| Abstract
| - Carbon dioxide catalyzed oxidative coupling of phenol by peroxynitrite occurs by two pathwaysdistinguished by the isomer ratio of 2,2‘- to 4,4‘-biphenols. As already established, at neutral pH andmoderate phenol concentrations, both biphenols are formed in comparable yields by the coupling of twophenoxyl radicals. However, at high pH and phenol concentration, 2,2‘-biphenol is the only identifiedcoupled product, and its formation does not involve phenoxyl radicals. Instead, under these conditions,a previously unreported long-lived (t1/2 ∼ 10 s at pH 10 and 1 mM phenol) diamagnetic intermediatewith an absorption maximum at 400 nm is observed. This intermediate is formed from phenolateconcomitantly with the decay of peroxynitrite and disappears via reaction with phenol [k = (2.4 ± 0.1)× 10 M-1 s-1 at pH 10.5] to form 2,2‘-biphenol. We also find that para-benzoquinone, previouslyunreported, is formed in up to 5% yield relative to the initial peroxynitrite concentration. The appearanceof an absorption band above 500 nm, which might be due to quinhydrone, indicates that hydroquinoneis a likely para-benzoquinone precursor. The dependence of para-benzoquinone yields on pH and phenolconcentration suggests that its formation is related to the nonradical pathway of 2,2‘-biphenol formation.This novel nonradical pathway of 2,2‘-biphenol formation might be relevant to the mechanisms of reactionof phenolic antioxidants with peroxynitrite. The existence of two distinct pathways of biphenol formationimplies that, apart from a CO3•-/NO2• radical pair, another reactive intermediate is formed during thecarbon dioxide catalyzed decay of peroxynitrite.
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