Inverse Solvent Effects in Carbocation Carbanion Combination Reactions:  The Unique Behavior of Trifluoromethylsulfonyl Stabilized Carbanions

Abstract

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. Brønsted 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.

Reactions of Substituted Phenylnitromethane Carbanions with Aromatic Nitro Compounds in Methanol: Carbanion Reactivity, Kinetic, and Equilibrium Studies

“…Evaluation of the correlation lines according to Eq. (2) yields the N and s parameters for carbanions 1a-d in methanol, as listed in Table VI. As expected, electron-withdrawing groups at the paraposition of the aromatic ring decrease the nucleophilicities N of the carbanions 1a-d. On the other hand, the solvent effects, as previously reported for the basicities of carbanions [35,36], have now been found for their nucleophilicities. Because the stabilization of most carbanions occurs to a large extent through hydrogen bonding, the basicities as well as the nucleophilicities of carbanions are usually smaller in protic than in aprotic solvents, in line with the observation that the nucleophilicities of carbanions 1ad are higher in DMSO than water and methanol.…”

Di‐ and Triarylmethylium Ions as Probes for the Ambident Reactivities of Carbanions Derived from 5‐Benzylated Meldrum’s Acid

“…In order to quantify the nucleophilic carbon reactivity of enolate anions, we have previously investigated the kinetics of their reactions with benzhydrylium ions (Ar 2 CH + ) and structurally related quinone methides. Analysis of the second‐order rate constants ( k ) of these reactions by the correlation Equation (1),2–6 in which E describes the reactivity of the reference electrophiles, provided the nucleophile‐specific, solvent‐dependent nucleophilicity parameters N (and sensitivities s N ) for enolates and other types of acceptor‐stabilized carbanions in DMSO,7–17 water,9, 10 and methanolic solutions 11. 18 …”

Understanding the Nucleophilic Character and Stability of the Carbanions and Alkoxides of 1-(9-Anthryl)ethanol and Derivatives

“…Negative δΔ acid G° values for 1b-1d indicate that the electron-releasing substituent is not efficient enough to delocalize the negative charge into the aromatic ring; the carbanions of 1b-1d are less stable than the alkoxides by 7.7-5.2 kcal mol −1 respectively. Therefore, the stability of the carbanions of 1 and 2 increases as the electron-acceptor capacity of the substituent increases [17]. The correlation between the electronic effect of X and the ionization reactions of Scheme 1, is evident from the good linear regression analysis obtained for Δ acid G°(CH) or Δ acid G°(OH) and the Hammett substituent constants in the gas σ p Table 2) [18].…”