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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
journal contribution
posted on 2016-02-19, 06:54 authored by Ik-Hwan Um, Ji-Sun Kang, Jong-Yoon ParkA 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 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 1a–j 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.