Iodine-Silver Benzoate


[7553-56-2]  · I2  · Iodine-Silver Benzoate  · (MW 253.80) (PhCO2Ag)

[532-31-0]  · C7H5AgO2  · Iodine-Silver Benzoate  · (MW 228.99)

(the mixture gives silver iododibenzoate as an active reagent which converts alkenes to cis-dibenzoates under mild conditions1)

Alternate Names: Prévost's reagent; silver iododibenzoate.

Physical Data: cream-colored solid.

Solubility: the most common solvent for this reagent is benzene. No data are available for the solubility of the dibenzoate reagent. PhCO2Ag: sol water to the extent of 2.66 g L-1 at 25 °C;2 slightly sol ethanol, 0.169 g L-1 (cold) and 0.465 g L-1 (reflux).2

Form Supplied in: prepared as needed.

Preparative Method: prepared by reaction of Iodine with 2 equiv of Silver(I) Benzoate.1 In general, 2 equiv of silver benzoate are refluxed in benzene with 1 equiv of iodine per mole of alkene to be reacted.3 If the reagent is to be isolated prior to reaction with the alkene, it is first washed with saturated aqueous ammonia to remove silver benzoate and then with saturated aqueous sodium bicarbonate to remove benzoic acid.

Purification: the crystalline complex can be purified by extraction with benzene in a Soxhlet apparatus.3

Handling, Storage, and Precautions: no special precautions have been reported for the dibenzoate.

Prévost Reaction.

The reaction of 2 equiv of silver benzoate and iodine initially forms PhCO2I, but this reacts further to form silver iododibenzoate, (PhCO2)2AgI, containing two benzoate groups.3 This intermediate can react further under the proper conditions to give the tribenzoate silver derivative, but the dibenzoate is taken to be the reactive species in reactions with alkenes, which is the most important reaction of this reagent. Prévost found that silver benzoate and iodine converts alkenes such as styrene to the 1,2-dibenzoate (eq 1), which can be saponified to the 1,2-diol.4a This transformation (formally a hydroxylation) is now called the Prévost reaction.4 Reaction with cyclic alkenes generates trans-diols, in contrast to permanganate and osmium hydroxylations which give cis-diols (see Potassium Permanganate and Osmium Tetroxide). This reaction is generally related to the Hunsdiecker reaction (the coupling reaction of silver carboxylate; see Iodine). When an alkene reacts with the silver iododibenzoate, an iodonium salt is generated and this is reacts with silver benzoate to form a cationic dioxolane species. Attack by a second molecule of benzoate gives the ester, a trans-dibenzoate. Saponification generates the trans-diol. A similar hydroxylation reaction, also called the Prévost reaction, uses silver acetate and iodine5 (see Iodine-Silver Acetate), although the acetate reaction (often called the Woodward-Prévost reaction) gives the cis-diol.

A synthetically useful example of the reaction with silver benzoate is the conversion of a dibenz[a,j]acridine derivative to the trans-dibenzoate in 35% yield (eq 2).6 With relatively simple systems (such as styrene), the yields are usually quite good (80-90%).

There are interesting variations in the reaction when other functional groups are present. When 5-isopropyl-5-allylbarbituric acid is treated with silver benzoate and iodine, for example, a 28% yield of a furopyrimidine is obtained (eq 3).7 The initially formed benzoate intermediate reacts with the proximate imide moiety to produce the final product.

In addition to quinone derivatives, silver iododibenzoate reacts with aromatic hydrocarbons to give different products.8 Electron-deficient benzene derivatives such as nitrobenzene give biphenyl derivatives, electron-rich benzene derivatives such as anisole give iodoanisole derivatives, and alkylbenzenes give essentially no reaction.8,9

Oxidative Cleavage of 1,2-Diols.

Using an excess of this reagent, 1,2-diols can be cleaved oxidatively.10 For example, the fatty chain diol in eq 4 is cleaved selectively using I2/PhCO2Ag.11

Allylic Oxidations.

In certain cases, oxidation of alkenes with I2/PhCO2Ag in benzene causes allylic oxidation (eq 5).12

Related Reagents.

Iodine-Silver Trifluoroacetate.

1. Hershberg, E. B. HCA 1934, 17, 351.
2. Linke, W. F. Solubilities of Inorganic and Metal-Organic Compounds; Van Nostrand: New Jersey, 1958, Vol 1, pp 46-47.
3. Ferretti, A.; Tessi, G. JCS 1965, 5203.
4. (a) Prévost, C. CR(C) 1933, 196, 1129. (b) Prévost, C. CR(C) 1935, 200, 942. (c) Prévost, C.; Lutz, R. CR(C) 1934, 198, 2264. (d) Wilson, C. V. OR 1957, 9, 332.
5. Birkenbach, L.; Goubeau, J.; Berninger, E. CB 1932, 65, 1339.
6. Rosario, C. A.; Holder, G. M.; Duke, C. C. JOC 1987, 52, 1064.
7. Smissman, E. E.; Robinson, R. A. JOC 1970, 35, 3532.
8. Bryce-Smith, D.; Clarke, P. JCS 1956, 2264.
9. Sletzinger, M.; Dawson, C. R. JOC 1949, 14, 670.
10. Raman, P. PIA(A) 1956, 44, 321.
11. Lüning, B.; Paulsson, L. ACS 1967, 21, 830.
12. Briggs, L. H.; Cain, B. F.; Cambie, R. C.; Davis, B. R.; Rutledge, P. S. JCS 1962, 1850.

Michael B. Smith

University of Connecticut, Storrs, CT, USA

Lars-G. Wistrand

Nycomed Innovation, Malmö, Sweden

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