Iodine-Silver Acetate1


[7553-56-2]  · I2  · Iodine-Silver Acetate  · (MW 253.80) (AgOAc)

[563-63-3]  · C2H3AgO2  · Iodine-Silver Acetate  · (MW 166.92)

(source of positive iodine for stereoselective 1,2-functionalization of alkenes)

Physical Data: AgOAc: mp 210 °C (dec). I2: see Iodine.

Solubility: AgOAc: slightly sol acetic acid (0.54 g L-1), methanol (0.17 g L-1).

Form Supplied in: AgOAc: white solid; widely available.

Analysis of Reagent Purity: Ag can be assayed conveniently by volumetric titration of AgI.

Purification: usually used as supplied, but Silver(I) Acetate can be purified by treatment with acetic acid for a prolonged period, filtering, and drying of the solid in a vacuum oven at 40 °C for 48 h.

Handling, Storage, and Precautions: protect from light.

1,2-Dihydroxylation of Alkenes.

In acetic acid, which is the solvent of choice for these reactions, electron-rich alkenes can be converted into 1,2-dihydroxylated derivatives. The stereochemical outcome is dependent upon the conditions; if the reaction is performed in dry solvent, trans-1,2-diacetates are formed (the Prévost reaction) (eq 2).2 On the other hand, if moist acetic acid is used as the solvent, monoacetylated cis-1,2-diols are formed (the Woodward-Prévost reaction) (eq 3).3 A mechanism involving the intermediate formation of an acetoxonium ion has been suggested (eq 1).

One interesting aspect of the cis-hydroxylation method is that it is complementary to oxidation with Osmium Tetroxide, which also gives cis-1,2-diols, but these are formed by attack on the least hindered face of the molecule (eq 4). Using the Woodward-Prévost conditions, the cis-1,2-diol resulting from attack on the most hindered face predominates (eq 5).

In the oxidation of 1-alkylindenes under both wet and dry conditions, identical facial selectivities were observed, supporting the proposed common acetoxonium ion intermediate (eqs 6 and 7). For example, the identical ratios (1)/(2) and (3)/(4), respectively, changed from 0.45 to >100 when going from R = Me to R = t-Bu, an effect of the increasing substituent size.4

Under certain conditions, 1,2-dihydroxylation does not occur; instead, intramolecular opening of the intermediate iodonium ion is observed, e.g. the lactam oxygen in (5) acts as a nucleophile to give (6) (eq 8).5

Oxidation of Enol Silyl Ethers.

Enol ethers, and in particular enol silyl ethers, can be transformed into the corresponding a-iodinated ketones by oxidation with I2/AgOAc in CH2Cl2 followed by cleavage of the silyl ether (eq 9).6

Related Reagents.

Iodine-Silver Benzoate; Iodine-Silver Trifluoroacetate.

1. (a) Wilson, C. V. OR 1957, 9, 332. (b) Gunstone, F. D. Adv. Org. Chem. 1960, 1, 122.
2. Prévost, M. C. CR(C) 1933, 197, 1661.
3. Woodward, R. B.; Brutcher, F. V., Jr. JACS 1958, 80, 209.
4. Cedheim, L.; Eberson, L. ACS 1975, B29, 904.
5. Subramanian, M.; Mohan, P. S.; Shanmugam, P.; Rajendra Prasa, K. J. ZN(B) 1992, 47, 1016.
6. Rubottom, G. M.; Mott, R. C. JOC 1979, 44, 1731.

Lars-G. Wistrand

Nycomed Innovation, Malmö, Sweden

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