Homogeneous Base-Metal-Catalyzed Transfer Hydrogenation of Unsaturated N-Containing Organic Compounds

Owing to the synthetic availability of imines, N-heteroarenes, nitriles, carboxamides, and nitroarenes, their catalytic hydrogenation is considered an attractive and atom-economical route to a diversity of amines, which find widespread applications specialty chemical industries. Although catalytic hydrogenation of unsaturated N-containing organic compounds with compressed H ₂ gas is well-established, such transformations require expensive high-pressure equipment and have associated H ₂ handling risks. In contrast, transfer hydrogenation protocols utilize nongaseous hydrogen sources, offering significant advantages in the operational cost and safety of transformations. Whereas many economical nonprecious metal catalysts have been described for efficient transfer hydrogenation of aldehydes and ketones, similar systems for selective transfer hydrogenation of unsaturated nitrogen-containing organic compounds have been developed relatively recently. This review aims to highlight current advances and challenges of base-metal-catalyzed (i.e., Mn, Fe, Co, Ni, and Cu) transfer hydrogenation of imines, N-heteroarenes, nitriles, nitro compounds, as well as carboxamides and related molecules to the corresponding amines. Mechanistic aspects of catalytic reactions, the substrate scope, and selectivity of the transformations are also discussed.