for transfer hydrogenation of a variety of quinoxalines, quinoxalinones, quinolines and indoles under aqueous conditions with formate as the hydrogen source is reported. This approach provides various tetrahydroquinoxalines, dihydroquinoxalinones, tetrahydroquinolines and indolines in good to excellent yields. The activity of the catalyst towards quinoxalines and quinoxalinones is excellent, with a substrate to catalyst ratio (S/C) of 10000 being feasible. The choice of ligand is critical to the catalysis, and the aqueous phase reduction is shown to be highly pH‐dependent, with acidic pH values needed for optimal reduction. The catalyst is easy to access, and the reaction is operationally simple without requiring an inert atmosphere. magnified image