Metal‐Catalyzed C─H Activation for the Synthesis and Applications of Polysubstituted Quaternary Ammonium Salts

Abstract Polysubstituted quaternary ammonium salts (PQAS) have attracted significant interest for their broad applications in pharmaceuticals, catalysis, and material science. Metal‐catalyzed C─H activation has emerged as a transformative, atom‐economical strategy for their synthesis, offering exceptional regio‐ and stereoselectivity. This review explores recent advancements in the use of Rh, Ru, Co, Pd, Cu, and Ir complexes for their efficient synthesis, presenting key mechanistic insights into the roles of metal centers in steps such as ligand‐directed activation, oxidative addition, reductive elimination, and regeneration of catalyst by external oxidant. Furthermore, we focus on poly‐substituted cationic N ‐heterocycles, which play a vital role in natural products and functional materials, while also highlighting the remarkable fluorescent emission properties of quaternary ammonium salts, ranging from visible to long‐wavelength regions, along with their diverse biological applications. It also addresses challenges in expanding substrate scope, improving catalytic efficiency, and adopting sustainable methodologies. Emerging applications, particularly in electrocatalysis and biomimetic systems, underscore the broader potential of metal‐catalyzed organic synthesis. By offering a comprehensive overview, this work aims to inspire advancements in green chemistry, catalyst design, and novel applications, providing readers with a solid foundation in this rapidly evolving field.