Kinetic Study on SNAr Reaction of 1-(Y-Substituted-phenoxy)-2,4-dinitrobenzenes with Cyclic Secondary Amines in Acetonitrile: Evidence for Cyclic Transition-State Structure

2014 ◽  
Vol 79 (15) ◽  
pp. 7025-7031 ◽  
Author(s):  
Ik-Hwan Um ◽  
Min-Young Kim ◽  
Tae-Ah Kang ◽  
Julian M. Dust
Keyword(s):  
Transition State ◽  
Kinetic Study ◽  
Secondary Amines ◽  
State Structure ◽  
Transition State Structure ◽  
Cyclic Transition ◽  
Cyclic Transition State ◽  
Snar Reaction

A Computational Study of the Hofmann Elimination Pathway for the MoritaBaylis-Hillman Reaction Under DABCO Catalysis. Participation of a Bridge-Head Ylide

2020 ◽  
Author(s):  
Veejendra Yadav
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Computational Study ◽  
State Structure ◽  
Transition State Structure ◽  
Cyclic Transition ◽  
Activation Free Energy ◽  
Cyclic Transition State ◽  
Elimination Pathway ◽  
Hofmann Elimination

In comparison to the popular pathway involving proton-transfer via a four-centred cyclic transition state structure, the recently proposed overall lower energy pathway involving proton-transfer via a seven-centred cyclic transition state structure followed by Hofmann elimination for the Me<sub>3</sub>N-catalyzed Morita-Baylis-Hillman reaction is applicable to the DABCO-catalyzed reaction equally well. This finding clearly establishes that the zwitterion at the bridge-head in DABCO is well tolerated. Also, the activation free energy of the rate-limiting aldol reaction under DABCO-catalysis is lower than that under Me<sub>3</sub>N-catalysis, suggesting that DABCO is likely a better catalyst to achieve faster conversion.

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

2017 ◽  
Vol 95 (12) ◽  
pp. 1273-1279 ◽  
Author(s):  
Ik-Hwan Um ◽  
Min-Young Kim ◽  
Julian M. Dust
Keyword(s):  
Transition State ◽  
Secondary Amines ◽  
Medium Effect ◽  
Strong Electron ◽  
Cyclic Transition ◽  
Cyclic Transition State ◽  
Snar Reaction ◽  
Electron Withdrawing Group ◽  
Catalyzed Reactions ◽  
Zwitterionic Intermediate

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 °C is reported. The plots of kobsd 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 Brønsted-type plots have been obtained for the uncatalyzed and catalyzed reactions of 1-(4-nitrophenoxy)-2,4-dinitrobenzene (1a) with βnuc = 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 ρ = 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.

A Computational Study of the Hofmann Elimination Pathway for the MoritaBaylis-Hillman Reaction Under DABCO Catalysis. Participation of a Bridge-Head Ylide

2020 ◽  
Author(s):  
Veejendra Yadav
Keyword(s):  
Proton Transfer ◽  
Transition State ◽  
Computational Study ◽  
State Structure ◽  
Transition State Structure ◽  
Cyclic Transition ◽  
Activation Free Energy ◽  
Cyclic Transition State ◽  
Elimination Pathway ◽  
Hofmann Elimination

In comparison to the popular pathway involving proton-transfer via a four-centred cyclic transition state structure, the recently proposed overall lower energy pathway involving proton-transfer via a seven-centred cyclic transition state structure followed by Hofmann elimination for the Me<sub>3</sub>N-catalyzed Morita-Baylis-Hillman reaction is applicable to the DABCO-catalyzed reaction equally well. This finding clearly establishes that the zwitterion at the bridge-head in DABCO is well tolerated. Also, the activation free energy of the rate-limiting aldol reaction under DABCO-catalysis is lower than that under Me<sub>3</sub>N-catalysis, suggesting that DABCO is likely a better catalyst to achieve faster conversion.

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