Synthesis, Development, and Applications of Chiral Salen Ligands in Asymmetric Catalysis

Chiral salen ligands, distinguished by their exceptional spatial and electronic tunability, serve as pivotal scaffolds in asymmetric catalysis, forming stable complexes with diverse metal ions through a robust [O,N,N,O] tetradentate coordination motif derived from the condensation of vicinal diamines with salicylaldehyde derivatives; this review consolidates advances over the past two decades, encompassing synthetic methodologies, structural evolution, and catalytic applications, where Mn-salen complexes facilitate enantioselective oxidations with high stereocontrol, exemplified by oxidative kinetic resolution of secondary alcohols and sulfide oxidations, Co-salen complexes exhibit unparalleled efficiency in hydrolytic kinetic resolution and CO2/epoxide copolymerization, achieving quantitative conversions and exceptional enantiomeric excesses (ee >99%), and Ti-salen systems deliver high enantioselectivity in sulfide oxidations across diverse substrates due to broad electronic tolerance. Innovative extensions include enantioselective olefin epoxidation, asymmetric nitroalkene cyanation, and sustainable polycarbonate synthesis from CO2, offering significant advantages such as low toxicity, recyclability, and operational efficiency in aqueous media, while computational studies provide mechanistic insights, elucidating atomic-level behavior and the electronic origins of catalytic performance; future research priorities emphasize machine learning-guided design of programmable derivatives, development of redox-tunable electrocatalytic platforms, and lifecycle-optimized synthesis to enhance stability, selectivity, and reusability, thereby underscoring the structural versatility of salen scaffolds in advancing green chemistry, pharmaceutical synthesis, and CO2 utilization, with interdisciplinary innovation addressing current challenges to unlock full potential in sustainable catalysis and refine performance metrics for practical implementation.