Kinetics and Mechanism of the Oxygenation of Potassium Flavonolate. Evidence for an Electron Transfer Mechanism

Abstract

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), DeltaH()/kJ mol(-1) = 29 +/- 2, DeltaS/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.