Dynamic Hydrogen Bonding Tuned Enantioselectivity Control in Cobaloxime-Chiral Amine Cooperative Catalysis

Cobaloxime is a versatile 3d-metal catalyst for dehydrogenative radical coupling reactions. Prior mechanistic studies of cobaloxime catalysis have revealed that cobaloxime(II)-catalyzed hydrogen atom transfer (HAT) process can control the site-selectivity and chemoselectivity of radical transformation. However, the relevance of HAT to the enantioselectivity is seldom reported. Herein, our density functional theory studies of the cobaloxime-chiral amine co-catalyzed enantioselective coupling of the enamine radical cation with alkene proved that it is the HAT step that determines the enantioselectivity, instead of the previously proposed radical addition. Because the metal-alkene-coupled (MAC) radical addition is a reversible process while the followed by HAT is the rate-determining step. Moreover, computational studies suggest that the dynamically changing hydrogen bonding between the radical cation and cobaloxime plays a significant role in harnessing the reactivity and tuning the enantiocontrol. The intra- and inter-molecular hydrogen bonding between the iminium and the cobaloxime fragments contributes to the lower energy barrier of MAC radical addition. Structural analysis and quantitative steric-electronic effect dissection jointly unveiled that the stronger intermolecular hydrogen bonding between the aminium and the cobaloxime fragments is the dominant factor that stabilizes the (R)-HAT transition state and guarantees high enantioselectivity.