Transition States and Energetics of Nucleophilic Additions of Thiols to Substituted α,β-Unsaturated Ketones: Substituent Effects Involve Enone Stabilization, Product Branching, and Solvation

2011 ◽  
Vol 76 (12) ◽  
pp. 5074-5081 ◽  
Author(s):  
Elizabeth H. Krenske ◽  
Russell C. Petter ◽  
Zhendong Zhu ◽  
K. N. Houk
Keyword(s):  
Transition States ◽  
Substituent Effects ◽  
Nucleophilic Additions ◽  
Unsaturated Ketones ◽  
Product Branching

Substituent effects of rate constants for thermal [4π + 2π] cycloreversion of spiro-fused β-lactam oxadiazolines

1993 ◽  
Vol 71 (6) ◽  
pp. 907-911 ◽  
Author(s):  
Michel Zoghbi ◽  
John Warkentin
Keyword(s):  
Thermal Decomposition ◽  
Rate Constant ◽  
Rate Constants ◽  
Transition States ◽  
Substituent Effects ◽  
Ground States ◽  
Phenyl Substituent ◽  
First Order ◽  
Average Value ◽  
Carbonyl Ylide

Twelve Δ3-1,3,4-oxadiazolines in which C-2 is also C-4 of a β-lactam moiety (spiro-fused β-lactam oxadiazoline system) were thermolyzed as solutions in benzene. Substituents in the β-lactam portion affect the rate constant for thermal decomposition of the oxadiazolines to N2, acetone, and a β-lactam-4-ylidene. The total spread of first-order rate constants at 100 °C was 47-fold and the average value was 6.7 × 10−4 s−1. A phenyl substituent at N-1 or at C-3 was found to be rate enhancing, relative to methyl. At C-3, H and Cl were also rate enhancing, relative to methyl. The data are interpreted in terms of the differential effects of substituents on the stabilities of the ground states, and on the stabilities of corresponding transition states for concerted, suprafacial, [4π + 2π] cycloreversion. The first products, presumably formed irreversibly, are N2 and a carbonyl ylide. The latter subsequently fragments to form acetone (quantitative) and a β-lactam-4-ylidene.

The log kexovs. log kendo correlation as a mechanistic criterion; on the mechanisms of solvolysis of norbornyl derivatives and base-catalyzed isotope exchange of norbornanones

1976 ◽  
Vol 54 (5) ◽  
pp. 678-684 ◽  
Author(s):  
Sujit Banerjee ◽  
Nick Henry Werstiuk
Keyword(s):  
Transition State ◽  
Exchange Rates ◽  
Isotope Exchange ◽  
Transition States ◽  
Substituent Effects ◽  
Proton Exchange ◽  
Hydrogen Deuterium Exchange ◽  
Rate Data ◽  
Hydrogen Deuterium ◽  
The Difference

Rate data for the acetolysis of exo-norbornyl-sulfonates have been correlated with those for the corresponding endo isomers. It is shown that the slopes of the log kexovs. log kendo plots reflect the difference in delocalization between the transition states derived from the exo and endo isomers, respectively. The log kexovs. log kendo plot, which is comprised of the parent norbornyl sufonate and its derivatives substituted at the 5, 6, and 7 positions, has a slope of 1.11 ± 0.08, which establishes that σ bridging is absent in the transition state obtained from the exo isomer. A similar analysis of base-catalyzed hydrogen–deuterium exchange rates of norbornanones reveals that exo proton exchange is more sensitive to substituent effects than the corresponding endo process.

Réduction catalytique de cétones α,β-éthyléniques à température modérée par

1979 ◽  
Vol 57 (2) ◽  
pp. 218-221 ◽  
Author(s):  
D. Beaupere ◽  
P. Bauer ◽  
R. Uzan
Keyword(s):  
Electrochemical Reduction ◽  
Reduction Potential ◽  
Initial Rate ◽  
Substituent Effects ◽  
Hydrogenation Reaction ◽  
Unsaturated Ketones ◽  
Hydride Ion ◽  
Inverse Effect ◽  
Electron Donating Groups

The use of the catalyst [Formula: see text] in the reduction of the ethylenic system of styryl ketones by 1-phenylethanol permits the hydrogenation reaction to proceed at 50 °C. Some substituent effects in the α,β-unsaturated ketones were demonstrated at this temperature. These showed an increase in the initial rate of reduction for electron-attracting groups and the inverse effect for electron-donating groups. The observation of a relation between the electrochemical reduction potential of these ketones and their rate of hydrogenation suggests a transfer of hydrogen to the ketone by a hydride ion intermediate. [Journal translation]

Reactivity of phenylsulfonyl isothiocyanates and transfer of substituent effects through the sulfonyl group

1991 ◽  
Vol 56 (12) ◽  
pp. 2872-2878 ◽  
Author(s):  
Gejza Suchár ◽  
Oľga Hritzová ◽  
Ján Imrich ◽  
Antónia Jasovská
Keyword(s):  
Spectroscopic Method ◽  
Substituent Effects ◽  
Stopped Flow ◽  
Nucleophilic Additions ◽  
Reaction Centre ◽  
Electronic Effects ◽  
Sulfonyl Group ◽  
Electronic Effects Of Substituents ◽  
Kinetics Of

Kinetics of nucleophilic additions of butylamine to phenylsulfonyl isothiocyanate was investigated by the stopped-flow UV spectroscopic method and the transfer of electronic effects of substituents on the reaction centre through the SO2 group was examined. The structure and the polar character of SO2NCS, NCS and SO2 groups is discussed.

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure

1989 ◽  
Vol 67 (1) ◽  
pp. 21-26 ◽  
Author(s):  
Zhu-Gen Lai ◽  
Kenneth Charles Westaway
Keyword(s):  
Nucleophilic Substitution ◽  
Isotope Effects ◽  
Transition States ◽  
Substituent Effects ◽  
Ion Pairing ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Substitution Reactions ◽  
Kinetic Isotope ◽  
Free Ion

The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.

Substituent effects on stability of oxiranes, oxirenes, and dioxiranes

2000 ◽  
Vol 78 (5) ◽  
pp. 562-567 ◽  
Author(s):  
Kuangsen Sung
Keyword(s):  
Substituent Effect ◽  
Transition States ◽  
Substituent Effects ◽  
Stabilization Energies ◽  
Homodesmotic Reaction

Homodesmotic reactions were designed to study substituent effects on stability of oxiranes, oxirenes, and dioxiranes. Good or fair correlation between their homodesmotic stabilization energies and Taft's dual-substituent-parameters has been found. Oxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on oxiranes are comparable to σ-effects for π-acceptor substituents and are much stronger than σ-effects for π-donor substituents. Oxirenes are stabilized by σ-donating, π-donating, and π-accepting substituents but destabilized by σ-accepting substituents. The π-effects on oxirenes are stronger than σ-effects. Oxirenes with strong π-donor substituents such as F, OH, and NH2, or strong σ-donor substituents such as Li and Na are neither real molecules nor transition states. Dioxiranes are stabilized by σ-donating and π-donating substituents but destabilized by σ-accepting and π-accepting substituents. The π-effects on dioxiranes are stronger than σ-effects for π-donor substituents, but weaker than σ-effects for π-acceptor substituentsKey words: oxirane, oxirene, dioxirane, homodesmotic reaction, substituent effect.

Elimination reactions. Substituent effects on positions in the E2H-E2C spectrum of transition states

1968 ◽  
Vol 9 (55) ◽  
pp. 5715-5718 ◽  
Author(s):  
D. Cook ◽  
A.J. Parker ◽  
M. Ruane
Keyword(s):  
Transition States ◽  
Substituent Effects ◽  
Elimination Reactions
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