[GRAPHICS] A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)(n)CO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar center dot+(CH2)(n)CO2H) or radical zwitterions (Ar center dot+(CH2)(n)CO2-) depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with -OH (k(-OH)) have been obtained. These values are similar to those obtained previously for the -OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.

Bietti, M., Capone, A. (2006). Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway. JOURNAL OF ORGANIC CHEMISTRY, 71(14), 5260-5267 [10.1021/jo060678i].

Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway

BIETTI, MASSIMO;
2006-01-01

Abstract

[GRAPHICS] A product and time-resolved kinetic study of the one-electron oxidation of ring-methoxylated phenylpropanoic and phenylbutanoic acids (Ar(CH2)(n)CO2H, n = 2, 3) has been carried out at different pH values. Oxidation leads to the formation of aromatic radical cations (Ar center dot+(CH2)(n)CO2H) or radical zwitterions (Ar center dot+(CH2)(n)CO2-) depending on pH, and pK(a) values for the corresponding acid-base equilibria have been measured. In the radical cation, the acidity of the carboxylic proton decreases by increasing the number of methoxy ring substituents and by increasing the distance between the carboxylic group and the aromatic ring. At pH 1.7 or 6.7, the radical cations or radical zwitterions undergo benzylic C-H deprotonation as the exclusive side-chain fragmentation pathway, as clearly shown by product analysis results. At pH 1.7, the first-order deprotonation rate constants measured for the ring-methoxylated arylalkanoic acid radical cations are similar to those measured previously in acidic aqueous solution for the alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations. In basic solution, the second-order rate constants for reaction of the radical zwitterions with -OH (k(-OH)) have been obtained. These values are similar to those obtained previously for the -OH-induced alpha-C-H deprotonation of structurally related ring-methoxylated alkylaromatic radical cations, indicating that under these conditions the radical zwitterions undergo benzylic C-H deprotonation. Very interestingly, with 3,4-dimethoxyphenylethanoic acid radical zwitterion, that was previously observed to undergo exclusive decarboxylation up to pH 10, competition between decarboxylation and benzylic C-H deprotonation is observed above pH 11.
2006
Pubblicato
Rilevanza internazionale
Articolo
Sì, ma tipo non specificato
Settore CHIM/06 - CHIMICA ORGANICA
English
Con Impact Factor ISI
Oxidation; pH effects; Positive ions; Protons; Rate constants; Solutions; Acid-base equilibria; Phenylbutanoic acids; Ring-methoxylated arylalkanoic acid radical cations; Zwitterions; Carboxylic acids; alkanoic acid; ampholyte; butyric acid derivative; carbon; cation; hydrogen; propionic acid derivative; radical; aqueous solution; article; chemical reaction kinetics; decarboxylation; electron; oxidation; pH; proton nuclear magnetic resonance; radiolysis; reaction analysis
Bietti, M., Capone, A. (2006). Reactivity and acid-base behavior of ring-methoxylated arylalkanoic acid radical cations and radical zwitterions in aqueous solution. Influence of structural effects and pH on the benzylic C-H deprotonation pathway. JOURNAL OF ORGANIC CHEMISTRY, 71(14), 5260-5267 [10.1021/jo060678i].
Bietti, M; Capone, A
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Utilizza questo identificativo per citare o creare un link a questo documento: https://hdl.handle.net/2108/39210
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