One-Electron Oxidation of 2-(4-Methoxyphenyl)-2-Methylpropanoic and 1-(4-Methoxyphenyl)cyclopropanecarboxylic Acids in Aqueous Solution. The Involvement of Radical Cations and the Influence of Structural Effects and pH on the Side-Chain Fragmentation Reactivity

Abstract

A product and time-resolved kinetic study on the one-electron oxidation of 2-(4-methoxyphenyl)-2-methylpropanoic acid (2), 1-(4-methoxyphenyl)cyclopropanecarboxylic acid (3), and of the corresponding methyl esters (substrates 4 and 5, respectively) has been carried out in aqueous solution. With 2, no direct evidence for the formation of an intermediate radical cation 2*+ but only of the decarboxylated 4-methoxycumyl radical has been obtained, indicating either that 2*+ is not formed or that its decarboxylation is too fast to allow detection under the experimental conditions employed (k > 1 x 10(7) s(-1)). With 3, oxidation leads to the formation of the corresponding radical cation 3*+ or radical zwitterion -3*+ depending on pH. At pH 1.0 and 6.7, 3*+ and -3*+ have been observed to undergo decarboxylation as the exclusive side-chain fragmentation pathway with rate constants k = 4.6 x 10(3) and 2.3 x 10(4) s(-1), respectively. With methyl esters 4 and 5, direct evidence for the formation of the corresponding radical cations 4*+ and 5*+ has been obtained. Both radical cations have been observed to display a very low reactivity and an upper limit for their decay rate constants has been determined as k < 10(3) s(-1). Comparison between the one-electron oxidation reactions of 2 and 3 shows that the replacement of the C(CH3)2 moiety with a cyclopropyl group determines a decrease in decarboxylation rate constant of more than 3 orders of magnitude. This large difference in reactivity has been qualitatively explained in terms of three main contributions: substrate oxidation potential, stability of the carbon-centered radical formed after decarboxylation, and stereoelectronic effects. In basic solution, -3*+ and 5*+ have been observed to react with -OH in a process that is assigned to the -OH-induced ring-opening of the cyclopropane ring, and the corresponding second-order rate constants (k-OH) have been obtained. With -3*+, competition between decarboxylation and -OH-induced cyclopropane ring-opening is observed at pH >or=10, with the latter process that becomes the major fragmentation pathway around pH 12.

Mechanism of Selective C–H Hydroxylation Mediated by Manganese Aminopyridine Enzyme Models

“…This linear relationship suggests that the reaction occurs via a common mechanism. 59−62 The sensitivity of the relative rates to BDE(C−H) 63,64 represented by the slope of ca. −0.32 in Figure 2b is similar to those previously reported by Tanaka (−0.3), 65 Que (−0.35), 62 Goldberg (−0.39), 56 and Mayer (−0.4) 60 for H abstractions by high-valence oxometal species.…”

“…Furthermore, the logarithms of relative rate constants ( SI ) for the oxidations of a series of hydrocarbons correlated linearly with the homolytic C–H bond dissociation energies (BDEs) of the substrates (Figure b). This linear relationship suggests that the reaction occurs via a common mechanism. − The sensitivity of the relative rates to BDE­(C–H) , represented by the slope of ca. −0.32 in Figure b is similar to those previously reported by Tanaka (−0.3), Que (−0.35), Goldberg (−0.39), and Mayer (−0.4) for H abstractions by high-valence oxometal species.…”

1,3-Dichloroacetone

“…23,24 In this procedure, the initially formed 2-substituted 1,3-dichloro-isopropoxide is reduced to the desired compounds by using low-valent iron. 25 Analogously, cyclopropenone derivatives are readily available from 1,3-dichloroacetone.…”

“…(E) 1,3-Dichloroacetone is useful as starting material for a one-pot synthesis of several 1-substituted cyclopropanols. 23,24 In this procedure, the initially formed 2-substituted 1,3-dichloro-isopropoxide is reduced to the desired compounds by using low-valent iron. 25…”

Reversible Bond/Cation-Coupled Electron Transfer on Phenylenediamine-Based Rhodamine B and Its Application on Electrochromism

“…Besides PCET and MCET, electron transfer coupled with covalent bond cleavage or formation, such as C–C − /C–O/C–S − /S–S/others, − also plays a critical role in photosensitization, , polymer synthesis, and other significant applications. , Even though these important bond-state changes during electron transfer have attracted much attention, and several convincing theories have been proposed and reviewed . Less attention has been paid to reversible bond (cleavage and formation) coupling with electron transfer, which is an urgent need for further understanding why enzymes are fully reversible during their dynamic processes involving structure and function alteration coupling with electron transfer.…”