.alpha.-Deuterium effects on the rates of solvolysis of a 2-adamantyl sulfonate ester

The results of calculations indicate that a previously proposed model for the transition state in "borderline" substitution reactions can be generalized and, as a result, the observed differences in the carbon-13 and deuterium isotope effects of S s l , Ss2, and "borderline" reactions rationalized. Although the conclusions may apply more generally, the standard reaction investigated is the solvolysis of benzyl bromide. The importance of resonance interaction with the phenyl ring, the significance of the product-or reactant-like character of the transition state, and the influence of the magnitude of force constants in determining isotope effects are examined. The temperamre dependence of kinetic isotope effects i n solvolysis is also investigated.Les rksultats des calculs indiquent que le mod2le proposk anttrieurement pour I'ktat de transition dans les rkactions de substitutions "limites" peut &tre gtnkralisk. En cons&-quence, les difftrences observkes dans les effets isotopiques du carbone-13 et du deutkrium dans les rkactions S r l , Ss2 et les rtactions "limites" peuvent &tre rationaliskes. La rtaction standard que I'on a Ctudike est la solvolyse du bromure de benzyle, bien que les conclusions peuvent &tre gknkralistes. On examine I'importance d e l'interaction de rksonance avec l e cycle du phtnyle, du caractkre de I'ttat de transition en fonction de sa ressemblance avec le produit initial ou final et l'influence de la grandeur des constantes d e force lors de l a dttermination des effets isotopiques. On examine d e plus la dipendance entre la tempkrature et les effets isotopiques sur la cinktique de solvolyse.[Traduit par le journal]Can. J. Chem.. 52, 903 (1974) Introduction as in ion pair formation (2), the overall isotope The mechanism of solvolytic reactions with effect is the product of an equilibrium and a characters between the two extremes of SN1 and kinetic isotope effect and unless one of the SN2 (I), the "borderline" cases, is still not isotope effects can be calculated o r measured clearly resolved. Reaction schemes of varying separately, the overall isotope effect becomes a complexity have been proposed (2-6).less useful parameter. The simplest mechanism involves a single T o explain experimental observations a model transition state with a high degree of charge for the transition state may be proposed. An separation (7). The other more complex mechan-example is Ingold's (1) model for second order isms for the borderline cases supposedly follow nucleophilic substitution reactions. a mixture of two competing reaction paths. The By use of well known computational methods first path leads via the traditional (1) SN2 (12-19) a suitable model of reasonable geometry transition state to products while the second may and bonding (force field) should then reproduce involve several carbonium ion-like species (2, 5). the observed isotope effects (12)(13)(14). For large The kinetic isotope effect method has been transition states it has been shown (12) that used extensively to obtain information about pa...

“…Reaction schemes of varying separately, the overall isotope effect becomes a complexity have been proposed (2)(3)(4)(5)(6).…”

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Model Calculations of Kinetic Isotope Effects in Nucleophilic Substitution Reactions

Bron¹

1974

“…In other words there is a significant nucleophilic participation by solvent in the solvolysis of these compounds in ethanolwater, although somewhat less in the case of 2,4,6-trimethylbenzyl chloride. The a-deuterium isotope effect for 2,4,6-trimethylbenzyl chloride in 80% trifluoroethanol-water (k,/kD = 1.16) actually exceeded the "upper limit" established by Shiner and Fisher for solvolysis of bengyl chlorides (13). The isotope effects for 2,4,6-tri-t-butylbenzyl chloride (Table 7) are also of considerable interest.…”

Section: Resultsmentioning

confidence: 86%

“…The 2,4,6-tri-t-butylbenzyl system are of interest, especially in view of some recent provides an interesting model for further work publications on the magnitude of a-deuterium in this field. In particular it is an interesting isotope effects (13,14). Our results are not of substrate for studying the magnitude of the --~-deuterium i s o t o~e effect for different leavine.…”

Section: Resultsmentioning

confidence: 99%

Sterically Hindered Aromatic Compounds. V. Solvolysis of p-Methyl-, 2,4,6-Trimethyl-, and 2,4,6-Tri-t-butylbenzyl Chloride. Effect of Solvent and α-Deuterium Substitution

Barclay¹,

Mercer²,

Hudson³

1972

Rates, activation parameters, and kinetic a-deuterium isotope effects are reported for solvolysis ofp-methylbenzyl chloride in 80% trifluoroethanol-water, and in ethanol-water and I-butyl alcohol-water of varying solvent compositions. The a-deuterium isotope effect in trifluoroethanol-water (kH/kD = 1.14 per D) indicates approximately limiting behavior. Results in the other solvents (kH/kD = 1.04-1.08) indicate considerable nucleophilic participation by solvent in the transition state. 2,4,6-Trimethylbenzyl chloride gave kH/kD = 1.09 in 80% ethanol-water but kH/kD = 1.16 in 80% trifluoroethanol-water. 2,4,6-Tri-I-butylbenzyl chloride showed kH/kD effects of 1.13-1 .I5 in I-butyl alcohol-water and 1.15-1.16 in ethanol-water. The higher kinetic isotope effects for solvolysis of these trialkylbenzyl chlorides are discussed in terms of steric hindrance to nucleophilic participation by solvent. N.m.r. chemical shifts give evidence for steric compression of the benzylic protons in 2,4,6-tri-I-butylbenzyl chlorides.Les vitesses, les parametres d'activation, et les effets isotopiques sur la cinetique du deuttrium a sont rapportis pour la solvolyse du p-m6thylbenzyle chlorure dans les systkmes trifluoroethanol 80%-eau et ethanol-eau, I-butanol-eau de compositions variables. L'effet isotopique de deuterium a dans le trifluorokthanol-eau (kH/kD = 1.14 par D) correspond approximativement a un comportement limite. Les resultats dans les autres solvants (kH/kD =

“…The predicted maximal values for α-SDKIEs at 25°C for solvolysis of secondary alkyl tosylates and bromides are approximately 1.225 and 1.125, respectively (20,21). Indeed, two separate research groups reported α-SDKIEs for solvolyses of two different 2-adamantyl sulfonates at 25°C that are indistinguishable from the predicted maximal value (22,23). In contrast, the documented α-SDKIE values for the aqueous ethanolysis of 1-OTs fall in the range of 1.14-1.17 at 25°C (10).…”

Section: Secondary Deuterium Kinetic Isotope Effectsmentioning

confidence: 90%

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

Huang¹,

Bennet²

2004

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.Résumé : On a examiné les réactions d'éthanolyse aqueuse du tosylate, du bromure et de l'iodure d'adamantylidène-adamantyle (1-OTs, 1-Br et 1-I) en fonction de la force ionique. On a observé des effets de sel spéciaux au cours des solvolyses de chacun des halogénures homoallyliques, mais ce n'est pas le cas pour le tosylate 1-OTs. On a déterminé que les effets isotopiques cinétiques du deutérium secondaire en α sur la solvolyse du 1-Br dans des mélanges 80:20 et 60:40 v/v d'éthanol et d'eau, à 25°C, sont respectivement de 1,110 ± 0,018 et 1,146 ± 0,009. Ces résultats sont en accord avec un mécanisme par lequel les halogénures homoallyliques réagissent par le biais d'un état de transition virtuel dans lequel la formation ainsi que la dissociation d'une paire d'ions séparés par le solvant sont toutes les deux partiellement cinétiquement déterminantes. Dans le cas du tosylate d'adamantylidèneadamantyle, l'état de transition correspondant implique toutefois une paire d'ions séparés par le solvant.Mots clés : effets de sel, effet isotopique cinétique, retour interne, solvolyse, paires d'ions.[Traduit par la Rédaction] Huang and Bennet 1340