The Mechanism of Nucleophilic Substitution Reactions at an Iodine Center: Single-Well or Double-Well Potential Energy Surface?

Nucleophilic substitution (SN) reactions belong to the most important transformations in both organic and inorganic chemistry. Although SN reactivity is well-known for many main group elements, its extension to halogen centers is restricted to a few examples, and the mechanism of such reactions lacks fundamental understanding. Recent achievements initiated our investigations to decipher the mechanism of the substitution reactions between various amide ions and elemental I2. Here, we present that these reactions follow an addition-elimination pathway through a charge-transfer complex (single-well potential energy surface), opposing a classical bimolecular mechanism via a central transition state (double-well potential energy surface), as reported recently. In addition, inclusion of a continuum solvent model for a polar solvent (such as water) remarkably impacts the thermodynamic feasibility of these reactions. Uncovering the mechanism of these SN2 reactions is a key step to a general understanding of SN reactions involving other attacking nucleophiles and to possible future extensions toward further halogens.