Markovnikov additions 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. 

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. 

Addition of alcohol to alkenes hy alkoxymercuration-reduction produces ethers via Markovnikov addition. This addition is similar to the acid-catalysed addition of an alcohol. For example, propene reacts with mercuric acetate in aqueous THF, followed hy reduction with NaBFl4, to yield methyl propyl ether. The second step is known as demercuration, where Flg(OAc) is removed hy NaBH4. Therefore, this reaction is also called alkoxymercura-tion-demercuration. The reaction mechanism is exactly the same as the oxymercuration-reduction of alkenes. 

Oxymercuration-demercuration of an unsymmetrical alkene generally gives Markovnikov orientation of addition, as shown by the oxymercuration of propene in the preceding example. The mercurinium ion has a considerable amount of positive charge on both of its carbon atoms, but there is more of a positive charge on the more substituted carbon atom, where it is more stable. Attack by water occurs on this more electrophilic carbon, giving Markovnikov orientation. The electrophile, +Hg(OAc), remains... 

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

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. 

Define each term, and give an example, (a) dimerization (d) stereospecific addition (g) Markovnikov addition (j) hydrogenation (m) heterogeneous catalysis (p) hydroxylation (s) hydroboration (v) oxymercuration-demercuration (y) alkoxymercuration-demercuration... 

Alcohols from Alkenes through Oxymercuration-Demercuration Markovnikov Addition 349... 

ALCOHOLS FROM ALKENES THROUGH OXYMERCURATION-DEMERCURATION MARKOVNIKOV ADDITION... 

In oxymercuration—demercuration, the net orientation of the addition of the elements of water, H — and — OH, is in accordance with Markovnikov s rule. The H — becomes attached to the carbon atom of the double bond with the greater number of hydrogen atoms. 

For example, the acid-catalyzed hydration (or oxymercuration-demercuration) of 1-hexene yields 2-hexanol, the Markovnikov addition product. 

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 (1) Hg(OAc)2,THF—H2O (2) NaBH4, HO Markovnikov addition Not controlled Seldom... 

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. 

STRATEGY AND ANSWER We recognize that synthesis by path (a) would require a Markovnikov addition of water to the alkene. So, we could use either acid-catalyzed hydration or oxymercuration—demercuration. 

Formation of the secondary alcohol 71 clearly indicates that reaction of the alkene and the Lewis acid gives the more stable secondary carbocation. Indeed, this is the mechanism for this transformation, as discussed in further detail later. The overall transformation is a hydration process that adds water to the more substituted carbon of the alkene (sometimes called a Markovnikov addition), followed by removal of mercury, and is called oxymercuration-demercuration or simply oxymercuration. 

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 ... 

The previous sections covered two different methods for achieving a Markovnikov addition of water across a it bond (1) acid-catalyzed hydration and (2) oxymercuration-demercuration. In this section, we will explore a method for achieving an ij wri-Markovnikov addition of water. This process, called hydroboration-oxidation, places the OH group at the less substituted carbon ... 

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. 

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. 

Oxymercuration-demercuration of olefins is a useful route to alcohols and ethers with Markovnikov orientation of addition. The relative reactivities of a number of olefins have been determined to assess the possibility of selective oxymercuration-demercuration of one olefin in the presence of another. 

In addition to the oxymercuration-demercuration method, which yields the Markovnikov product, a complementary method that yields the non-Markovnikov product is also useful. Discovered in 1959 by H.C. Brown and called hydroboration, the reaction involves addition of a B-H bond of borane, BH3, to an alkene to yield an organoborane intermediate, RBH2. Oxidation of the organoborane by reaction with basic hydrogen peroxide, H2O2, then gives an alcohol. For example ... 

One of the features that makes the hydroboration reaction so useful is the regiochemistry that results when an unsymmetrical alkene is hydroborated. For example, hydroboration-oxidation of 1-methylcyclopentene yields trans-2-methylcyclopentanol. Boron and hydrogen add to the alkene from the same face of the double bond—that is, with syn stereochemistry, the opposite of anti— with boron attaching to the less highly substituted carbon. During the oxidation step, the boron is replaced by an OH with the same stereochemistry, resulting in an overall syn non-Markovnikov addition of water. This stereochemical result is particularly useful because it is complementary to the Markovnikov regiochemistry observed for oxymercuration-demercuration. 

When predicting the product of a reaction, you have to recall what you know about the kind of reaction being carried out and then apply that knowledge to the specific case you re dealing with. In the present instance, recall that the two methods of hydration— hydroboratlon-oxidation and oxymercuration-demercuration—give complementary products. Hydroboration-oxidation occurs with syn stereochemistry and gives the non-Markovnikov addition product oxymercuration-demercuration gives the Markovnikov product. 

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 an oxymercuration-demercuration reaction, an alkene is treated with mercury(II) acetate, Hg(OAc)2, and the product is treated with sodium borohydride. The net result is a Markovnikov addition product in which the OH group bonds to the more substituted carbon atom of the alkene. 

Students confuse oxymercuration-demercuration (Synthetic Transformation 1) with anti-Markovnikov addition of H and OH (S5mthetic Transformation 8.3), perhaps because they both result in delivery of an H and OH to an alkene (though to different carbons ) or because they both involve boron (but in different steps for different purposes). It is worth your time to put the note cards with these reactions side by side and figure out the differences. 

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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