Acyloxymercuration

Carboxylic esters are produced by the addition of carboxylic acids to olefins, a reaction that is usually acid-catalyzed (by proton or Lewis acids182) and similar in mechanism to 5-4. Since Markovnikov s rule is followed, hard-to-get esters of tertiary alcohols can be prepared from olefins of the form R2C=CHR.183 When a carboxylic acid that contains a double bond in the chain is treated with a strong acid, the addition occurs internally and the product is a y- and/or a 8-lactone, regardless of the original position of the double bond in the chain, since strong acids catalyze double bond shifts (2-2).184 The double bond always migrates to a position favorable for the reaction, whether this has to be toward or away from the carboxyl group. Carboxylic esters have also been prepared by the acyloxymercuration-demercuration of olefins (similar to the procedures mentioned in 5-2 and 5-4).185... 

The acyloxymercuration-demercuration of alkenes provides an alternative route to esters which is probably less prone to caibon skeleton rearrangements than the direct addition of carboxylic acids to alkenes (equation 282). This reaction has recently been reviewed.477 The reaction is most commonly run using mercury(II) acetate in acetic acid, though other mercury salts may be used and aprotic solvents can also be employed. Equilibria have been measured for the reaction of mercury(II) trifluoroacetate and alkenes in tetrahydrofuran, and were found to be solvent dependent.478... 

A number of alkenoic acids undergo intramolecular acyloxymercuration when treated with mercury(II) acetate or mercury(II) chloride (equations 286 and 287).477... 

Dienes and polyenes undergo acyloxymercuration. Nonconjugated dienes afford either mono- or di-mercurated species, depending on the stoichiometry of the reaction (equation 288).511-513 Products involving carbon-carbon bond formation are occasionally observed during this process.512 Allenes afford vinylmercurials (equation 289).514,515... 

The acyloxymercuration of alkynes has been reported to produce a wide variety of products. Terminal alkynes afford either dialkynylmercurials516,517 or polymercurated products whose structures have not been well established (equation 290). Internal alkynes usually afford vinylic mercurials in which the mercury(II) salt has added in an anti fashion (equation 291 ).518-520 Only sodium borohydride has been used to demercurate a few of these mercurials.520... 

Ethanol reacts like MeOH however, as the nucleophilicity of the alcohol decreases, so the potential for competing acetoxymercuration to provide RCH(OAc)CH2HgOAc increases. Such competing reactions become serious when t-BuOH is used. Hence, for this and other weakly nucleophilic reagents, Hg(OCOCF3)2 has advantages , because the [CF3CO2] ion is too weak a nucleophile to compete with the alkoxide, and so no acyloxymercuration results. 

It is also advantageous to use Hg(OCOCF3)2 rather than Hg(OAc)2 for peroxymercuration . This is not only to circumvent acyloxymercuration, but to enable the reactions to be carried out with moderate amounts of peroxide. 

Acyloxymercurations are carried out in the carboxylic acid media or in nonparticipating media if Hg carboxylates are used as the electrophile. 

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