Hydrogenation of N-Heterocycles by a Well-Defined Phosphine-Free Manganese Catalyst Using Ammonia Borane

Efficient hydrogenation of N-heterocycles employing low-cost and easy-to-access catalysts is highly desirable as the corresponding hydrogenated N-heterocycles have significant importance. Herein, we disclosed the well-defined phosphine-free Mn-catalyzed effective transfer hydrogenation (TH) of various N-heterocycles using ammonia borane as the hydrogen source. In this work, a series of phosphine-free Mn(I) complexes were synthesized to investigate the importance of the ligand backbone containing benzimidazole N–H, amine N–H, and a soft sulfur atom. Reactivity studies of these Mn complexes revealed that the proton-responsive Mn1 complex bearing the N–H proton of benzimidazole, amine, and hemilabile sulfur arm was found to be most effective. The capability of this procedure was verified by the synthesis of several hydrogenated N-heterocycles and biologically active compounds. Mechanistic pathways were established by performing several control reactions and kinetic experiments.