The Control Effect of Different Scaffolds in Chiral Phosphoric Acid: A Case Study of Enantioselective Asymmetric Arylation
Chiral phosphoric acids (CPAs) catalysis with BINOL or SPINOL backbone has attracted immense attentions in recent years. Despite a number of successful studies, the theoretical elucidation of the CPAs scaffold role is rarely considered and its effect on enantioselectivity is still far from being understood. The structure inspection shows that the axially chiral scaffolds induce a stereogenic central phosphate, and the phosphoric acid functional groups in (R/S)-BINOL are identical to those in (S/R)-SPINOL skeleton. Therefore, a hypothesis is proposed that each orientation of phosphoric acid functional groups regulated by different (R/S) BINOL- and SPINOL-derived scaffolds may control the signs of enantioselectivity (positive or negative of ee value) by changing the substrates combination modes. In other words, the sign of enantioselectivity (positive or negative of ee value) can be tuned by (R or S) of BINOL- or (S or R) SPINOL-derived backbone, respectively. Thus, we elaborate an in-depth mechanistic case study of newly reported CPA-catalyzed enantioselective asymmetric arylation reactions (a-c) catalyzed by CPAs with two different backbones. We found that although the origin of high enantioselectivity for these three case reactions a-c can be ascribed to favorable C−H•••O interactions, catalyst and substrates distortion interactions, and electrostatic interactions in the preferential TSs leading to the major products, the signs of enantioselectivity still follow our hypothesis. Our general applicability finding encourages the further development of more effective CPA catalysts and can be used to guide strategic choice of CPA skeletons.