Effect of HMPA on the Ground and Excited State Reactivity of SmI₂

Samarium Diiodide (SmI₂) is a privileged reducing agent in organic synthesis. Tunability of reactivity and selectivity of SmI₂ through additives made this reagent an attractive choice for organic chemists. Present investigation showed contrasting effects of hexamethylphosphoramide (HMPA) on the ground and excited states of reactions of SmI₂ with haloalkanes. While HMPA enhances the rate of reduction of haloalkanes in the ground state, a deleterious impact was found for excited-state reactions. Detailed investigations with kinetics, element effect, known BDE of C-H bonds, and computed C-X bond distance indicated that while ground state reactivity is governed by stability of radicals and weakening of C-X bond due to HMPA and SmI₂ coordination, excited state reactions are predominantly governed by inner sphere nature of electron transfer. The critical role of HMPA in facilitating the reduction of haloalkanes was confirmed using density functional theory-based calculations.