Polyoxometalate (POM) oxidation of phenols: Effect of aromatic substituent groups on reaction mechanism

The kinetics of the oxidation of a series of substituted phenols by polyoxometalate (POM), K-5[SiVW11O40]center dot 12H(2)O were investigated. Under all conditions studied second-order kinetics were observed. Electron-transfer from neutral phenols (k(1)) was slower than that from the corresponding phenoxide anions (k(2)), and the reaction rate was highly dependent on the nature of the substituent groups. The observed rate constants decreased in the order: p-methoxyphenol (1) > p-methylphenol (2) > m-methylphenol (4) > m-methoxyphenol (3) > phenol (5) > p-chlorophenol (6) > p-bromophenol (7) > m-chlorophenol (8) > m-nitrophenol (9). A Hammett plot of the data revealed a better correlation with sigma(+) than with sigma, suggesting a reaction involving the formation of an electron-deficient radical intermediate with the rate-determining step being an electron-transfer from a neutral substrate. Depending on the concentration of POM, oxidized phenolic and oxidative coupling products were detected.