Hydroxymercuration alkenes

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. 

In view of the many limitations inherent in the direct acid-catalyzed hydration of alkenes, indirect hydration via hydroxymercuration-demeicuration has become a very valuable method for the preparation of alcohols. This process has recently been thoroughly reviewed.311... 

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. 

The stereochemistry of addition to acyclic, cyclic and polycyclic alkenes is essentially identical to that of hydroxymercuration wherever the two processes have been compared. Fewer data have been accumulated for alkoxymercuration however. 

The kinetics of alkoxymercuration have received considerable attention 415 The relative reactivity of various alkenes parallels that reported for hydroxymercuration. 

We saw in Section 7.4 that alkenes 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 alkoxymercuration reaction occurs when an alkcne is treated with an lilcohol in the presence of mercuric acetate or, even better, mercuric trilluoroacetate, (CF3C02)2Hg. Demercura-tion by reaction with NaBIi4 then yields an ether. The net result is Markovnikov addition of the alcohol to the alkene. 

T lie mechanism of tire alkoxymercuration reaction is similar to that described in Section 7.4 for hydroxymercuration. T he reaction is initiated by electrophilic addition of to the alkene, followed by reaction of the intermediate cation with alcohol and reduction of the C-1 Ig bond by NaBH4. A variety of alcohols and alkenes can be used in tlic alkoxymercuration reaction. Primary, secondary, and even tertiary alcohols react v -ell, but ditertiary ethers can t be prepared because of steric hindrance to reaction. 

As was already mentioned, the standard procedure for acid catalyzed alkene hydration exhibits a rather low selectivity. On the other hand, the use of a hydroxymercuration-reduction sequence leads to the exclusive formation of Markovnikov s alcohols. A nearly exclusive anti-Markovnikov s hydration is achieved via a hydroboration-oxidation reaction (see Section 2.4). The result in both these cases is the net addition of H2O, but the basic differences in the reaction mechanisms unambiguously determine a reversed regioselectivity pattern. 

Heathcock has observed that secondary and tertiary alkyl azides are obtained in good yields from terminal alkenes, but that non-terminal alkenes are relatively unreactive, except where reactivity is enhanced by steric strain as in norbornene. The order of reactivity in this reaction is similar to that in hydroxymercuration. A correlation between the reactivity sequence for azidomercuration and the known order of alkene-Ag stability constants was also apparent zmd Heathcock therefore proposed a mechanism which entails a rapid equilibrium involving the ion 197 which affords the organomercury... 

The order of reactivity in hydroxymercurations of simple alkyl-substituted alkenes " is R2C=CH2 > RCH=CH2 > cis-RCH=CHR > trans-RCH=CHR > R2C=CHR > R2C=CR2, which arises from steric and electronic effects. The reactivity of alkenes is reduced by branching within R or by conjugation of the double bond with an aryl group. The reactivities of alkenes are linked with electronic (a ) and steric (Ej) effects of substituents ... 

The mechanism of the alkoxymercuration reaction is similar to that described in Section 7.4 for hydroxymercuration. The reaction is initiated by electrophilic addition of to the alkene, followed by reaction of the... 

Synthesis of alkyl azides.1 Terminal alkenes and strained cyclic alkenes react with the reagent to give a mercurial intermediate, which on reduction with sodium borohydride gives an azide. Internal olefins do not react. The method is an extension of the hydroxymercuration reaction of Brown (2,265-267). 

Electrophilic Addition to Alkenes, Hydroxy- and alkoxymercurations of alkenes have been performed in micellar SDS. Hydroxymercuration of (1) with mercury(II) acetate, followed by reduction with sodium borohydride, gave a greatly enhanced yield of (2) in micellar SDS (90%) relative to that obtained in THF-H2O (20-25%) (eq 2). Also, the reactions of (1) and the related limonene gave greater cyclic ether diol product ratios in the SDS environment than in aq THF. Both the enhanced 3delds and ratios were attributed to anisotropic solubilization of the alkylmercurial intermediate in a relatively H2O poor mIceUar microenvironment. The hydroxymercuration of an aromatic diene, /Mliallylbenzene, did not display enhanced chemoselectivlty (mono vs. diol formation) in micellar SDS relative to that obtained in THF-H2O. This result suggests that the mIceUar solubilization sites of aromatic substrates and reaction intermediates are more HzO-rich than those of aliphatic systems. 

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