Microwave-accelerated catalysis with Ru(II)-protic-NHC complexes for selective N-methylation of anilines using methanol as a sustainable C1 source

A sustainable and atom-economic strategy for the selective N -methylation of anilines using methanol as a benign C1 source has been explored with Ru(II)-protic-NHC complexes, resulting in excellent performance under microwave irradiation. A series of Ru(II)-protic-N-heterocyclic carbene complexes bearing unsymmetrical CNN pincer ligands with benzimidazolylidene-based ( Ru1-Ru4 ), triazolylidene-based ( Ru5-Ru7 ), and newly synthesized imidazolylidene-based complexes ( Ru8 & Ru9 ) have been evaluated, among which the benzimidazolylidene complex Ru1 exhibited outstanding catalytic performance. Under optimized conditions, Ru1 achieved >99% conversion and excellent mono- N -methylation selectivity utilizing methanol as both the methyl donor and hydrogen source. Remarkably, microwave irradiation accelerated the reaction, reducing the time from 12 h (thermal) to just 35 min at 40°C with comparable efficiency. The method displays broad substrate scope, tolerating electron-donating, electron-withdrawing, heteroaromatic, and sterically hindered substrates to afford quantitative N -methylated products. Gram-scale synthesis confirmed the robustness and scalability of the process. Mechanistic studies, supported by NMR, mass spectrometry, and GC-MS analyses, along with control experiments, identified N-phenylmethanimine and N-phenylformamide as key intermediates in a borrowing-hydrogen pathway. The synergistic Ru-protic-NHC framework, combined with microwave-acceleration, provides a green, rapid, and scalable approach for the efficient N-methylation of anilines.