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

Abstract

As part of studies aimed at clarifying conflicting reports concerning the acidifying effects exerted by the SO₂CF₃vs. NO₂ moieties with respect to carbanion stabilities, we have investigated the ionization equilibria of an extended set of benzyltriflones and have determined both pK_a 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 α-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 α-NO₂ and α-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.