α-Sulfonyl Carbanions:  Combined Kinetic, Thermodynamic, and NMR Approaches for the Study of the Ionization of Benzyltriflones in Me₂SO and H₂O−Me₂SO Mixtures

The ionization of a series of benzyltriflones ArCH 2 SO 2 CF 3 (Ar = C 6 H 5, 4-CNC 6 H 4, 4-NO 2 C 6 H 4, 2,4-(NO 2 ) 2 C 6 H 3, 2,4,6-(NO 2 ) 3 C 6 H 2 ) has been studied kinetically and thermodynamically as well as by 1 H, 13 C, and 19 F NMR in H 2 O−Me 2 SO mixtures and/or Me 2 SO. The findings that the acidity of the triflones is enhanced on going from hydroxylic solvents to Me 2 SO and that their deprotonation is associated with high Marcus intrinsic reactivities are two major results indicating the presence of a large polarizable charge density at C α of the carbanionic species. Comparison of the results obtained with those for related arylnitromethane and arylacetonitrile compounds strongly supports this conclusion. NMR data add to the evidence that polarization rather than d−p π-resonance or negative hyperconjugation is the dominant factor in accounting for the capability of the strongly electron withdrawing SO 2 CF 3 group to stabilize negative charge. Altogether, our results lead to a structural picture of the triflone carbanions in solution which does not totally fit the conclusions reached on the basis of solid-state or computed gas-phase studies.