Alkoxy-substituted benzenes, oxidation

TABLE 12.4 Voltammetric Oxidation Potentials and Peak Currents (pA/mM) of Alkoxy-Substituted Benzenes, Phenols, and Benzyl Alcohols, and Their Derivatives in MeCN (0.1 M TEAP) at a Glassy-Carbon Electrode... 

Table 12.4 summarizes the voltammetric oxidation potentials and peak currents for l,4-(MeO)2Ph and other alkoxy-substituted benzenes, phenols, and benzyl alcohols. Only the 1,4-(MeO)2PhX members of the series exhibit an initial irreversible anodic cyclic voltammogram via the sequence of Eq. (12.37). These plus the l,2-(MeO)2Ph isomer yield a metastable product from the second oxidation [species A, Eq. (12.37)] that undergoes a reversible reduction. Thus, the two-electron oxidation of dimethoxy benzenes yields the corresponding quinone. 

As Schaffer has found 2.4.6-triphenyl-X -phosphorin 22 and other 2.4.6-tri-substituted X -phosphorins react smoothly with aryl diazonium salts in benzene. Nitrogen develops and the aryl residue bonds with the phosphorus. In presence of alcohols as nucleophiles, l-alkoxy-l-aryl-2.4.6-triphenyl-X -phosphorins 100 can be isolated. The aryl diazonium-tetrafluoroborate without any nucleophile in DMOE yields l-aiyl-l-fluoro-2.4.6-triphenyl-X -phosphorin 70i. As with other oxidants like halogen or mercury-Il-acetate, we suppose that in the first step triphenyl-X -phosphorin radical cation is formed. This could be shown by ESR spectroscopy. The next step may be a radical-radical addition to the X -phosphorin cation or a nucleophileradical addition respectively ... 

Reaction with Allylic and Benzylic Electrophiles. The acyl radicals can be trapped with halogen- and silicon-based electrophiles. a -Allylation of a, -unsaturated ketones is done while using Mn(OAc)3 dihydrate and allyl bromide in refluxing benzene (eq 29). Better yields are usually observed for cyclopen-tenones compared to cyclohexenones. a -Benzylation is also possible using benzyl bromide as the electrophilic partner (eq 30). Both methods tolerate a range of substitution, including 8-alkoxy-a, -unsaturated ketones. It is possible to perform a sequential allylation/cyclization with an excess of allyltrimethylsilane (eq 31). Mn(OAc)3 offers good conversions, but the use of ceric ammonium nitrate (CAN) as co-oxidant improves yields. 

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