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.

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

1988 ◽  
Vol 66 (5) ◽  
pp. 1263-1271 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Zhu-Gen Lai
Keyword(s):  
Transition State ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Ion Pair ◽  
State Structure ◽  
Transition State Structure ◽  
Substitution Reactions ◽  
Sn2 Reactions ◽  
Kinetic Isotope

Spectroscopic and conductivity studies of sodium thiophenoxide solutions in four different solvents and the secondary α-deuterium kinetic isotope effects found in the presence of 15-crown-5 ether demonstrate that the secondary α-deuterium kinetic isotope effect and transition state structure for the SN2 reaction between sodium thiophenoxide and n-butyl chloride are significantly different, depending on whether the ionic reactant is a solvent-separated ion-pair complex or a free ion. In all three solvents in which the form of the ionic reactant changes, a smaller isotope effect and tighter transition state are found for the reaction with the ion-pair complex.

Solvent effects on SN2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

1989 ◽  
Vol 67 (2) ◽  
pp. 345-349 ◽  
Author(s):  
Kenneth Charles West Away ◽  
Zhu-Gen Lai
Keyword(s):  
Transition State ◽  
Solvent Effects ◽  
Isotope Effects ◽  
Ion Pairs ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
State Structure ◽  
Butyl Chloride ◽  
Transition State Structure ◽  
Kinetic Isotope

Identical secondary α-deuterium kinetic isotope effects (transition state structures) in the SN2 reaction between n-butyl chloride and a free thiophenoxide ion in aprotic and protic solvents confirm the validity of the Solvation Rule for SN2 Reactions. These isotope effects also suggest that hydrogen bonding from the solvent to the developing chloride ion in the SN2 transition state does not have a marked effect on the magnitude of the chlorine (leaving group) kinetic isotope effects. Unlike the free ion reactions, the secondary α-deuterium kinetic isotope effect (transition state structure) for the SN2 reaction between n-butyl chloride and the solvent-separated sodium thiophenoxide ion pair complex is strongly solvent dependent. These completely different responses to a change in solvent are rationalized by an extension to the Solvation Rule for SN2 Reactions. Finally, the loosest transition state in the reactions with the solvent-separated ion pair complex is found in the solvent with the smallest dielectric constant. Keywords: ion pairs, transition state, solvent effects, nucleophilic substitution, isotope effects.

Isotope effects in nucleophilic substitution reactions. II. Secondary α-deuterium kinetic isotope effects: a criterion of mechanism?

1979 ◽  
Vol 57 (9) ◽  
pp. 1089-1097 ◽  
Author(s):  
Kenneth Charles Westaway ◽  
Syed Fasahat Ali
Keyword(s):  
Transition State ◽  
Nucleophilic Substitution ◽  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Substitution Reactions ◽  
Nucleophilic Substitution Reactions ◽  
Kinetic Isotope ◽  
Steric Crowding

A very large secondary α-deuterium kinetic isotope effect of 1.179 ± 0.007 (1.086 ± 0.003 per α-deuterium) has been observed for the SN2 reaction of thiophenoxide ion with benzyldimethylphenylammonium ion in DMF at 0°C. This large isotope effect which is far outside the range reported for SN2 reactions, is attributed to the fact that the extraordinarily large steric crowding around the Cα—H bonds in the substrate is reduced in the SN2 transition state. The structure of the transition state is shown to be consistent with this hypothesis.

Kinetics of the reduction of nicotinonitrile cations by 1,4-dihydronicotinamides

1985 ◽  
Vol 63 (6) ◽  
pp. 1245-1249 ◽  
Author(s):  
John W. Bunting ◽  
John C. Brewer
Keyword(s):  
Ionic Strength ◽  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Spectral Studies ◽  
Close Correspondence ◽  
Charge Generation ◽  
Kinetic Isotope ◽  
Hammett Σ Constants ◽  
Kinetics Of

The rates of reduction of a series of 1-(Z-benzyl)nicotinonitrile cations by a series of 1-(X-benzyl)-1,4-dihydronicotinamides have been studied at 25 °C in 20% CH3CN – 80% H2O (pH 7.0 (5 mM phosphate), ionic strength 1.0 (KCl)). Spectral studies indicate the formation of 1,4-dihydronicotinonitrile products, without the formation of the isomeric 1,2-dihydro- or 1,6-dihydro-nicotinamide intermediates. Second-order rate constants (k2) for these reductions are closely correlated with the Hammett σ constants for X and Z. Thus, for X = H, log k2 = 0.63σz − 1.05, while for Z = 4-CN, log k2 = −0.64σx − 0.65. The close correspondence between these ρx and ρz values indicates that charge neutralization on the nicotinonitrile cation exactly balances charge generation on the nicotinamide cation product in the rate-determining transition state. Thus the migrating hydrogen species is electrically neutral in the rate-determining transition state, which contrasts with the hydridic transition states previously reported in the reduction of isoquinolinium cations by 1,4-dihydronicotinamides. When 1-benzyl-4,4-dideuterio-1,4-dihydronicotinamide is used as the reductant, primary kinetic isotope effects of 3.0 and 2.7 are observed for the reduction of the 1-methylnicotinonitrile and 1-(4-cyanobenzyl)-nicotinonitrile cations, respectively. These data are evaluated in terms of the various mechanistic possibilities for hydride transfer.

SOME DEUTERIUM KINETIC ISOTOPE EFFECTS: IV. β-DEUTERIUM EFFECTS IN WATER SOLVOLYSIS OF ETHYL, ISOPROPYL, AND tert-BUTYL COMPOUNDS

1960 ◽  
Vol 38 (11) ◽  
pp. 2171-2177 ◽  
Author(s):  
K. T. Leffek ◽  
J. A. Llewellyn ◽  
R. E. Robertson
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Alkyl Halides ◽  
Tert Butyl ◽  
Kinetic Isotope ◽  
Deuterium Isotope Effects ◽  
Intramolecular Forces

The secondary β-deuterium isotope effects have been measured in the water solvolytic reaction of alkyl halides and sulphonates for primary, secondary, and tertiary species. In every case the kinetic isotope effect was greater than unity (kH/kD > 1). This isotope effect may be associated with varying degrees of hyperconjugation or altered non-bonding intramolecular forces. The experiments make it difficult to decide which effect is most important.

Solvolysis of specifically deuterated 7-chloro- and 7-bromo-exo-2-norbornyl brosylates; unusually large γ-kinetic isotope effects in the absence of σ-participation

1980 ◽  
Vol 58 (16) ◽  
pp. 1738-1750 ◽  
Author(s):  
Nick Henry Werstiuk ◽  
George Timmins ◽  
Frank Peter Cappelli
Keyword(s):  
Isotope Effect ◽  
Kinetic Isotope Effect ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Kinetic Isotope

A series of specifically deuterated syn-7-chloro-, anti-7-chloro-, syn-7-bromo-, and anti-7-bromo-exo-2-norbornyl brosylates have been prepared and solvolyzed in NaOAc-buffered 80:20 EtOH–H2O. For solvolysis at 25 °C the γ-kinetic isotope effects (KIE's) for syn-7-chloro-exo-2-norbornyl brosylate-endo-6-d (1e), anti-7-chloro-exo-2-norbornyl brosylate-endo-6-d (2c), syn-7-bromo-exo-2-norbornyl brosylate-endo-6-d (1f), anti-7-bromo-exo-2-norbornyl brosylate-endo-6-d (2d), syn-7-chloro-exo-2-norbornyl brosylate-exo,exo-5,6-d2 (1g), anti-7-chloro-exo-2-norbornyl brosylate-exo,exo-5,6-d2 (2e) are 1.125 ± 0.007, 1.128 ± 0.005, 1.063 ± 0.008, 1.149 ± 0.020, 1.119 ± 0.011, and 1.115 ± 0.013, respectively. There is no detectable γ-kinetic isotope effect for solvolysis of anti-7-chloro-endo-2-norbornyl brosylate-endo-6-d(3a) and the β-KIE for anti-7-chloro-exo-2-norbornyl brosylate-exo-3-d(4a) is 1.111 ± 0.011. From a consideration of the possible sources of the unusually large secondary KIE's, we conclude that the exo-6-d and endo-6-d γ-KIE's likely are derived from a combination of effects rather than from participation of the C1—C6 bond in the ionization step.

SECONDARY KINETIC ISOTOPE EFFECTS IN THE SOLVOLYSES OF 2-(2,4-DIMETIIOXYPHENYL)-1,1-d2-ETHYL AND 2-(3,5-DIMETHOXYPHENYL)-1,1-d2-ETIIYL p-BROMOBENZENESULFONATES

1966 ◽  
Vol 44 (21) ◽  
pp. 2491-2495
Author(s):  
C. C. Lee ◽  
L. Noszkó
Keyword(s):  
Transition State ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Ion Pair ◽  
Similar Data ◽  
Kinetic Isotope ◽  
Ion Stability ◽  
Transition State Structures

The secondary α-deuterium kinetic isotope effects in the acetolyses and formolyses of 2-(2,4-dimethoxyphenyl)-ethyl and 2-(3,5-dimethoxyphenyl)-ethyl p-bromobenzenesulfonates (I-OBs and II-OBs, respectively) and their corresponding 1,1-dideuterio analogues (I-OBs-1-d2 and II-OBs-1-d2) were determined. The observed kH/kD values are compared with similar data from the literature for 2-phenylethyl and 2-p-anisylethyl p-toluenesulfonates (III-OTs and IV-OTs, respectively). In the formolyses of III-OTs, IV-OTs, and I-OBs, which proceed either chiefly or exclusively by way of bridged ions as intermediates, the isotope effects appear to increase slightly with increasing bridged-ion stability. For I-OBs and I-OBs-1-d2, acetolyses gave smaller kH/kD values than formolyses because of deuterium scrambling caused by ion-pair returns during the acetolysis. Acetolyses and formolyses of II-OBs and II-OBs-1-d2 gave lower isotope effects than the corresponding reactions with I-OBs and I-OBs-1-d2. The magnitudes of the observed isotope effects in relation to transition-state structures are discussed.

Transition-state structure for the hydronium-ion-promoted hydrolysis of α-d-glucopyranosyl fluoride

2015 ◽  
Vol 93 (4) ◽  
pp. 463-467 ◽  
Author(s):  
Jefferson Chan ◽  
Ariel Tang ◽  
Andrew J. Bennet
Keyword(s):  
Transition State ◽  
Kinetic Isotope Effect ◽  
Bond Cleavage ◽  
Isotope Effects ◽  
Kinetic Isotope Effects ◽  
Theoretical Modelling ◽  
Kinetic Isotope ◽  
Hydronium Ion ◽  
Anomeric Carbon ◽  
Hydrolysis Of

The transition state for the hydronium-ion-promoted hydrolysis of α-d-glucopyranosyl fluoride in water has been characterized by combining multiple kinetic isotope effect measurements with theoretical modelling. The measured kinetic isotope effects for the C1-deuterium, C2-deuterium, C5-deuterium, anomeric carbon-13, and ring oxygen-18 are 1.219 ± 0.021, 1.099 ± 0.024, 0.976 ± 0.014, 1.014 ± 0.005, and 0.991 ± 0.013, respectively. The transition state for the hydronium ion reaction is late with respect to both C–F bond cleavage and proton transfer.

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.

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