Electronic Nature of Substituent X Governs Reaction Mechanism in Aminolysis of 4-Pyridyl X-Substituted-Benzoates in Acetonitrile

Kinetic Study on Pyridinolysis of Aryl Benzenesulfonates: Factors Governing Regioselectivity and Reaction Mechanism

A kinetic study is reported for reactions of 2,4-dinitrophenyl X-substituted-benzenesulfonates (1a-1f) and Y-substituted-phenyl benzenesulfonates (2a-2f) with Z-substituted-pyridines. The reactions proceed through S‒O and C‒O bond scissions competitively. The Yukawa-Tsuno plot for the reactions of 1a-1f with 4-oxypyridine (S‒O bond fission) exhibits an excellent linearity. The Brønsted-type plot for the reactions of 2,4-dinitrophenyl benzenesulfonate (1d) with pyridines (S‒O bond fission) is also linear with βnuc = 0.62. The Brønsted-type plot for the reactions of 2a-2f with 4-oxypyridines (S‒O bond fission) is linear with βlg= ‒1.17. Thus, the reactions have been concluded to proceed through a concerted mechanism, in which leaving-group expulsion is significantly more advanced than bond formation between the electrophilic center and nucleophile at the transition state. The Hammett plot for the reactions of 1a-1f with 4-oxypyridine (C‒O bond fission) exhibits scattered points with ρX= 0.98. The Brønsted-type plot for the reactions of 1d with Z-substituted-pyridines (C‒O bond fission) results in an excellent linear correlation with βnuc = 0.38. Thus, the reactions (C‒O bond fission) have been concluded to proceed through a stepwise mechanism with a Meisenheimer complex, in which expulsion of the leaving group occurs after the rate-determining step. Factors governing regioselectivity and reaction mechanism are discussed.

Kinetic study on aminolysis of aryl X-substituted-cinnamates in acetonitrile: differential medium effect determines reactivity and reaction mechanism

A kinetic study on nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted-cinnamates (1a–1f) and Y-substituted-phenyl cinnamates (2a–2g) with a series of alicyclic secondary amines in MeCN at 25.0 ± 0.1 °C is reported. The Brønsted-type plots for the reactions of 1a–1f are linear with βnuc = 0.47∼0.50, indicating that the bond formation between the amine nucleophile and the electrophilic center is advanced slightly in the transition state. The Brønsted-type plot for the reactions of 2a–2g with piperidine is also linear with βlg = –0.66, which is a typical βlg value for reactions reported previously to proceed through a concerted mechanism. Furthermore, the Hammett plot correlated with σ– constants results in much better linearity than that correlated with σo constants, implying that expulsion of the leaving group is advanced in the rate-determining step (RDS). Thus, the reactions are concluded to proceed through a concerted mechanism. The Hammett plots for the reactions of 1a–1f consist of two intersecting straight lines, whereas the corresponding Yukawa–Tsuno plots exhibit excellent linear correlations with ρX = 0.62∼0.71 and r = 0.65∼0.68. Apparently, the nonlinear Hammett plots are not due to a change in the reaction mechanism (or the RDS) but are caused by stabilization of the substrate possessing an electron-donating group (EDG) in the cinnamoyl moiety through resonance interactions between the EDG and the C=O bond of the substrate. Medium effects on reactivity and reaction mechanism are also discussed.

Unexpected medium effect on the mechanism for aminolysis of aryl phenyl carbonates in acetonitrile and H2O: transition-state structure in the catalytic pathway

Upward curvature in the kinetic plots of pseudo first-order rate constants (kobsd) vs. [amine] for the aminolysis of aryl phenyl carbonates (5a–5j) in MeCN demonstrates that these reactions proceed via a zwitterionic tetrahedral intermediate (T±) that partitions between catalyzed and uncatalyzed routes to give the products. Yukawa–Tsuno plots for the reactions of 5a–5j with piperidine result in excellent linear correlations with ρY = 4.82 and r = 0.47 for the uncatalyzed reaction versus ρY = 2.21 and r = 0.21 for the catalyzed reaction. Brønsted plots for reactions of 4-(ethoxycarbonyl)-phenyl phenyl carbonate (5f) with a series of cyclic secondary amines exhibit excellent linear correlations with βnuc = 0.87 and 0.58 for the uncatalyzed and catalyzed reactions, respectively. The ΔH‡ and ΔS‡ values are 0.92 kcal/mol and –50.1 cal/mol K, respectively, for the catalyzed reaction of 5f with piperidine. Deuterium kinetic isotope effects found for reactions of 5f with piperidine/deuterated piperidine are 0.84 (uncatalyzed) and 1.42 (catalyzed). Multi-parameter analysis supports a concerted catalytic pathway involving a six-membered cyclic transition state rather than a traditionally accepted stepwise pathway with an anionic intermediate. The current unexpected results, where T± is the essential central intermediate in this aminolysis, contrast with previous calculation studies that deemed T± unstable in gas phase or MeCN.

Alkali-metal ion catalysis and inhibition in SNAr displacement: relative stabilization of ground state and transition state determines catalysis and inhibition in SNAr reactivity

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<EtONa<EtOK<EtO(-)<EtOK/18C6. This indicates that the reaction is inhibited by Li(+), Na(+), and K(+) ions but is catalyzed by 18C6 K(+) ion. The reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes have been proposed to proceed through a stepwise mechanism, in which expulsion of the leaving group occurs after the rate-determining step based on the kinetic result that σ(o) constants exhibit a much better Hammett correlation than σ(-) constants. Alkali-metal ion catalysis or inhibition has been discussed in terms of differential stabilization of ground-state and transition-state complexes through a qualitative energy profile. A π-complexed transition-state structure is proposed to account for the kinetic results.

Exploration of the α-effect by substitution on hydroxylamine anions. I. Effects of alkyl- and fluoroalkylation

Many explanations for the α-effect have been suggested. Most are simple models conjecturing various sources affecting the nucleophilic behavior. This paper provides a comprehensive study of electronic effects using the single α-atom lone electron pair of substituted hydroxyl amine anions. This substituent scheme shows that the interaction of this lone pair with the negatively charged oxygen atom is an important indicator of reactivity.

Comparison of aminolysis of 2-pyridyl and 4-pyridyl x-substituted benzoates in acetonitrile: evidence for a concerted mechanism involving a cyclic transition state

A kinetic study on reactions of 2-pyridyl X-substituted benzoates (6a-i) with a series of cyclic secondary amines in MeCN is reported. The Hammett plot for the reaction of 6a-i with piperidine consists of two intersecting straight lines while the Yukawa-Tsuno plot exhibits an excellent linear correlation with ρX = 1.28 and r = 0.63, indicating that the nonlinear Hammett plot is not caused by a change in the rate-determining step but rather by resonance stabilization of substrates possessing an electron-donating group (EDG) in the benzoyl moiety. The Brønsted-type plots are linear with βnuc = 0.59 ± 0.02, which is typical of reactions reported to proceed through a concerted mechanism. A cyclic transition state (TS), which forces the reaction to proceed through a concerted mechanism, is proposed. The deuterium kinetic isotope effect of 1.3 ± 0.1 is consistent with the proposed mechanism. Analysis of activation parameters reveals that ΔH(‡) increases linearly as the substituent X changes from an electron-withdrawing group (EWG) to an EDG, while TΔS(‡) remains nearly constant with a large negative value. The constant TΔS(‡) value further supports the proposal that the reaction proceeds through a concerted mechanism with a cyclic TS.