Secondary α hydrogen–deuterium kinetic isotope effects in the syn-elimination reaction of 2-phenylethyldimethylamine oxide

1987 ◽  
Vol 65 (9) ◽  
pp. 2149-2153 ◽  
Author(s):  
Peter James Smith ◽  
Kenneth Charles Westaway
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Thermal Reaction ◽  
Bond Rupture ◽  
Elimination Process ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium ◽  
Relative Transition

The secondary α-deuterium kinetic isotope effect has been measured for the thermal reaction of 2-phenylethyldimethylamine oxide in 90 mol% DMSO–H2O at 60.0 °C. A large secondary α-deuterium isotope effect of 1.158 per α-D was found, which indicates significant [Formula: see text] bond rupture and very little double bond formation at the transition state for this concerted syn-elimination process. The observed large normal value for (kH/kD)α is discussed in terms of the use of secondary α-D isotope effects for the determination of stereochemistry for a concerted elimination process. Various isotope effects found for the syn elimination of the above amine oxide and those for the anti elimination of 2-arylethyltrimethylammonium salts with ethoxide are considered with respect to the relative transition state structures for syn- and anti-elimination reactions.

Isotope Effect Studies on Elimination Reactions. IX. The Nature of the Transition State for the E2 Reaction of 2-Arylethylammonium Ions with Ethoxide in Ethanol

1974 ◽  
Vol 52 (5) ◽  
pp. 749-760 ◽  
Author(s):  
P. J. Smith ◽  
A. N. Bourns
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
Ammonium Ions ◽  
Nitrogen Effect ◽  
Kinetic Isotope ◽  
Leaving Group ◽  
Hydrogen Deuterium

Kinetic isotope effects have been determined for the E2 reaction of some 2-arylethyltrimethyl-ammonium ions with ethoxide in ethanol at 40°. The nitrogen effect, (k14/k15 − 1)100, decreased with increasing electron-withdrawing ability of the para substituent; i.e. 1.37, 1.33, 1.14, and 0.88 for p-OCH3, p-H, p-Cl, and p-CF3, respectively. Furthermore, the primary hydrogen–deuterium isotope effects increased for the same substituents, respectively; i.e. kH/kD = 2.64, 3.23, 3.48, and 4.16. A large positive ρ value of 3.66 was found as well as a small secondary α-deuterium effect of 1.02 for p-H. In addition, the nitrogen isotope effect decreased with increasing strength of the abstracting base for the reaction of ethyltrimethylammonium ion; i.e. 1.86 and 1.41 at 60° for reaction with EtO−–EtOH and t-BuO−–t-BuOH, respectively. The results are discussed in terms of recent theoretical treatments of the effect of base, substituents, and nature of the leaving group on the nature of the transition state for an E2 process. The conclusion is reached that any structural change which causes one bond (C—H) to be weakened more at the transition state will have a corresponding effect on the other bond [Formula: see text]

Isotope Effect Studies on Elimination Reactions. X. The Nature of The Transition State for the ECO2 Reaction of Para- substituted Benzyl Nitrates with Base in 90% by Volume Ethanol–Water

1975 ◽  
Vol 53 (9) ◽  
pp. 1319-1326 ◽  
Author(s):  
Peter James Smith ◽  
Carol Audrey Pollock ◽  
Arthur Newcombe Bourns
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Nitrogen Isotope ◽  
Kinetic Isotope Effects ◽  
The Other ◽  
Theoretical Treatment ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium ◽  
Deuterium Isotope Effects

Kinetic isotope effects have been determined for the Eco2 reaction of para-substituted benzyl nitrates with ethoxide in 90 vol.% ethanol–water at 20°. The nitrogen isotope effect, (k14/k15−1)100 decreased with increasing electron-withdrawing ability of the para-substituent; i.e. 2.26, 1.95, 1.60, and 0.84 for p-CH3, p-H, p-CF3, and p-NO2, respectively. Furthermore, the primary hydrogen–deuterium isotope effects increased also for electron-withdrawing substituents; i.e. kH/kD = 5.78, 6.06, 6.40, 6.67, and 7.05 for p-CH3, p-H, p-Br, p-CF3, and p-NO2, respectively. The results are discussed in terms of a recent theoretical treatment dealing with the effect of substituents on the nature of the transition state for a concerted E2 process. The conclusion is reached that any structural change which causes one bond (carbon–hydrogen) to be weakened more at the transition state will have a corresponding effect on the other bond (oxygen–nitrogen).

The effect of the nature of the amine leaving group on the nature of the E2 transition state for the reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol

1989 ◽  
Vol 67 (9) ◽  
pp. 1457-1467 ◽  
Author(s):  
Peter James Smith ◽  
Md. Amin
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Sodium Ethoxide ◽  
Elimination Process ◽  
Leaving Group ◽  
Hydrogen Deuterium ◽  
Mechanism Transition ◽  
Leaving Groups ◽  
Leaving Group Ability

To investigate the effect of the leaving group on the elimination reaction of 1-phenylethylammonium ions with sodium ethoxide in ethanol at 60 °C, the reaction of seven different quaternary ammonium salts and their β-deuterated analogues with trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, N,N-dimethylbenzylamine, tripropylamine, and N,N-diethylbenzylamine as leaving groups has been studied. In all cases the elimination, which was shown to proceed via the concerted E2 process, was accompanied by competing substitution reactions. Although a significant dependence of the rate of the elimination process on the nature of the leaving group was noted, there was not any linear correlation with the basicity of the amine leaving group. The primary hydrogen–deuterium kinetic isotope effect for the elimination process, (kH/kD)E, was found to increase initially with an increase of reaction rate, [Formula: see text] for substrates containing the leaving groups trimethylamine, N-methylpiperidine, N-methyldiethylamine, triethylamine, and N,N-dimethylbenzylamine; i.e., (kH/kD)E = 5.03, 5.26, 5.40, 5.83, and 5.85, respectively. A further increase in rate, using substrates with tripropylamine and N,N-diethylbenzylamine as leaving groups resulted in a decrease of the magnitude of the hydrogen–deuterium isotope effect; i. e., (kH/kD)E = 5.42 and 4.67, respectively. It is concluded that steric effects mainly determine leaving group ability. As well, it is concluded that the leaving group ability of the amine determines the structure of the E2 transition state. For the reaction of the poorer leaving groups, trimethylamine, N-methylpiperidine, and N-methyldiethylamine, the proton is morethan one-half transferred at the transition state while for reaction involving the two best leaving groups, tripropylamine and N,N-diethylbenzylamine, the Cβ—H bond is lessthan one-half broken at the transition state. The conclusions are considered in the light of the More O'Ferrall – Jencks potential energy surface diagram. Keywords: elimination mechanism, transition state, isotope effects, leaving group, quaternary salts.

Using Incoming Nucleophile Primary Hydrogen−Deuterium Kinetic Isotope Effects To Model the SN2 Transition State

2000 ◽  
Vol 122 (30) ◽  
pp. 7342-7350 ◽  
Author(s):  
Terry Koerner ◽  
Yao-ren Fang ◽  
Kenneth Charles Westaway
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope ◽  
Hydrogen Deuterium

Additions and corrections: Transverse compression and the secondary H/D isotope effects in intramolecular SN2 methyl-transfer reactions

1998 ◽  
Vol 76 (3) ◽  
pp. 359-370 ◽  
Author(s):  
Saul Wolfe ◽  
Chan-Kyung Kim ◽  
Kiyull Yang ◽  
Noham Weinberg ◽  
Zheng Shi
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
High Energy ◽  
Retention Mechanism ◽  
Sigmatropic Rearrangement ◽  
Orbital Theory ◽  
Bending Vibration ◽  
Kinetic Isotope ◽  
Methyl Transfer

Using ab initio molecular orbital theory mainly at the 3-21+G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3 kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3 -O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3 kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.

Isotope Effect Studies on Elimination Reactions. XI. The Nature of the Transition State for the E2 Reaction of 2-Phenylethyldimethylanilinium Salts Containing Substituents in the Aniline Ring with Sodium Ethoxide in Ethanol

1975 ◽  
Vol 53 (23) ◽  
pp. 3513-3525 ◽  
Author(s):  
Peter Schmid ◽  
Arthur Newcombe Bourns
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Structural Changes ◽  
Isotope Effects ◽  
Sodium Ethoxide ◽  
Nitrogen Isotope ◽  
Structural Variations ◽  
Hydrogen Deuterium ◽  
Elimination Reactions ◽  
Pass Through

Kinetic isotope effects have been determined for the E2 reactions of a series of 2-phenylethyldimethylanilinium salts containing substituents in the aniline ring with sodium ethoxide in ethanol at 40 °C. The nitrogen isotope effect, (k14/k15−1)100, is not very sensitive to substituent changes but appears to increase slightly with increasing electron-withdrawing ability of the substituents, i.e., 1.19 ± 0.07, 1.13 ± 0.06, 1.12 ± 0.08, 1.30 ± 0.07, and 1.32 ± 0.06 for p-OCH3, p-CH3, p-H, p-Cl, and, m-CF3, respectively. The hydrogen–deuterium isotope effects pass through a minimum in the region of the unsubstituted compound and increase both with increasing electron-donating as well as with electron-withdrawing power of the substituents, i.e. kH/kD = 4.70 ± 0.06, 4.61 ± 0.04, 4.51 ± 0.04, 4.53 ± 0.09, 5.00 ± 0.07, and 5.39 ± 0.07 for p-OCH3, p-CH3, p-H, p-Cl, m-CF3, and p-CF3, respectively. The results are discussed in terms of recent theoretical treatments of the effect of structural variations in the reactants on the nature of the transition state of E2 elimination reactions. The conclusion is reached that the transition states in the present reaction series can be characterized as 'central with slight carbanion character' and that the effect of a change in the ability of the leaving group on the structure of the transition state manifests itself mainly in the direction perpendicular to the reaction coordinate. A simple novel hypothesis is formulated which emphasizes the importance of the location of the transition state in a More O'Ferrall-type potential energy diagram in determining its sensitivity to structural changes in the reactants.

scholarly journals Isotope effect analyses provide evidence for an altered transition state for RNA 2′-O-transphosphorylation catalyzed by Zn2+

2016 ◽  
Vol 52 (24) ◽  
pp. 4462-4465 ◽  
Author(s):  
Shuming Zhang ◽  
Hong Gu ◽  
Haoyuan Chen ◽  
Emily Strong ◽  
Edward W. Ollie ◽  
...  
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Base Catalysis ◽  
Kinetic Isotope

Solvent D2O and18O kinetic isotope effects on RNA 2′-O-transphosphorylation catalyzed by Zn2+demonstrate an altered transition state relative to specific base catalysis.

The nature of the transition state in diarylmethyl cation – nucleophile combination reactions as probed by secondary α-deuterium isotope effects

2001 ◽  
Vol 79 (12) ◽  
pp. 1887-1897
Author(s):  
Thuy Van Pham ◽  
Robert A McClelland
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Rate Constant ◽  
Rate Constants ◽  
Flash Photolysis ◽  
Isotope Effects ◽  
Equilibrium Constants ◽  
Bond Formation ◽  
Kinetic Isotope ◽  
Rate Limiting

Transition-state structures for the carbocation–nucleophile combination reactions of (4-substituted-4'- methoxydiphenyl)methyl cations with water, chloride, and bromide ions in acetonitrile–water mixtures have been investigated by measuring the secondary α-deuterium kinetic and equilibrium isotope effects. Rate constants in the combination direction were measured with laser flash photolysis. Equilibrium constants were measured for the water reaction by a comparison method in moderately concentrated sulfuric acid solutions, for the bromide reaction via the observation of reversible combination, and for the chloride reaction from the ratio of the combination rate constant and the rate constant for the ionization of the diarylmethyl chloride product. The fraction of bond making in the transition state has been calculated as the ratio log (kinetic isotope effect):log (equilibrium isotope effect). For the water reaction, there is 50–65% bond making in the transition state; this is also true for cations that are many orders of magnitude less reactive. The same conclusions, 50–65% bond formation in the transition state independent of reactivity, have previously been made in correlations of log kw vs. log KR. Thus, two quite different measures of transition structure provide the same result. The kH:kD values for the halide combinations in 100% acetonitrile are within experimental error of unity. This is consistent with suggestions that these reactions are occurring with diffusional encounter as the rate-limiting step. Addition of water has a dramatic retarding effect on the halide reactions, with rate constants decreasing steadily with increased water content. Small inverse kinetic isotope effects are observed (in 20% acetonitrile:80% water) indicating that carbon—halogen bond formation is rate-limiting. Comparison of the kinetic and equilibrium isotope effects shows ~25 and ~40% bond formation in the transition states for the reactions with bromide and chloride, respectively.Key words: carbocation, isotope effect, transition state, halide.

Unequal primary kinetic isotope effects in the base-catalysed H–D exchange of diastereotopic protons

1980 ◽  
Vol 58 (1) ◽  
pp. 72-78 ◽  
Author(s):  
Robert R. Fraser ◽  
Philippe J. Champagne
Keyword(s):  
Transition State ◽  
Exchange Reaction ◽  
Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Deuterium Isotope Effect ◽  
Kinetic Isotope

Primary kinetic isotope effects have been measured for the base-catalyzed exchange reaction of 4′,1″-dimethyl-1,2,3,4-dibenzcyclohepta-1,3-diene-6-one, 1. It was found that the isotope effects kH/kT and kD/kT for the faster exchanging protons (13.6 and 3.8 respectively) are significantly larger than the corresponding values for the slower exchanging protons (4.6 and 1.6 respectively). These differences could result from truly unequal isotope effects due to transition state differences or intrusion of a second pathway for exchange of the less reactive proton in the dedeuteration reaction. The data appear to support the latter interpretation. The secondary deuterium isotope effect was found to be 1.18.

Kinetic evidence for the importance of solvent-separated ion pairs during the solvolyses of adamantylideneadamantyl derivatives

2004 ◽  
Vol 82 (9) ◽  
pp. 1336-1340
Author(s):  
Xicai Huang ◽  
Andrew J Bennet
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Salt Effects ◽  
Kinetic Isotope

The aqueous ethanolysis reactions of adamantylideneadamantyl tosylate, -bromide, and -iodide (1-OTs, 1-Br and 1-I) were monitored as a function of ionic strength. Special salt effects are observed during the solvolyses of both homoallylic halides, but not in the case of the tosylate 1-OTs. The measured α-secondary deuterium kinetic isotope effects for the solvolysis of 1-Br in 80:20 and 60:40 v/v ethanol–water mixtures at 25 °C are 1.110 ± 0.018 and 1.146 ± 0.009, respectively. The above results are consistent with the homoallylic halides reacting via a virtual transition state in which both formation and dissociation of a solvent-separated ion pair are partially rate-determining. While the corresponding transition state for adamantylideneadamantyl tosylate involves formation of the solvent-separated ion pair.Key words: salt effects, kinetic isotope effect, internal return, solvolysis, ion pairs.

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