Silver(I) Carbonate on Celite

Ag2CO3/Celite
(Ag2CO3)

[534-16-7]  · CAg2O3  · Silver(I) Carbonate  · (MW 275.75)

(mild oxidizing agent, which operates under neutral and heterogeneous conditions1-3)

Physical Data: Ag2CO3: mp 210 °C (dec); d 6.077 g cm-3.

Solubility: insol water, organic solvents; destroyed by acids.

Form Supplied in: green-yellow powder. Commercially available reagent contains ca. 50 wt% Ag2CO3.

Preparative Method: Silver(I) Nitrate (30 g) is dissolved in 200 mL of distilled water, and Celite (30 g) is added. To the stirred mixture, a solution of Sodium Carbonate (Na2CO3.10H2O) (30 g) in distilled water (300 mL) is slowly added. The yellow-green precipitate is filtered and washed to neutrality with distilled water. It is then dried (4 h) by rotatory evaporation on a steam bath, preferably in the dark. The reagent can be stored in the dark at room temperature for several years without significant loss of activity. When prepared according to this procedure, 0.6 g of this reagent contains approximately 1 mmol of Ag2CO3. Since an excess of silver carbonate on Celite is necessary for the reaction to proceed at a reasonable rate, recovery of silver is often required. This can be easily achieved by dissolving the used reagent in nitric acid. The filtered solution after concentration gives back silver nitrate (poorly soluble in nitric acid).

Handling, Storage, and Precautions: can be safely handled in the dry state. Moisture or exposure to light are not critical; can be stored for years in the dark. Silver salts are toxic.

Introduction.

Silver carbonate on Celite oxidizes primary alcohols to aldehydes, and secondary alcohols to ketones. 1,2-Diols are either cleaved or oxidized to a-hydroxy ketones or sometimes to a-diketones; 1,4-, 1,5-, and 1,6-diols with one primary hydroxyl group may give lactones. Lactols are converted to lactones. Hydroquinones and catechols afford quinones. Steric factors play an important role in determining the outcome of these reactions, including the generally high regioselectivity in polyol oxidations.

Oxidation, rearrangement, or glycosylation with the help of silver carbonate on Celite are usually carried out in aromatic solvents, sometimes in chloroform or methylene chloride, and, in the case of carbohydrates, in methanol. Reactions are easily monitored by TLC. As soon as the reaction is over, the solid is filtered off, and the solution evaporated. The price of the reagent makes it difficult to use on a large scale, unless the recovery of silver is contemplated. The mildness and the simplicity of the workup may, in some cases, overcome this drawback. Many protective groups (acetals, formate, acetate, tetrahydropyranyl, silyl derivatives), as well as sensitive functionality such as furan, indole, and N-substituted pyrrole ring systems, are not affected by the reagent. A critical evaluation of several solid supported reagents has been published.52

Oxidation of primary alcohols.

Primary alcohols (saturated, allylic, polyunsaturated, benzylic) are normally converted into aldehydes in good to excellent yield. A few examples out of many reported are listed in Table 1.4-6

In contrast to the oxidation of other primary alcohols, a-hydroxymethyl cyclic ethers give lactones instead of the expected aldehydes (Table 2).7,8

Due to a strong isotope effect, tritiated primary alcohols are oxidized to aldehydes, with virtually no loss of tritium (eq 1).9

Ketones from Secondary Alcohols.

A large number of secondary alcohols, usually bearing other functional groups sensitive to acids and oxidizing agents, has been transformed into ketones.3 A few typical examples are recorded in Table 3.10-13

Allylic alcohols are rapidly oxidized. In sharp contrast, oxidation of a homoallylic alcohol such as cholesterol (in carefully deoxygenated benzene, under argon) leads to an intractable mixture of unidentified products. Celite (the best inert support tested so far) is essential. Thus codeine has been oxidized either by standard silver carbonate, or by silver carbonate on Celite, but only the latter reagent could oxidize dihydrocodeine. Some strained cyclobutanols afford g-lactones in addition to the expected ketone (eq 2).14 A case of molecular rearrangement has been mentioned (eq 3).15

Oxidation of Tertiary Alcohols.

Tertiary alcohols are generally quite stable towards the reagent, with the exception of ethynyl carbinols which are cleaved in quantitative yield (Table 4)16 at a rate comparable to that of allylic alcohol oxidation. Cyanohydrins behave similarly. The use of the ethynyl group as a ketone protective group has been suggested.

Oxidation of Diols.

Depending upon their structure and stereochemistry, and the experimental conditions (reaction time) chosen, 1,2-diols are either cleaved (to dialdehydes or keto aldehydes) or oxidized to a-hydroxy ketones. In some cases, a-diketones have been obtained in reasonable yield (Table 5).17-23

Oxidation of 1,3-diols with silver carbonate on Celite leads to b-hydroxy ketones. Retroaldolization has occasionally been observed.29 In benzene as solvent, a secondary hydroxyl group seems to be oxidized somewhat faster than a primary one. Aliphatic 1,4-, 1,5-, and 1,6-diols behave in the same manner, unless one of the hydroxyl groups is primary. Many diols of this type, in which one of the functions is primary, have been oxidized to lactones, generally in good yield (Table 6).24-28

Quite often, oxidations of polycyclic polyols with silver carbonate on Celite are highly regioselective. A tentative mechanism has been proposed which explains most of the results obtained so far3,30 and has therefore reasonable predictive power for the synthetic organic chemist. A few examples are listed in Table 7.1,31-34

Oxidation of Lactols to Lactones.

Silver carbonate oxidation of lactols is faster than the oxidation of any other type of hydroxyl derivative, and can therefore be achieved without protection of alcohols in the same molecule. Some representative examples are shown in Table 8.35-37

Oxidation and Degradation of Carbohydrates.

A considerable amount of work has been devoted to the oxidation and degradation of carbohydrates using silver carbonate on Celite. If the hydroxyl at C-1 is free, and the reaction is carried out in benzene, toluene, or a mixture of benzene and DMF, then the corresponding lactone is obtained. However, if there is an unprotected OH group at C-2, and especially in methanol, a cleavage between C-1 and C-2 takes place as illustrated in eq 4.38 Hydrolysis of the intermediate formate leads to D-threose.

Methyl Esters from Aldehydes.

With the exception of a-substituted aldehydes of the type reported in Table 2, the oxidation of aldehydes in benzene does not lead to carboxylic acid derivatives. Those which can spontaneously give hemiacetals with another hydroxyl group of the molecule are not an exception, since lactols are very rapidly oxidized to lactones. It is therefore noteworthy that, in methanol, some aldehydes can be converted to methyl esters in modest yield (Table 9).39,40

Oxidation of Phenols.

Hydroquinones and catechols are oxidized to p-quinones and o-quinones, respectively, with this reagent.41,42 4,4-Dihydroxybiphenyls and 4,4-dihydroxystilbenes lead to diphenoquinones and stilbenequinones. Hindered phenols (e.g. 2,6-dimethylphenol, 2,4,6-trimethylphenol) are dimerized, and give diphenoquinones or stilbenequinones in high yield, as illustrated in eq 5. Unhindered phenols give complex mixtures. For instance, p-cresol affords a 65% yield of Pummerer's ketone (eq 6).43 A systematic survey of silver carbonate on Celite oxidation of phenols has been published.44

Oxidation of Amines.

Very little is known so far on the oxidation of aliphatic amines. Anilines give azobenzenes in modest yield (Table 10).3,45

Oxidation of Hydrazines and Hydrazones.

Symmetrically disubstituted hydrazines and hydrazides are rapidly oxidized to the corresponding azo compounds (Table 10). Hydrazones are converted in a few minutes into diazoalkanes, generally in high yield (Table 10).45,46

Nitroso Compounds from Hydroxylamines.

N-Monosubstituted hydroxylamines lead to nitroso compounds, or their dimers, in good yield upon treatment with silver carbonate on Celite.47 If a double bond is suitably placed, a cyclization of the nitroso intermediate gives a nitroxide free radical (Table 11).48

Nitrile Oxides from Oximes.

Benzaldoximes react with silver carbonate on Celite to give nitrile oxides, which undergo 1,3-dipolar cycloaddition with the original oxime. The nitrile oxides can also be trapped by other dipolarophiles such as nitriles and ethylenic compounds (Table 11).46-48

Miscellaneous Reactions.

Halohydrins are smoothly converted by this reagent into epoxides or rearranged into aldehydes or ketones.49,50 Silver carbonate on Celite has also been proposed to improve O-glycosylation51 (Koenigs-Knorr reaction).


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Marcel Fétizon

Institut de Chimie des Substances Naturelles, Gif-sur-Yvette, France



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