Superacidifiers: Assessing the activation and the mode of charge transmission of the extraordinary electron-withdrawing SO2CF3 and S(O)(=NSO2CF3)CF3 substituents in carbanion stabilization

摘要

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 (a) 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 benzy1triflone in Me2SO solution, (corresponding to remarkably high Hammett sigma values sigma(p) approximate to 1.35, sigma(p-) approximate to 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 alpha-nitro activated carbon acids but a large increase in acidity of alpha-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 C alpha carbon, implying a major role of a polarizability effect. H-1, C-13, and F-19 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.