5633-96-5Relevant articles and documents
Ambident Reactivity of Phenolate Anions Revisited: A Quantitative Approach to Phenolate Reactivities
Mayer, Robert J.,Breugst, Martin,Hampel, Nathalie,Ofial, Armin R.,Mayr, Herbert
, p. 8837 - 8858 (2019/07/08)
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 (Aryl2CH+) 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 diffusion-limited 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, sN) 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 single-electron transfer manifests that these reactions proceed via polar mechanisms.
A kinetic study on nucleophilic displacement reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides: Metal ion effect and reaction mechanism
Um, Ik-Hwan,Seo, Ji-Yoon,Kang, Ji-Sun,An, Jun-Sung
, p. 1007 - 1013,7 (2020/08/24)
Pseudo-first-order rate constants (kobsd) have been measured for reactions of phenyl Y-substituted-phenyl carbonates with alkali metal ethoxides (EtOM, M = Li, Na, and K). The plot of kobsd vs. [EtOM] curves upward for the reaction of diphenyl carbonate with EtOM but is linear for that with EtOK in the presence of 18-crown-6-ether (18C6), indicating that the reaction is catalyzed by M+ ions and the catalytic effect disappears in the presence of 18C6. The kobsd values for the reactions with EtOK have been dissected into fEtO- and kEtOK, i.e., the second-order rate constants for the reactions with dissociated EtO- and ion-paired EtOK, respectively. The Hammett plots correlated with σ- and σ-0 constants exhibit highly scattered points, while the Yukawa-Tsuno plots result in an excellent linear correlation with p = 2.11 and r = 0.21 for kEtO-, and P = 1.62 and r = 0.26 for kEtOK, implying that the reaction proceeds through a concerted mechanism. The catalytic effect (i.e., the kEtOK/kEtOr ratio) is independent of the electronic nature of the substituent Y. Thus, it has been concluded that K+ ion catalyzes the reaction by increasing the electrophilicity of the reaction center.
Combined dual substituent constant and activation parameter analysis assigns a concerted mechanism to alkaline ethanolysis at phosphorus of Y-substituted phenyl diphenylphosphinates
Um, Ik-Hwan,Park, Jee Eun,Shin, Young-Hee
, p. 3539 - 3543 (2008/09/20)
Second-order rate constants have been measured for reactions of Y-substituted phenyl diphenylphosphinates (1a-h) with EtO-K + in anhydrous ethanol. A linear Bronsted-type plot is obtained with βLg = -0.54, a typical βLg value for reactions which proceed through a concerted mechanism. The Hammett plots correlated with σo and σ- constants are linear but exhibit many scattered points, while the corresponding Yukawa-Tsuno plot results in excellent linear correlation with r = 0.41. The r value of 0.41 indicates that the leaving group departs at the rate-determining step (RDS) whether the reactions proceed through either a concerted or a stepwise mechanism. However, a stepwise mechanism in which departure of the leaving group occurs at the RDS is excluded since the incoming EtO- ion is much more basic and a poorer leaving group than the leaving aryloxide. The ΔH? values determined in the current reactions are strongly dependent on the nature of the substituent Y, while the ΔS ? values remain constant on changing the substituent Y in the leaving group, i.e., from Y = H to Y = 4-NO2 and Y = 3,4-(NO 2)2. These ΔH? and ΔS ? trends also support a concerted mechanism. The Royal Society of Chemistry.
