Redox Active vs Redox Neutral in Ru/Pd-Catalyzed Sulfonylation: Theoretical Insights into Structure–Activity Relationship between Metal Centers and Regio-Selectivity

The structure-activity relationship between the metal center and regio-selectivity is persistently a pivotal scientific issue. To address this, we select the 2-phenylpyridine sulfonylation reactions catalyzed by ruthenium and palladium as research subjects. An extensive theoretical study has been conducted on their reaction mechanisms, the sources of regio-selectivity, and the evolution of electronic structures. The distinct electronic structures lead to completely different catalytic mechanisms and electronic structure evolution processes for ruthenium and palladium. Ruthenium tends to form six-coordinate octahedral complexes, thus undergoing an inner-sphere redox active Ru(II)-Ru(III)-Ru(IV)-Ru(II) catalytic cycle. In contrast, palladium tends to form four-coordinate planar quadrilateral complexes, hence undergoing an outer-sphere redox neutral Pd(II) catalytic cycle. The distinct electronic structure evolution processes fundamentally differentiate the radical attack modes in the sulfonation process, thereby determining the regio-selectivity of the reaction. In the Ru-catalyzed system, the meta-selectivity arises mainly from a more stable Schrock carbene-type meta-intermediate. For the Pd-catalyzed system, the ortho-selectivity mainly comes from the stabilizing effect of the Pd(II) center on the single electron. This study provides novel insights into how the electronic structure of metal centers influences the reaction mechanism and selectivity, making a theoretical contribution to a deeper comprehension of the mechanism and regio-selectivity underlying aromatic functionalization reactions.