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

2016-02-19T06:54:37Z (GMT) by Ik-Hwan Um Ji-Sun Kang Jong-Yoon Park
A kinetic study is reported for the Michael-type reactions of X-substituted β-nitrostyrenes (1aj) with a series of cyclic secondary amines in MeCN. The plots of pseudo-first-order rate constant kobsd vs [amine] curve upward, indicating that the reactions proceed through catalyzed and uncatalyzed routes. The dissection of kobsd 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 βnuc = 0.51 and 0.61, and the Hammett plots for the reactions of 1aj are also linear with ρX = 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 βnuc and ρX obtained in this study are discussed.