Oxymercuration-Reduction of Alkenes

Oxymercuration-reduction converts alkenes to alcohols through two sequential steps. First, the oxymercuration step reacts the alkene with mercuric acetate in tetrahydrofuran and water, forming an organo-mercury adduct. Second, the demercuration step reduces this adduct using sodium borohydride to yield the final alcohol product.

The bridged-mercurinium ion is a resonance hybrid bearing a partial positive charge distributed between the more substituted carbon and mercury, rather than a full charge on one carbon. This partial charge distribution minimizes carbocation rearrangement, unlike acid-catalyzed hydration where a regular carbocation intermediate can undergo rearrangements.

The carbon-mercury bond to the more substituted carbon is longer than to the less substituted carbon, making it easier to break. This structural feature, combined with the partial positive charge distribution in the mercurinium ion, favors nucleophilic attack by water exclusively at the more substituted position, ensuring regioselective alcohol formation.

The oxymercuration step is stereospecific, resulting in anti addition of the hydroxyl group across the bridged-mercurinium intermediate. Water attacks the three-membered ring from the opposite face of the mercury, opening the ring and establishing the stereochemistry of the organo-mercury adduct.

During demercuration, sodium borohydride reduces the organo-mercury adduct, but the hydrogen replacing mercury can add in either syn or anti fashion relative to the hydroxyl group. This loss of stereospecificity produces a racemic mixture of syn- and anti-addition products, even though the initial oxymercuration step was stereospecific.

Tetrahydrofuran acts as a solvent, dissolving the alkene and the aqueous mercuric acetate solution to create a homogeneous reaction medium. Water functions as both a reactant, providing the nucleophilic hydroxyl group, and a solvent for mercuric acetate, enabling the reaction to proceed efficiently.

Oxymercuration-reduction avoids carbocation rearrangement because the bridged-mercurinium ion intermediate distributes charge across mercury and carbon, preventing the full positive charge concentration that triggers rearrangement. In contrast, acid-catalyzed hydration forms a regular carbocation intermediate prone to rearrangement, making oxymercuration-reduction more reliable for certain substrates.