Super acidifiers: the origin of the exceptional electron transmission capability of the SO2CF3 group in carbanion stabilization.

As part of studies aimed at clarifying conflicting reports concerning the acidifying effects exerted by the SO2CF3 vs. NO2 moieties with respect to carbanion stabilities, we have investigated the ionization equilibria of an extended set of benzyltriflones and have determined both pKa values of the carbon acids and 1H and 13C NMR parameters of the resulting carbanions. Acidities determined in H2O-Me2SO mixtures and in pure Me2SO show a contrasting behaviour between 4-X-substituted benzyl triflones and related arylnitromethanes. While the latter exhibit a decreasing acidity on going from H2O to Me2SO media, the benzyltriflone analogues show in fact increasing acidity in Me2SO. This opposing trend suggests that the exocyclic alpha-SO2CF3 moiety is strongly stabilizing the negative charge of the carbanions through polarizability effects favored by the dipolar aprotic Me2SO solvent. As a result, inversions in the acidity sequences of alpha-NO2 and alpha-SO2CF3 activated carbon acids are observed on going from H2O to Me2SO. 1H and 13C NMR data are in full accord with the conclusion that only little negative charge is transferred to the 4-X-substituted phenyl ring upon ionization. Increasing further the ring substitution by electronegative groups to 2,4- and 2,4,6- patterns, enhances the charge transfer but this nevertheless remains moderate even with the most activated 2,4,6- trinitro or 2,6-dinitro-4-SO2CF3 sequences. Altogether, our results provide convincing evidence of the unusual electron transmission ability of the very strongly acidifying SO2CF3 group.