Iodine alkene hydroxylation

The Woodward Reaction (or Woodward c/s-Hydroxylation) allows the synthesis of syn-diols from alkenes by the addition of iodine followed by nucleophilic displacement with acetate in the presence of water. Hydrolysis of the intermediate ester gives the desired diol. The Prevost Reaction gives crn/z -diols. 

Alkenes from diols. Reaction of v/c-diols with two secondary hydroxyls or one primary and one secondary alcohol with chlorodiphenylphosphine (2 equiv.), imidazole (4 equiv.), and iodine (2 equiv.) results in alkenes. The reaction presumably involves a vic-iododiphenylphosphinate, which can be isolated in some cases and converted to an alkene with zinc in acetic acid. 

Syn hydroxylation from the more hindered face of a ir-system can be effected using Woodward s pro-cedure in which an alkene is treated with iodine-silver acetate in acetic acid containing water. Variants of this method avoid the use of silver salts.A versatile pocedure by which syn hydroxyla-tion can be performed on either the more hindered or less hindered face of an alkene relies on stereoselective formation of the appropriate traru-bromohydrin from the alkene. ... 

Anti hydroxylation of an alkene is readily achieved with peroxycarboxylic adds. Add-catalyzed ring opening of the irutial produd, an oxirane (epoxide), forms the monoester of a 1,2-diol, hydrolysis of which affords the parent diol. Alternative reagents whi ate often used for anti hydroxylation of alkenes are hydrogen peroxide with oxides of tungsten > 4.2i gj. selenium, 444i and iodine-silver benzoate (Pr6vost reaction).. ... 

Several related procedures for syn hydroxylation of alkenes involve a halohydrin ester (32) as the key intermediate. In Woodward s procedure an alkene in glacial acetic acid is treated with iodine and silver acetate. Acetyl hypoiodite, MeCOjI, formed by reaction of the latter two reagents attacks the alkene, R R 0=CR R in an electrophilic manner, from the less hindered side to give, by overall anti addition. 

Since silver salts are expensive, other cheaper reagents have been sought which can bring about the same type of conversion, and the iodine- tassium iodate- tassium acetate combination has been found to provide a useful alternative. Thallium(I) acetate may be used in place of silver acetate in the Woodward procedure, and syn hydroxylation of steroidal alkenes with dialliumflll) acetate in acetic acid has been performed. ... 

Hydrolysis of the ester does not change the configuration. The Woodward modification of the Prevost reaction is similar, but results in overall syn hydroxylation. The alkene is treated with iodine and silver acetate in a 1 1 molar ratio in acetic acid containing water. Here again, the initial product is a (3-halo ester the addition is anti and a nucleophilic replacement of the iodine occurs. However, in the presence of water, neighboring-group participation is prevented or greatly decreased by solvation of the ester function, and the mechanism is the normal Sn2 process, ... 

Thallium monoacetate, TIOCOCH3 (hygroscopic), behaves similarly to silver acetate or silver benzoate. In the presence of iodine, it accomplishes stereoselective hydroxylations of alkenes [410]. 

Prevost method The overall //7//-hydroxylation of an alkene achieved by treating the alkene with a 1 2 molar ratio of iodine and silver benzoate, followed by hydrolysis. Overall. syw-hy droxy 1 ation may be achieved by the method of Woodward. 

Woodward method The syn-hydroxylation of an alkene the first step involves treating the alkene with iodine and silver acetate in a 1 1 molar ratio in wet acetic acid. The hydrolysis of the intermediate c/////-P-halogenoester is via a normal SN2 reaction in which there is no neighbouring group effect because the ester function is solvated by the water. Overall //-addition may be achieved by the method of Prevost. 

In practice 136 was not made from proline. Breaking open the other, more functionalised, ring 136a is possible with the idea of a radical cyclisation onto an alkene 137. FGI of hydroxyl for iodine reveals the elements of another amino acid, serine 140 and an allylic bromide 139. 

That surfaces of e.g. alumina play a role in the hydroiodination of alkenes and alkynes had been observed earlier . In refluxing low boiling petroleum ether a mixture of 1 -hexyne, iodine and dehydrated alumina afforded in 2 h a 62% yield of the Markovnikov addition product 2-iodo-l-hexene. The authors assume that iodine reacts with surface hydroxyl groups to form HI. The presence of alumina is required for the reaction to occur. 

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