Oxymercuration of an Alkene

Oxymercuration of an Alkene Step I Electrophilic attack forms a mercurinium ion. 

Step 2 Water opens the ring to give an oiganomercurial alcohol. 

Demercuration replaces the mercuric fragment with hydrogen to give the alcohol. 

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Figure 7.3 Mechanism of the oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similar to that of halohydrin formation. The product of the reaction is the more highly substituted alcohol, corresponding to Markovnikov regiochemistry.

Mechanism 8-4 Acid-Catalyzed Hydration of an Alkene 338 8-5 Hydration by Oxymercuration-Demercuration 340 Mechanism 8-5 Oxymercuration of an Alkene 340 8-6 Alkoxymercuration-Demercuration 342 8-7 Hydroboration of Alkenes 343... 

This halonium ion mechanism can be used to explain and predict a wide variety of reactions in both nucleophilic and non-nucleophilic solvents. The halonium ion mechanism is similar to the mercurinium ion mechanism for oxymercuration of an alkene, and both give Markovnikov orientation (Section 8-5). 

CHAPTER 8 Ionic Addition of HX to an Alkene 332 Free-Radical Addition of HBr to Alkenes 334 Acid-Catalyzed Hydration of an Alkene 338 Oxymercuration of an Alkene 340 Hydroboration of an Alkene 345 Addition of Halogens to Alkenes 350 Formation of Halohydrins 352 Epoxidation of Alkenes 360 Acid-Catalyzed Opening of Epoxides 362 Olefin Metathesis 376... 

When the oxymercuration of an alkene is executed in an alcohol solvent, demercuration gives an ether, as shown in the margin. 

The electrophile in oxymercuration reactions, HgX or Hg " , is a soft acid and strongly polarizing. It polarizes the n electrons of an alkene to the extent that a three-center, two-... 

Hydration of an alkene—the addition of water—is carried out by either of two procedures, depending on the product desired. Oxymercuration involves electrophilic addition of Hg2+ to an alkene, followed by trapping of the cation intermediate with water and subsequent treatment with NaBH4. Hydroboration involves addition of borane (BH3) followed by oxidation of the intermediate organoborane with alkaline H202- The two hydration methods are complementary oxymercuration gives the product of Markovnikov addition, whereas hydroboration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Oxidation of an Alcohol- Section 10.14 Figure 10.8 Oxymercuration-Reduction Section 11.6 Figure 11.5 Ozonolysis of an Alkene Section 11.11... 

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. 

Cis additions occur exclusively to trans-cyclooctene and trans-cyclononane . For these two alkenes, trans addition is hindered. The tendency of an alkene to oxymercurate via a trans mechanism can be related directly to its ability to form the normal anti transition state . If for either steric or twist-strain reasons this transition state is made energetically unfavorable, cis-addition prevails. 

Where do mercuration reactions fit into this picture A mercurinium ion has both similarities and differences, as compared with the intermediates that have been described for other electrophilic additions. The electrophile in oxymercuration reactions, +HgX or Hg +, is a soft Lewis acid and polarizes the TT-electrons of an alkene to the extent that a three-center two-electron bond is formed between mercury and the two carbons of the double bond. However, there is also back bonding from Hg +(i orbitals to the alkene tt orbital. There is weaker bridging in the mercurinium ion than in the three-center four-electron bonding of the bromonium ion. 

The structure of the intermediate in oxymercuration has received much attention and can be approached by considering what is likely to happen when the electrophile HgOAc reacts with the double bond of an alkene. 

Alcohols can be prepared from a variety of other functional groups, including by reduction of a carbonyl, hydration of an alkene, or substitution of a leaving group. Remember, regiochemistry is a concern when starting with an alkene, and water can be added with either Markovnikov (oxymercuration-demercuration) or anti-Markovnikov (hydroboration-oxidation) orientation. 

Hydration of an alkene—the addition of water—is carried out in the laboratory by either of two complementary procedures, depending on the product desired. Oxymercuration gives the product of Markovnikov addition, whereas hydrohoration/oxidation gives the product with non-Markovnikov syn stereochemistry. 

Oxymercuration (Section 8.4) A method for double-bond hydration by reaction of an alkene with aqueous mercuric acetate followed by treatment with NaBH4. 

The compound YXC=CHCH2C(CH2HgCl)CH2CH2CH2CH=>C(S02Tol) undergoes an intramolecular Michael reaction in the presence of R4NX and a Lewis acid. The stereochemical control in the intramolecular oxymercurations of 5-alken-l-ols, derived from tri-O-acetyl-D-glucal, have been studied. ... 

The last example in Table 12-2 is an electrophilic addition of a mercuric salt to an alkene. The reaction is called mercuration, and the resulting compound is an alkyl-mercury derivative, from which the mercury can be removed in a subsequent step. One particularly useful reaction sequence is oxymercuration-demercuration, in which mercuric acetate acts as the reagent. In the first step (oxymercuration), treatment of an alkene with this species in the presence of water leads to the corresponding addition product. 

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. 

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... 

In Chapter 15, we saw that we can convert an alkene to an alcohol by oxymercuration-demercuration. If we perform oxymercuration-demercuration of an alkene in an alcohol as the solvent, the product is an ether. In this reaction, the alcohol, rather than water, acts as the nucleophile. This process, called alkoxymercuration, occurs by a mechanism analogous to oxymercuration. First, electrophihc addition of Hg(OAc)2 to the carbon-carbon double bond forms a mercurinium ion intermediate, which is subsequently attacked by the nucleophilic oxygen atom of the alcohoL... 

Oxymercuration-demercuration achieves hydration of an alkene without... 

In the laboratory, alkenes are often hydrated by the oxymercuration procedure. When an alkene is treated with mercury(II) acetate Hg(02CCH3)2, usually abbreviated Hg(OAc)2l in aqueous tetrahydrofuran (THF) solvent, electrophilic addition of Hg2+ to the double bond rapidly occurs. The intermediate orgnnomercury compound is then treated with sodium borohydride, NaBH4, and an alcohol is produced. For example ... 

In addition to the oxymercuration 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 cailed 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 ... 

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