Mercurinium ion intermediate

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.

The stereochemistry of oxymercuration has been examined in a number of systems. Conformationally biased cyclic alkenes such as 4-r-butylcyclohexene and 4-f-butyl-l-methycyclohexene give exclusively the product of anti addition, which is consistent with a mercurinium ion intermediate.17,22... 

The most synthetically valuable method for converting alkynes to ketones is by mercuric ion-catalyzed hydration. Terminal alkynes give methyl ketones, in accordance with the Markovnikov rule. Internal alkynes give mixtures of ketones unless some structural feature promotes regioselectivity. Reactions with Hg(OAc)2 in other nucleophilic solvents such as acetic acid or methanol proceed to (3-acetoxy- or (3-methoxyalkenylmercury intermediates,152 which can be reduced or solvolyzed to ketones. The regiochemistry is indicative of a mercurinium ion intermediate that is opened by nucleophilic attack at the more positive carbon, that is, the additions follow the Markovnikov rule. Scheme 4.8 gives some examples of alkyne hydration reactions. 

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. 

Asymmetrically substituted C=C double bonds are hydrated according to the same mechanism (Figure 3.38). The regioselectivity is high, and the explanation for this is that the mercurinium ion intermediate is distorted in the same way as the bromonium ion in Figure 3.34. The H20 preferentially breaks the stretched and therefore weakened Csec—Hg+ bond by a backside attack and does not affect the shorter and therefore more stable Cprim—Hg+ bond. 

These reactions are believed to proceed through a mercurinium ion intermediate (15-XXI) which in turn reacts with the alcohol. 

However, the alkenylmercury acetate (10) undergoes mercury-hydrogen exchange with HCl with inversion. A mercurinium ion intermediate has been proposed, where the vicinal phenyl groups provide the driving force for isomerization (Scheme 22). ... 

Oxymercuration, followed by reductive demercuration of methylenecyclohexanes, yielded tertiary alcohols, following axial solvent attack which takes place trans to a substituent in the 2-position except for acetoxy-derivatives. In the latter case, cis-alcohols are formed preferentially by intramolecular attack of the acetoxy-group or the mercurinium ion intermediate. 

Attack of the alkene tt bond onto AcOHg+ (an electrophile) forms a bridged mercurinium ion intermediate. 

The fact that oxymercuration occurs without rearrangement indicates that the intermediate formed in Step 2 is not a true carbocation, but rather a resonance hybrid largely with the character of a bridged mercurinium ion intermediate. 

Step 3 Make a new bond between a nucleophile and an electrophile. Anti attack of water (a nucleophile) on the bridged mercurinium ion intermediate (an electrophile) occurs at the more substituted carbon to open the three-membered ring. 

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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