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

Abstract

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.