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), 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.
Barhács
L
L
Department of Organic Chemistry, University of Veszprém, 8201 Veszprém, Hungary.
Kaizer
J
J
Speier
G
G
D004578:Electron Spin Resonance Spectroscopy; D004583:Electrons; D005419:Flavonoids / Q000138:chemical synthesis*; D007700:Kinetics; D010100:Oxygen / Q000737:chemistry; D011188:Potassium / Q000737:chemistry; D013055:Spectrophotometry, Infrared; D013056:Spectrophotometry, Ultraviolet
D005419:Flavonoids; D011188:Potassium; D010100:Oxygen