Hydration of Alkynes
Hydratation of Alkynes - Hg(II) Catalysis
Just as alkenes, alkynes can be hydrated by two different methods. The direct addition of water catalyzed by mercury(II) salts yields the Markovnikov product. In contrast, the indirect hydration by the reaction sequence of hydroboration, oxidation and hydrolysis results in the anti-Markovnikov product.
Water can be added to triple bonds with the help of mercury(II) catalysts, too.
The first mechanistic steps of the reaction are the same as in the oxymercuration of alkenes. But the hydroxy mercury compound intermediately formed is an alkenyl mercury and not an alkyl mercury compound. Therefore, no reductive demercuration is necessary because the mercury is expelled by protonation of the double bond and subsequent elimination of a mercuric acetate ion (HgOAc+). This demercuration yields an enol which is subsequently transformed into a ketone by tautomerism, a sequence of protonation, intramolecular electron shift and deprotonation.