Regioselectivity oxymercuration-demercuration

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. 

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. 

The overall oxymercuration-demercuration procedure is highly regioselective, with the nucleophile being introduced at the most highly substituted carbon ... 

Steric effects may play a role in the regiochemistry of the reaction as well. While the oxymercuration-demercuration reaction is generally considered to give only Markovnikov hydration, as shown by predominant formation of 43, this regioselectivity is not absolute. For example, methoxymercuration of 3,3-dimethyl-l-butene produced 2% of 3,3-dimethylbutyl methyl ether (44, equation 9.42). ... 

Oxymercuration-demercuration and hydroboration-oxidation have complementary regioselectivity. 

In the first step, boron and hydrogen undergo syn addition to the alkene in the second step, treatment with hydrogen peroxide and base replaces the boron with — OH with retention of configuration. The net addition of —H and —OH occurs with autl-Markovnikov regioselectivity and syn stereoselectivity. Hydroboration-oxidation, therefore, serves as a useful regiochemical complement to oxymercuration-demercuration. 

In Summary Oxymercuration-demercuration is a synthetically useful method for converting alkenes regioselectively (following the Markovnikov rule) into alcohols or ethers. Carbocalions are not involved therefore, rearrangements do not occur. 

Because borane additions to double bonds and subsequent oxidation are so selective, this sequence allows the stereospeeific and regioselective synthesis of alcohols from alkenes. The anti-Markovnikov legioselectivity of the hydroboration-oxidation sequence complements that of acid-catalyzed hydration and oxymercuration-demercuration. In addition, hydroboration, like oxymercuration, occurs without the participation of carbocations therefore, rearrangements are not observed. 

In the oxymercuration step, water and mercuric acetate add to the double bond in the demercuration step, sodium borohydride reduces the acetoxymercury group and replaces it with hydrogen. The net addition of H— and —OH takes place with Markovnikov regioselectivity and generally takes place without the complication of rearrangements, as sometimes occurs with acid-catalyzed hydration of aUcenes. The overall alkene hydration is not stereoselective because even though the oxymercuration step occurs with anti addition, the demercuration step is not stereoselective (radicals are thought to be involved), and hence a mixture of syn and anti products results. 

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