Hydration of acylimidazoles: tetrahedral intermediates in acylimidazole hydrolysis and nucleophilic attack by imidazole on esters. The question of concerted mechanisms for acyl transfers

Abstract

. Can. J . Chem. 65, 1951Chem. 65, (1987.Heats of hydrolysis have been measured for three acylimidazole acetals. From these results free energies of formation in aqueous solution have been calculated for the acetals and the corresponding tetrahedral intermediates. Having free energies of formation for the tetrahedral intermediates allows a detailed analysis of the energetics of both the unobservable nucleophilic reaction of imidazole with ethyl acetate and also the observable reaction of imidazole with p-nitrophenyl acetate. pK, values of 1-(3-aminopropy1)-imidazole and 1-(2-aminoethy1)-imidazole have been determined to allow calculation of the a* value for the imidazolyl substituent. The dependence of imidazole pK, values on the electron-withdrawing properties of the 1-substituent was also determined. There is a linear free energy relation between the free energy change for replacement of OH in a carbonyl hydrate by imidazolyl and the sum of the a* values for the other substituents. The implications of these results for the question of concerted versus stepwise mechanisms for the reactions of imidazole with aryl acetates have been examined. An equilibrium constant has been calculated for the addition of imidazole to p-nitrophenyl acetate. A simple extension of Marcus theory allows the free energy surface for a three-dimensional reaction coordinate diagram to be calculated using the energy levels of the tetrahedral intermediate determined in this work, the energy level of the acylium ion derived from literature data, and intrinsic barriers for the edge reactions. It is shown that the reaction of imidazole with p-nitrophenyl acetate probably follows a concerted path. General conclusions from this theory of concerted reactions are discussed.

The hydration of ketene and carbon dioxide: an examination of the kinetics in terms of No Barrier Theory

“…Thus the free energy change upon covalent hydration of ketene to form ketene hydrate is ∆G°= -1.57 ± 2.03 kcal/mol, and the free energy change for hydration of ketene to acetic acid is ∆G°= -28.02 ± 1.16 kcal/mol. The free energy of formation of acetylium ion was estimated from that of acetic acid (15) with the pK R value calculated (19) from data of Deno et al (20). The extrapolation involved was long and consequently uncertainties are large but it is not clear how to estimate them.…”

Rate constants for decarboxylation reactions calculated using no barrier theory

“…For protonated malonic acid to neutral, we start with acetic acid for which pK BH + = -6.56 103 and correct for the effect of the neighboring COOH as was done for acetoacetic acid. 49 This leads to a pK BH + = -8.84 for protonated malonic acid.…”

Energetics of acetic acid enol in aqueous solution. A new method for thermodynamic estimation

“…Estimation of the solvent deuterium kinetic isotope effect by the method of Schowen (21-23) leads to a kinetic isotope effect of 0.69, and a revised estimate of the rate constant-of log kH+ = -10.14. The rate-determining step is presumably deprotonation at carbon of acetic acidium ion, for which the pK, is -6.56 (24,25); see Scheme 1. The rate constant for deprotonation of acetic acidium ion by solvent water is then log k = -3.58.…”