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Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines

Title
Medium effect (water versus MeCN) on reactivity and reaction pathways for the SNAr reaction of 1-aryloxy-2,4-dinitrobenzenes with cyclic secondary amines
Authors
Um, Ik-HwanKim, Min-YoungDust, Julian M.
Ewha Authors
엄익환
SCOPUS Author ID
엄익환scopusscopus
Issue Date
2017
Journal Title
CANADIAN JOURNAL OF CHEMISTRY
ISSN
0008-4042JCR Link1480-3291JCR Link
Citation
vol. 95, no. 12, pp. 1273 - 1279
Keywords
SNAr reactionBronsted-type plotYukawa-Tsuno plotzwitterionic intermediatesix-membered cyclic transition state
Publisher
CANADIAN SCIENCE PUBLISHING, NRC RESEARCH PRESS
Indexed
SCI; SCIE; SCOPUS WOS
Abstract
A kinetic study on SNAr reactions of 1-aryloxy-2,4-dinitrobenzenes(1a-1h) with a series of cyclic secondary amines in 80 mol% water - 20 mol% DMSO at 25.0 +/- 0.1 degrees C is reported. The plots of k(obsd) versus amine concentration curve upward except for the reactions of substrates possessing a strong electron-withdrawing group in the leaving aryloxide with strongly basic piperidine. The curved plots indicate that the reactions proceed through both uncatalytic and catalytic routes. Linear Bronstedtype plots have been obtained for the uncatalyzed and catalyzed reactions of 1-(4-nitrophenoxy)- 2,4-dinitrobenzene (1a) with beta(nuc) = 0.84 and 0.78, respectively. The Yukawa-Tsuno plot for the uncatalyzed reactions of 1a-1h with piperidine results in an excellent linear correlation with rho = 1.66 and r = 0.31. In contrast, rate constants for catalyzed reactions are independent of the electronic nature of the substituent in the leaving group. The current SNAr reactions have been proposed to proceed via a zwitterionic intermediate (MC +/-) that partitions to products through uncatalytic and catalytic routes. The catalyzed reaction from MC +/- has been concluded to proceed through a concerted mechanism with a six-membered cyclic transition state (TScycl) rather than via a stepwise pathway with a discrete anionic intermediate (MC-), the traditionally accepted mechanism. Medium effects on the reactivity and reaction mechanism are discussed. Particularly, hydrogen bonding of the amines to water precludes formation of kinetically significant dimers found in some aprotic solvents; no explicit role for water in the catalytic transition state is required or proposed. The specific stabilization of the leaving aryloxides substituted with strong electron-withdrawing groups accounts for the lack of the catalytic pathway in these systems (1a-1c) with piperidine nucleophile.
DOI
10.1139/cjc-2017-0454
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자연과학대학 > 화학·나노과학전공 > Journal papers
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