Mechanisms of base-catalyzed alkene-forming 1,2-eliminations

Gandler, Joseph R.

doi:10.1002/9780470772249.ch12

Cited by 9 publications

(26 citation statements)

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“…The calculated k - 1 value is approximately 10 4 -fold larger than k 2 . The result is consistent with the (E1cb) rev mechanism in which the departure of the leaving group is rate-limiting …”

Section: Resultssupporting

confidence: 85%

Kinetics of Polymerization Reaction of α,α‘-Bis(tetrahydrothiophenio)-p-xylene Dichloride

1998

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Polymerization reactions of α,α‘-bis(tetrahydrothiophenio)-p-xylene dichlorides with OH- in H2O have been studied kinetically. The reactions proceeded via the α-tetrahydrothiophenio-p-xylylene intermediates. Results of the H−D exchange experiments and kinetic studies reveal that the 1,6-elimination forming the intermediates proceeds reversibly via the ylide intermediate. The observed rate of disappearance of the intermediate was increased by the addition of OH-, tetrahydrothiophene, and S2O8 2- as well as by the photoirradiation with a tungsten lamp in the presence of Rose Bengal and was inhibited by TEMPO. However, the rate was found to be independent of ylide concentration. From these results, an elimination−free radical polymerization mechanism is proposed. The microscopic rate constants for each step of this mechanism were calculated from the change in the intermediate concentration with time. All of the rate data showed excellent correlations with the substituent constants in the Hammett plot.

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Section: Resultssupporting

confidence: 85%

Kinetics of Polymerization Reaction of α,α‘-Bis(tetrahydrothiophenio)-p-xylene Dichloride

1998

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“…Similarly, the Hammett ρ value also decreased from 2.20 ± 0.11 to 1.90 ± 0.05 with the same variation of the substituent in the benzoyl group . These results can be described by a negative p yy ‘ interaction coefficient, p yy ‘ = −∂β lg /∂σ - = −∂ρ/∂p K lg , that describes the interaction between the leaving group and the β-aryl substituent . The negative p yy ‘ coefficients are not consistent an E1cb mechanism for which p yy ‘ = 0 is expected but provide a strong support for the E2 mechanism .…”

Section: Referencesmentioning

confidence: 90%

“…These results can be described by a negative p yy ‘ interaction coefficient, p yy ‘ = −∂β lg /∂σ - = −∂ρ/∂p K lg , that describes the interaction between the leaving group and the β-aryl substituent . The negative p yy ‘ coefficients are not consistent an E1cb mechanism for which p yy ‘ = 0 is expected but provide a strong support for the E2 mechanism . Therefore, it seems reasonable to ascribe the same E2 mechanism to the eliminations from 1 .…”

Section: Referencesmentioning

confidence: 94%

“…An excellent understanding of electronic effects and the effects of changes in reactant structure and reaction conditions on the structure of the E2 transition state for alkene- and alkyne-forming anti eliminations has evolved. − Similarly, the complementary syn eliminations have also been studied. − The results of these studies reveal that the anti elimination is more facile and proceeds through the transition state with less carbanionic character and smaller extents of C β −H bond cleavage than the corresponding syn elimination reactions. In contrast, little is known about the differences between the syn and anti eliminations forming carbon−nitrogen triple bonds.…”

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confidence: 99%

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Elimination Reactions of (E)- and (Z)-Benzaldehyde O-Pivaloyloximes. Transition-State Differences for the Syn and Anti Eliminations Forming Nitriles

Cho

Lee

1997

J. Org. Chem.

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Elimination reactions of (E)- and (Z)-benzaldehyde O-pivaloyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett rho and k(H)/k(D) values, and an E2 mechanism is evident. The rate of elimination from 2 is approximately 20 000-fold faster than that from 1. For reactions of 1 with DBU in MeCN, a Hammett rho value of 2.4 +/- 0.1, k(H)/k(D) = 2.7 +/- 0.3, DeltaH() = 12.5 +/- 0.2 kcal/mol, and DeltaS() = -31.0 +/- 0.6 eu have been determined. The corresponding values for 2 are rho = 1.4 +/- 0.1, k(H)/k(D) = 7.8 +/- 0.3, DeltaH() = 8.8 +/- 0.1 kcal/mol, and DeltaS() = -23.6 +/- 0.4 eu, respectively. The results indicate that the nitrile-forming anti eliminations from 2 proceed via a more symmetrical transition state with a smaller degree of proton transfer, less negative charge development at the beta-carbon, and greater extent of triple-bond formation than that for the syn elimination.

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“…Since the reactions produced only elimination products and exhibited second-order kinetics, all but bimolecular pathways can be ruled out. In addition, an E1cb mechanism is negated by the substantial values of k H / k D and |β 1g |. , …”

Section: Discussionmentioning

confidence: 99%

Elimination Reactions of (Z)-Thiophene- and (Z)-Furan-2-carbaldehydeO-Benzoyloximes. Effect of β-Aryl Group upon the Nitrile-Forming Anti Transition State

Cho

Song

et al. 1998

J. Org. Chem.

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Elimination reactions of (Z)-thiophene- and (Z)-furan-2-carbaldehyde O-benzoyloximes 1 and 2 with DBU in MeCN have been investigated kinetically. The reactions are second order and exhibit substantial values of Hammett ρ and k H/k D values, and an E2 mechanism is evident. The relative rates of elimination from (Z)-ArCHNOC(O)C6H4Y are 1/1.1/0.6 for Ar = phenyl/thienyl/furyl, respectively. For reactions of 1 with DBU in MeCN, k H/k D = 8.2 ± 0.1, Hammett ρ = 1.22 ± 0.19, β1g = −0.43 ± 0.01, ΔH ⧧ = 5.9 ± 0.1 kcal/mol, and ΔS ⧧ = −28.5 ± 0.3 eu have been determined. The corresponding values for 2 are k H/k D = 8.8 ± 0.2, ρ = 1.87 ± 0.05, β1g = −0.55 ± 0.10, ΔH ⧧ = 6.5 ± 0.1 kcal/mol, and ΔS ⧧ = −29.0 ± 1.5 eu. The k H/k D, Hammett ρ, and |β1g| values increase as the β-aryl group is changed in the order phenyl < thienyl < furyl. The results indicate that the transition state structure for the nitrile-forming elimination changes slightly toward product-like with the change of the β-aryl group.

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Mechanisms of base-catalyzed alkene-forming 1,2-eliminations

Cited by 9 publications

References 0 publications

Kinetics of Polymerization Reaction of α,α‘-Bis(tetrahydrothiophenio)-p-xylene Dichloride

Kinetics of Polymerization Reaction of α,α‘-Bis(tetrahydrothiophenio)-p-xylene Dichloride

Elimination Reactions of (E)- and (Z)-Benzaldehyde O-Pivaloyloximes. Transition-State Differences for the Syn and Anti Eliminations Forming Nitriles

Elimination Reactions of (Z)-Thiophene- and (Z)-Furan-2-carbaldehydeO-Benzoyloximes. Effect of β-Aryl Group upon the Nitrile-Forming Anti Transition State

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