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

摘要

A product and time-resolved kinetic study on the one-electron oxidation of 2-(4-methoxyphenyl)-2methylpropanoic 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(center dot+) but only of the decarboxylated 4-methoxycumyl radical has been obtained, indicating either that 2(center dot+) 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(center dot+) or radical zwitterion -3(center dot+) depending on pH. At pH 1.0 and 6.7, 3(center dot+) and -3(center dot+) 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(center dot+) and 5(center dot+) 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(center dot+) and 5(center dot+) 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(center dot+), competition between decarboxylation and -OH-induced cyclopropane ring-opening is observed at pH >= 10, with the latter process that becomes the major fragmentation pathway around pH 12.