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.