Ketone formation, phenoxyl radical

As a typical example of coupling reactions the oxidation of p-cresol (VII) to Pummerer s ketone (VIII) is presented in Figure 5, showing the mesomeric nature of the phenoxyl radical. According to Barton et al. 2) the ketone is formed by coupling at one 0-carbon atom and at one p-carbon atom and subsequent formation of an ether linkage. 

The kinetics of combination of radical pairs formed by quenching of the triplet excited state of B by 2,4,6-trimethylphenol (HR) in films of rigid and plasticized polyvinylchloride (PVC) were studied by the method of laser flash photolysis by Levin et al. At high concentrations of the phenol quencher ( 15 wt%), most of the triplet excited state of the ketone is quenched and the formation and disappearance of the ketyl and phenoxyl radicals can be followed from changes in their characteristic transient absorptions at 540 and 390 nm, respectively. 

Phenols are quite sensitive to oxidation. On the one hand, they are easily oxidized to quinols and on further oxidation with, for example, iron(III) chloride, chromic acid or silver(I) oxide give p-quinones. However, under one-electron transfer conditions the phenoxide anion is oxidized to the phenoxyl radical. This shows free radical reactivity on the oxygen atom and at the ortho and para positions (Scheme 4.20a). The phenoxyl radical may readily dimerize. This is exemplified by the formation of Pummerer s ketone from p-cresol (Scheme 4.20b). 

To mimic the function of GAO, a large number of copper(II)-phenoxide complexes have been synthesized, and these catalyze the aerobic oxidation of alcohols to aldehydes and/or ketones via formation of intermediate Cu(II)-phenoxyl radical complexes, followed by production of H2O2 as a coproduct. Stack and coworkers reported the first functional models (Fig. 11) (116,117). Wieghardt and co-workers reported on a dinuclear Cu(II)-phenoxyl complex, which can catalyze the oxidation of primary alcohols as well as secondary alcohols to ketones (118). Futhermore, Wieghardt and co-workers have synthesized various mononuclear copper complexes with imino-, thio-, and semiquinonate ligands, which affect similar chemistry (118-120). 

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