Influence of counterion on the methylation of some ambident nucleophiles: DFT study

Abstract

The reactions of ambident anions YCHX⁻ and nucleophiles YCHXNa (X, Y = CH₂, O, and S) with CH₃F were investigated in the gas phase and THF solution. For this purpose the M06-2X/6-311+G(d,p) theoretical model was engaged in combination with the CPCM solvation model. The goal of the work was to examine the influence of counterion (Na⁺ in this case) to the reaction systems. It was found that the order of reactivity of non-ambident anions in both media is as follows: ${\text{CH}}_{2} {\text{CHCH}}_{2}^{ - }$ > OCHO⁻ > SCHS⁻. On the other side, the reactivity of non-ambident nucleophiles follows different order: CH₂CHCH₂Na > SCHSNa > OCHONa. The latter order of reactivity is in agreement with chemical intuition as it is inversely proportional to the order of hardness of the respective nucleophilic atoms: C < S < O. Ambident nucleophiles behave differently in the two media, thus indicating that solvents also influence the examined reactions. Namely, HOMO–LUMO gaps are smaller in the gas-phase, whereas charge distribution is more favorable in THF solution. Owing to these facts, C-methylation is more favorable than S- and O-methylations, and S-methylation is less unfavorable than O-methylation in the gas-phase. In THF solution O- and S-methylations are kinetically controlled, and competitive C-methylations are thermodynamically controlled. In this way, the applicability of the hard–soft acid–base theory to the examined reaction systems is confirmed. Furthermore, the findings of this work offer robust confirmation to our approach which involves counterions in examinations. Counterions do exist in reaction systems, and their influence should not be neglected.