Inverse solvent effects in carbocation carbanion combination reactions: The unique behavior of trifluoromethylsulfonyl stabilized carbanions

Second-order rate constants for the reactions of the trifluoromethylsulfonyl substituted benzyl anions 1a-e (CF3SO2CH--C6H4-X) with the benzhydrylium ions 2f-j and structurally related quinone methides 2a-e have been determined by UV-vis spectroscopy. The reactions proceed approximately 10-40 times faster in methanol than in DMSO leading to the unique situation that these carbocation carbanion combinations are faster in protic than in dipolar aprotic media. The pK(a) values of some benzyl trifluoromethylsulfones were determined in methanol (1c-H, 17.1; 1d-H, 16.0; 1e-H, 15.0) and found to be 5 units larger than the corresponding values in DMSO. Rate and equilibrium measurements thus agree that the trifluoromethylsulfonyl substituted benzyl anions 1a-e are more effectively solvated by ion-dipole interactions in DMSO than by hydrogen bonding in methanol. Bronsted correlations show that in DMSO the trifluoromethylsulfonyl substituted carbanions 1 are less nucleophilic than most other types of carbanions of similar basicity, indicating that in DMSO the intrinsic barriers for the reactions of the localized carbanions 1 are higher than those of delocalized carbanions, including nitroalkyl anions. The situation is reversed in methanol, where the reactions of the localized carbanions 1 possess lower intrinsic barriers than those of delocalized carbanions as commonly found for proton-transfer processes. As a consequence, the relative magnitudes of intrinsic barriers are strongly dependent on the solvent.