LIFETIMES OF IMINIUM IONS IN AQUEOUS SOLUTION

摘要

Iminium cations have been generated in aqueous solution from the solvolysis of anilinothioethers ArN(CH3)CH(2)SR at 25 degrees C. Common ion inhibition of the solvolysis of anilinothioethers was observed when the thiolate anion leaving group was added to the reaction mixture. This inhibition results from trapping of the iminium ion by RS(-) to regenerate the anilinothioether. Rate constants, k(H2O), for hydration of the iminium ions were determined from common ion inhibition of the solvolysis of anilinothioethers in the presence of added thiolate ions and a rate constant of 5 x 10(9) M(-1) s(-1) for diffusion-controlled trapping of the iminium ions by RS(-), which regenerates the starting material. The values of k(H2O) for H2C=N+(CH3)C(6)H(4)X are 1.0 x 10(8), 3.1 x 10(7), 3.0 x 10(7), 5.5 x 10(6), 4.5 x 10(6), and 3.1 x 10(6) s(-1) for X = 4-NO2, 4-CN, 3-NO2, 3-Cl, 4-COO-, and 4-Cl, respectively. Rate constants for the solvolysis of anilinothioethers, k(solv) depend on the pK(a) of the thiolate leaving group with beta(1g) = -0.93 +/- 0.09, which shows that the dissociation reaction has a very late transition state. This result is consistent with rate-limiting diffusion-controlled encounter of RS(-) with H2C=N+(CH3)Ar in the reverse direction. The value of beta(nuc) = 1.00-0.93 = 0.07 +/- 0.09 approximate to 0 is expected for a diffusion-controlled addition reaction. The solvolysis reaction is sensitive to the basicity of the electron-donating nitrogen atom of the aniline with a value of beta(dg) = 0.79 +/- 0.06; i.e., there is a large amount of electron donation from the nitrogen atom that provides the driving force for cleavage of the CH2-S bond in the late transition state. Hammett correlations of the rate constants for the solvolysis reaction, k(solv), show a better correlation with sigma(-) than with sigma for substituents on the aniline and follow rho(-) = -3.3 +/- 0.3. This also indicates a large amount of electron donation and an important contribution of resonance in the transition state of the solvolysis reaction. Reaction of the iminium ions with water is accelerated by electron-withdrawing substituents on the aniline ring and follows rho(-) = 1.5 +/- 0.2 and beta(dg) = -0.35 +/- 0.03; i.e., the rate of reaction of the iminium cation with water is approximately half as sensitive to polar substituents as the solvolysis reaction. [References: 30]