Additive-free N-methylation of amines with methanol over supported iridium catalyst

Abstract

An efficient and versatile zinc oxide-supported iridium (Ir/ZnO) catalyst was developed to catalyze the additive-free N-methylation of amines with methanol. Mechanistic studies suggested that the high catalytic reactivity is rooted in the small sizes (1.4 nm) of Ir nanoparticles and the high ratio (93%) of oxidized iridium species (IrO_x, Ir³⁺ and Ir⁴⁺) on the catalyst. Moreover, the delicate cooperation between the IrO_x and ZnO support also promoted its high reactivity. The selectivity of this catalytic N-methylation was controllable between dimethylation and monomethylation by carefully tuning the catalyst loading and reaction solvent. Specifically, neat methanol with high catalyst loading (2 mol% Ir) favored the formation of N,N-dimethylated amine, while the mesitylene/methanol mixture with low catalyst loading (0.5 mol% Ir) was prone to producing mono-N-methylated amines. An environmentally benign continuous flow system with a recycled mode was also developed for the efficient production of N-methylated amines. With optimal flow rates and amine concentrations, a variety of N-methylamines were produced with good to excellent yields in this Ir/ZnO-based flow system, providing a starting point for the clean and efficient production of N-methylamines with this cost-effective chemical process.