Halogen Cation Transfer in Halogen Bonding Catalysis

The potential of fluorine to participate in halogen bonding (XB) has remained a subject of ongoing debate. The discovery of N-fluoropyridinium triflate as a unique "F-cation organocatalyst" for aziridine synthesis is considered to be the first reported example of F-halogen bonding catalysis. Nevertheless, the mechanistic details of this reaction have remained elusive, and compelling evidence supporting the F-halogen bond catalysis has been lacking. In this study, we present an in-depth computational investigation of the mechanism of this reaction to gain insight into the intriguing role of the F-cation organocatalyst. Our results, however, are inconsistent with the previous prevalent F-halogen bonding catalysis mechanism but instead, bring to light a new fluorine cation transfer mechanism. Control experiments provide further support for this alternative mechanism. Moreover, preliminary results suggest that other reactions traditionally attributed to ‘classic’ halogen bonding catalysis might also involve a halogen cation transfer process. These mechanistic insights have important far reaching implications for the rational design of halogen bonding catalytic systems.