Hydration reactions oxymercuration-demercuration

Oxymercuration/demercuration provides a milder alternative for the conventional acid-catalyzed hydration of alkenes. The reaction also provides the Markovnikov regiochemistry for unsymmetrical alkenes.33 Interestingly, an enantioselective/inverse phase-transfer catalysis (IPTC) reaction for the Markovnikov hydration of double bonds by an oxymercuration-demercuration reaction with cyclodextrins as catalysts was recently reported.34 Relative to the more common phase-transfer... 

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. 

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). ... 

Addition reactions that will produce alcohols include acid-catalyzed hydration, oxymercuration-demercuration, and hydroboration-oxidation. 

Both alkyl groups of this disubstituted alkene are part of the ring and are bonded to the same carbon atom. The CH, unit has the less substituted carbon atom. An oxymercuration-demercuration reaction places a hydrogen atom at the CH, site and a hydroxyl group on the ring carbon atom. This product is the predicted Markovnikov product of indirect hydration of an alkene... 

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. 

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. 

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. 

Hydration of the double bond in 3,4-unsaturated sugars by the oxymercuration-demercuration procedure should lead either to 3- or to 4-deoxy sugars. It was found that this reaction applied to methyl 3,4-dideoxy- and 3,4,6-trideoxy-a-DL-hex-3-enopyranosides proceeded regio- and stereospecifically and furnished methyl 3-deoxy- and 3,6-dideoxy-DL-hexopyranosides. From substrates of the a-erythro configuration only a-xylo products (374) were obtained, a-threo substrates gave products (375) of the lyxo configuration. 

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. 

Among the synthetically useful reactions of Hg(II) salts with organic compounds, the most familiar is a two-stage procedure for alkene hydration called oxymercuration-demercuration. Its application in the conversion of 3,3-dimethyl-l-butene to 3,3-dimethyl-2-butanol illustrates the procedure. 

Oxymercuration-demercuration is commonly used for the Markovnikov hydration of olefins. In order to improve the selectivity of the reaction with dienes, the use of various mercuric salts was envisaged, but the choice of commercially available compounds is limited. A simple sonochemical preparation was found (p. 164), and the in situ formation from mercuric oxide and a carboxylic acid in the presence of a diene provided increased selectivities. The case of the hydration of limonene is illustrative (Eq. 8). 

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 both SnI and Sn2 reactions, the leaving group is the halogen of an alkyl halide or the sulfonate group of a sulfonate ester. Both alkyl halides and sulfonate esters are prepared from alcohols. In Chapter 10, alcohols were prepared by the hydration reaction of alkenes, by oxymercuration-demercuration of alk-enes, or by hydroboration of alkenes. Other methods can be used to prepare alcohols, and they will be discussed at a later time. This section will describe several of the reactions used to convert alcohols to halides or sulfonate esters. 

Predict the product for each reaction, and predict the products if an acid-catalyzed hydration had been performed rather than an oxymercuration-demercuration ... 

In the laboratory, alkenes are often hydrated by the oxymercuration-demercuration procedure. Oxymercuration involves electrophilic addition of Hg2+ to the alkene on reaction with mercury(II) acetate [(CH3C02)2Hg, often abbreviated Hg(OAc)2] in aqueous tetrahydrofuran (THF) solvent. When the intermediate organomercury compound is then treated with sodium borohydride, NaBH4, demercuration occurs to produce an alcohol. For example ... 

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. 

Alcohols can he prepared hy hydration of alkenes. Because the direct hydration of alkenes with aqueous acid is generally a poor reaction in the laboratory, two indirect methods are commonly used. Hydroboration-oxidation yields the product of syn, non-Markovnikov hydration (Section 8.5), whereas oxymercuration-demercuration yields the product of Markovnikov hydration (Section 8.4). 

Oxymercuration/demercuration has thus been suggested as a mild procedure for the Markovnikov hydration of carbon-carbon double bonds. As shown in Chapter 3, hydroboration followed by oxidation can be used for their hydration contrary to Markovnikov s rule. The two reaction sequences are compared below for 1-hexene ... 

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. 

The first stage, oxymercuration, involves addition to the carbon-carbon double bond of —OH and —HgOAc. Then, in demercuration, the —HgOAc is replaced by —H. The reaction sequence amounts to hydration of the alkene, but is much more widely applicable than direct hydration. 

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