Direct Access to meta-Alkylated N-Arylazolium Salts via Ruthenium Catalyzed Electronically Controlled Site Selective Functionalization: Scope and Mechanistic Aspect

Effective access to azolium salts with varying stereoelectronic properties has always fascinated the organometallic chemists. Herein, we described a Ru(II)-catalyzed one-step access to diverse meta-alkylated N-aryl (benz)imidazolium salts (>40 examples) via σ-bond activation strategy starting from the readily available azolium salts using alkyl bromide. Notably, the present method is compatible with a wide range of azolium salts and secondary/tertiary alkyl bromides, including the biologically relevant motifs. Various control experiments along with the DFT calculation established the reaction pathway of orthometalated Ru(II)-NHC complex (6) formation followed by the generation of a Ru(III)-intermediate, which controls the observed meta-selectivity, via single electron transfer. Crucially, detailed insight of the reaction mechanism helped to comprehend why some substrates are challenging for this methodology.