Alcohols oxymercuration-demercuration

On the basis of the mechanistic pattern for oxymercuration-demercuration, predict the structure and stereochemistry of the alcohol(s) to be expected by application of the reaction to each of the following substituted cyclohexenes. 

Alcohols from Alkenes through Oxymercuration-Demercuration... 

Oxymercuration-demercuration is a useful laboratory method for the synthesis of small quantities of alcohol. Like the catalytic hydration reaction, this process is an example of Markovnikov addition. It s a useful procedure because it tends to result in high yields and rearrangements rarely occur. 

Problem 13.43 Write the structural formulas for the alcohols formed by oxymercuration-demercuration from... 

Mechanism. The reaction is analogous to the addition of bromine molecules to an alkene. The electrophilic mercury of mercuric acetate adds to the double bond, and forms a cyclic mercurinium ion intermediate rather than a planer carbocation. In the next step, water attacks the most substituted carbon of the mercurinium ion to yield the addition product. The hydroxymercurial compound is reduced in situ using NaBH4 to give alcohol. The removal of Hg(OAc) in the second step is called demer-curation. Therefore, the reaction is also known as oxymercuration-demercuration. 

Silver(I) catalyzed cyclizations of allenic alcohols (202) lead to 2,5-dihydrofurans (203) (79S743), whilst another mild method for the synthesis of tetrahydrofurans is the intramolecular oxymercuration-demercuration process. Geraniol, when treated with mer-cury(II) acetate and subsequently with sodium borohydride, gave a tetrahydrofuran. 

Various allylic amines and protected allylic alcohols were tested using different cyclodextrins. Although only low to moderate enantioselectivity was obtained, the method demonstrated for the first time an enantioselective inverse phase-transfer catalysis hydration reaction via an oxymercuration-demercuration process. 

Oxymercuration-demercuration is another method for converting alkenes to alcohols with Markovnikov orientation. Oxymercuration-demercuration works with many alkenes that do not easily undergo direct hydration, and it takes place under milder conditions. No free carbocation is formed, so there is no opportunity for rearrangements or polymerization. 

Problem 13.37 Write the structural formulas for the alcohols formed by oxymercuration-demercuration from (a) l-heptene, (b) 1-methylcyclohexene, (c) 3,3-dimethyl-1-butene. M... 

Water, alcohols, hydroperoxides or carboxylates can be added regioselectively to a,/ -unsaturat-ed carbonyl compounds by the oxymercuration demercuration protocol. In this sequence, the oxymercuration step yields the -addition product. Depending on whether the demercura-tion is performed with retention or inversion of configuration, two sets of diastereomeric pairs of enantiomers (racemic mixtures) are obtained. 

Hydroboration-oxidation, then, converts alkenes into alcohols. Addition is highly regiospecific the preferred product here, however, is exactly opposite to the one formed by oxymercuration-demercuration or by direct acid-catalyzed hydration. For example ... 

Acid catalyzed hydration of alkenes is not well suited for laboratory preparation of alcohols. Since the reaction proceeds via carbocation intermediates, mixtures of alcohols may be formed. However, oxymercuration-demercuration of alkenes provides a simple tool for regioselective hydration of alkenes whereby rearrangements are seldom observed. 

Markownikoff hydration of olefins (2, 265-267). Brown and Geoghegan1 have reported a detailed study of the oxymercuration-demercuration procedure. Yields of alcohols in the case of terminal olefins, RCH=CH2 and R2C=CH2, and disubstituted internal olefins, RCH=CHR are practically quantitative. Tri-substituted olefins of the type R2C=CHR show a wide variation in reactivity. 1 -Phenylcyclopentene and 1 -phenylcy clohexene, for example, are unreacti ve. 

In the oxymercuration process, the electrophilic addition of the mercuric species occurs resulting in a mercurinium ion which is a three-membered ring. This is followed by the nucleophilic attack of water and as the proton leaves, an organomercuric alcohol (addition product) is formed. The next step, demercuration, occurs when sodium borohydride (NaBH ) substitutes the mercuric acetate substituent with hydrogen. If an alkene is unsymmetric, Oxymercuration-demercuration results in Markovnikov addition. The addition of mercuric species and water follows an anti (opposite side) addition pattern. This reaction has good yield, since there is no possibility of rearrangement unlike acid-catalyzed hydration of alkenes. 

The answer is B. The oxymercuration-demercuration reaction converts an alkene to an alcohol. The hydration follows Markovnikov s rule and thus the hydrogen adds to the carbon atom with greater number of hydrogens. 

Which alkene would be expected to give the following alcohol by oxymercuration-demercuration ... 

Oxymercuration-demercuration of allyl alcohol gives 1,2-propanediol. 

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