The effect of conjugation on the magnitude of secondary alpha deuterium kinetic isotope effects

Waszczylo, Zbigniew; Westaway, Kenneth Charles

doi:10.1016/s0040-4039(00)86769-6

Cited by 3 publications

(3 citation statements)

References 8 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…Conjugation between the a carbon and the phenyl ring, which reduces the magnitude of secondary a-deuterium kinetic isotope effects, only appears to be important when strongly electrondonating groups are on the benzene ring on the a carbon (15). Thus, changes in conjugation would not seem to be the cause of the unexpected isotope effects.…”

Section: Resultsmentioning

confidence: 99%

“…0.014) than the isotope effect for the corresponding free ion reaction. Although other factors, such as a change in the amount of conjugation to the phenyl ring on the a carbon ( 10,15), changes in the reaction coordinate motion ( 16,17), tunneling (16,17), and changes in the product composition (18), can affect the magnitude of secondary a-deuterium kinetic isotope effects, the simplest explanation is that the free ion transition state is tighter than the ion pair transition state when the nucleophile is para-chlorothiophenoxide ion, Fig. 3, and that some factor(s) other than the charge on the sulfur of the nucleophile is important in determining transition state structure.…”

Section: Resultsmentioning

confidence: 99%

See 1 more Smart Citation

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Lai¹,

Westaway²

1989

Can. J. Chem.

Self Cite

View full text Add to dashboard Cite

ZHU-GEN LAI and KENNETH CHARLES WESTAWAY. Can. J. Chem. 67, 21 (1989).The secondary a-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 p values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulfur-a-carbon bond in the ion pair and free ion transition states. On a trouvt que, lorsque le nucltophile est un ion libre, les effets isotopiques cinttiques secondaires des atomes deuterium en a ainsi que I'effet de substituant qui ont Ctt observes lors de reactions SN2 d'une strie de thioph6nolates de sodium substituts en para avec l'ion benzyldimtthylphtnylammonium sont beaucoup plus importants que lorsque ce nuclCophile est une paire d'ions complexe stparte par le solvant. On a trouvt que les ttats de transition sont beaucoup plus compacts lorsqu'on utilise un mauvais nuclCophile tant dans des reactions d'ions libres que dans des rtactions de paires d'ions. Par comparaison avec les rtactions se produisant avec I'ion libre, les ttats de transition de tous les substituants, 2 I'exception d'un, sont plus compacts pour les reactions avec la paire d'ions complexe stparte par le solvant. I1 ne semble pas que les valeurs p de Hammett que l'on trouve en faisant varier la nature du substituant attach6 au nucleophile soient utiles pour determiner la longueur de la liaison entre le soufre et le carbone en a , tant dans la paire d'ions que dans les etats de transition de l'ion libre.Mots cl6s : Effets isotopiques, couplage ionique, substitution nucltophile, rtactions SN2, ttats de transition.[Traduit par la revue] Introduction Recent work by Westaway and Lai (1) has shown that the form of the reacting anion has a significant effect on the magnitude of the secondary a-deuterium kinetic isotope effect and transition state structure of an SN2 reaction. For example, changing the form of the reacting anion from a solventseparated ion pair complex (ref. 1, vide infra) to a free ion in the SN2 reaction between n-butyl chloride and sodium thiophenoxide in DMSO at 20°C, eq. [I], changes the secondary a-deuterium kinetic isotope effect from 1.034 t. 0.003 to 1.118 +. 0.009 (1). In fact, this change in the form of the reacting nucleophile seems to be a general phenomenon because the secondary a-deuterium kinetic isotope effect changes in at least three other SN2 reactions when the form of the nucleophile is altered ( I ) .~ These results clearly demonstrate that the magnitude of the secondary a-deuterium kinetic isotope effect and transition state struct...

show abstract

Section: Resultsmentioning

confidence: 99%

Section: Resultsmentioning

confidence: 99%

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Lai¹,

Westaway²

1989

Can. J. Chem.

Self Cite

View full text Add to dashboard Cite

show abstract

“…Although other factors such as a change in the amount of conjugation to the phenyl ring on the a carbon (3,12), changes in the reaction coordinate motion (13,14), tunnelling (13,14), and changes in product composition (15) can affect the magnitude of secondary a-deuterium kinetic isotope effects, the simplest explanation for the identical isotope effects in the free ion reaction is preferred because (i) the substrate is saturated, Can (5). dStandard deviation on the average isotope effect.…”

Section: Testing the Solvation Rule For Sn2 Reactionsmentioning

confidence: 99%

Solvent effects on S_N2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

Away¹,

Lai²

1989

Can. J. Chem.

View full text Add to dashboard Cite

. Can. J. Chem. 67, 345 (1989).Identical secondary a-deuterium kinetic isotope effects (transition state structures) in the SN2 reaction between 11-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 a-deuterium kinetic isotope effect (transition state structure) for the SN2 reaction between 11-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.Key words: ion pairs, transition state, solvent effects, nucleophilic substitution, isotope effects.KENNETH CHARLES WESTAWAY et ZHU-GEN LAI. Can. J. Chem. 67, 345 (1989).Les effets isotopiques cinetiques secondaires des deuterium en a (Ctats de transition ressemblant aux structures) qui sont observes au cours de la rkaction SN2 du chlorure de n-butyle avec l'ion thiophenolate libre sont identiques que les solvants soient aprotique ou protique; ces resultats confirment la validite de la regle de solvatation pour les rkactions SN2. Ces effets isotopiques suggkrent aussi que la liaison hydrogkne qui se developpe, dans I'ktat de transition SN2, entre le solvant et l'ion chlorure qui se forme n'a pas d'influence marquee sur I'amplitude des effets isotopiques cinktiques du chlore (groupement nucleofuge). Par opposition avec ce qui est observe lors des reactions des ions libres, l'effet isotopique cinktique secondaire des deuterium en a (structure ressemblant a 1'Ctat de transition) qui caractirise la rkaction SN2 entre le chlorure de n-butyle et la paire d'ions complexe de I'ion thiophenolate de sodium sCpar6 par le solvant depend fortement sur le solvant. Pour rationaliser ces reponses complktement diffkrentes i un changement de solvant, on fait appel i une extension de la rkgle de solvatation pour les reactions SN2. Enfin, dans les rkactions comportant une paire d'ions complexe skparke par le solvant, c'est avec le solvant possedant la plus faible constante diklectrique qu'on observe 1'Ctat de transition le plus liche.

show abstract

Theoretical studies on the gas-phase nucleophilic substitution reactions of benzyl chlorides with phenoxides and thiophenoxides

Kim

Ryu

Han

et al. 1998

J. Phys. Org. Chem.

View full text Add to dashboard Cite

Gas‐phase nucleophilic substitution reactions of Y‐benzyl chlorides with X‐phenoxide and X‐thiophenoxide nucleophiles were investigated theoretically using the PM3 semi‐empirical MO method. The Leffler–Grunwald rate‐equilibrium and Brønsted correlations predict that the degree of bond formation in the transition state (TS) is approximately 45 and 40% on the reaction coordinate for the phenoxides and thiophenoxides, respectively. For a weaker nucleophile, a later TS is obtained with an increased bond making and breaking. The variation of the TS structure with substituents in the nucleophile is thermodynamically controlled and is well correlated by rate–equilibrium relationships. In contrast, the TS variation (a tighter TS) with substituent (for a stronger acceptor Y) in the substrate is dependent only on variations of the intrinsic barrier and so cannot be correlated by such thermodynamically based rate–equilibrium relationships. The gas phase ρX and ρY values are much greater in magnitude than those in solution. A similar gas‐phase theoretical cross‐interaction constant, ρXY (ca −0.60), is obtained for both phenoxides and thiophenoxides, which is in good agreement with the experimental value (−0.62) for the thiophenoxide reactions in MeOH at 20.0 °C. The oxy and sulfur anion bases lead to a similar TS structure, but a lower reactivity for the former is due to a greater endothermicity of the reaction. A relatively wide range variation of the reaction energies, ΔG°, can be ascribed to the loss of resonance stabilization of anion nucleophiles upon product formation. © 1998 John Wiley & Sons, Ltd.

show abstract

The effect of conjugation on the magnitude of secondary alpha deuterium kinetic isotope effects

Cited by 3 publications

References 8 publications

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Solvent effects on S_N2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure

Theoretical studies on the gas-phase nucleophilic substitution reactions of benzyl chlorides with phenoxides and thiophenoxides

Contact Info

Product

Resources

About

The effect of conjugation on the magnitude of secondary alpha deuterium kinetic isotope effects

Cited by 3 publications

References 8 publications

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

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

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

Theoretical studies on the gas-phase nucleophilic substitution reactions of benzyl chlorides with phenoxides and thiophenoxides

Contact Info

Product

Resources

About

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on S_N2 transition state structure

Solvent effects on S_N2 transition state structure. II: The effect of ion pairing on the solvent effect on transition state structure