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. 2021 Sep 22;26(19):5738.
doi: 10.3390/molecules26195738.

Origin of Salt Effects in SN2 Fluorination Using KF Promoted by Ionic Liquids: Quantum Chemical Analysis

Young-Ho Oh  1 , Sungyul Lee  1
Affiliations

Origin of Salt Effects in SN2 Fluorination Using KF Promoted by Ionic Liquids: Quantum Chemical Analysis

Young-Ho Oh et al. Molecules. .

Abstract

Quantum chemical analysis is presented, motivated by Grée and co-workers' observation of salt effects [Adv. Synth. Catal. 2006, 348, 1149-1153] for SN2 fluorination of KF in ionic liquids (ILs). We examine the relative promoting capacity of KF in [bmim]PF6 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]PF6 achieved by adding KPF6. We find that the anion PF6- 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. KPF6 would further activate SN2 fluorination.

Keywords: SN2 fluorination; ionic liquid; mechanism; salt effect.

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Conflict of interest statement

The authors declare no conflict of interest.

Figures

Scheme 1
Scheme 1
SN2 fluorination of trichlorotoluene by KF [28].
Figure 1
Figure 1
Fluorination by KF in ILs [28].
Figure 2
Figure 2
Calculated (a) transition states and (b) energetics of SN2 fluorination in [bmim]PF6 with substrate: KF = 1:1.
Figure 3
Figure 3
Calculated (a) transition states and (b) energetics of SN2 fluorination in [bmim]Cl with substrate: KF = 1:1.
Figure 4
Figure 4
Calculated (a) transition states and (b) energetics of SN2 fluorination in [bmim]PF6 activated by KPF6, with substrate:KF:KPF6 = 1:1:1.
Figure 5
Figure 5
Calculated (a) transition state and (b) energetics of SN2 fluorination in [bmim]PF6 with substrate:KF = 1:2.
Figure 6
Figure 6
Calculated (a) transition state and (b) energetics of SN2 fluorination in [bmim]PF6 with substrate: KF:KPF6 = 1:2:1.
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