Mercury-Mediated Nucleophilic Addition to Alkenes

You may remember that the acid-catalyzed addition of H2O to alkenes is usually a difficult reaction to carry out because of the side reactions that can occur under the strongly acidic conditions required for the reaction. However, in the presence of Hg(II) salts such as HgCl2, H2O and other nucleophiles add readily 

The alcohol is nucleophilic, so the alkene must be made electrophilic. The HgCla first coordinates to the alkene to form a tt complex. The coordination of the Hg(II) makes the alkene electrophilic. (In principle one could draw a metal-lacyclopropane resonance structure for the tt complex, but in this resonance structure Hg would be in the (TV) oxidation state, and Hg(IV) is a high-energy oxidation state.) The coordination weakens the C=C tt bond and makes the C s electrophilic. The nucleophile then attacks one of the C s of the tt complex, and the electrons from the C=C tt bond move to form a cr bond between Hg(II) and the other C to give a stable, isolable organomercury compound. 

An alternative mechanism might begin with formation of an O—Hg bond. Insertion of the C=C TT bond could then follow. 

Mercury-mediated reactions of alkenes have largely been superseded by other, more efficient, less toxic procedures. 

Palladium salts also promote the addition of nucleophiles to alkenes, but because alkylpalladium complexes are unstable toward )8-hydride elimination and because Pd salts are so expensive, the Pd is always used catalytically. The Pd-catalyzed additions of nucleophiles to alkenes are discussed later in this chapter. 

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