SnOx-Decorated Pt Catalysts for the Reductive N-Methylation of Quinoline with Methanol

A direct and efficient synthesis of N-methyl-1,2,3,4-tetrahydroquinolines (MTHQs) is achieved through the one-pot reductive N-methylation of quinolines. The SnOx-decorated Pt/Al2O3 (Pt-SnOx/Al2O3) catalyst proves to be highly effective in the transformation from quinolines to MTHQs, using methanol as the exclusive source for hydrogen (H) and methyl (CH3), without the addition of additional bases or additives. Mechanistic and kinetic studies reveal a tandem reaction pathway: (1) the dehydrogenation of methanol, (2) reduction of quinolines to 1,2,3,4-tetrahydroquinolines (THQs), and (3) N-methylation of THQs to MTHQs, with the dehydrogenation of methanol identified as the rate-determining step. Characterization and catalytic performance results demonstrate that SnOx modifies the morphology and electronic properties of Pt-based metal active sites, which dramatically facilitates the cleavage of O–H and less reactive C–H bonds in methanol, and maintains a delicate balance between methanol dehydrogenation and reductive N-methylation of quinolines, thus obtaining a high reactivity for the total transformation.