The effect of CF3 functional group substituent on bifunctional activation model and enantioselectivity for BINOL N-triflylphosphoramides catalyzed rearrangement reaction

The acidity of the chiral Bronsted acid catalysts is crucial for the catalytic efficiency, the highly acidic BINOL-derived chiral N-triflylphosphoramides can provide effective asymmetric catalysis for rearrangement reactions. Despite multiple interactions between the basic/acidic sites of the functional group and the substrates have been evaluated, the origin of high catalytic activity and excellent enantioselelctivity of N-triflylphosphoramides catalyst is still challenging. Especially the role of CF3 groups in controlling activation mode and determining enantioselectivity. Thus, two effective case reactions of pinacol and acyloin rearrangement catalyzed by BINOL N-triflyphosphoramide were employed to provide basis understanding for these issues. Our calculations reveal that the P(=NTf)OH tautomer of N-triflyphosphoramide is an active catalyst form for the rearrangement reaction, differing from previous proposed model involving P(=O)NHTf group. This can be ascribed to the additional C-F center dot center dot center dot pi interaction between the CF3 group of catalyst and the migration group of substrate in the preferred activation model. Furthermore, we found that the CF3 substituent on the central functional group effectively aids the bifunctional activation and improves catalytic activity of N-triflyphosphoramide catalyst. More importantly, the CF3 substituent and the orientation of migration groups play a significant role in controlling the enantioselectivity by contributing different strength multiple C-H center dot center dot center dot F, C-F center dot center dot center dot pi, and C-H center dot center dot center dot pi interactions between catalyst and substrate. Overall, our findings on the factors affecting the stereochemical control may show utility for other challenging asymmetric reactions catalyzed by chiral N-triflylphosphoramides. (C) 2020 Elsevier Inc. All rights reserved.