FeCl3 as an Ion-Pairing Lewis Acid Catalyst. Formation of Highly Lewis Acidic FeCl2+ and Thermodynamically Stable FeCl4- To Catalyze the Aza-Diels-Alder Reaction with High Turnover Frequency.

The aza-Diels-Alder reaction of nonactivated dienes and imines was realized through the action of the ion-paired Lewis acid catalyst [FeCl2]+[FeCl4]- generated by the in situ disproportionation of FeCl3. The uniquely high reactivity of [FeCl2]+[FeCl4]- was attributed to both the highly Lewis acidic FeCl2+ and thermodynamically stable FeCl4- acting as an ion-paired catalyst. Synchrotron-based X-ray absorption fine structure measurements provided fundamental insights into the disproportionation and structure of the resulting ion-paired iron complex. A theoretical study was performed to analyze the catalytic reaction and better understand the "ion-pairing effect" which transforms simple FeCl3 into a high turnover frequency Lewis acid catalyst in the aza-Diels-Alder reaction of nonactivated dienes and imines.