[20667-12-3] · Ag2O · Silver(I) Oxide · (MW 231.74)
(oxidizing reagent for conversion of hydroquinones to quinones,1 alkylphenols to quinone methides,5 and aldehydes to acids;7 oxidative coupling reactions;12,13 a Lewis acid with halides18,20 and thioethers;24 Wolff rearrangement26)
Physical Data: dec at about 200 °C; d 7.22 g cm-3.
Solubility: practically insol alcohol; sol in 40 000 parts H2O; sol dilute nitric acid, ammonia; moderately sol NaOH.
Form Supplied in: brownish-black, heavy, odorless powder; widely available.
Handling, Storage, and Precautions: protect from light. Reduced by hydrogen, carbon monoxide, and most metals. Skin contact with this toxic reagent should be carefully avoided.
This reagent is a powerful oxidizing reagent for the conversion of hydroquinones to the corresponding quinones. 1,4-Hydroquinones are generally converted to quinones by treatment with Ag2O (eq 1).1 Other reagents (e.g. Cerium(IV) Ammonium Nitrate) are less expensive and easier to handle. However, Ag2O in nonhydroxylic solvents can be used to produce electron-deficient quinones which are unstable to nucleophiles. For example, Ag2O oxidation of nitro hydroquinones affords nitro quinones (eq 2).2
A one-pot technique has been developed to oxidize 1,4-hydroquinones and utilize the quinone products as dienophiles in Diels-Alder reactions (eq 3).3 Tetrahydrophenanthrene-9,10-quinones have also been prepared by a one-pot double Diels-Alder reaction (eq 4).4
1,4-Quinone methides can be obtained by the reaction of p-alkylphenols with Ag2O. These reactive compounds may undergo subsequent transformations, for example, a Lewis acid promoted cyclization (eq 5).5 1,2-Quinone methides can also be formed by Ag2O oxidation of appropriately substituted phenols (eqs 6 and 7).6
Ag2O oxidizes aldehydes to acids,7 and conjugated keto aldehydes to conjugated keto acids.8 For example, Ag2O was used to selectively oxidize one of two formyl substituents of an intermediate in the total synthesis of inhibitor K-76 (eq 8).9 The unreactive formyl substituent is conjugated with the hydroxyl and alkoxy substituents.
Radicals produced by Ag2O oxidation can undergo coupling reactions. For example, 2-(vinyloxy)phenols undergo Ag2O oxidation-induced coupling.10 Acylacetates and monosubstituted malonates are oxidatively dimerized in the presence of Ag2O and DMSO (eq 9).11
Radical coupling followed by nucleophilic attack of hydroxyl on a quinone methide intermediate is postulated as the mechanism of the key step in the syntheses of silybin and eusiderin (eq 10).12 1,4-Diketones are produced in the reaction of silyl enol ethers with Ag2O in DMSO (eq 11).13
Ag2O can activate halide as a leaving group by coordination. Methylation of carbohydrates14 and 5-benzylidenebarbituric acid15 have been carried out with Ag2O and Iodomethane in DMF. Triphenyltin trifluoroacetate was obtained from the reaction of triphenyltin iodide and trifluoroacetic acid in the presence of silver oxide.16 Ag2O converts trans-halohydrins to epoxides (eq 12)17 or rearranged products (eq 13)18. It is reported that the reaction is nonstereoselective, and that trans-epoxides are the major products when cis-alkenes are the reactants.19
2-Arylpropionaldehydes are produced from 1-aryl-1-propene by oxidative rearrangement with Iodine and Ag2O in dioxane-H2O (eq 14).20 The mechanism may involve the 1,2-shift of the aryl group through a bridged phenonium ion in the iodohydrin intermediate.
In the total synthesis of mycophenolic acid, electrophilic substitution was promoted by Ag2O (eq 15).21 a,b-Unsaturated nitriles are formed from the corresponding g-bromo-b-oxo nitriles with Ag2O (eq 16).22 Arylsulfenylation and arylselenenylation at the 5-position of uracils are promoted by Ag2O (eq 17).23
Ag2O has been applied to the 1,2-cleavage of penicillins in a strong nonnucleophilic base, such as 1,5-Diazabicyclo[4.3.0]non-5-ene,24 and to the hydrolysis of thioacetals.25
The Wolff rearrangement of diazo ketones is promoted by Ag2O (eq 18).26
Kathlyn A. Parker & Dai-Shi Su
Brown University, Providence, RI, USA