8.7: Oxymercuration-Reduction of Alkenes
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.
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.
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.
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.
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.