Comparison of Aminolysis of 2-Pyridyl and 4-Pyridyl X-Substituted Benzoates in Acetonitrile: Evidence for a Concerted Mechanism Involving a Cyclic Transition State

2014 ◽  
Vol 79 (3) ◽  
pp. 1206-1212 ◽  
Author(s):  
Ik-Hwan Um ◽  
Ae-Ri Bae ◽  
Tae-Il Um
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
Cyclic Transition State

Hydrazinolysis of aryl cinnamates and related esters: the α-effect arises from stabilization of five-membered cyclic transition state

2019 ◽  
Vol 97 (1) ◽  
pp. 7-12 ◽  
Author(s):  
Ik-Hwan Um ◽  
Ae-Ri Bae ◽  
Julian M. Dust
Keyword(s):  
Transition State ◽  
Linear Correlation ◽  
Primary Amines ◽  
Substitution Reactions ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
Bonding Interaction ◽  
Leaving Group ◽  
Cyclic Transition State ◽  
Electron Withdrawing Group

A kinetic study is reported for nucleophilic substitution reactions of Y-substituted-phenyl cinnamates (1a–1h) with a series of primary amines including hydrazine in H2O containing 20 mol % DMSO at 25.0 °C. The Brønsted-type plot for the reaction of 2,4-dinitrophenyl cinnamate (1a) is linear with βnuc = 0.57 except hydrazine, which exhibits positive deviation from the linear correlation (i.e., the α-effect). The Brønsted-type plots for the reactions of 1a–1h with hydrazine and glycylglycine (glygly) are also linear with βlg = –0.71 and –0.87, respectively, when 1a is excluded from the linear correlation. Thus, the reactions have been concluded to proceed through a concerted mechanism on the basis of the linear Brønsted-type plots and magnitudes of the βnuc and βlg values. The α-effect shown by hydrazine is dependent on electronic nature of the substituent Y in the leaving group, e.g., it increases as the substituent Y becomes a weaker electron-withdrawing group (or as basicity of the leaving aryloxide increases), indicating that the α-effect is not due to destabilization of the ground state but mainly due to stabilization of the transition state. A five-membered cyclic TS structure, which could increase nucleofugality of the leaving aryloxide through H-bonding interaction, has been proposed to account for the leaving-group dependent α-effect found in this study. The theories suggested previously to rationalize the α-effect found for the related systems are also discussed.

Kinetic Studies of [4n+2]π-Thermal Cyclodimerization of 1-(3-Pyridazinyl)-3-oxidopyridinium Betaines

1992 ◽  
Vol 57 (9) ◽  
pp. 1951-1959 ◽  
Author(s):  
Madlene L. Iskander ◽  
Samia A. El-Abbady ◽  
Alyaa A. Shalaby ◽  
Ahmed H. Moustafa
Keyword(s):  
Energy Gap ◽  
Activation Parameters ◽  
Kinetic Studies ◽  
Cyclic Transition ◽  
Concerted Mechanism ◽  
First Order ◽  
Thermodynamic Activation Parameters ◽  
Cyclic Transition State ◽  
Complete Dissociation ◽  
Homo Lumo

The reactivity of the base induced cyclodimerization of 1-(6-arylpyridazin-3-yl)-3-oxidopyridinium chlorides in a pericyclic process have been investigated kinetically at λ 380 nm. The reaction was found to be second order with respect to the liberated betaine and zero order with respect to the base. On the other hand dedimerization (monomer formation) was found to be first order. It was shown that dimerization is favoured at low temperature, whereas dedimerization process is favoured at relatively high temperature (ca 70 °C). Solvent effects on the reaction rate have been found to follow the order ethanol > chloroform ≈ 1,2-dichloroethane. Complete dissociation was accomplished only in 1,2-dichloroethane at ca 70 °C. The thermodynamic activation parameters have been calculated by a standard method. Thus, ∆G# has been found to be independent on substituents and solvents. The high negative values of ∆S# supports the cyclic transition state which is in favour with the concerted mechanism. MO calculations using SCF-PPP approximation method indicated low HOMO-LUMO energy gap of the investigated betaines.

Divergent Reactivity of Stannane and Silane in the Trifluoromethylation of PdII: Cyclic Transition State versus Difluorocarbene Release

2018 ◽  
Vol 130 (46) ◽  
pp. 15301-15305 ◽  
Author(s):  
Maoping Pu ◽  
Italo A. Sanhueza ◽  
Erdem Senol ◽  
Franziska Schoenebeck
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State

ChemInform Abstract: A Cyclic Transition State for the Darzen Reaction.

ChemInform ◽  
2010 ◽  
Vol 28 (5) ◽  
pp. no-no
Author(s):  
A. YLINIEMELAE ◽  
G. BRUNOW ◽  
J. FLUEGGE ◽  
O. TELEMAN
Keyword(s):  
Transition State ◽  
Cyclic Transition ◽  
Cyclic Transition State
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