• Bulletin of the Korean Chemical Society
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• v.33 no.9
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• pp.2971-2975
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• 2012

Second-order rate constants $k_N$ have been measured spectrophotometrically for nucleophilic substitution reactions of t-butyl 4-pyridyl carbonate 8 with a series of alicyclic secondary amines in $H_2O$ at $25.0{\pm}0.1^{\circ}C$. The Br${\emptyset}$nsted-type plot for the reactions of 8 is linear with ${\beta}_{nuc}$ = 0.84. The ${\beta}_{nuc}$ value obtained for the reactions of 8 is much larger than that reported for the corresponding reactions of t-butyl 2-pyridyl carbonate 6 (i.e., ${\beta}_{nuc}$ = 0.44), which was proposed to proceed through a forced concerted mechanism. Thus, the aminolysis of 8 has been concluded to proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate $T^{\pm}$, in which expulsion of the leaving-group from $T^{\pm}$ occurs at the rate-determining step (RDS). In contrast, aminolysis of benzyl 4-pyridyl carbonate 7 has been reported to proceed through two intermediates, $T^{\pm}$ and its deprotonated form $T^-$ on the basis of the fact that the plots of pseudo-first-order rate constant $k_{obsd}$ vs. amine concentration curve upward. The current study has demonstrated convincingly that the nature of the leaving and nonleaving groups governs the reaction mechanism. The contrasting reaction mechanisms have been rationalized in terms of an intramolecular H-bonding interaction, steric acceleration, and steric inhibition.

• Bulletin of the Korean Chemical Society
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• v.35 no.1
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• pp.93-97
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• 2014

A kinetic study is reported on nucleophilic substitution reactions of 4-nitrophenyl X-substituted-2-methylbenzoates (5a-e) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The Hammett plots for the aminolysis of 5a-e are nonlinear, e.g., substrates possessing an electron-donating group (EDG) in the benzoyl moiety deviate negatively from the linear line composed of substrates bearing no EDG. In contrast, the Yukawa-Tsuno plots for the same reactions exhibit excellent linear correlations with ${\rho}_X$ = 0.30-0.59 and r = 0.90-1.15, indicating that the nonlinear Hammett plots are caused by stabilization of the substrates possessing an EDG through resonance interactions but are not due to a change in the rate-determining step (RDS). The Br${\phi}$nsted-type plots are linear with ${\beta}_{nuc}$ = 0.66-0.82. Thus, the aminolysis of 5a-e has been suggested to proceed through a stepwise mechanism in which departure of the leaving group occurs at the RDS. The ${\rho}_X$ and ${\beta}_{nuc}$ values for the aminolysis of 5a-e increase as the reactivity of the substrates and amines increases, indicating that the reactivity-selectivity principle is not applicable to the current reactions.

• Bulletin of the Korean Chemical Society
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• v.35 no.4
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• pp.1128-1132
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• 2014

A kinetic study on aminolysis of 4-chloro-2-nitrophenyl X-substituted-benzoates (6a-i) in MeCN is reported. The Hammett plot for the reactions of 6a-i with piperidine consists of two intersecting straight lines, while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ${\rho}_X$ = 1.03 and r = 0.78. The nonlinear Hammett plot is not due to a change in rate-determining step (RDS) but is caused by the resonance stabilization of substrates possessing an electron-donating group in the benzoyl moiety. The Br${\phi}$nsted-type plot for the reactions of 4-chloro-2-nitrophenyl benzoate (6e) with a series of cyclic secondary amines is linear with ${\beta}_{nuc}$ = 0.69, an upper limit for reactions reported to proceed through a concerted mechanism. The aminolysis of 6e in aqueous medium has previously been reported to proceed through a stepwise mechanism with a change in RDS on the basis of a curved Br${\phi}$nsted-type plot. It has been concluded that instability of the zwitterionic tetrahedral intermediate ($T^{\pm}$) in MeCN forces the reaction to proceed through a concerted mechanism. This is further supported by the kinetic result that the amines used in this study are less reactive in MeCN than in $H_2O$, although they are more basic in MeCN over 7 $pK_a$ units.

• Bulletin of the Korean Chemical Society
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• v.29 no.8
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• pp.1459-1463
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• 2008

Second-order rate constants (kN) have been measured spectrophotometrically for reactions of 3,4-dinitrophenyl 2-thiophenecarboxylate (2) with a series of alicyclic secondary amines in 80 mol % $H_2O$/20 mol % dimethyl sulfoxide at 25.0 ${^{\circ}C}$. The Brønsted-type plot exhibits a downward curvature for the aminolysis of 2. The curved Brønsted-type plot is similar to that reported for the corresponding reactions of 2,4-dinitrophenyl 2- thiophenecarboxylate (1). The reactions of 1 and 2 have been suggested to proceed through the same mechanism, i.e., through a zwitterionic tetrahedral intermediate ($T^{\pm}$) with a change in the rate-determining step. Substrate 2 is less reactive than 1 toward weakly basic amines (e.g., $pK_a$ < 10.4) but becomes more reactive as the basicity of amines increases further. Dissection of kN into the microscopic rate constants has revealed that the reaction of 2 results in a smaller $k_2/k_{-1}$ ratio but larger $k_1$ than the corresponding reaction of 1. Steric hindrance exerted by the ortho-nitro group has been suggested to be responsible for the smaller $k_1$ value found for the reactions of 1.

• Bulletin of the Korean Chemical Society
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• v.18 no.5
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• pp.523-527
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• 1997

Apparent second-order rate constants (kapp) have been measured spectrophotometrically for the reaction of 2,4-dinitrophenyl benzoate (DNPB) with 6 secondary cyclic amines in H2O containing 20 mole% DMSO at 25.0±0.1 ℃. The Bronsted-type plot (log kapp vs. pKa) shows a break at pKa near 9.1, e.g. two straight lines with βapp values of 0.67 and 0.44 for the low basic (pKa < 9.1) and the highly basic (pKa > 9.1) amines, respectively. Using an estimated k2 value of 3×109 sec-1, all the other microconstants (k1, k-1 and K) involved in the present aminolysis have been calculated. The k value decreases with increasing the basicity of amines while k1 and K values increase with increasing the amine basicity, as expected. Good linear Bronsted-type plots have been obtained for these microconstants of the present aminolysis of DNPB. The magnitudes of the slope of the Bronsted-type plots, k1 and k-1 have been calculated to be 0.43 and - 0.24, respectively, indicating the k-1 step is about two folds less sensitive than the k1 step to the amine basicity. The K value has been calculated to be 0.66, which appears to be much smaller than the one for other aminolyses showing general base catalysis. The small K value has been attributed to the absence of general base catalysis in the present aminolysis of DNPB.

• Bulletin of the Korean Chemical Society
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• v.18 no.9
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• pp.911-913
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• 1997

• Bulletin of the Korean Chemical Society
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• v.17 no.3
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• pp.211-212
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• 1996

• Bulletin of the Korean Chemical Society
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• v.32 no.5
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• pp.1539-1542
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• 2011

The aminolysis of O-methyl S-aryl thiocarbonates with benzylamines are studied in acetonitrile at -45.0$^{\circ}C$. The ${\beta}_X$(${\beta}_{nuc}$) values are in the range 0.62-0.80 with a negative cross-interaction constant, ${\rho}_{XZ}$ = -0.42, which are interpreted to indicate a concerted mechanism. The kinetic isotope effects involving deuterated benzylamine nucleophiles ($XC_6H_4CH_2ND_2$) are large, $k_H/k_D$ = 1.29-1.75, suggesting that the N-H(D) bond is partially broken in the transition state by forming a hydrogen-bonded four-center cyclic structure. The concerted mechanism is enforced by the strong push provided by the MeO group which enhances the nucleofugalities of both benzylamine and arenethiolate from the putative zwitterionic tetrahedral intermediate.

• Bulletin of the Korean Chemical Society
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• v.32 no.2
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• pp.483-488
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• 2011

Present work comprises of synthesis various analogues of ciprofloxacin by introducing new functionality at carboxylic group position via ester aminolysis reaction. For this purpose the carboxylic group at C-3 was esterified and later subjected to nucleophilic attack at the carbonyl carbon by various aromatic amines. Structure of the analogues was confirmed by different techniques i.e. IR, $^1H$ NMR and mass spectrometry. The antibacterial activity of the derivatives was also assessed with the parent against a series of Gram-positive and Gram-negative bacteria. The synthesized compounds showed diverse antimicrobial profile among which most compounds possessed a comparable or better activity in comparison to the ciprofloxacin. Additionally unlike ciprofloxacin, some of the derivatives were also found to show antifungal activity.

• Bulletin of the Korean Chemical Society
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• v.32 no.1
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• pp.137-140
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• 2011

The aminolysis reactions of phenyl N-benzyl thiocarbamate with benzylamines in acetonitrile at $50.0^{\circ}C$ are investigated. The reactions are first order in both the amine and the substrate. Under amine excess, pseudo-first coefficient ($k_{obs}$) are obtained, plot of $k_{obs}$ vs free amine concentration are linear. The signs of ${\rho}_{XZ}$ (< 0) are consistent with concerted mechanism. Moreover, the variations of $\rho_X$ and $\rho_Z$ with respect to the sustituent in the substrate and large ${\rho}_{XZ}$ value indicate that the reactions proceed concerted mechanism. The normal kinetic isotope effects ($k_H/k_D$ = 1.3 ~ 1.5) involving deuterated benzylamine nucleophiles suggest a hydrogen-bonded, four-centered-type transition state. The activation parameters, ${\Delta}H^\ddagger$ and ${\Delta}S^\ddagger$, are consistent with this transition state structure.