Unconventional Pathways in Nitrogen-Centered S_N2 Reactions: From Roundabout to Hydride Transfer

The mechanisms and dynamics of bimolecular nucleophilic substitution (S_N2) reactions are complex and influenced by the nature of the central atom. In this study, we explore S_N2 at a nitrogen center (S_N2@N) by investigating the reaction of chloramine (NH₂Cl) with methoxide ion (CH₃O^-) using ab initio classical trajectory simulations at the MP2(fc)/aug-cc-pVDZ level of theory. We observe that, in addition to the expected S_N2 product formation (CH₃ONH₂ + Cl^-), a high-energy proton-transfer pathway leading to CH₃OH and NHCl^- dominates, with near-quantitative agreement between simulations and experimental data. Notably, we identify a novel hydride-transfer pathway yielding NH₃, H₂CO, and Cl^-, revealing alternative reactivity channels previously uncharacterized in nitrogen-centered S_N2 reactions. Mechanistic analysis uncovers unconventional roaming-mediated and roundabout pathways alongside the traditional direct rebound and indirect mechanisms. Additionally, an umbrella inversion of the NH₂ group resulting in retention of configuration in the CH₃ONH₂ product was observed in a fraction of trajectories.