Ligand-free iron carbonyl catalyzed transfer hydrogenation strategy: synthesis of N-heterocycles

Abstract

Transfer hydrogenation (TH) systems have been extensively developed using a variety of metal catalysts. Herein, we report a ligand-free TH protocol that proceeds via iron carbonyl complex catalysis for the efficient synthesis of diverse N-heterocycles. Using benzyl alcohol and 2-nitrobenzamide, quinazolinone scaffolds were obtained in high yields via TH with subsequent condensation and annulation steps. Notably, 2-nitrobenzonitriles can also be converted into quinazolinones using this protocol. Furthermore, the method was successfully applied to the synthesis of other heterocycles, benzoxazoles and pyrrolo[1,2-α]quinoxalines. The protocol is operationally simple, cost-effective and does not require auxiliary ligands, providing a sustainable route to complex N-heterocycles. Control experiments suggested that molecular hydrogen is generated during alcohol oxidation and participates in the subsequent reduction of nitro groups to amines. DFT calculations supported a mechanism in which an iron tricarbonyl complex mediates hydrogen transfer, facilitating the generation of molecular hydrogen from benzyl alcohol.