27062-55-1Relevant articles and documents
Steric effects in the hydrolysis reactions of N-acylimidazoles. Effect of aryl substitution in the leaving group
Lee,Bembi, Ramesh,Fife, Thomas H.
, p. 2872 - 2876 (1997)
The kinetic and mechanistic effects of aryl substitution in the leaving group have been determined in the hydrolysis reactions of N-acylimidazoles. N-acyl derivatives of 2,4,5-triphenylimidazole hydrolyze rapidly in OH- and water reactions. The latter reactions are pH independent from pH 4 to 9. The N-acetyl derivative hydrolyzes with rate constants similar to those of N-acetylimidazole in the OH- reaction but 40-fold larger in the pH-independent reaction. N-(trimethylacetyl)-2,4,5-triphenylimidazole hydrolyzes at 15°C with k(OH), the second-order rate we constant for the OH-reaction, 26-fold larger than the rate constant for alkaline hydrolysis of the corresponding N-acetyl derivative, even though steric hindrance to approach of a nucleophile is extreme in the former reaction. The pH-independent reaction of the N-trimethylacetyl compound is 4-fold faster than that of the N-acetyl derivative and is characterized by a D2O solvent isotope effect (κ(H2O)/κ(D2O)) of 2.0. A phenyl substituent in the 2-position of the imidazole ring exerts a small rate-retarding effect in the hydrolysis reactions. N-(trimethylacetyl)-4,5-diphenylimidazole hydrolyzes 10- and 55-fold faster in the OH- and water reactions, respectively, at 15°C, than N-(trimethylacetyl)benzimidazole at 30°C, although the pK(a) of the leaving group is identical in the two cases. The additive nature of the steric rate-accelerating effects in the acyl group and the leaving group indicates an effect on the ease of C-N bond breaking; the hydrolysis reactions very likely proceed in a concerted manner without the formation of a stable tetrahedral intermediate.
