• Bulletin of the Korean Chemical Society
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• v.20 no.10
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• pp.1177-1180
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• 1999

The ab initio SCF MO and density functional theory (DFT) studies are carried out on the electrophilic (1a) and electron transfer (1b) addition reactions to the vinyl double bond of aryl vinyl sulfides and ethers. In the electrophilic addition processes, a double bond shift from C3 = C4 to X = C3 occurs with occupation number (1.97) close to the normal two. Due to this shift direct conjugation between the cationic center, X = S or O, and the para electron-donor substituent becomes impossible so that the reaction energies (or log K) are correlated with σ rather than σ+. By contrast, radical cation formation leads to delocalization of the SOMO, a lone-pair πorbital on X, with four major resonance structures in which cationic charge as well as spin density is delocalized over C4 , X and C7 atoms. As a result, partial πbonds are formed over C1 -X and C3 - C4 with occupation numbers (0.82) lower than one. In two of the cannonical structures, III(Ⅹ) and III(X+), direct conjugation between the cationic center, X, and the para substituent is achieved so that a better correlation with σ+ rather than σis obtained. The SCF MO energies at the HF/3-21G* and HF/6-31G* levels lead to very much inferior Hammett correlations in the σ/ σ+ diagnostic criterion. In contrast, the ρvalues evaluated with the DFT energies can give reliable diagnostic distinction between the two addition mechanisms.

• Bulletin of the Korean Chemical Society
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• v.32 no.4
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• pp.1165-1169
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• 2011

Second-order rate constants ($k_N$) have been measured spectrophotometrically for nucleophilic substitution reactions of 2,4-dinitrophenyl phenyl thionocarbonate 4 with a series of Z-substituted pyridines in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The Br${\o}$nsted-type plot for the reactions of 4 exhibits downward curvature (i.e., ${\beta}_1$ = 0.21 and ${\beta}_2$ = 1.04), indicating that the reactions proceed through a stepwise mechanism with a change in rate-determining step. It has been found that 4 is less reactive than its oxygen analogue, 2,4-dinitrophenyl phenyl carbonate 3, although the thionocarbonate is expected to be more electrophilic than its oxygen analogue. The $pK_a$ at the center of the Br${\o}$nsted curvature, defined as $pK_a^o$, has been analyzed to be 6.6 for the reactions of 4 and 8.5 for those of 3. Dissection of $k_N$ into the microscopic rate constants $k_1$ and $k_2/k_{-1}$ ratio has revealed that the reactions of 4 result in smaller $k_1$ values but larger $k_2/k_{-1}$ ratios than the corresponding reactions of 3. The larger $k_2/k_{-1}$ ratios have been concluded to be responsible for the smaller $pK_a^o$ found for the reactions of 4.

• Bulletin of the Korean Chemical Society
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• v.34 no.4
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• pp.1115-1119
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• 2013

A kinetic study is reported for nucleophilic substitution reactions of benzyl 2-pyridyl thionocarbonate (5b) and t-butyl 2-pyridyl thionocarbonate (6b) with a series of alicyclic secondary amines in $H_2O$ at $25.0^{\circ}C$. General-base catalysis, which has often been reported to occur for aminolysis of esters possessing a C=S electrophilic center, is absent for the reactions of 5b and 6b. The Br${\o}$nsted-type plots for the reactions of 5b and 6b are linear with ${\beta}_{nuc}$ = 0.29 and 0.43, respectively, indicating that the reactions of 5b proceed through a stepwise mechanism with formation of a zwitterionic tetrahedral intermediate ($T^{\pm}$) being the rate-determining step while those of 6b proceed through a concerted mechanism. The reactivity of 5b and 6b is similar to that of their oxygen analogues (i.e., benzyl 2-pyridyl carbonate 5a and t-butyl 2-pyridyl carbonate 6a, respectively), indicating that the effect of modification of the electrophilic center from C=O to C=S (i.e., from 5a to 5b and from 6a to 6b) on reactivity is insignificant. In contrast, 6b is much less reactive than 5b, indicating that the replacement of the $PhCH_2$ in 5b by the t-Bu in 6b results in a significant decrease in reactivity as well as a change in the reaction mechanism (i.e., from a stepwise mechanism to a concerted pathway). It has been concluded that the contrasting reactivity and reaction mechanism for the reactions of 5b and 6b are not due to the electronic effects of $PhCH_2$ and t-Bu but are caused by the large steric hindrance exerted by the bulky t-Bu in 6b.

• Bulletin of the Korean Chemical Society
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• v.32 no.1
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• pp.179-182
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• 2011

Second-order rate constants ($k_{OH^-}$) have been measured spectrophotometrically for reactions of Y-substituted phenyl phenyl thionocarbonates (4a-i) with $OH^-$ in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The $k_{OH^-}$ values for the reactions of 4a-i have been compared with those reported previously for the corresponding reactions of Y-substituted phenyl phenyl carbonates (3a-i) to investigate the effect of changing the electrophilic center from C=O to C=S on reactivity and mechanism. Thionocarbonates 4a-i are less reactive than the corresponding carbonates 3a-i although 4a-i are expected to be more reactive than 3a-i. The Bronsted-type plot for reactions of 4a-i is linear with $\beta_{lg}$ = -0.33, a typical $\beta_{lg}$ value for reactions reported to proceed through a stepwise mechanism with formation of an intermediate being the rate-determining step (RDS). Furthermore, the Hammett plot correlated with $\sigma^o$ constants results in much better linearity than that correlated with $\sigma^-$ constants, indicating that expulsion of the leaving group is not advanced in the RDS. Thus, alkaline hydrolysis of 4a-i has been concluded to proceed through a stepwise mechanism with formation of an intermediate being RDS, which is in contrast to the forced concerted mechanism reported for the corresponding reactions of 3a-i. Enhanced stability of the intermediate upon modification of the electrophilic center from C=O to C=S has been concluded to be responsible for the contrasting mechanisms.

• Journal of the Korean Chemical Society
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• v.30 no.3
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• pp.265-272
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• 1986

Kinetic studies for the methanolysis of tert-butyl halides (t-BuCl, t-BuBr, t-BuI) were carried out in MeOH-1,1,2,2-tetrachloroethane mixtures. The solvatochromic comparison method was used with six indicators to analyze solvent effects on the ionizations of tert-butyl halides. It was shown that the cooperative effect of solvent polarity-polarizability was the most important factor influenced on the methanolysis rates of tert-butyl halides, but the electrophilic assistance for halide leaving group and the nucleophilic assistance for tert-butylium ion were considerably influential, too. And it was found that the electrophilic assistance caused by hydrogen bonding and the nucleophilic assistance for carbon center were stronger for more basic leaving group ($I^-) and more polarizable leaving group(t-BuCl

• Bulletin of the Korean Chemical Society
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• v.26 no.3
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• pp.457-460
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• 2005

• Bulletin of the Korean Chemical Society
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• v.31 no.1
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• pp.234-237
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• 2010

• Bulletin of the Korean Chemical Society
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• v.29 no.12
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• pp.2509-2512
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• 2008

• Bulletin of the Korean Chemical Society
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• v.24 no.6
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• pp.829-831
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• 2003

Various theoretical studies of σ-bond activation of organic molecules by transition metal complexes arereviewed. In the homolytic σ-bond activation, the d orbital energy level of the central metal is an importantfactor, as well known. At the same time, the electron-withdrawing substituent which stabilizes the sp3 orbitalaccelerates the homolytic σ-bond activation. In the heterolytic C-H σ-bond activation of RH by $MXL_n$, the XHbond formation is an important driving force, where $MRL_n$ and HX are formed as products. The heterolytic σ-bond activation is also understood in terms of the electrophilic attack of the metal center to the substrate.

• Proceedings of the Korea Environmental Mutagen Society Conference
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• 2002.05a
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• pp.25-35
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• 2002

Transcription factor Nrf2 is essential for the antioxidant responsive element (ARE)-mediated induction of phase II detoxifying and oxidative stress enzyme genes. Detailed analysis of differential Nrf2 activity displayed in transfected cell lines ultimately led to the identification of a new protein, which we named Keap1, that suppresses Nrf2 transcriptional activity by specific binding to its evolutionarily conserved amino-terminal regulatory domain. The closest homolog of Keap1 is a Drosophila actin-binding protein called Kelch, implying that Keap1 might be a Nrf2 cytoplasmic effector. We then showed that electrophilic agents antagonize Keap1 inhibition of Nrf2 activity in vivo, allowing Nrf2 to traverse from the cytoplasm to the nucleus and potentiate the ARE response. We postulate that Keap1 and Nrf2 constitute a crucial cellular sensor for oxidative stress, and together mediate a key step in the signaling pathway that leads to transcriptional activation by this novel Nrf2 nuclear shuttling mechanism. The activation of Nrf2 leads in turn to the induction of phase II enzyme and antioxidative stress genes in response to electrophiles and reactive oxygen species.