Oxymercuration & Borane

1. Oxymercuration/Demercuration of Alkenes

2. Borane

1) Oxymercuration/Demercuration of Alkenes: Markovnikov Hydration

Treating alkenes with mercuric acetate in water or an alcohol solvent is a mild method of obtaining Markovnikov hydration or alkoxylation products.

No rearrangements or polymerization is observed in this process, since there is no carbocation intermediate.

The reaction occurs through the intermediacy of a mercurinium ion, formed from the reaction of the alkene with +Hg(OAc).

Because of the presence of a partial positive charge on the more substituted carbon of the mercurinium ion intermediate, the nucleophile (water or an alcohol) attacks the more substituted carbon to give the anti-hydroxy-organomercurial intermediate.

The C-Hg bond is reduced with NaBH4 in NaOH to give the alcohol/ether product, in which Markovnikov regiochemistry is observed.

Note : The anti relationship between the OH (OR) and Hg(OAc) in the intermediate.

Yields in the oxymercuration & demercuration process are high, the reaction occurs under mild conditions, and no rearrangements or polymerizations are observed.

Oxomercuration :

Convert alkene to organomercurial alcohol in aqueous solvent.

Demercuration :

Transforms organomercurial alcohol to corresponding alcohol.

Solvomercuration :

Transfer organomercurial ether to product ether as per the solvent used.

Example :

Alkenes react with alcohols in the presence of an acid catalyst to form ethers.

Just as the addition of water works better in the presence of mercuric acetate than in the presence of a strong acid, the addition of an alcohol works better in the presence of mercuric acetate. [Mercuric trifluoroacetate, works even better.]

This reaction is called Alkoxymercuration–Reduction.