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-H(2)O 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 degrees C. The rate law -d[flaH]/dt = k(obs) [OH(-)][flaH][O(2)] (k(obs) = kK(1)/[H(2)O]) 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/kJ mol(-1)= 59 +/- 4, DeltaS/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.