22719-89-7Relevant academic research and scientific papers
Kinetics and mechanism of the aminolysis of ethyl aryl carbonates in acetonitrile
Koh, Han Joong,Lee, Ji-Won,Lee, Hai Whang,Lee, Ikchoon
, p. 710 - 716 (2007/10/03)
The aminolysis reactions of ethyl aryl carbonates with benzylamines in acetonitrile at 25.0°C are investigated. The base-catalyzed path, k2, disappears when strong nucleophiles (X = p-CH3O and p-CH3) react with a substrate activated by a strong nucleofuge (Z = p-NO2). The large magnitude of ρ(x) (-1.7 to -2.5), ρ(z) (3.4 to 4.3), and ρ(xz) (1.4) values, and relatively large k(H)/k(D) (1.6 to 1.8) found for the uncatalyzed path (k1) can be accounted for in terms of a stepwise mechanism with rate-limiting expulsion of the phenoxide leaving group. The catalyzed process (k2) is characterized by the much smaller magnitude of ρ(x) (-1.0 to -1.7), ρ(z) (0.4 to 0.7), and ρ(xz) (0.2), the larger k(H)/k(D) (2.1 to 2.5) values, and the lower ΔH(+) values (1.8-1.9 kcal mol-1) than those of the uncatalyzed process (k1) with large negative ΔS(+) values (-65 to -67 cal K-1 mol- 1). These results are consistent with four- and six-centered transition states for the two processes, k1 and k2, respectively.
Effect of Nucleophile Basicity on Intramolecular Nucleophilic Aminolysis Reactions of Carbonate Diesters
Fife, Thomas H.,Hutchins, J. E. C.
, p. 4194 - 4199 (2007/10/02)
The rates of phenol release from para-substituted phenyl 2-pyridylethyl carbonates have been measured in H2O at 25 deg C (μ 0.5 M).The pH-rate constant profiles are sigmoidal, showing participation by the pyridine neutral base species.The D2O solvent isotope effect is nearly unity, indicating that participation is by a nucleophilic reaction.The Hammett ρ value for intramolecular pyridine assisted phenol release is +2.2, and the fit is better with ?- than with ?, indicating considerable C-O bond breaking in the transition state.The effective molarity of the neighboring pyridine of p-nitrophenyl 2-pyridylethyl carbonate is 81 M in comparison with pyridine acting as a bimolecular catalyst in the hydrolysis of ethyl p-nitrophenyl carbonate.Intramolecular nucleophilic attack is 217 times more favorable when a five-membered ring is formed with phenyl 2-pyridylmethyl carbonate than in the case of phenyl 2-pyridylethyl carbonate where the reaction proceeds via a six-membered ring.The effective molarity of neighboring pyridine in the 2-pyridylmethyl series is 2x103 M.Sigmoidal pH-rate constant profiles were also obtained with neighboring imidazole, N,N-dimethylamino, and N-methylpiperidine nucleophiles.Values of the limiting rate constants (kB) are similar in spite of pKapp values which vary from 3.9 to 10.The effective molarity of the dimethylamino group of p-nitrophenyl N,N-dimethylaminopropyl carbonate is only 32 M in comparison to reaction of triethylamine with ethyl p-nitrophenyl carbonate.On the other hand, the effective molarity of the dimethylamino group of p-nitrophenyl o-(N,N-dimethylamino)phenyl carbonate (pKapp=3.9) is > 105 M.The most efficient intramolecular nucleophiles in reactions of p-nitrophenyl carbonate diesters are those of low pKa.In contrast, with analogous carboxylate esters the converse is the case, even though the rate constants for bimolecular aminolysuis of p-nitrophenyl acetate and ethyl p-nitrophenyl carbonate are closely similar.These results may indicate that C-N bond formation is not complete in the transition states of the intramolecular reactions.
