65767-24-0Relevant articles and documents
Dioxygen Transfer from 4a-Hydroperoxyflavin Anion. 2. Oxygen Transfer to the 10 Position of 9-Hydroxyphenanthrene Anions and to 3,5-Di-tert-butylcatechol Anion
Muto, Shigeaki,Bruice, Thomas C.
, p. 4472 - 4480 (1980)
The reaction of the peroxy anion of N5-ethyl-4a-hydroperoxy-3-methyllumiflavin (4a-FlEtO2-) with the anions of 3,5-di-tert-butylcatechol (VIII), 10-ethoxy-9-phenanthrol (Ia), and 10-methyl-9-phenanthrol (Ib) has been investigated.All products may be accountable through the transfer of O2 from the 4a-FlEtO2- reactant to the phenolate anions with the production of reduced flavin anion (FlEt-) and a hydroperoxycyclohexadienone.From VIII- (t-BuOH) there was obtained 3,5-di-tert-butyl-o-quinone (IX) and Ib- yielded (t-BuOH or CH2CN) 10-hydroxy-10-methyl-9,10-dihydro-9-phenanthrone (IIIb), while Ia- provided both 9,10-phenanthrenequinone (V) and monoethyl 1,1'-diphenate (IVa) (the ratio of V:IVa being solvent dependent).The mechanisms for the decomposition of intermediate peroxide anions to products are discussed.The conversion of Ia- to IVa by oxygen transfer from 4a-FlEtO2- amounts to a catalysis by FlEt- of the reaction of 3O2 with Ia and serves as a biomimetic reaction of flavoenzyme dioxygenase.The kinetic for the reaction of VIII- with 4a-FlEtO2- require the formation of an intermediate.Since the rate constants for the reaction of both VIII- and 2,6-di-tert-butyl-4-methylphenolate anion with 4a-FlEtO2- are identical under saturating conditions by these phenolate ions, it is concluded that the intermediate is formed in a unimolecular reaction from 4a-FlEtO2- (k = 0.36 s-) as in eq 19.Dissociation of 4a-FlEtO2- to FlEt-+ O2 and reaction of phenolate ions with O2 may be discounted since the second-order rate constants for the reaction of phenolate ions with O2 are less than required for the kinetic competency of this process.Dissociation of 4a-FlEtO2- to yield neutral flavin radical (FlEt.) + O2-. followed by reduction of FlEt. by fenolate ion to provide FlEt- and phenoxy radical with the coupling of the latter with O2-. is also improbable.Thus, though the second-order rate constants for 1e- reduction of FlEt. by the various phenolate species are sufficiently large to allow the kinetic competency of this step, there exists no evidence that O2-. can couple with any radical species to provide a hydroperoxide.The oxygen-donating intermediate formed from 4a-FlEtO2- is suggested to be the 4a,10-dioxetane (XII) or an oxygen molecule more loosely associated with FlEt-.The equilibrium constant for the formation of such an intermediate may be as small as 10-5 if the rate of reaction of phenolate ion with this species approaches a diffusion-controlled process.
Redox reactions of 3,5-di-tert-butyl-1,2-benzoquinone. Implications for reversal of paper yellowing
Jovanovic, S. V.,Konya, K.,Scaiano, J. C.
, p. 1803 - 1810 (2007/10/03)
One- and two-electron transfer reactions of 3,5-di-tert-butylcatechol and the corresponding quinone were studied by fast kinetic spectroscopy coupled with laser photolysis and pulse radiolysis, and by cyclic voltammetry in aqueous solutions.Photoionization of catechol yields the same semiquinone radical as the formate-radical-induced quinone reduction in the pulse radiolysis experiment.The neutral semiquinone radical deprotonates at pKr = 6.0 +/- 0.1, as deduced from the pH induced changes in the radical spectra.The two-electron reduction potentials of quinone and catechol were measured by cyclic voltammetry in aqueous solutions containing 25percent methanol in the pH range 3-14.The pH dependence has two linear parts with slopes of 0.056 and 0.03 V/pH, intersecting at pKa = 10.0.This is in excellent agreement with pKa = 10.05 +/- 0.05 obtained spectrophotometrically.The reduction potential of 3,5-di-tert-butyl quinone, E7 = 0.01 +/- 0.04 V, was obtained from the one-electron transfer equilibrium of the quinone with the oxygen/superoxide redox couple by pulse radiolysis.The rate constant of the reaction of quinone with the superoxide radical, k = 1.2 x 109 M-1 s-1, is higher than those of the superoxide reduction by catechols and phenols (typically ca. 105 M-1 s-1); thus, lignin o-quinones could be efficient scavengers of superoxide under the conditions relevant for the photo-induced yellowing of lignin-rich paper.The reaction with O2.- effectively bleaches yellow quinone and generates metastable furanone, which hydrolyses to muconic acid, thus completely eliminating yellow quinone. 3,5-Di-tert-butylquinone also undergoes rapid bleaching with ascorbate, k = 600 +/- 100 M-1 s-1, in methanol.The reaction has a 1:1 stoichiometry and leads to complete reduction of quinone to catechol with concomitant oxidation of ascorbate to dehydroascorbate.This unusual selectivity and the fact that the reaction of the milder oxidant 3,5-di-tert-butylquinone is an order of magnitude faster than that of stronger oxidant p-benzoquinone (k = 60 +/- 10 M-1 s-1) suggest that a nucleophilic attack of quinone at the ascorbate double bond initiates the reaction. - Keywords: superoxide; lignin; photochemistry; quinone
