88563-51-3Relevant academic research and scientific papers
Dynamics of reaction of ion pairs in aqueous solution: Racemization of the chiral ion pair intermediate of solvolysis of (S)-1-(4-methylphenyl)ethyl pentafluorobenzoate
Tsuji, Yutaka,Mori, Tetsuo,Toteva, Maria M.,Richard, John P.
, p. 484 - 490 (2007/10/03)
The chiral ester (S)-1-(4-methylphenyl)ethyl pentafluorobenzoate, (S)-1-OC(O)C6F5, was prepared and a value of k rac = 8.5 × 10-7 s-1 was determined for the rate constant for its racemization to (R)-1-OC(O)C6F5 in 50:50 (v/v) trifluoroethanol-water (I = 0.50, NaClO4). This rate constant is 12 times smaller than ksolv = 1.06 × 10 -5 s-1 for the stepwise solvolysis of 1-OC(O)C 6F5, which shows that inversion of the ion-pair intermediate is a relatively rare event during solvolysis in a largely aqueous solvent, and two times smaller than kiso = 1.6 × 10 -6 s-1 for degenerate isomerization which exchanges the position of the ester bridging and non-bridging oxygens of 1-OC(O)C 6F5. A simple relationship is derived between the empirical rate constant ratio krac/kiso and the microscopic rate constants for reaction of the ion-pair intermediate within its solvation shell. Substitution of estimated rate constants for reactions of the 1-(4-methylphenyl)ethyl carbocation-pentafluorobenzoate anion pair into this equation gives ki = 1.5 × 1010 s-1 for inversion of the chiral ion pair, which is similar to k-d = 1.6 × 1010 s-1 for its separation to free ions. Copyright
Formation and Stability of Ring-Substituted 1-Phenylethyl Carbocations
Richard, John P.,Rothenberg, Marc E.,Jencks, William P.
, p. 1361 - 1372 (2007/10/02)
The solvolysis of 1-phenylethyl derivatives with electron-donating 4-substituents in 50:50 trifluoroethanol:water(v:v) occurs at a rate that is independent of azide concentration but gives yields of the corresponding azide adducts of up to 98percent by trapping a carbocation intermediate.Rate constants for reactions of the cations with solvent range from 2 x 103 s-1 (4-Me2N) to 4 x 109 s-1 (4-Me), assuming a diffusion-controlled rate constant of 5 x 109 M-1 s-1 for their reactions with azide and thiol anions.Correlation of the rate constants following the Yukawa-Tsuno treatment gives ρn = 2.5, ρr = 5.2, and r+ = 2.1 for the reaction with trifluoroethanol, and ρn = 2.7, ρr = 4.9, and r+ = 1.8 for the reaction with water.The reverse reaction, acid-catalyzed cleavage of substituted 1-phenylethyl alcohols to give the corresponding carbocation, follows ρn = -4.9, ρr = -4.4, and r+ = 0.9.This gives values of ρn = -7.6, ρr = -9.3, and r+ = 1.2 for formation of the cations at equilibrium.There is an imbalance in the development of resonance delocalization, analogous to the "nitroalkane anomaly", that is consistent with a dependence of the fraction of maximal resonance delocalization on the fraction of rehybridization or C-X bond cleavage.Solvent effects on carbocation stability in aqueous-organic mixtures are relatively small.They depend mainly on the nucleophilicity of the solvent components and a specific solvent effect of trifluoroethanol on the reactivity of hydroxylic nucleophiles, including trifluoroethanol itself.The "ionizing power" of the solvent has only a small effect on cation stability, and there is little effect of the concentration or nature of added salts.
