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

摘要

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 kobsd vs [amine] curve upward, indicating that the reactions proceed through catalyzed and uncatalyzed routes. The dissection of k_(obsd) into Kk_2 and Kk _3 (i.e., the rate constants for the uncatalyzed and catalyzed routes, respectively) revealed that Kk_3 is much larger than Kk_2, 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 Br?nsted-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 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 β_(nuc) and ρ_X obtained in this study are discussed.