Asymmetric Aza-Diels-Alder Reaction with Ion-Paired-Iron Lewis Acid-Brønsted Acid Catalyst.

The development and use of a multiple-activation catalyst with ion-paired Lewis acid and Brønsted acid in an asymmetric aza-Diels-Alder reaction of simple dienes (non-Danishefsky-type electron-rich dienes) was achieved by utilizing the [FeBr2 ]+ [FeBr4 ]- combination prepared in situ from FeBr3 and chiral phosphoric acid. Synergistic effects of the highly active ion-paired Lewis acid [FeBr2 ]+ [FeBr4 ]- and a chiral Brønsted acid are important for promoting the reaction with high turnover frequency and high enantioselectivity. The multiple-activation catalyst system was confirmed using synchrotron-based X-ray absorption fine structure measurements, and theoretical studies. This study reveals that the developed catalyst promoted the reaction not only by the interaction offered by the ion-paired Lewis acid and the Brønsted acid but also noncovalent interactions.