Facile Synthesis of a Novel Heterogeneous Rh/COF Catalyst and Its Application in Tandem Selective Transfer Hydrogenation and Monomethylation of Nitro Compounds with Methanol

Abstract

The development of transition-metal heterogeneous catalysts for economical and effective synthesis of N-methylamine, especially for the monomethylation of amines, is still challenging. Herein, two Rh-supported covalent organic framework (COF) heterogeneous catalysts Rh/MelCOF were facile-synthesized by a Schiff base reaction using melamine as a precursor, and, for the first time, it was successfully applied to the effective and high selective tandem reaction of transfer hydrogenation and monomethylation of nitroaromatic hydrocarbons, with methanol as a C1 and hydrogenation source. A series of nitroaromatic hydrocarbons, including heterocyclic or sterically hindered derivatives, can be well tolerated, and the catalyst could also be reused four times without losing significant reactivity. At the same time, the study of the Rh/MelCOF mechanism supports the hydrogen-borrowing mechanism and puts forward the reaction pathway of azobenzene as an intermediate, which is better than the hydrogen-transfer pathway from N-phenylhydroxylamine to aniline directly. This work expands the application of COF catalysts and provides an effective way to obtain mono-N-methylated amines from nitroaromatic hydrocarbons, as well as the detailed mechanism of Rh/COF-catalyzed tandem transfer hydrogenation and monomethylation of amines.