Rearrangements oxymercuration-demercuration

Oxymercuration-demercuration allows the Markovnikov addition of H-and -OH without rearrangements. 

Oxymercuration-demercuration gives Markovnikov addition of H- and -OH to an alkene, yet it is not complicated by rearrangement. 

Rearrangements of the carbon skeleton seldom occur in 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. 

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. 

Similarly, oxymercuration-demercuration of 3,3-dimethylbut-l-ene gives the Markovnikov product, 3,3-dimethylbutan-2-ol, in excellent yield. Contrast this unrearranged product with the rearranged product formed in the acid-catalyzed hydration of the same alkene in Section 8-4B. Oxymercuration-demercuration reliably adds water across the double bond of an alkene with Markovnikov orientation and without rearrangement. 

Of the methods we have seen for Markovnikov hydration of alkenes, oxymercuration-demercuration is most commonly used in the laboratory. It gives better yields than direct acid-catalyzed hydration, it avoids the possibility of rearrangements, and it does not involve harsh conditions. There are also disadvantages, however. Organomercurial compounds are highly toxic. They must be used with great care and then must be disposed of properly. 

The net addition of H—OH to alkenes is cis, anti-Markovnikov, and free from rearrangement. 4. Oxymercuration-Demercuration of Alkenes... 

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. 

Oxymercuration-demercuration gives the same products as aqueous sulfuric acid. The carbocations formed from these substrates and H+ are not particularly prone to rearrangement. All chiral products are formed as racemic mixtures. 

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. 

Oxymercuration-demercuration is not prone to hydride or aikanide rearrangements. 

Oxymercutation-demercutation occurs with Markovnikov regiochemistry and results in hydration of alkenes without complication from carbocation rearrangement. It is often the preferred choice over acid-catalyzed hydration for Markovnikov addition. The overall stereochemistry of addition in acid-catalyzed hydration and oxymercuration-demercuration is not controlled—they both result in a mixture of cis and trans addition products. 

Oxymercuration-demercuration of 3-phenyl-l-pentene gives an alcohol that is formed without rearrangement. Draw both possible products and use differences in the IR and proton NMR to compare the unrearranged and the rearranged products in order to distinguish them. 

In cases where protonation of the alkene ultimately leads to carbocation rearrangements, acid-catalyzed hydration is an inefficient method for adding water across the alkene. Many other methods can achieve a Markovnikov addition of water across an alkene without carbocation rearrangements. One of the oldest and perhaps best known methods is called oxymercuration-demercuration ... 

If rearrangement were possible, then oxymercuration-demercuration would have been the preferred route. 

Acid-catalyzed hydration proceeds with Markovnikov addition (Section 9.4). That is, the hydroxyl group is positioned at the more substituted carbon. It is a useful method if the substrate is not susceptible to carbocation rearrangements (Section 6.11). In a case where the substrate can possibly rearrange, oxymercuration-demercuration can be employed. This approach also proceeds via Markovnikov addition, but it does not involve carbocation rearrangements. Hydroboration-oxidation is used to achieve an Markovnikov addition of water. 

The alcohol obtained after demercuration is the same as the product of Markovnikov hydration (Section 12-4) of the starting material. However, oxymercuration-demercuration is a valuable alternative to acid-catalyzed hydration, because no carbocation is involved therefore oxymercuration-demercuration is not susceptible to the rearrangements that commonly occur under acidic conditions (Section 12-3). Its use is limited by the expense of the mercury reagent and its toxicity, which requires careful removal of mercury from the product and safe disposal. 

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. 

Oxymercuration-demercuration gives the product that would result from direct hydration of an alkene. However, the reactions occur with a higher yield than the direct hydration reaction because the competing reverse reaction, dehydration, does not occur. Because most of the positive charge in the mercurinium ion is on the mercury atom, the mercurinium ion has little carbocation character, and rearrangement reactions do not occur. 

Synthetic Transformation 8.1 Addition of H and OH without rearrangement (Oxymercuration-Demercuration)... 

This conversion requires the Markovnikov addition of water without carbocation rearrangement. This can be achieved via oxymercuration-demercuration ... 

Addition of H and OH across this alkene will provide an alcohol. Markovnikov addition will give a secondary alcohol, but we must be careful. Protonation of the alkene will generate a secondary carbocation which can rearrange (via a methyl shift) to give a more stable, tertiary carbocation. Therefore, acid-catalyzed hydration cannot be used. Instead, the desired product can be obtained via oxymercuration-demercuration, which will install the OH group at the more substituted position without carbocation rearrangements. 

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