Phenoxy radical generation process

The reversible radical generation process mediated by HRP represented in Figure 2 involves radical-producing forward enzymatic reactions and backward reverse electron transfer processes in which phenoxy radicals are re-converted to dieir original substrate forms. The forward reactions produce phenoxy radicals (AH ) via single-electron transfers from phenol substrates (AH2) to the enzyme (5, d). The phenoxy radicals dius produced can form radical-pairs with the enzyme interme tes and abstract electrons from die enzyme via reverse electron transfer, essentially reversing the radical-producing reaction, as demonstrated by Taraban et al. (28). 

The oxidation generates highly delocalized phenoxy radicals (PhO, Scheme 2.21), which may initiate (i) a radical polymerization process, trapping the reactant (CF) to give a benzyl radical intermediate (QMR), or it may (ii) follow a radical coupling to produce the p-QM p-O-QM, which being a reactive electrophile could undergo cationic polymerization. 

Figure 6.12 A mechanism for the generation of lignin phenoxy radicals due to direct oxidation by laccase and a mediation process involving colloidal lignin fragments, according to Felby etal. (1997b).

Until recently, the mechanism of the inhibition of light-induced yellowing was subject to speculation. However, solid state ESR and CIDEP have provided insight into the mode of inhibition by thiols (Wan, J.K.S., et al, J. Wood Chem. Technol., in press). Near-uv irradiation of unbleached and peroxide bleached thermomechanical pulp impregnated with thiols caused a rapid increase of the thiol radicals. The time resolved CIDEP spectrum, however, shows a symmetric broad band characteristic of the polarized phenoxy radical. This result suggests that thiols quench triplet generated phenoxy radicals in a secondary thermal process. 

Bond cleavage from excited radicals has also been observed in a laser jet study of the fc/s-ether (219). Thus, the excited 9-anthrylmethyl radical is suggested to undergo loss of phenoxy radical to yield the 9,10-bis-anthrylmethyl biradical (Scheme 13). This is a three-photon process, with two photons required to produce the monoradical and an additional photon needed to generate the biradical. The authors note that the biradical itself may have photochemistry, forming the 9,10-bis-methylether (220) upon further photolysis in methanol [132]. 

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