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 (1a–j) with a series of cyclic secondary amines in MeCN. The plots of pseudo-first-order rate constant k_obsd vs [amine] curve upward, indicating that the reactions proceed through catalyzed and uncatalyzed routes. The dissection of k_obsd into Kk₂ and Kk₃ (i.e., the rate constants for the uncatalyzed and catalyzed routes, respectively) revealed that Kk₃ is much larger than Kk₂, implying that the reactions proceed mainly through the catalyzed route when [amine] > 0.01 M. Strikingly, the reactivity of β-nitrostyrene (1g) 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 1g are linear with β_nuc = 0.51 and 0.61, and the Hammett plots for the reactions of 1a–j 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.