• Bulletin of the Korean Chemical Society
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• 제35권7호
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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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• 제24권7호
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• pp.993-998
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• 2003

In the aminolysis of phenacyl bromides ($YC_6H_4COCH_2Br$) with benzylamines ($XC_6H_4CH_2NH_2$) in acetonitrile, the Bronsted βx (βnuc) values observed are rather low ( βX = 0.69-0.73). These values are similar to those (βx $^~_=$ 0.7) for other aminolysis reactions of phenacyl compounds with anilines and pyridines, but are much smaller than those ( βx = 1.1-2.5) for the aminolysis of esters with benzylamines which are believed to proceed stepwise with rate-limiting expulsion of the leaving group. The relative constancy of the βx values (βx $^~_=$ 0.7) irrespective of the amine, leaving group and solvent can be accounted for by a bridged type transition state in the rate-limiting expulsion of the leaving group. Thus the aminolysis of phenacyl derivatives are proposed to proceed stepwise through a zwitterionic tetrahedral intermediate ($T^{\pm}$), with rate-limiting expulsion of the leaving group from $T^{\pm}$. In the transition state, the amine is bridged between the carbonyl and α-carbons, which leads to negligible effect of amine on the leaving group expulsion rate.

• Bulletin of the Korean Chemical Society
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• 제29권4호
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• pp.851-855
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• 2008

The reactions of O,O-diphenyl Z-S-phenyl phosphorothiolates with X-pyridines have been studied kinetically in acetonitrile at $35.0{^{\circ}C}$. The Hammett plots for substituent (Z) variations in the leaving group (log $k_2$ vs. $\sigma$ Z) are biphasic concave downwards with breaks at Z = H. The large magnitudes of ${\rho}X(\rho_{nuc})$, ${\beta}X(\rho_{nuc})$, and the cross-interaction constant, $\rho$XZ, suggest frontside nucleophilic attack toward the leaving group. The sign reversal of $\rho$Z from positive in $\sigma$ Z $\leq$ 0 to negative in $\sigma$ Z $\geq$ 0 is interpreted as the change in mechanism from concerted to stepwise with rate-limiting expulsion of the leaving group. The anomalous negative sign of $\rho$ Z for leaving groups with electron-withdrawing substituents is interpreted as the intramolecular ligand exchange process of the leaving group from the equatorial position in the intermediate to the apical position in the TS.

• Bulletin of the Korean Chemical Society
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• 제35권8호
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• pp.2371-2374
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• 2014

A kinetic study is reported on SNAr reaction of 1-fluoro-2,4-dinitrobenzene with a series of primary amines including hydrazine in $H_2O$ at $25.0^{\circ}C$. The plots of $k_{obsd}$ vs. [amine] are linear and pass through the origin, indicating that general-base catalysis by a second amine molecule is absent. The Br${\o}$nsted-type plot exhibits an excellent linear correlation with ${\beta}_{nuc}$ = 0.46 when hydrazine is excluded from the correlation. The reaction has been suggested to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step (RDS). Hydrazine is ca. 10 times more reactive than similarly basic glycylglycine (i.e., the ${\alpha}$-effect). A five-membered cyclic intermediate has been suggested for the reaction with hydrazine, in which intramolecular H-bonding interactions would facilitate expulsion of the leaving group. However, the enhanced leaving-group ability is not responsible for the ${\alpha}$-effect shown by hydrazine because expulsion of the leaving group occurs after RDS. Destabilization of the ground-state of hydrazine through the electronic repulsion between the nonbonding electron pairs is responsible for the ${\alpha}$-effect found in the current $S_NAr$ reaction.

• 대한화학회지
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• 제19권6호
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• pp.449-453
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• 1975

Benzyl arenesulfonate와 디메틸아닐린을 아세톤 중에서 $35^{\circ}C$로 반응시켜 離脫基의 置換基效果를 硏究한 結果 다음과 같은 結論을 얻었다. (1) 親核試藥이 피리딘에서 N,N-디메틸아닐린으로 바뀌어도 離脫基의 치환기效果는 변화가 없다. (2) p-MeO의 치환기정수는 아세톤에서 -0.35가 적당했다. (3) 치환 디메틸아닐린의 ) 親核力이 弱할 수록 전이상태에서는 N에서 C로 電子移動이 보다 크며 C${\ldots}$O結合 開裂은 보다 진행된 상태라고 생각된다.

• 대한화학회지
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• 제30권3호
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• pp.265-272
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• 1986

MeOH-1,1,2,2-tetrachloroethane 혼합용매에서 할로겐화삼차부틸(t-BuCl, t-BuBr, t-BuI)의 가메탄올 분해반응을 속도론적으로 연구하였으며, 이온화에 미치는 용매효과를 고찰하기 위하여 6가지 indicater를 이용한 분광용매화비교법을 적용하였다. 이결과 할로겐화삼차부틸의 가메탄올 분해반응에 미치는 용매화의 작용은 용매의 극상-편극성에 기인되는 상호작용이 주된 것이긴 하지만 halide leaving group의 living ability에 미치는 electrophilic assistance와 t-Butyl 양이온쪽에 대한 nucleophilic assistance도 상당히 작용함을 알 수 있다. 특히 hydrogen bonding에 의한 electrophilic assistance는 basicity가 큰 leaving group일수록 커지며 ($I^-), 탄소 중심일수록 커짐을 밝힐 수 있었다. (t-BuCl

• Bulletin of the Korean Chemical Society
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• 제22권4호
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• pp.375-378
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• 2001

The palladium catalyzed dehalogenative homocoupling of haloarylsulfonates under reductive conditions has proceeded selectively depending on the type of the halogen. Thus, an iodo or a bromo leaving group of haloarylsulfonates was homocoupled to gi ve symmetrical biaryls in good yields with the sulfonate group intact, whereas a chloro leaving group gave no reaction under the conditions used. When the more reactive nickel catalyst was employed instead of the palladium catalyst in the reaction, both dehalogenative and desulfonative homocouplings of haloarylsulfonates occurred regardless of the type of the halogen used.

• Bulletin of the Korean Chemical Society
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• 제30권10호
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• pp.2393-2397
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• 2009

Second-order rate constants ($k_{HOO}$‒) have been measured spectrophotometrically for nucleophilic substitution reactions of Y-substituted phenyl benzenesulfonates (1a-g) with $HOO^-$ ion in $H_2O$ at $25.0\;{\pm}\;0.1\;{^{\circ}C}$. The Br$\phi$nsted-type plot is linear with ${\beta}_{lg}$ = ‒0.73. The Hammett plot correlated with with ${\sigma}^-$ constants results in much better linearity than ${\sigma}^o$ constants, indicating that expulsion of the leaving group occurs in the rate-determining step (RDS) either in a stepwise mechanism or in a concerted pathway. However, a stepwise mechanism in which departure of the leaving group occurs in the RDS has been excluded since $HOO^-$ ion is more basic and a poorer leaving group than the leaving Y-substituted phenoxide ions. Thus, the reactions of 1a-g with $HOO^-$ ion have been concluded to proceed through a concerted mechanism. The $\alpha$-nucleophile $HOO^-$ ion is more reactive than its reference nucleophile $OH^-$ ion although the former is ca. 4 p$K_a$ units less basic than the latter (i.e., the $\alpha$-effect). TS stabilization through intramolecular H-bonding interaction has been suggested to be irresponsible for the $\alpha$-effect shown by $HOO^-$ ion, since the magnitude of the $\alpha$-effect is independent of the electronic nature of substituent Y in the leaving group. GS destabilization through desolvation of $HOO^-$ ion has been concluded to be responsible for the $\alpha$-effect found in the this study.

• Bulletin of the Korean Chemical Society
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• 제35권7호
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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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• 제34권7호
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• pp.2023-2028
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• 2013

A kinetic study is reported for the nucleophilic substitution reactions of phenyl Y-substituted-phenyl carbonates (5a-5k) with piperidine in 80 mol % $H_2O$/20 mol % DMSO at $25.0{\pm}0.1^{\circ}C$. The plots of $k_{obsd}$ vs. [piperidine] for the reactions of substrates possessing a strong electron-withdrawing group (EWG) in the leaving group (i.e., 5a-5i) are linear and pass through the origin. In contrast, the plots for the reactions of substrates bearing a weak EWG or no substituent (i.e., 5j or 5k) curve upward, indicating that the electronic nature of the substituent Y in the leaving group governs the reaction mechanism. Thus, it has been suggested that the reactions of 5a-5i proceed through a stepwise mechanism with a zwitterionic tetrahedral intermediate (i.e., $T^{\pm}$) while those of 5j and 5k proceed through a stepwise mechanism with two intermediates (i.e., $T^{\pm}$ and its deprotonated form $T^-$). The slope of the Br${\o}$nsted-type plot for the second-order rate constants (i.e., $k_N$ or $Kk_2$) changes from -0.41 to -1.89 as the leaving-group basicity increases, indicating that a change in the rate-determining step (RDS) occurs. The reactions of 5a-5k with piperidine result in larger $k_1$ values than the corresponding reactions with ethylamine.