Secondary stereocontrolling interactions in chiral Bronsted acid catalysis: study of a Petasis-Ferrier-type rearrangement catalyzed by chiral phosphoric acids

Chiral phosphoric acids have emerged as promising asymmetric Bronsted acid catalysts that harness hydrogen bonding interactions as key stereocontrolling elements. A new approach to chiral phosphoric acid catalysis through ion-pairing interactions between the anionic conjugate base of the catalyst and a cationic electrophile has recently attracted attention. However, the mechanism of stereocontrol through ion-pairing interactions is still elusive. As a probe reaction for studying the stereocontrolling element involved in such catalytic reactions, we investigated the Petasis-Ferrier-type rearrangement of a 7-membered cyclic vinyl acetal catalyzed by chiral phosphoric acids. DFT calculations suggested that non-classical C-H center dot center dot center dot O hydrogen bonds between the catalyst and the substrate play an important role in determining the stereosetectivity. In addition, pi-pi stacking interactions were found to be a key factor for stereocontrol when using a 9-anthryl group-bearing catalyst.