Alkenes hydroxymercuration-demercuration

While many different procedures have been reported for the hydroxymercuration—demercuration of alkenes, the most useful procedure uses mercury(II) acetate in aqueous THF, followed by in situ alkaline sodium borohydride reduction (equation 205).312-313 Virtually all substitution patterns about the C—C double bond are accommodated. 

Numerous dienes and polyenes have been subjected to hydroxymercuration-demercuration.311 With nonconjugated dienes, the products can usually be predicted by applying what one has learned from the corresponding simple alkenes. Isolated double bonds are more reactive than conjugated double bonds and frequently one of the double bonds is sufficiently more reactive than the others that monohydroxy-lated products can be obtained. Improvements in selectivity have been reported by using mercury(II) tri-fluoroacetate339 or by adding sodium lauryl sulfate.340... 

The hydroxymercuration-demercuration of conjugated dienes generally does not afford monohydration products selectivity, but diols can sometimes be obtained in reasonable yield. The direction of addition of H—OH is that expected by extrapolation from simple alkenes and allylic alcohols. 

Thus good yields of -alkoxy alcohols can be obtained, albeit as diastereomeric mixtures, but unfortunately hydroxymercurated alkenes under similar conditions do not lead to useful products. Despite this apparent limitation, alkoxymercuration-oxidative demercuration has been very effective in a number of systems described below, and there is no doubt it is a procedure worth consideration for hydroxy group introduction. 

We saw in Section 7.4 that aikenes react with water in the presence of mercuric acetate to yield a hydroxymercuration product. Subsequent treatment with NaBH4 breaks the C-Hg bond and yields the alcohol. A similar alkosymercuration reaction occurs when an alkene is treated with an alcohol in the presence of mercuric acetate. [Mercuric trifluoroacetate, (CF3C02)2Hg, works even better.] Demercuration by reaction with NaBH4 then yields an ether. As indicated by the following examples, the net result is Markovnikov addition of the alcohol to the alkene. 

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