The Role of Ligand Dearomatization for Metal-Ligand Cooperation in Mn(I)-Catalyzed Hydrophosphination

Metal-ligand cooperation (MLC) via ligand dearomatization has been proposed as a key mechanistic feature in transition-metal-catalyzed hydrofunctionalizations; however, its necessity and generality remain under debate. In this work, we investigate the role of a dearomatization-driven MLC pathway in hydrophosphination reaction using a chiral manganese(I) complex that is also known to be a highly efficient catalyst for (transfer) hydrogenation reactions. Through comprehensive speciation, kinetic, and spectroscopic studies, we established the structure of the Mn(I) catalyst in solution and identify key reaction intermediates, including the isolation of both dearomatized and nondearomatized catalytically active species. We show that although multiple mechanistic pathways are accessible in this catalytic system, MLC via ligand dearomatization is not responsible for enantioselective hydrophosphination. Despite forming catalytically competent intermediate capable of H-P bond activation, this pathway is intrinsically coupled to free phosphide formation, which erodes enantioselectivity. In contrast, coordination of HPPh₂ to the Mn center followed by base-assisted deprotonation leads to enantioenriched product. A distinct NH-based mode of MLC contributes to catalytic turnover, particularly under conditions employing irreversible bases. We show that the base plays multiple noninnocent roles, controlling catalyst speciation and promoting competing pathways that affect enantioselectivity of the reaction. Together these experiments provide clear evidence that ligand dearomatization is not a mechanistic necessity for enantioselective hydrophosphination. These results clarify the role of dearomatization-based MLC in Mn(I)-catalyzed H-P bond activation, provide strategies for obtaining high stereoselectivity under mild conditions, and contribute to the broader discussion of how MLC operates in catalysis.