Quinones reactions with phenoxy radicals

The catalytic cycle of laccase includes several one-electron transfers between a suitable substrate and the copper atoms, with the concomitant reduction of an oxygen molecule to water during the sequential oxidation of four substrate molecules [66]. With this mechanism, laccases generate phenoxy radicals that undergo non-enzymatic reactions [65]. Multiple reactions lead finally to polymerization, alkyl-aryl cleavage, quinone formation, C> -oxidation or demethoxylation of the phenolic reductant [67]. 

Time-resolved resonance Raman spectroscopy of 25 in 50% aqueous CH3CN proved that the final product 26 appears with a rate constant of 2.1 x 109 s 1 following pulsed excitation of 25.207 The appearance of 26 was slightly delayed with respect to the decay of (25), A = 3.0 x 109s, that was determined independently by optical pump probe spectroscopy in the same solvent. The intermediate that is responsible for the delayed appearance of 26, t 0.5 ns, is attributed to the triplet biradical 327.462 It shows weak, but characteristic, absorption bands at 445 and 420 nm, similar to those of the phenoxy radical. ISC is presumably rate limiting for the decay of 327, which cyclizes to the spiro-dienone 28. The intermediate 28 is not detectable its decay must be faster than its rate of formation under the reaction conditions. Decarbonylation of 28 to form p-quinone methide (29) competes with hydrolysis to 26 at low water concentrations. Hydrolysis of 29 then yields p-hydroxybenzyl alcohol (30) as the final product. 

In most instances, phenoxy radicals do not engage in propagation steps in chain reactions, consistent with their use as free radical inhibitors. They are, however, reactive enough to oxidize ascorbic acid or to reduce some quinones (Neta and Steenken, 1981). 

Prooxidant activity of phenolic compounds Phenolic antioxidants can initiate an autoxidation process and act like prooxidants under conditions that favor their autoxidation. Instead of terminating a free-radical chain reaction by reacting with a second radical, the phenoxy radical may also interact with oxygen and produce quinones (P = 0) and superoxide anion (02 ). PO -I-O2—>P = 0- -02 . Small phenolic compounds which are easily oxidized, such as quercetin, gallic acid, possess prooxidant activity while high-molecular weight phenolic compounds, such as condensed and hydrolyzable tannins, have little or no prooxidant activity. 

The mechanism by which BHT, a simple phenolic, functions is shown in Figure 14.3. A hindered phenol, when used alone, will react with two radicals. It then becomes spent and it wiU no longer protect the polymer. These reactions are only simplihed overviews. More complex and undesirable side reactions can occur. For example, quinone formation by the reaction of an alkoxy radical at the para position of the phenoxy radical leads to color development because of the conjugated diene structure [18]. 

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