Acidity of Hydrocarbons. X. Exchange Rates of Ring-Substituted Toluene-α-t's with Lithium Cyclohexylamide in Cyclohexylamine

Streitwieser, Andrew; Koch, H. Fred

doi:10.1021/ja01057a023

Cited by 61 publications

(21 citation statements)

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“…The negative charge for the 9-PhFl -anion would not be p-delocalized into the phenyl ring, and a Hammett plot using the four 9-YPhFl compounds results in a rho of 2.1, which is similar to that obtained for the hydrolysis of methyl benzoates. A plot with the six Y-I compounds results in a rho value of 4.9, which is similar that obtained for the protodetritiation reactions of ring-substituted toluene-a-t's with lithium cyclohexylamide in cyclohexylamine [14].…”

supporting

confidence: 79%

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Koch

2006

Hydrogen‐Transfer Reactions

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This chapter is divided into three sections: Proton transfer from carbon acids to methoxide ion; proton ttransfer from methanol to carbanion intermediates; proton transfer associated with methoxide promoted dehydrohalogenation reactions. Proton Transfer from Carbon Acids to Methoxide IonKnowledge of equilibrium pK a values of carbon acids is important for an understanding of organic chemistry; however, they are not always reliable indicators of relative rates for proton transfer reactions. Ritchie [1] predicted in 1969 that carbon acids whose conjugate bases have localized charge will show "kinetic acidities" greater than their thermodynamic acidities. This is illustrated by the weaker acid pentafluorobenzene-t [PFB-t], pK a = 25.8 [2], with a methanolic sodium methoxide catalyzed protodetritiation rate, k = 2.57 · 10 -2 M -1 s -1 [3], that is 15 times faster at 25 C than that for 9-phenylfluorene-9-t [9-PhFl-9-t], pK a = 18.5 and k = 1.54 · 10 -3 M -1 s -1 [4]. Primary kinetic isotope effects [PKIE] can be used to determine C-H bond breaking in the rate limiting steps of any reaction mechanism. Normal hydrogen isotope effects are largely due to zero-point energy differences of the stretching frequencies of C-H vs. C-D bonds. If only stretching frequencies are considered the value for k H /k D would be 6.2 for the C-i H bond. With allowance for bending vibrations this could result in a k H /k D = 10, k H /k T = 27 and k D /k T = 2.7 at 25 C [5]. Melander [6] and Westheimer [7] suggested that lower values of k H /k D could be due to residual zero-point energy in an asymmetric transition structure. For this reason the magnitude of k H /k D was often used to assign early or late transition structures for hydron-transfer reactions. Smaller values due to asymmetric transition structures should still obey the Swain-Schaad relationship [8], k H /k T = (k H /k D ) 1.442 . The k D /k T = 1.0 associated with the exchange reactions of PFB i H suggest that the C-H bond is not broken in the rate limiting step and this differs significantly from the k D /k T of 2.54 measured from the reactions of 9-PhFl-9-i H.

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confidence: 79%

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Koch

2006

Hydrogen‐Transfer Reactions

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“…Decarboxylation leading to the immediate carbanion in the Breslow intermediate rather than to the subsequent enol would result in a much greater sensitivity to substituents (see Figure ). ρ values >3 are typical for E1 cb reactions with rate-determining formation of carbanions. , Transfers of benzylic protons give a ρ of 4.0, and the anionic polymerization of styrene gives a ρ of 5 . In contrast, the deprotonation of 1-phenylnitromethane, in which the developing charge is internally neutralized, has a ρ of 1.2 .…”

Section: Resultsmentioning

confidence: 99%

“…ρ values >3 are typical for E1 cb reactions with ratedetermining formation of carbanions. 29,30 Transfers of benzylic protons give a ρ of 4.0, 31 and the anionic polymerization of styrene gives a ρ of 5. 32 In contrast, the deprotonation of 1phenylnitromethane, in which the developing charge is internally neutralized, has a ρ of 1.2.…”

Section: ■ Results and Discussionmentioning

confidence: 99%

Charge Dispersion and Its Effects on the Reactivity of Thiamin-Derived Breslow Intermediates

2018

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The enzymic decarboxylation of 2-ketoacids proceeds via their C2-thiazolium adducts of thiamin diphosphate (ThDP). Loss of CO from these adducts leads to reactive species that are known as Breslow intermediates. The protein-bound adducts of the 2-ketoacids and ThDP are several orders of magnitude more reactive than the synthetic analogues in solution. Studies of enzymes are consistent with formulation of protein-bound Breslow intermediates with localized carbanionic character at the reactive C2α position, reflecting the charge-stabilized transition state that leads to this form. Our study reveals that nonenzymic decarboxylation of the related thiamin adducts proceeds to the alternative charge-dispersed enol form of the Breslow intermediate. These differences suggest that the greatly enhanced rate of decarboxylation of the precursors to Breslow intermediates in enzymes arises from maintenance of the carbanionic character at the position from which the carboxyl group departs, avoiding charge dispersion by stabilizing electrostatic interactions with the protein as formulated by Warshel. Applying Guthrie's "no-barrier" addition to Marcus theory also leads to the conclusion that maintaining the tetrahedral carbanion at C2α of the resulting adduct minimizes associated kinetic barriers by avoiding rehybridization as part of steps to and from the intermediate. Finally, maintenance of the reactive energetic carbanion agrees with the concepts of Albery and Knowles as the outcome of evolved enzymic processes.

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“…A typical example was our study of the exchange reaction of ArCH 2 D with lithium cyclohexylamide in cyclohexylamine, in which the Ar groups are polycyclic aromatic rings 13,14. The transition state for the reaction has a high degree of carbanionic character 15. The π–system model that could be calculated by HMO theory was that of an arene going to an arylmethyl anion.…”

Section: Early Theorymentioning

confidence: 99%

Perspectives on Computational Organic Chemistry

Streitwieser

2009

J. Org. Chem.

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The author reviews how his early love for theoretical organic chemistry led to experimental research and the extended search for quantitative correlations between experiment and quantum calculations. The experimental work led to ion pair acidities of alkali-organic compounds and most recently to equilibria and reactions of lithium and cesium enolates in THF. This chemistry is now being modeled by ab initio calculations. An important consideration is the treatment of solvation in which coordination of the alkali cation with the ether solvent plays a major role.

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Acidity of Hydrocarbons. X. Exchange Rates of Ring-Substituted Toluene-α-t's with Lithium Cyclohexylamide in Cyclohexylamine

Cited by 61 publications

References 2 publications

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Formation of Hydrogen‐bonded Carbanions as Intermediates in Hydron Transfer between Carbon and Oxygen

Charge Dispersion and Its Effects on the Reactivity of Thiamin-Derived Breslow Intermediates

Perspectives on Computational Organic Chemistry

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