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Oxymercuration–reduction of alkenes is one of the major reactions converting alkenes to alcohols. It involves the hydration of alkenes with mercuric acetate in a mixture of tetrahydrofuran and water, forming an organomercury adduct. This is followed by a demercuration step in which the adduct is reduced to an alcohol using sodium borohydride.

Figure1

In the mixture of water and tetrahydrofuran, tetrahydrofuran acts as a solvent dissolving the alkene and the aqueous mercuric acetate solution, while water functions as a reactant and a solvent for mercuric acetate.

Oxymercuration–Demercuration Mechanism

Consider the conversion of 2-methyl-2-butene to yield 2-methyl-2-butanol.

The mechanism proceeds with the dissociation of mercuric acetate, forming an electrophilic mercuric cation and an acetate anion. The alkene π bond attacks the electrophilic mercuric cation, resulting in a bridged-mercurinium-ion intermediate.

Figure2

Regiochemical and Stereochemical Outcome

The bridged-mercurinium-ion intermediate is a resonance hybrid of a carbocation and a bridged mercurinium ion. The partial positive charge is shared between the more substituted carbon atom and the mercury atom, minimizing the chance of a carbocation rearrangement. Furthermore, the carbon–mercury bond to the more substituted carbon is longer and can be easily broken.

Figure3

The factors mentioned above lead to the nucleophilic attack by water exclusively at the more substituted carbon, opening the three-membered ring.

The oxymercuration step is stereospecific, as the attack by water on the bridged mercurinium ion leads to the anti addition of the hydroxyl group. A proton transfer completes the oxymercuration step, forming an organomercury compound.

Lastly, the oxymercuration adduct is treated with sodium borohydride through a process called demercuration to yield an alcohol with Markovnikov's orientation.

During the demercuration step, as the hydrogen can replace the mercury species in either a syn or an anti fashion with respect to the hydroxyl group, the overall reaction produces a racemic mixture of two enantiomeric alcohols.

Tags

Oxymercuration ReductionAlkenesAlcoholsMercuric AcetateTetrahydrofuranWaterOrganomercury AdductDemercurationSodium BorohydrideSolventReactantMechanismElectrophilic Mercuric CationAcetate AnionBridged mercurinium ion IntermediateRegiochemical OutcomeStereochemical OutcomeCarbocation RearrangementNucleophilic Attack

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8.7 : Oxymercuration-Reduction of Alkenes

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8.1 : Regioselektivität elektrophiler Additionen-Peroxid-Effekt

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8.2 : Kettenreaktion und Polymerisation von Alkenen durch freie Radikale

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8.3 : Halogenierung von Alkenen

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8.4 : Bildung von Halohydrin aus Alkenen

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8.5 : Säurekatalysierte Hydratation von Alkenen

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8.6 : Regioselektivität und Stereochemie der säurekatalysierten Hydratation

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8.8 : Hydroborierung-Oxidation von Alkenen

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8.9 : Regioselektivität und Stereochemie der Hydroborierung

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8.10 : Oxidation von Alkenen: Syn-Dihydroxylierung mit Osmiumtetraoxid

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8.12 : Oxidation von Alkenen: Antidihydroxylierung mit Peroxysäuren

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8.13 : Oxidative Spaltung von Alkenen: Ozonolyse

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8.14 : Reduktion von Alkenen: Katalytische Hydrierung

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8.15 : Reduktion von Alkenen: Asymmetrische katalytische Hydrierung

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