We report here the first observation of alkali-metal ion catalysis and inhibition in SNAr reactions. The plot of kobsd versus [alkali-metal ethoxide] exhibits downward curvature for the reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene with EtOLi, EtONa, and EtOK, but upward curvature for the corresponding reaction with EtOK in the presence of 18-crown-6-ether (18C6). Dissection of kobsd into the second-order rate constants for the reactions with the dissociated EtO(-) and the ion-paired EtOM (i.e., k EtO - and kEtOM , respectively) has revealed that the reactivity increases in the order EtOLi
A kinetic study is reported for the S N Ar reaction of 1-Y-substituted-phenoxy-2,4-dinitrobenzenes (1a-1h) with OH -in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 o 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 Brønsted-type plot for the reactions of 1a-1h is linear with β lg = -0.16, indicating that the reactivity of substrates 1a-1h is little affected by the leaving-group basicity. A linear Brønsted-type plot with β lg = -0.3 ± 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 σ Y o constants results in a much better correlation than that correlated with σ 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 N Ar 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.
A kinetic study on S N Ar reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with various anionic nucleophiles in 80 mol% water -20 mol% DMSO at 25.0°C is reported. The Brønsted-type plot for the reaction of 1a with a series of substituted phenoxides and HOO − results in an excellent linear correlation with  nuc = 1.17. However, OH − exhibits dramatic negative deviation from the Brønsted-type plot, while N 3 − , C 6 H 5 S − , and butane-2,3-dione monoximate (Ox − ) deviate positively from linearity. HOO − is 680-fold more reactive than OH − but does not exhibit the ␣-effect. In contrast, Ox − is 166-fold more reactive than isobasic 4-Cl − C 6 H 4 O − and exhibits the ␣-effect. Differential solvation effects have been suggested to be responsible for the ␣-effect in this study, i.e., Ox − exhibits the ␣-effect, since it is 5.7 kcal/mol less strongly solvated than 4-Cl − C 6 H 4 O − in the reaction medium, while HOO − does not show the ␣-effect due to a strong requirement for partial desolvation before nucleophilic attack. The highly enhanced reactivity of polarizable N 3 − and C 6 H 5 S − and extremely decreased reactivity of nonpolarizable OH − are in accord with the hard-soft acid and base principle.Résumé : Nous présentons une étude cinétique des réactions S N Ar entre le 1-(4-nitrophénoxy)-2,4-dinitrobenzène (1a) et divers nucléophiles anioniques dans un mélange constitué d'eau à 80 % molaire et de DMSO à 20 % molaire à une température de 25,0°C. La courbe de type Brønsted pour les réactions du composé 1a avec une série de phénoxydes substitués et avec l'ion HOO − donne lieu à une excellente corrélation linéaire dont le coefficient  nuc = 1,17. Cependant, les ions OH − sont associés à une déviation négative importante de la courbe de type Brønsted, tandis que les ions N 3 − , C 6 H 5 S − et butane-2,3-dionemonoximate (Ox − ) dévient de la linéarité de façon positive. L'ion HOO − est 680 fois plus réactif que l'ion OH − , mais ne montre pas d'effet ␣. À l'inverse, l'ion Ox − est 166 fois plus réactif que l'ion isobasique 4-ClC 6 H 4 O − et présente l'effet ␣. Nous avons proposé des effets de solvatation variables d'un composé à l'autre pour expliquer l'effet ␣ observé dans cette étude, c.-à -d. que l'ion Ox − présente l'effet ␣ parce qu'il est moins fortement solvaté dans le milieu réactionnel que l'ion 4-Cl-C 6 H 4 O − (différence de 5,7 kcal/mol), tandis que l'ion HOO − ne montre pas d'effet ␣ en raison de fortes contraintes à la désolvatation partielle préalable à l'attaque nucléophile. La réactivité fortement accrue des ions polarisables N 3 − et C 6 H 5 S − et la réduction extrême de la réactivité de l'ion non polarisable OH − sont des phénomènes en accord avec le principe de dureté chimique des acides et des bases. [Traduit par la Rédaction]
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