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Michael-type reactions of 1-(X-substituted phenyl)-2-propyn-1-ones with alicyclic secondary amines in MeCN and H2O: Effect of medium on reactivity and transition-state structure

Title
Michael-type reactions of 1-(X-substituted phenyl)-2-propyn-1-ones with alicyclic secondary amines in MeCN and H2O: Effect of medium on reactivity and transition-state structure
Authors
Kim S.-I.Hwang S.-J.Park Y.-M.Um I.-H.
Ewha Authors
엄익환
SCOPUS Author ID
엄익환scopusscopus
Issue Date
2010
Journal Title
Bulletin of the Korean Chemical Society
ISSN
0253-2964JCR Link
Citation
vol. 31, no. 5, pp. 1199 - 1203
Indexed
SCI; SCIE; SCOPUS; KCI WOS scopus
Abstract
Second-order rate constants (kN) have been measured spectrophotometrically for Michael-type reactions of 1-(X-substituted phenyl)-2-propyn-1-ones (2a-f) with a series of alicyclic secondary amines in MeCN at 25.0 ± 0.1 °C. The kN value increases as the incoming amine becomes more basic and the substituent X changes form an electron-donating group (EDG) to an electron-withdrawing group (EWG). The Brønsted-type plots are linear with βnuc = 0.48 - 0.51. The Hammett plots for the reactions of 2a-f exhibit poor correlations but the corresponding Yukawa-Tsuno plots result in much better linear correlations with ρ = 1.57 and r = 0.46 for the reactions with piperidine while ρ = 1.72 and r = 0.39 for those with morpholine. The amines employed in this study are less reactive in MeCN than in water for reactions with substrates possessing an EDG, although they are ca. 8 pKa units more basic in the aprotic solvent. This indicates that the transition state (TS) is significantly more destabilized than the ground state (GS) in the aprotic solvent. It has been concluded that the reactions proceed through a stepwise mechanism with a partially charged TS, since such TS would be destabilized in the aprotic solvent due to the electronic repulsion between the negative-dipole end of MeCN and the negative charge of the TS. The fact that primary deuterium kinetic effect is absent supports a stepwise mechanism in which proton transfer occurs after the rate-determining step.
DOI
10.5012/bkcs.2010.31.5.1199
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자연과학대학 > 화학·나노과학전공 > Journal papers
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