[1309-60-0] · O2Pb · Lead(IV) Oxide · (MW 239.19)
(oxidation of phenols1 and hydroquinones2 to quinones; oxidation of methyl groups in methylated phenols to carboxyl groups;3 oxidative cyclization4 and oxidative decarboxylation;5 formation of hydrazyl radical6)
Alternate Name: lead dioxide.
Physical Data: evolves oxygen when heated, first forming Pb3O4, and then PbO at high temperatures; d 9.38 g cm-3.
Solubility: insol water; sol HCl with evolution of Cl2; sol dil HNO3 in the presence of H2O2; sol oxalic acid; sol alkaline iodide solutions with liberation of iodine; sol hot caustic alkaline solutions.
Form Supplied in: black powder; widely available.
Handling, Storage, and Precautions: harmful by inhalation, in contact with skin, and if swallowed; irritating to eyes and respiratory system; danger of cumulative effects; reproductive hazard; contact with combustible material may cause fire; should be handled in a fume hood.
Historically, PbO2 was used to effect a variety of oxidative transformations. However, because of its general toxicity and because of the inconvenience and expense of disposing of lead byproducts, PbO2 has largely been replaced by other oxidizing agents, such as Cerium(IV) Ammonium Nitrate, Silver(I) Oxide, and Salcomine.
This reagent oxidizes phenols to the corresponding p-benzoquinones and/or diphenoquinones in polar solvents, e.g. acetic acid or formic acid (eq 1).1a The product distribution depends on the mole ratio of phenol/PbO2 and reaction conditions. A mechanism in which the initial step is the formation of the metal phenoxide has been proposed. In nonpolar solvents the oxidation yields mainly polymeric ethers.1b
PbO2 can oxidize catechol and its derivatives to the corresponding quinones. In nucleophilic solvents, addition and a subsequent oxidation lead to substituted o-quinones. For example, when catechol is treated with PbO2 in methanol which contains sodium methoxide, 4,5-dimethoxy-o-benzoquinone is obtained (eq 2).3
The intermediate o-quinone can also be
trapped by annulation with o-phenylenediamine in acetic acid-benzene to form a-methoxyphenazine after a second oxidation.9 PbO2 also oxidizes 2,6-dihydroxynaphthalene to 2,6-naphthoquinone (eq 3),10 and 2,6-naphthylenedibenzenesulfonamide to 2,6-naphthoquinone dibenzosulfonimide.11
Oxidation of a substituted p-hydroxybenzophenone with PbO2 afforded dehydrogriseofulvin (eq 4), and oxidation of a related o-carboxydiphenyl ether gave a spiranic lactone.4
PbO2 oxidizes 1,2-dicarboxylic acids and their anhydrides to the doubly decarboxylated alkenes (eq 5).5
In the presence of PbO2, nepetonic acid (a b-carboxy ketone) is oxidized with decarboxylation to (-)-3-methylcyclopenten-1-yl methyl ketone (eq 6).12 Caution: The reactants should be well mixed and air should be excluded from the hot residue of the decarboxylation reaction.
The methyl groups of methylated phenols can be oxidized to carboxyl groups by PbO2 (eq 7).3 The oxidation of 1,1,4,4-tetraphenyl-2,3-dibenzoyltetrazane with PbO2 affords the hydrazyl radical.6
Kathlyn A. Parker & Dai-Shi Su
Brown University, Providence, RI, USA