• Bulletin of the Korean Chemical Society
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• v.28 no.12
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• pp.2219-2225
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• 2007

The reaction mechanisms for the cyclizations of N-methyl-2-(2-chloropyridin-3-yloxy)acetamide to 1-methylpyrido[ 3,2-b][1,4]oxazin-2-one and 1-methyl-pyrido[2,3-b][1,4]oxazin-2-one were investigated using ab initio Hartree-Fock, second-order Moller-Plesset perturbation, single point coupled cluster with both single and double substitution, and density functional theory methods. The 5-membered spiro intermediate (2) is optimized from the cyclization of the acyclic reactants through the proton-transfer reaction, and this intermediate proceeds continuously to the 6-membered intermediate through either a stepwise or a concerted reaction. In the stepwise reaction, an N-bridge-type intermediate as a stable structure is optimized, whereas, in the concerted reaction, the O-bridge-type intermediate is not optimized.

• Analytical Science and Technology
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• v.18 no.6
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• pp.460-468
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• 2005

The biotin-avidin complex, as a model recognition system, has been constructed through N-hydroxysuccinimide(NHS) reaction on a variety of substrates such as a smooth Au film, electrochemically roughened Au electrode and chemically modified mica. Stepwise self-assembled monolayers (SAMs) of biotin-avidin system were characterized by surface-enhanced resonance Raman scattering (SERRS) spectroscopy, atomic force microscopy (AFM) and surface plasmon resonance (SPR). A strong SERRS signal of rhodamine tags labeled in avidin from the SAMs on a roughened gold electrode indicated the successful complex formation of stepwise biotin-avidin recognition system. AFM images showed the circular shaped avidin aggregates (hexamer) with ca. $60{\AA}$ thick on the substrate, corresponding to one layer of avidin. The surface coverage and concentration of avidin molecules were estimated to be 90% and $7.5{\times}10^{-12}mol/cm^2$, respectively. SPR technique allowed one to monitor the surface reaction of the specific recognition with high sensitivity and precision.

• Bulletin of the Korean Chemical Society
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• v.35 no.7
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• pp.2135-2138
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• 2014

A kinetic study is reported for the SNAr reaction of 1-Y-substituted-phenoxy-2,4-dinitrobenzenes (1a-1h) with OH- in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The second-order rate constant ($k_{OH^-}$) increases as the substituent Y in the leaving group changes from an electron-donating group (EDG) to an electronwithdrawing group (EWG). The Br${\o}$nsted-type plot for the reactions of 1a-1h is linear with ${\beta}_{lg}$ = -0.16, indicating that the reactivity of substrates 1a-1h is little affected by the leaving-group basicity. A linear Br${\o}$nsted-type plot with ${\beta}_{lg}=-0.3{\pm}0.1$ is typical for reactions reported previously to proceed through a stepwise mechanism in which formation of a Meisenheimer complex is the rate-determining step (RDS). The Hammett plot correlated with ${\sigma}_Y{^{\circ}}$ constants results in a much better correlation than that correlated with ${\sigma}_Y{^-}$constants, implyng that no negative charge is developing on the O atom of the leaving group (or expulsion of the leaving group is not advanced at all in the TS). This excludes a possibility that the $S_NAr$ reaction of 1a-1h with $OH^-$ proceeds through a concerted mechanism or via a stepwise pathway with expulsion of the leaving group being the RDS. Thus, the current reactions have been concluded to proceed through a stepwise mechanism in which expulsion of the leaving group occurs rapidly after the RDS.

• Bulletin of the Korean Chemical Society
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• v.35 no.7
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• pp.2081-2085
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• 2014

Second-order rate constants ($k_N$) have been measured spectrophotometrically for the reactions of phenyl 2- pyridyl carbonate (6) with a series of cyclic secondary amines in MeCN at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plot for the reaction of 6 is linear with ${\beta}_{nuc}$ = 0.54, which is typical for reactions reported previously to proceed through a concerted mechanism. Substrate 6 is over $10^3$ times more reactive than 2-pyridyl benzoate (5), although the reactions of 6 and 5 proceed through the same mechanism. A combination of steric hindrance, inductive effect and resonance contribution is responsible for the kinetic results. The reactions of 6 and 5 proceed through a cyclic transition state (TS) in which H-bonding interactions increase the nucleofugality of the leaving group (i.e., 2-pyridiniumoxide). The enhanced nucleofugality forces the reactions of 6 and 5 to proceed through a concerted mechanism. In contrast, the corresponding reaction of 4-nitrophenyl 2-pyridyl carbonate (7) proceeds through a stepwise mechanism with quantitative liberation of 4-nitrophenoxide ion as the leaving group, indicating that replacement of the 4-nitrophenoxy group in 7 by the PhO group in 6 changes the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway) as well as the leaving group (i.e., from 4-nitrophenoxide to 2-pyridiniumoxide). The strong electron-withdrawing ability of the 4-nitrophenoxy group in 7 inhibits formation of a H-bonded cyclic TS. The presence or absence of a H-bonded cyclic TS governs the reaction mechanism (i.e., a concerted or stepwise mechanism) as well as the leaving group (i.e., 2-pyridiniumoxide or 4-nitrophenoxide).

• Bulletin of the Korean Chemical Society
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• v.22 no.10
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• pp.1123-1126
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• 2001

• Bulletin of the Korean Chemical Society
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• v.18 no.12
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• pp.1288-1291
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• 1997

4-Allylanisole was polymerized with AlCl3 as a catalyst. The polymerization was carried out in nitroethane at various temperatures with changing the ratio of the initiator to the monomer concentration. The weight averge molecular weights measured by gel permeation chromatography in chloroform with polystyrene standards were between 1,500 and 4,700. 1H NMR spectroscopy showed that the polymerization proceeded through a stepwise aromatic electrophilic substitution reaction along with a minor chain-reaction, resulting in a branched polymer. 4-Chloromethylanisole was also polymerized with AlCl3 in nitroethane through an aromatic electrophilic substitution reaction to give a high molecular weight polymer (Mw=88,000).

• Bulletin of the Korean Chemical Society
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• v.33 no.7
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• pp.2269-2273
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• 2012

Pseudo-first-order rate constants $k_{amine}$ have been measured spectrophotometrically for the reactions of benzyl 4-pyridyl carbonate 6 with a series of alicyclic secondary amines in $H_2O$ at $25.0^{\circ}C$. The plots of $k_{amine}$ vs. [amine] curve upward, indicating that the reactions proceed through a stepwise mechanism with two intermediates, a zwitterionic tetrahedral intermediate $T^{\pm}$ and its deprotonated form $T^-$. This contrasts to the report that the corresponding reactions of benzyl 2-pyridyl carbonate 5 proceed through a forced concerted pathway. The $k_{amine}$ values for the reactions of 6 have been dissected into the second-order rate constant $Kk_2$ and the thirdorder rate constant $Kk_3$. The Br${\o}$nsted-type plots are linear with ${\beta}_{nuc}=0.94$ and 1.18 for $Kk_2$ and $Kk_3$, respectively. The $Kk_2$ for the reaction of 6 is smaller than the second-order rate constant $k_N$ for the corresponding reaction of 5, although 4-pyridyloxide in 6 is less basic and a better nucleofuge than 2-pyridyloxide in 5.

• Bulletin of the Korean Chemical Society
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• v.26 no.11
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• pp.1875-1877
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• 2005

• Bulletin of the Korean Chemical Society
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• v.23 no.5
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• pp.715-720
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• 2002

Kinetic studies of the reaction of Z-aryl cyclobutanecarboxylates with X-pyridines in acetonitrile at $55.0^{\circ}C$ have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterion ic tetrahedral intermediate, T $\pm$ . These mechanistic conclusions are drawn based on (i) the large magnitude of ${\rho}X$ and $\rhoZ$, (ⅱ) the positive sign of ${\rho}XZ$ and the larger magnitude of $\rhoXZ$ than normal SN2 processes, (ⅲ) a small positive enthalpy of activation, ${\Delta}H{\neq}$, and a large negative, ${\Delta}S{\neq}$, and lastly (iv) adherence to the reactivity-selectivity principle (RSP) in all cases.

• Bulletin of the Korean Chemical Society
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• v.24 no.1
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• pp.91-94
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• 2003

Kinetic studies of the reaction of O-methyl-S-phenylthiocarbonates with benzylamines in methanol at 45.0 ℃ have been carried out. The reaction proceeds by a stepwise mechanism in which the rate-determining step is the breakdown of the zwitterionic tetrahedral intermediate, $T^{\pm}$, with a hydrogen-bonded four-center type transition state (TS). These mechanistic conclusions are drawn based on (ⅰ) the large magnitude of ${\rho}_X\;and\;{\rho}_Z$, (ⅱ) the normal kinetic isotope effects $(k_H/k_D\;>\;1.0)$ involving deuterated benzylamine nucleophiles, (ⅲ) the positive sign of ${\rho}_{XZ}$ and the larger magnitude of ${\rho}_{XZ}$ than that for normal $S_N2$ processes, and lastly (ⅳ) adherence to the reactivity-selectivity principle (RSP) in all cases.