Superacidifiers: Assessing the Activation and the Mode of Charge Transmission of the Extraordinary Electron-Withdrawing SO₂CF₃and S(O)(NSO₂CF₃)CF₃Substituents in Carbanion Stabilization

Abstract

We report on a structural (multinuclear NMR), thermodynamic (pK a), and kinetic (Marcus intrinsic reactivity) study of the ionization of benzylic carbon acids activated by an exocyclic (α) SO2CF3 group and SO2CF3 or S(O)(NSO2CF3)CF3 in the para position of the phenyl ring. The latter exerts an enormous acidifying effect of ca. 8 pK units as compared with 4-H benzyltriflone in Me2SO solution, (corresponding to remarkably high Hammett σ values σp ≈ 1.35, σp - ≈ 2.30). In considering the origin of this effect, important information was derived in comparing medium effects on pK a's for NO2, SO2CF3, and S(O)(NSO2CF3)CF3 activated carbon acids. Highly contrasting behavior was thus induced by H2O → Me2SO transfer, with a large decrease in acidity of α-nitro activated carbon acids but a large increase in acidity of α-SO2CF3 analogues, leading to remarkable inversions in C−H acidity. These results support the view that in the case of the triflones the carbanion negative charge resides for the most part at the exocyclic Cα carbon, implying a major role of a polarizability effect. 1H, 13C, and 19F NMR data fully support this proposal. Most importantly, the intrinsic reactivity (log k 0) positioning 9 and 10 on the Marcus scale for carbon acids could be kinetically measured in 50%H2O−50%Me2SO; for 9, log k 0 = 3.80 and for 10, log k 0 = 4.20. Such high log k 0 values correspond to low intrinsic barriers which can only be reconciled on the basis of minimum electronic and structural reorganization in formation of the conjugate carbanions. This further emphasizes polarization as the predominant mechanistic mode of charge stabilization in these species.

Farewell to the HSAB Treatment of Ambident Reactivity

“…(3)]. Promising approaches to elucidate the magnitude and origin of intrinsic barriers as described by Bernasconi, [172] Bordwell, [173] Hoz, [23] Kreevoy, [20d] Lewis, [174] Murdoch, [175] Richard, [16a] and Terrier [176] should, therefore, be further developed.…”

Substituent effects on the reactions of aromatic triflones with para‐X‐anilines in methanol: Low intrinsic reactivity and transition state imbalances

“…Our results show that the replacement of NO 2 by SO 2 CF 3 is accompanied by an appreciable decrease in intrinsic reactivity ( log k o = −0.70). This is certainly a reflection of the preferential stabilization of the negative charge of the transition state by a para-SO 2 CF 3 group [28,29]. On the other hand, a recent study of the σ -complexation of 1a and 1b and other anisole series 2,4,6-trinitro-and 4-SO 2 CF 3 -2,6-dinitro-has revealed that the especially high capacity of resonance stabilization of the negative charge of the adducts through an Fπ -type polarization effect is a major factor that accounts for the strong activation provided by SO 2 CF 3 groups [4].…”

Abschied vom HSAB‐Modell ambidenter Reaktivität

“…(3)]. Vielversprechende Ansätze, die Größe und den Ursprung intrinsischer Barrieren zu ermitteln, wie von Bernasconi, [172] Bordwell, [173] Hoz, [23] Kreevoy, [20d] Lewis, [174] Murdoch, [175] Richard [16a] oder Terrier [176] beschrieben, sollten daher weiterentwickelt werden. …”