Kinetics of the Solvolyses of Benzhydryl Derivatives: Basis for the Construction of a Comprehensive Nucleofugality Scale

Denegri, Bernard; Streiter, André; Jurić, Sandra; Ofial, Armin R.; Kronja, Olga; Mayr, Herbert

doi:10.1002/chem.200690066

Cited by 18 publications

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“…Intramolecular competition : In an argon-filled glovebox, a solution of 1 -NBu 4 in 0.75 mL of anhydrous CD 3 CN (distilled over CaH 2 and stored over molecular sieves) was added to an equimolar amount of the solid benzhydryl chlorides 2 ( n–u )-Cl or the benzhydryl tosylate 2u -OTs . The resulting solution was mixed, then transferred into an NMR tube, sealed, and submitted to analysis ( 1 H NMR, 1 H, 13 C-HSQC).…”

Section: Experimental Sectionmentioning

confidence: 99%

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: Experimental Sectionmentioning

confidence: 99%

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

et al. 2019

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“…1,14,23 Equation 3, which characterizes electrofuges by one parameter (E f ) and nucleofuges by two (N f and s f ) can analogously be used to correlate ionization rate constants of substrates with widely variable reactivity. 13 As solvent variation affects the leaving group abilities of different groups Xquite differently, the nucleofuge-specific parameters N f and s f are generally defined for pairs of leaving groups and solvents, for example, for Clin EtOH or for CH 3 CO 2 in 60% aqueous acetonitrile. As most solvolysis rate constants have been measured at 25 °C, the reference temperature for eq 3 differs from that for eq 2.…”

Section: Correlation Analysismentioning

confidence: 99%

“…Recently, we have suggested to solve this problem by developing comprehensive nucleofugality and electrofugality scales, using an approach similar to that which gave access to the most comprehensive nucleophilicity and electrophilicity scales presently available. , By defining differently substituted benzhydrylium ions as reference electrofuges, the steric environment of the reaction center was kept constant while the stabilization of the resulting benzhydrylium ions was varied by many orders of magnitude.…”

Section: Introductionmentioning

confidence: 99%

A Practical Guide for Estimating Rates of Heterolysis Reactions

et al. 2010

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Chemists are well trained to recognize what controls relative reactivities within a series of compounds. Thus, it is well-known how the rate of ionization of R-X is affected by the stabilization of the carbocation R(+), the nature of the leaving group X(-), or the solvent ionizing power. On the other hand, when asked to estimate the half-life of the ionization of a certain substrate in a certain solvent, most chemists resign. This question, however, is crucial in daily laboratory practice. Can a certain substrate R-X be handled in alcoholic or aqueous solution without being solvolyzed? Can a biologically active tertiary amine or azole be released by ionization of a quaternary ammonium ion? In this Account, we describe a straightforward means of addressing such experimental concerns. A semiquantitative answer to these questions is given by the correlation equation log k(25 °C) = s(f)(N(f) + E(f)), in which carbocations R(+) are characterized by the electrofugality parameter E(f), and leaving groups X(-) in a certain solvent are characterized by the nucleofugality parameter N(f) and the nucleofuge-specific sensitivity parameter s(f). As s(f) is typically around 1 (0.8 < s(f) < 1.2), ionization half-lives of around 1 h at 25 °C can be expected when E(f) + N(f) = -4. This correlation equation is formally analogous to the linear free energy relationship that was used to derive the most comprehensive nucleophilicity and electrophilicity scales presently available (Mayr, H.; Bug, T.; Gotta, M. F.; Hering, N.; Irrgang, B.; Janker, B.; Kempf, B.; Loos, R.; Ofial, A. R.; Remennikov, G.; Schimmel, H. Reference Scales for the Characterization of Cationic Electrophiles and Neutral Nucleophiles. J. Am. Chem. Soc. 2001, 123, 9500-9512). By subjecting 628 solvolysis rate constants k(25 °C) for different benzhydryl derivatives (aryl(2)CH-X) to a least-squares minimization on the basis of the correlation equation, we obtained and tabulate here (i) the electrofugality parameters E(f) for 39 benzhydrylium ions and (ii) the nucleofuge-specific parameters N(f) and s(f) for 101 combinations of common leaving groups and solvents. We show that the E(f) parameters of the reference electrofuges can be used to determine N(f) and s(f) for almost any combination of leaving group and solvent. The nucleofuge-specific parameters of the reference systems can analogously be used to derive the electrofugalities E(f) of other types of carbocations. While it has long been recognized that good nucleophiles are not necessarily poor nucleofuges, it is now reported that there is also no general inverse relationship between electrophilicity and electrofugality. Although more electrophilic methyl- and methoxy-substituted benzhydrylium ions are generally weaker electrofuges, the inverse relationship between electrophilicity and electrofugality breaks down in the series of amino-substituted benzhydrylium ions. Because neither differential solvation of the carbocations nor steric effects are explicitly considered by this treatment, predictions for substrates ...

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“…This indicator has also proved useful in other contexts and studies [13,10,9,15]. The idea behind this indicator was that the nucleofugality of a leaving group may be described as the energy difference between the anion and some ideal charge q ðidealÞ a .…”

Section: Initial Indicator (Indicator 1)mentioning

confidence: 97%

“…These properties are closely related to the energy cost of accepting an electron, the charge of the leaving group, and its susceptibility to electron transfer. As a result there are several indicators in the literature that attempt to order the experimentally observed leaving ability [8][9][10][11][12][13][14]. To model nucleofugality we consider the leaving groups either in complete isolation or bonded to an sp 3 hybridised carbon of a simple molecule.…”

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confidence: 99%

Predicting the quality of leaving groups in organic chemistry: Tests against experimental data

Anderson

Liu

Thomson

et al. 2010

Journal of Molecular Structure: THEOCHEM

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Kinetics of the Solvolyses of Benzhydryl Derivatives: Basis for the Construction of a Comprehensive Nucleofugality Scale

Cited by 18 publications

References 0 publications

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

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

A Practical Guide for Estimating Rates of Heterolysis Reactions

Predicting the quality of leaving groups in organic chemistry: Tests against experimental data

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