Kinetics of ionisation of 3-phenylcoumaran-2-one and reprotonation of the resulting carbanion

Heathcote, David M.; Atherton, John H.; Boos, G. A.; Page, Michael I.

doi:10.1039/a707945a

Cited by 5 publications

(5 citation statements)

References 22 publications

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“…These results indicate a strong enolate ion-stabilizing effect of the phenyl group without significantly changing the p K E (Table ). It was also reported that the log k o value for the deprotonation of 8H - O by a series of aminosulfonic acids in 50% dioxane−50% water (v/v) at 25 °C is 2.60 . This is somewhat lower than the intrinsic rate constant for the deprotonation of 6H - O by aliphatic amines (log k o = 2.77) or secondary alicyclic amines (log k o = 3.23) in water.…”

Section: Discussionmentioning

confidence: 98%

“…A comparison between 6H - O and its phenyl-substituted analogue (3-phenylcoumaran-2-one, 8H - O ) is interesting. Heathcote et al report a = 8.39 and < 6.39 in water; in 50% dioxane−50% water (v/v) = 8.86, = 6.0 and p K E = 2.9. These results indicate a strong enolate ion-stabilizing effect of the phenyl group without significantly changing the p K E (Table ).…”

Section: Discussionmentioning

confidence: 99%

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Kinetics of Proton Transfer from Benzo[b]-2,3-dihydrofuran-2-one and Benzo[b]-2,3-dihydrothiophene-2-one. Effect of Anion Aromaticity on Intrinsic Barriers

Bernasconi

Zheng

2006

J. Org. Chem.

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Rates of the reversible deprotonation of benzo[b]-2,3-dihydrofuran-2-one (6H-O) and benzo[b]-2,3-dihydrothiophene-2-one (6H-S) by OH-, primary aliphatic amines, secondary alicyclic amines, and carboxylate ions have been determined in water at 25 degrees C. As noted earlier by Kresge and Meng, 6H-S (pKa = 8.82) is considerably more acidic than 6H-O (pKa = 11.68), which mainly reflects the greater aromatic stabilization of the conjugate base of 6H-S (thiophene derivative) compared to that of 6H-O (furan derivative). The main focus of this paper is to assess how the difference in the aromaticity of the two enolate ions affects the intrinsic barrier to the proton transfer. These intrinsic barriers were determined from Brønsted plots for the reactions with the amines and carboxylate ions or calculated on the basis of the Marcus equation for the reactions with OH-. They are consistently somewhat higher for the reactions of 6H-S than for the reactions of 6H-O, implying that the aromaticity in the anion enhances the intrinsic barrier. This contrasts with a previous report on the deprotonation of some cyclic rhenium Fischer-type carbene complexes where the reaction that leads to the most aromatic conjugate base (thiophene derivative) has a lower intrinsic barrier than the reaction that leads to the less aromatic furan analogue. We are offering a detailed analysis of other potential factors that may affect the intrinsic barriers and which could explain these contradictory results.

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Section: Discussionmentioning

confidence: 98%

Section: Discussionmentioning

confidence: 99%

Kinetics of Proton Transfer from Benzo[b]-2,3-dihydrofuran-2-one and Benzo[b]-2,3-dihydrothiophene-2-one. Effect of Anion Aromaticity on Intrinsic Barriers

Bernasconi

Zheng

2006

J. Org. Chem.

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“…Recently Page et al . have extended a rate−equilibrium correlation for proton transfer to a hydroxide ion from carbon acids activated by a mono carbonyl group. The linearity of this Bronsted relationship now spans 19 p K a units and covers proton transfers in both thermodynamically favorable and unfavorable directions.…”

Section: Discussionmentioning

confidence: 99%

“…This extended linearity is not consistent with a constant Marcus intrinsic barrier but could be interpreted in terms of varying intrinsic barriers. Recently Page et al 50 have extended a rate-equilibrium correlation for proton transfer to a hydroxide ion from carbon acids activated by a mono carbonyl group. The linearity of this Bronsted relationship now spans 19 pK a units and covers proton transfers in both thermodynamically favorable and unfavorable directions.…”

Section: Evaluation Of Equilibriummentioning

confidence: 99%

Nitroso Group Transfer from SubstitutedN-Methyl-N-nitrosobenzenesulfonamides to Amines. Intrinsic and Apparent Reactivity

et al. 2001

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We have studied the nitroso group transfer from substituted N-methyl-N-nitrosobenzenesulfonamides to primary and secondary amines, observing that the rate of the reaction increases as a consequence of the presence of electron withdrawing groups on the aromatic ring of the nitrosating agents. The rate constants determined for the nitroso group transfer, ktr, give good Bronsted-type relationships between log ktr (rate constant for nitroso group transfer) and pKaR2NH2+ and pKaleaving group. The study of the nitrosation processes of secondary amines catalyzed by ONSCN and denitrosation catalyzed by SCN-, in combination with the formation equilibrium of ONSCN, has enabled us to calculate the value of the equilibrium constant for the loss of the NO+ group from a protonated N-nitrosamine (pKNOR2N+HNO), which can be defined by analogy with pKaR2NH2+. The value of pKNOX-NO for the loss of the NO+ group from an N-methyl-N-nitrosobenzenesulfonamide was obtained in a similar way. By using values of delta pKNO = pKNOR2N+HNO - pKNOX-NO, we were able to calculate the equilibrium constant for the nitroso group transfer and characterize the transition state. On the basis of Bronsted-type correlations, we have obtained values of beta nuclnorm and alpha lgnorm approximately equal to 0.55, showing a perfectly balanced transition state. In terms of the Marcus theory, the calculation of the intrinsic barriers for the nitroso group transfer reaction shows that the presence of electron withdrawing groups on the aromatic ring of the N-methyl-N-nitrosobenzenesulfonamides does not cause these barriers to vary.

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“…The pK a of ethyl acetate in water is 25.6 5 and a-phenyl and a-Nacylamino substituents both lower the pK a of carbon acids by about 4 units each. 6,7 An estimated pK a of RCON(Me)CH(Ph-)CO 2 Et is thus about 17, and as RSO 2 NR-is slightly more electron-withdrawing than RCONR-the pK a of ( 1) is expected to be about 15. The pK a of the quaternary ammonium salt of the methyl ester of glycine, Me 3 N + CH 2 CO 2 Me, has recently been reported to be 18.0.…”

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confidence: 99%

Hydrolysis of a sulfonamide by a novel elimination mechanism generated by carbanion formation in the leaving group

et al. 2002

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Kinetics of ionisation of 3-phenylcoumaran-2-one and reprotonation of the resulting carbanion

Cited by 5 publications

References 22 publications

Kinetics of Proton Transfer from Benzo[b]-2,3-dihydrofuran-2-one and Benzo[b]-2,3-dihydrothiophene-2-one. Effect of Anion Aromaticity on Intrinsic Barriers

Kinetics of Proton Transfer from Benzo[b]-2,3-dihydrofuran-2-one and Benzo[b]-2,3-dihydrothiophene-2-one. Effect of Anion Aromaticity on Intrinsic Barriers

Nitroso Group Transfer from SubstitutedN-Methyl-N-nitrosobenzenesulfonamides to Amines. Intrinsic and Apparent Reactivity

Hydrolysis of a sulfonamide by a novel elimination mechanism generated by carbanion formation in the leaving group

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