Redox Active vs Redox\nNeutral in Ru/Pd-Catalyzed Sulfonylation:\nTheoretical Insights into Structure–Activity Relationship between\nMetal Centers and Regio-Selectivity

The\nstructure–activity relationship between the metal center\nand regio-selectivity is persistently a pivotal scientific issue.\nTo address this, we select the 2-phenylpyridine sulfonylation reactions\ncatalyzed by ruthenium and palladium as research subjects. An extensive\ntheoretical study has been conducted on their reaction mechanisms,\nthe sources of regio-selectivity, and the evolution of electronic\nstructures. The distinct electronic structures lead to completely\ndifferent catalytic mechanisms and electronic structure evolution\nprocesses for ruthenium and palladium. Ruthenium tends to form six-coordinate\noctahedral complexes, thus undergoing an inner-sphere redox active\nRu(II)–Ru(III)–Ru(IV)–Ru(II) catalytic cycle.\nIn contrast, palladium tends to form four-coordinate planar quadrilateral\ncomplexes, hence undergoing an outer-sphere redox neutral Pd(II) catalytic\ncycle. The distinct electronic structure evolution processes fundamentally\ndifferentiate the radical attack modes in the sulfonation process,\nthereby determining the regio-selectivity of the reaction. In the\nRu-catalyzed system, the meta-selectivity arises mainly from a more\nstable Schrock carbene-type meta-intermediate. For the Pd-catalyzed\nsystem, the ortho-selectivity mainly comes from the stabilizing effect\nof the Pd(II) center on the single electron. This study provides novel\ninsights into how the electronic structure of metal centers influences\nthe reaction mechanism and selectivity, making a theoretical contribution\nto a deeper comprehension of the mechanism and regio-selectivity underlying\naromatic functionalization reactions.