[1301-96-8] · AgO · Silver(II) Oxide · (MW 123.87)
(oxidizing reagent for conversion of hydroquinone ethers1 and alkoxy anilines5 to quinones, of alcohols to aldehydes or ketones,11 of allylic alcohols to conjugated acids,12 of aldehydes to acids, of amines to azo compounds, and of amino acids to aldehydes;13 radical coupling reactions;8,9 utilized in oxidative decarboxylation of carboxylic acids10)
Physical Data: dec above 100 °C; d 7.483 g cm-3.
Solubility: insol H2O; sol alkali; sol NH4OH (with decomposition and evolution of N2).
Form Supplied in: charcoal-gray powder; widely available.
Handling, Storage, and Precautions: protect from light. Highly irritating to skin, eyes, mucous membranes, and respiratory tract. Avoid contact with skin, organic matter, strong ammonia, and alkali.
This reagent effects the oxidative demethylation of hydroquinone ethers to afford para-quinones in acidic media (eq 1).1 A mechanism has been proposed in which oxidative hydration takes place at the aromatic carbon.
AgO in dioxane-nitric acid solution also oxidizes hydroquinone mono- and diesters to the corresponding quinones (eq 2).2 The order of reaction is 4-acetoxyphenols > hydroquinone dimethyl ethers > hydroquinone diacetates.2
AgO is effective in oxidizing electron-deficient 1,4,5-trimethoxy-9,10-anthraquinone to the corresponding diquinone, an intermediate in anthracyclinone synthesis (eq 3).3 This bifunctional dienophile undergoes the Diels-Alder reaction at the internal double bond with electron-rich dienes (e.g. 2-ethoxybutadiene), and at the external double bond with slightly electron-poor or unsubstituted dienes (e.g. 1,3-butadiene and 2-acetoxybutadiene).4
In the presence of AgO in aqueous nitric acid, p-methoxyanilines are oxidized with hydrolysis to quinones (eq 4).5
Tautomerization of a tetracyclic keto quinone obtained from AgO oxidation affords an aromatized anthracycline intermediate (eq 5).6 The crude product subsequently undergoes tautomerization at rt in acetone with a few drops of concd. HCl.
The trimethoxyphenyl lactam derivatives of hydroquinone methyl ethers are oxidized by AgO in an acidic milieu to give the lactam-substituted quinones (eq 6).7
The addition of acetone to terminal alkenes is accomplished by the action of AgO (eq 7).8 2-Alkanones are formed in 73-83% yield. Internal alkenes are less reactive. A radical mechanism is proposed for this transformation in which AgO acts as a
heterogeneous initiator for the hydrogen atom abstraction from acetone to generate an acetonyl radical; this is followed by the addition of this radical to the alkene.
The oxidative dimerization of 2-(vinyloxy) phenols by AgO under anhydrous conditions proceeds with rearrangement. A [3,3]-sigmatropic process involving an intermediate cyclohexadienol radical has been proposed (eq 8).9
AgO has been utilized in the oxidative decarboxylation of carboxylic acids such as pivalic acid, isobutyric acid, and n-butyric acid.10 The reaction proceeds significantly faster when acetonitrile is utilized as a cosolvent. AgO oxidizes primary alcohols to aldehydes, secondary alcohols to ketones,11 allylic alcohols to conjugated acids,12 aromatic hydrocarbons to aromatic aldehydes and ketones, a-amino acids and a-amino esters to aldehydes (with decarboxylation), aldehydes to acids,13 and amines, aniline, p-toluidine, and N,N-dimethyl-p-phenylenediamine to the corresponding azo derivatives.14
Kathlyn A. Parker & Dai-Shi Su
Brown University, Providence, RI, USA