Substitution at a saturated carbon atom. XVII. Organic ion pairs as intermediates in nucleophilic substitution and elimination reactions

Sneen, Richard A.

doi:10.1021/ar50062a002

Cited by 160 publications

(59 citation statements)

References 29 publications

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“…See ref. 46 j attack on a preformed ion-molecule pair (29). This may indeed be the case for a primary benzylic system, but we will show later that an SN2 explanation is untenable for our secondary substrates.…”

Section: Transition State Variationmentioning

confidence: 78%

Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

McLennan¹,

Stein²,

Dobson³

1986

Can. J. Chem.

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Kinetic chlorine isotope effects attending the solvolysis of several ring-substituted 1-phenylethyl chlorides in alcohol-water solvent mixtures are reported. The k35/k37 values are insensitive to the identity of ring substituents and to solvent composition. Results are interpreted in terms of an SN1 heterolytic process incorporating a significant amount of internal return. Theoretical calculations suggest that the incipient chloride ion in the transition state may be strongly hydrogen-bonded. On a mesurt les effets isotopiques cinttiques du chlore qui accompagnent les solvolyses de plusieurs chlorures de phtnyl-1 Cthyle substituts sur le cycle. Les valeurs de k35/k37 sont insensibles h la nature des substituants sur le cycle ainsi qu'h la composition du solvant. On interprbte les rtsultats en fonction d'un processus htttrolytique SN1 incorporant une part importante de retour interne. Des calcultes thkoriques suggbrent que I'ion chlorure qui attaque dans l'ttat de transition pourrait Ctre fortement lit par des liaisons hydrogbnes.[Traduit par la revue]

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Section: Transition State Variationmentioning

confidence: 78%

Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

McLennan¹,

Stein²,

Dobson³

1986

Can. J. Chem.

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“…Optimized structures for the reactants (1, 2), ion-dipole complexes (4,5), and TSs (6, 7), together with the thermodynamic parameters of complex formation and the activation parameters for the exchange reactions of benzyl bromides (1) and 1-aryl-1-bromoethanes (2) with Br - (3), were calculated at the DFT(B3LYP)/6-31G(d) level in vacuo and in acetone solution (see details in the Computational Methods Section). The computed activation parameters were compared with the results of kinetic measurements on the reactions of benzyl bromides [25] (1) and 1-aryl-1-bromoethanes [14] (2) with LiBr in ethane-1,2-diyl diacetate or acetone as solvent, respectively (Scheme 1).…”

Section: Results and Discussion Exchange Reactions Of Benzyl Bromidementioning

confidence: 99%

“…[14] This pathway has also been taken into consideration in other cases. [3,[15][16][17] In this paper we report on DFT computations relating to nucleophilic substitutions of benzyl bromides with the charged Br -and the uncharged pyridine as nucleophiles. Optimized structures of the reactants and TSs and also the activation parameters were calculated for the reactions of compounds containing e-d and e-w substituents in the substrate or in the nucleophile, in different solvents and in vacuo.…”

Section: Introductionmentioning

confidence: 99%

Computational Study of Reactivity and Transition Structures in Nucleophilic Substitutions on Benzyl Bromides

2006

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Keywords: Nucleophilic substitutions / Benzyl bromides / Substituent effect / Solvent effect / Density functional calculations / Activation parameters DFT computations on the mechanisms of nucleophilic substitutions on benzyl bromides were performed and the calculated activation parameters were compared with experimentally acquired data. In vacuo, the presence of electron-withdrawing (e-w) groups on the benzyl bromides accelerated the reactions and the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ plots were linear, while in solvents there were breaks in the calculated and measured ∆G ‡ /∆H ‡ vs. σ + plots of the reactions between the benzyl bromides and Br -, both with electron-donating (e-d) and with e-w substituents accelerating the reactions. The calculated ∆S ‡ values appeared to be independent of the substituents. In solvents, the calculated ∆G ‡ /∆H ‡ /∆S ‡ vs. σ + plots for the reactions between benzyl bromides and pyridine were linear, whereas breaks were observed in the plots of the measured data. These reactions were promoted by e-d substituents, but the measured reactivities of sub-

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“…The structures for attack on the 1-phenylethyl group of the R and S cations are shown in Figure 6. As we have noted before [39], the transition states of these reactions tend to be relatively loose (ion-pair like [47]) and consequently, the two chiral centers are far apart. In this case, the transition state is exceptionally loose and is best described as a carbocation trapped between the nucleophile and leaving group (S N 1-like).…”

Section: Resultsmentioning

confidence: 68%

Stereoselectivity in the collision-activated reactions of gas phase salt complexes

Gronert

Fagin

Okamoto

2004

J. Am. Soc. Mass Spectrom.

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The collision-activated dissociations (CAD) of gas phase salt complexes composed of chiral ions were studied in a quadrupole ion trap mass spectrometer. Because both partners in the salt are chiral, diastereomeric complexes can be formed (e.g., RR, RS). Two general types of complexes were investigated. In the first, the complex was composed of deprotonated binaphthol and a chiral bis-tetraalkylammonium dication. CAD of these complexes leads to the transfer of a proton or an alkyl cation to the binaphtholate leading to a singly-charged tetraalkylammonium cation. During CAD, diastereomeric complexes give significantly different product distributions indicating reasonable stereoselectivity in the process. In the second system, the complexes involved a peptide dianion and a chiral tetraalkylammonium cation. These systems may be viewed as very simple models for the interactions of peptides/proteins with small chiral molecules. Again, stereoselectivity was evident during CAD, but the extent was dependent on the nature of the peptide and not observable in some cases. To better understand the structural features needed to achieve stereoselectivity in gas phase salt complexes, representative transition states were modeled computationally. The results suggest that it is critical for the asymmetry of the nucleophile (i.e., anion) to be well represented in the vicinity of its reactive center. T he high sensitivity and rapid analysis capabilities of mass spectrometry make it an attractive methodology for solving a wide range of analytical problems. However, stereochemical issues are a challenge because a mass-to-charge ratio provides no direct information with regards to the stereochemistry of a species. Nonetheless, many workers have developed novel approaches for gaining stereochemical information from mass spectrometric data [1,2]. Much of this work has focused on the development of methods that could be used to determine the enantiomeric purity of a product mixture . Like many condensed phase approaches to enantiomeric analysis, the methods rely on complexing the analyte with a substrate of known chirality. This leads to the formation of a pair of diastereomers (assuming that the analyte is not enantiomerically pure). The diastereomers are not equivalent and as a result have different stabilities, kinetic reactivities, and dissociation patterns. These differences can be exploited (i.e., by comparison to data for known standards) and used to determine the enantiomeric purity of a sample. Cooks [3][4][5][6][7][8][9][10][11][12][13][14][15][16], Dearden , , Speranza [27][28][29][30][31][32][33][34][35], and others have provided useful examples of this approach to stereochemical analysis.Along with methods for analyzing the enantiomeric purity of products, there is a growing need for the development of new reaction processes that are stereoselective. In general, these processes have been probed in the condensed phase, but there is no reason why mass spectrometry could not also be used to screen gas phase reactions for stereo...

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Substitution at a saturated carbon atom. XVII. Organic ion pairs as intermediates in nucleophilic substitution and elimination reactions

Cited by 160 publications

References 29 publications

Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

Chlorine isotope effects in the solvolysis of substituted 1-phenylethyl chlorides

Computational Study of Reactivity and Transition Structures in Nucleophilic Substitutions on Benzyl Bromides

Stereoselectivity in the collision-activated reactions of gas phase salt complexes

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