Ru-Catalyzed Transfer Hydrogenation of N-Heterocycles Using Ammonium Formate as a Hydrogen Source.

The development of efficient transfer hydrogenation protocols using inexpensive and nontoxic molecular hydrogen donors is highly desirable for the synthesis of medicinally important piperidine derivatives. This study presents a ruthenium-catalyzed transfer hydrogenation of pyridinium salts using inexpensive ammonium formate as the hydrogen source. The method selectively produces tetrahydropyridine derivatives from 4-substituted pyridiniums and fully reduced piperidines from 3-substituted pyridiniums with high efficiency. Additionally, quinolinium and isoquinolinium salts can be reduced under metal-free conditions using ammonium formate. The procedure is scalable, as demonstrated by a gram-scale reaction, and enables subsequent synthetic transformations.