• Bulletin of the Korean Chemical Society
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• v.22 no.4
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• pp.383-387
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• 2001

The kinetics and mechanism of the reactions of thiophenyl dimethylacetates (TDA) and trimethylacetates (TTA) with benzylamines in acetonitrile are studied. The reactions are first order in both the amine and the substrate. Relatively large values of ${\beta}X(\betanuc$ = 1.1-1.5; TDA and 1.1-1.5; TTA) and ${\beta}Z({\beta}lg$ = -1.8~-2.0; DTA and -1.3~-1.6; TTA) for benzylamines, significantly large kH/kD values (=1.2-1.5; DTA and 1.2-1.5; TTA) involving deuterated benzylamines, and large ${\rho}XZ$ (=0.82; TDA and 1.05; TTA) values are interpreted to indicate stepwise acyl transfer mechanism, but with the hydrogen bonded four center type transition state for benzylamine. The relatively greater magnitudes of ${\rho}XZ$ and the secondary kinetic isotope effects involving deuterated nucleophiles are in line with the proposed mechanism.

• Bulletin of the Korean Chemical Society
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• v.35 no.1
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• pp.225-230
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• 2014

A kinetic study on nucleophilic substitution reactions of 4-nitrophenyl X-substituted-benzoates (7a-i) with EtOK in anhydrous ethanol at $25.0{\pm}0.1^{\circ}C$ is reported. The plots of pseudo-first-order rate constants ($k_{obsd}$) vs. [EtOK] curve upward. Dissection of $k_{obsd}$ into the second-order rate constants for the reactions with the dissociated $EtO^-$ and ion-paired EtOK (i.e., $k_{EtO^-}$ and $k_{EtOK}$, respectively) has revealed that the ion-paired EtOK is more reactive than the dissociated $EtO^-$. Hammett plots for the reactions of 7a-i with the dissociated $EtO^-$ and ion-paired EtOK exhibit excellent linear correlations with ${\rho}_X$ = 3.00 and 2.47, respectively. The reactions have been suggested to proceed through a stepwise mechanism in which departure of the leaving-group occurs after the RDS. The correlation of the $k_{EtOK}/k_{EtO^-}$ ratio with the ${\sigma}_X$ constants exhibits excellent linearity with a slope of -0.53. It is concluded that the ion-paired EtOK catalyzes the reaction by increasing the electrophilicity of the reaction center rather than by enhancing the nucleofugality of the leaving group.

• Journal of Photoscience
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• v.9 no.2
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• pp.44-46
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• 2002

In the vertebrate retina, rods mediate twilight vision and cones daylight vision. Rods have been purified easily from the retina, and thus the phototransduction mechanism in rods is now well documented. However, it has not been possible to purify cones in large quantities, and therefore, the knowledge on the mechanism in cones is limited. Here we report purification of carp (Cyprinus carpio) cones with a stepwise Percoll gradient. Using purified cells, we compared the phototransduction mechanism between rods and cones. The results showed that both transducin activation and phosphodiesterase activation are less effective, and visual pigment phosphorylation is faster in cones. These differences explain lower light-sensitivity and briefer photoresponse time course in cones.

• Bulletin of the Korean Chemical Society
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• v.33 no.3
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• pp.1042-1046
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• 2012

The kinetics and mechanism of the pyridinolysis ($XC_5H_4N$) of diethyl isothiocyanophosphate are investigated in acetonitrile at $55.0^{\circ}C$. The Hammett and Bronsted plots for substituent X variations in the nucleophiles exhibit the two discrete slopes with a break region between X = 3-Ac and 4-Ac. These are interpreted to indicate a mechanistic change at the break region from a concerted to a stepwise mechanism with rate-limiting expulsion of the isothiocyanate leaving group from a trigonal bipyramidal pentacoordinated intermediate. The relatively large ${\beta}_x$ values with more basic and less basic pyridines imply much greater fraction of frontside nucleophilic attack TSf than that of backside attack TSb.

• Bulletin of the Korean Chemical Society
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• v.35 no.9
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• pp.2797-2802
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• 2014

Nucleophilic substitution reactions of Y-aryl methyl (8) and Y-aryl propyl (10) chlorothiophosphates with substituted anilines and deuterated anilines are investigated kinetically in acetonitrile at $55.0^{\circ}C$. A concerted mechanism is proposed for 8 based on the negative ${\rho}_{XY}$ (= -0.23) value, while a stepwise mechanism with a rate-limiting leaving group departure from the intermediate is proposed for 10 based on the positive ${\rho}_{XY}$ (= +0.68) value. The deuterium kinetic isotope effects (DKIEs; $k_H/k_D$) are 0.89-1.28 and 0.62-1.20 with 8 and 10, respectively. Primary normal and secondary inverse DKIEs are rationalized by a frontside attack involving hydrogen bonded, four-center-type transition state and backside attack involving in-line-type transition state, respectively.

• Bulletin of the Korean Chemical Society
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• v.34 no.11
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• pp.3218-3222
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• 2013

The aminolyses, anilinolysis and pyridinolysis, of bis(2-oxo-3-oxazolidinyl) phosphinic chloride (1) have been kinetically investigated in acetonitrile at 55.0 and $35.0^{\circ}C$, respectively. For the reactions of 1 with substituted anilines and deuterated anilines, a concerted SN2 mechanism is proposed based on the selectivity parameters and activation parameters. The deuterium kinetic isotope effects ($k_H/k_D$) invariably increase from secondary inverse to primary normal as the aniline becomes more basic, rationalized by the transition state variation from a backside to a frontside attack. For the pyridinolysis of 1, the authors propose a stepwise mechanism with a rate-limiting step change from bond breaking for more basic pyridines to bond formation for less basic pyridines based on the selectivity parameters and activation parameters. Biphasic concave upward free energy relationship with X is ascribed to a change in the attacking direction of the nucleophile from a frontside attack with more basic pyridines to a backside attack with less basic pyridines.

• Bulletin of the Korean Chemical Society
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• v.33 no.7
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• pp.2260-2264
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• 2012

The kinetics and mechanism of the pyridinolysis ($XC_5H_4N$) of dimethyl isothiocyanophosphate are investigated in acetonitrile at $55.0^{\circ}C$. The Hammett and Br$\ddot{o}$nsted plots for substituent X variations in the nucleophiles exhibit two discrete slopes with a break region between X = 3-Ac and 4-Ac. These are interpreted to indicate a mechanistic change at the break region from a concerted to a stepwise mechanism with a rate-limiting expulsion of the isothiocyanate leaving group from the intermediate. The relatively large ${\beta}x$ values imply much greater fraction of frontside nucleophilic attack TSf than that of backside attack TSb. The steric effects of the two ligands play an important role to determine the pyridinolysis rates of isothiocyanophosphates.

• Bulletin of the Korean Chemical Society
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• v.35 no.2
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• pp.638-640
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• 2014

• Bulletin of the Korean Chemical Society
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• v.27 no.4
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• pp.503-507
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• 2006

The rhodium(II)-catalyzed reactions of diazo compound derived from Meldrum's acid with a variety of styrenes have been examined. These reactions provide a rapid route to the preparation of cyclopropanes with a variety of substituents on the benzene ring. The mechanistic pathway for the formation of these products has been also described in terms of a stepwise mechanism.

• Bulletin of the Korean Chemical Society
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• v.20 no.9
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• pp.1017-1020
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• 1999

The aminolysis reactions of thiophenyl phenylacetates with benzylamines are investigated in acetonitrile at 55.0℃. Relatively large selectivity parameters, βx≒ 1.5, βz = -1.5~-1.8 and βxz = 0.92 together with the valid reactivity-selectivity principle are consistent with stepwise acyl transfer mechanism with rate limiting expulsion of the leaving group, thiophenolate anion, from the tetrahedral intermediate, T ± . The first order kinetics with respect to the benzylamine concentration and the realtively large secondary kinetic isotope effect (kH / kD = 1.2-1.7) involving deuterated benzylamine nucleophiles suggest a four center type transition state in which concurrent leaving group departure and proton transfer are involved.