Kinetics and mechanism of the base-catalyzed oxygenation of flavonol in DMSO-H2O solution

The kinetics of the base-catalyzed oxygenation of flavonol have been investigated in 50% DMSO-H2O solution in the pH range 6.4-10.8 and an ionic strength of 0.1 mol L-1 using spectrophotometric techniques at temperatures between 70 and 90 degreesC. The rate law -d[flaH]/dt = k(obs) [OH-][flaH][O-2] (k(obs) = kK(1)/[H2O]) describes the kinetic data. The rate constant, activation enthalpy, and entropy at 353.16 K are as follows: k/mol(-1) L s(-1) = (4.53 +/- 0.07) x 10(-2), DeltaH(double dagger)/kJ mol(-1) = 59 +/- 4, DeltaS(double dagger)/J mol(-1) K-1 = -110 +/- 11. The reaction showed specific base catalysis. It fits a Hammett linear free energy relationship for 4'-substituted flavonols and electron-releasing substituents enhanced the reaction rate, The linear correlation between the oxidation potential of the flavonols and the rate constants supports that a higher electron density on the flavonolate ion makes them more nucleophilic and the electrophilic attack of O-2 easier.