Isotope effects in nucleophilic substitution reactions X. The effect of changing the nucleophilic atom on ion-pairing in an S&lt;sub&gt;N&lt;/sub&gt;2 reaction

Fang, Yuqiang; Lai, Zhu-Gen; Westaway, Kenneth Charles

doi:10.1139/cjc-76-6-758

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“…Our product studies as well as comprehensive kinetic and equilibrium measurements agree with the early work by Kornblum, who showed that allylations and benzylations of phenolates, which proceed via S N 2 mechanisms, give exclusively products of O attack in most solvents. We conclude, therefore, that the prediction of the principle of hard and soft bases, “changing from S N 1 to S N 2 conditions should favor C attack by phenolate ions” is misleading.…”

Section: Discussionmentioning

confidence: 85%

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Ambident Reactivity of Phenolate Anions Revisited: A Quantitative Approach to Phenolate Reactivities

et al. 2019

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Prompted by the observation that the regioselectivities of phenolate reactions (C versus O attack) are opposite to the predictions by the principle of hard and soft acids and bases, we performed a comprehensive experimental and computational investigation of phenolate reactivities. Rate and equilibrium constants for the reactions of various phenolate ions with benzhydrylium ions (Aryl 2 CH + ) and structurally related quinone methides have been determined photometrically in polar aprotic solvents. Quantum chemical calculations at the SMD(MeCN)/M06-2X/6-31+G(d,p) level confirmed that O attack is generally favored under kinetically controlled conditions, whereas C attack is favored under thermodynamically controlled conditions. Exceptions are diffusionlimited reactions with strong electrophiles, which give mixtures of products arising from O and C attack, as well as reactions with metal alkoxides in nonpolar solvents, where oxygen attack is blocked by strong ion pairing. The Lewis basicity (LB) and nucleophilicity (N, s N ) parameters of phenolates determined in this work can be used to predict whether their reactions with electrophiles are kinetically or thermodynamically controlled and whether the rates are activation-or diffusion-limited. Comparison of the measured rate constants for the reactions of phenolates with carbocations with the Gibbs energies for singleelectron transfer manifests that these reactions proceed via polar mechanisms.

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Section: Discussionmentioning

confidence: 85%

“…Phenolate ions are ambident nucleophiles which may react with electrophiles either at oxygen or at carbon, as indicated by the resonance structures in Scheme . − …”

Section: Introductionmentioning

confidence: 99%