Computational Exploration of Dirhodium Complex-Catalyzed Selective Intermolecular Amination of Tertiary vs. Benzylic C−H Bonds

Abstract

The mechanism and origins of site-selectivity of Rh2(S-tfpttl)4-catalyzed C(sp3)–H bond aminations were studied using density functional theory (DFT) calculations. The synergistic combination of the dirhodium complex Rh2(S-tfpttl)4 with tert-butylphenol sulfamate TBPhsNH2 composes a pocket that can access both tertiary and benzylic C–H bonds. The nonactivated tertiary C–H bond was selectively aminated in the presence of an electronically activated benzylic C–H bond. Both singlet and triplet energy surfaces were investigated in this study. The computational results suggest that the triplet stepwise pathway is more favorable than the singlet concerted pathway. In the hydrogen atom abstraction by Rh–nitrene species, which is the rate- and site-selectivity-determining step, there is an attractive π–π stacking interaction between the phenyl group of the substrate and the phthalimido group of the ligand in the tertiary C–H activation transition structure. By contrast, such attractive interaction is absent in the benzylic C–H amination transition structure. Therefore, the DFT computational results clearly demonstrate how the synergistic combination of the dirhodium complex with sulfamate overrides the intrinsic preference for benzylic C–H amination to achieve the amination of the nonactivated tertiary C–H bond.

Catalyst-Controlled Intermolecular Homobenzylic C(sp3)–H Amination for the Synthesis of β-Arylethylamines

“…Therefore, it appears that the key factor controlling the regioselectivity of the reaction is the degree of interaction between the catalyst and the substrate. In fact, weak interactions were recently described to control the tertiary vs benzylic selectivity in rhodium-catalyzed C–H amination with tert -butylphenol sulfamate . The tertiary C–H amination was found to be assisted by the establishment of attractive π–π stacking interactions between the substrate and the catalyst, while such noncovalent interactions were absent from the benzylic C–H amination transition state.…”

“…In fact, weak interactions were recently described to control the tertiary vs benzylic selectivity in rhodium-catalyzed C−H amination with tert-butylphenol sulfamate. 40 The tertiary C−H amination was found to be assisted by the establishment of attractive π−π stacking interactions between the substrate and the catalyst, while such noncovalent interactions were absent from the benzylic C−H amination transition state. In our case, the analysis of noncovalent interaction plots featured in S ts1 B and S ts1 H indicates the presence of stabilizing π−π stacking interactions at both transition states.…”