Effect of Solvation on the Nucleophilic Reaction of Stable Carbanions with Diaryl Disulfides

Article abstract of DOI:10.1021/ja00543a028

Stable carbanions react with diaryl disulfides in aqueous solution by direct displacement (S_N2) reaction to yield an arylthiol anion and the corresponding sulfide as products.The reaction of 1,3-dicarbonyl carbanions with 5,5'-dithiobis(2-nitrobenzoic acid) is characterized by Broensted β_C value of 0.5.Nitroalkane carbanions react 10²-10⁴ slower than 1,3-dicarbonyl carbanions of the same pK and are correlated by a Broensted β_C of 0.95.The second-order rate constants for the reaction of nitroalkane carbanions increase by factors of 10⁴-10⁶ as the solvent is changed from water to dimethyl sulfoxide.Smaller increases are observed in the rate constants for reaction of 2,4-pentadienone carbanion (10²) and malonitrile carbanion (10^0.6) with the same sulfide.A linear correlation is found between log (k_Me2SO4/k_HOH) and the increase in pK for the carbon acid ionization on changing the solvent from water to dimethyl sulfoxide.The effect is attributed to large differences in ground-state carbanion solvation which are reduced or absent in the transition state.Parallels are drawn between the nucleophilic reaction of stable carbanions with diaryl disulfides and proton-transfer reactions of the same carbanions.