Oxymercuration-demercuration mechanism

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. 

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... 

Another reaction that is relevant to the mechanism of the formation of the alkaloid erysodienone in vitro is the K3[Fe(CN)6]-induced para-para oxidative coupling of the phenol (13) to the azoninone derivative (14). ° Oxymercuration-demercuration [by Hg(OAc)2 and then NaBH4] of the cyclic lactam (15) resulted in the regio- and stereo-specific transannular formation of (16). The conformation of (15) is represented by (17), in which there is no steric crowding by the methyl group. The ring is believed to be responsible for the course of the reaction. 

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 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... 

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. 

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