Kinetic Study on Michael-Type Reactions of β‑Nitrostyrenes with Cyclic Secondary Amines in Acetonitrile: Transition-State Structures and Reaction Mechanism Deduced from Negative Enthalpy of Activation and Analyses of LFERs

A kinetic study is reported for the Michael-type reactions of X-substituted β-nitrostyrenes (<b>1a</b>–<b>j</b>) with a series of cyclic secondary amines in MeCN. The plots of pseudo-first-order rate constant <i>k</i>_obsd vs [amine] curve upward, indicating that the reactions proceed through catalyzed and uncatalyzed routes. The dissection of <i>k</i>_obsd into <i>Kk</i>₂ and <i>Kk</i>₃ (i.e., the rate constants for the uncatalyzed and catalyzed routes, respectively) revealed that <i>Kk</i>₃ is much larger than <i>Kk</i>₂, implying that the reactions proceed mainly through the catalyzed route when [amine] > 0.01 M. Strikingly, the reactivity of β-nitrostyrene (<b>1g</b>) toward piperidine decreases as the reaction temperature increases. Consequently, a negative enthalpy of activation is obtained, indicating that the reaction proceeds through a relatively stable intermediate. The Brønsted-type plots for the reactions of <b>1g</b> are linear with β_nuc = 0.51 and 0.61, and the Hammett plots for the reactions of <b>1a</b>–<b>j</b> are also linear with ρ_X = 0.84 and 2.10 for the uncatalyzed and catalyzed routes, respectively. The reactions are concluded to proceed through six-membered cyclic transition states for both the catalyzed and uncatalyzed routes. The effects of the substituent X on reactivity and factors influencing β_nuc and ρ_X obtained in this study are discussed

A Kinetic Study on Nucleophilic Displacement Reactions of Aryl Benzenesulfonates with Potassium Ethoxide: Role of K⁺ Ion and Reaction Mechanism Deduced from Analyses of LFERs and Activation Parameters

Pseudofirst-order rate constants (<i>k</i>_obsd) have been measured spectrophotometrically for the nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted benzenesulfonates <b>4a</b>–<b>f</b> and Y-substituted phenyl benzenesulfonates <b>5a</b>–<b>k</b> with EtOK in anhydrous ethanol. Dissection of <i>k</i>_obsd into <i>k</i>_EtO^– and <i>k</i>_EtOK (i.e., the second-order rate constants for the reactions with the dissociated EtO^– and ion-paired EtOK, respectively) shows that the ion-paired EtOK is more reactive than the dissociated EtO^–, indicating that K⁺ ion catalyzes the reaction. The catalytic effect exerted by K⁺ ion (e.g., the <i>k</i>_EtOK/<i>k</i>_EtO^– ratio) decreases linearly as the substituent X in the benzenesulfonyl moiety changes from an electron-donating group (EDG) to an electron-withdrawing group (EWG), but it is independent of the electronic nature of the substituent Y in the leaving group. The reactions have been concluded to proceed through a concerted mechanism from analyses of the kinetic data through linear free energy relationships (e.g., the Brønsted-type, Hammett, and Yukawa–Tsuno plots). K⁺ ion catalyzes the reactions by increasing the electrophilicity of the reaction center through a cyclic transition state (TS) rather than by increasing the nucleofugality of the leaving group. Activation parameters (e.g., Δ<i>H</i>^‡ and Δ<i>S</i>^‡) determined from the reactions performed at five different temperatures further support the proposed mechanism and TS structures

Mechanistic Assessment of S_NAr Displacement of Halides from 1‑Halo-2,4-dinitrobenzenes by Selected Primary and Secondary Amines: Brønsted and Mayr Analyses

Pseudo-first-order rate constants (<i>k</i>_obsd) have been measured spectrophotometrically for nucleophilic substitution reactions of 1-X-2,4-dinitrobenzenes (<b>1a</b>–<b>d</b>, X = F, Cl, Br, I) with various primary and secondary amines in MeCN and H₂O at 25.0 ± 0.1 °C. The plots of <i>k</i>_obsd vs [amine] curve upward for reactions of <b>1a</b> (X = F) with secondary amines in MeCN. In contrast, the corresponding plots for the other reactions of <b>1b</b>–<b>d</b> with primary and secondary amines in MeCN and H₂O are linear. The Brønsted-type plots for reactions of <b>1a</b>–<b>d</b> with a series of secondary amines are linear with β_nuc = 1.00 for the reaction of <b>1a</b> and 0.52 ± 0.01 for those of <b>1b</b>–<b>d</b>. Factors governing reaction mechanisms (e.g., solvent, halogen atoms, H-bonding interactions, amine types) have been discussed. Kinetic data were also analyzed in terms of the Mayr nucleophilicity parameter for the amines with each aromatic substrate. Provisional Mayr electrophilicity parameter (<i>E</i>) values for 1-X-2,4-dinitrobenzenes have been determined: <i>E</i> = −14.1 for X = F, <i>E</i> = −17.6 for X = Cl and Br, and <i>E</i> = −18.3 for X = I. These values are consistent with the range and order of <i>E</i> values for heteroaromatic superelectrophiles and normal 6-π aromatic electrophiles