Origin of Salt Effects in S(N)2 Fluorination Using KF Promoted by Ionic Liquids: Quantum Chemical Analysis

Quantum chemical analysis is presented, motivated by Grée and co-workers’ observation of salt effects [Adv. Synth. Catal. 2006, 348, 1149–1153] for S(N)2 fluorination of KF in ionic liquids (ILs). We examine the relative promoting capacity of KF in [bmim]PF(6) vs. [bmim]Cl by comparing the activation barriers of the reaction in the two ILs. We also elucidate the origin of the experimentally observed additional rate acceleration in IL [bmim]PF(6) achieved by adding KPF(6). We find that the anion PF(6)(−) in the added salt acts as an extra Lewis base binding to the counter-cation K(+) to alleviate the strong Coulomb attractive force on the nucleophile F(−), decreasing the Gibbs free energy of activation as compared with that in its absence, which is in good agreement with experimental observations of rate enhancement. We also predict that using 2 eq. KF together with an eq. KPF(6) would further activate S(N)2 fluorination