Kinetics and mechanism of the oxygenation of potassium flavonolate.: Evidence for an electron transfer mechanism

The oxygenation of the potassium salt of flavonol (flaH) in absolute DMF leads to potassium O-benzoylsalicylate and carbon monoxide in 95% yield at 40 degrees C. Kinetic measurements resulted in the rate law -d[flaK]/dt = k(2)[flaK][O-2]. The rate constant, activation enthalpy, and entropy at 313.16 K are as follows: k(2)/M-1 s(-1) = (3.28 +/- 0.10) x 10(-1), Delta H double dagger/kJ mol(-1) = 29 +/- 2, Delta S double dagger/J mol(-1) K-1 = -161 +/- 6. The reaction fits a Hammett linear free energy relationship for 4'-substituted flavonols, and electron-releasing groups make the oxygenation reaction faster. The anodic oxidation wave potentials E-a of the 4'-substituted flavonolates correlate well with reaction rates. At more negative E-a values faster reaction rates were observed. EPR spectrum of the reaction mixture (g = 2.0038, dH = 1.8 G, a(H) = 0.9 G) showed the presence of flavonoxyl radical as a result of a SET from the flavonolate to dioxygen.