Mechanism alkene oxymercuration

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). 

The mechanism of the mercurydD-catalyzed alkyne hydration reactioi is analogous to the oxymercuration reaction of alkenes (Section 7.4). Elec trophilic addition of mercury(II) ion to the alkyne gives a vinylic cation which reacts with water and loses a proton to yield a mercury-containii enol intermediate. In contrast to alkene oxymercuration, no treatment widi NaBH4 is necessary to remove the mercury the acidic reaction conditions alone are sufficient to effect replacement of mercury by hydrogen (Figure 8.3). 

FIGURE 8.3 Mechanism ofthe oxymercuration of an alkene to yield an alcohol. The reaction involves a mercurinium ion intermediate and proceeds by a mechanism similarto that of halohydrin formation. The product ofthe reaction is the more highly substituted alcohol, corresponding to Mar-kovnikov regiochemistry. 

Although alkyl groups in general increase the rates of electrophilic addition, we have already mentioned (p. 974) that there is a different pattern depending on whether the intermediate is a bridged ion or an open carbocation. For brominations and other electrophilic additions in which the first step of the mechanism is rate determining, the rates for substituted alkenes correlate well with the ionization potentials of the alkenes, which means that steric effects are not important. Where the second step is rate determining [e.g., oxymercuration (15-3), hydroboration (15-17)], steric effects are important. ... 

The relative reactivity profile of the simple alkenes toward Wacker oxidation is quite shallow and in the order ethene > propene > 1-butene > Zi-2-butene > Z-2-butene.102 This order indicates that steric factors outweigh electronic effects and is consistent with substantial nucleophilic character in the rate-determining step. (Compare with oxymercuration see Part A, Section 5.8.) The addition step is believed to occur by an internal ligand transfer through a four-center mechanism, leading to syn addition. 

Mechanism. The reaction is analogous to the addition of bromine molecules to an alkene. The electrophilic mercury of mercuric acetate adds to the double bond, and forms a cyclic mercurinium ion intermediate rather than a planer carbocation. In the next step, water attacks the most substituted carbon of the mercurinium ion to yield the addition product. The hydroxymercurial compound is reduced in situ using NaBH4 to give alcohol. The removal of Hg(OAc) in the second step is called demer-curation. Therefore, the reaction is also known as oxymercuration-demercuration. 

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. 

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... 

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. 

In analogy with other electrophilic additions, the mechanism of the oxymercuration reaction can be discussed in terms of a cationic intermediate. The cationic intermediate can be bridged mercurinium ion) or open, depending on the structure of the particular alkene. The intermediates can be detected by NMR in nonnucleophilic solvents. ° The addition is completed by attack of a nucleophile at the more positive carbon. 

In the first step of the oxymercuration mechanism, the electrophilic mercury of mercuric acetate adds to the double bond. (Two of mercury s 5d electrons are shown.) Because carbocation rearrangements do not occur, we can conclude that the product of the addition reaction is a cyclic mercurinium ion rather than a carbocation. The reaction is analogous to the addition of Br2 to an alkene to form a cyclic bromonium ion. 

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. 

The fact that the oxymercuration step is both regioselective and anti stereoselective has led chemists to propose the following mechanism, which is closely analogous to that for the addition of Btj and CI2 to an alkene (Section 6.3D). 

Oxymercuration-Reduction (Section 6.3F) Oxymercuration-reduction is used to convert alkenes to alcohols. The mechanism involves reaction of the alkene tt bond with an HgOAc+ to give a bridged mercurinium ion intermediate (a cation) that is, in turn, attacked by HjO from the backside to give a new intermediate, which loses a proton. In a second step, NaBH is added to replace the Hg atom with H.The first step is anti stereoselective because HgOAc and OH add from opposite faces of the alkene. However, the NaBH reduction step scrambles the stereochemistry as H replaces Hg, so the overall process is scrambled... 

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... 

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