Chapter 14: Organometallic Compounds

Oxymercuration-Demercuration of Alkenes

Summary

• Overall transformation C=C to H-C-C-OH
• This is an alternative method for hydrating alkenes to give alcohols
• Typical reagents are mercury acetate, Hg(OAc)₂ in aqueous THF
• Unfortunately, mercury compounds are generally quite toxic
• Regioselectivity predicted by Markovnikov's rule (most highly substituted alcohol)
• The reaction is not stereoselective
• Reaction proceeds via the formation of a cyclic mercurinium ion (compare with bromination of alkenes)

• The mercurinium ion is opened by the attack of water to complete the oxymercuration.
• When the water attacks, it does so at the more highly substituted carbon.
• Demercuration is effected by a reduction using sodium borohydride, NaBH₄
• If the reaction is carried out in the presence of an alcohol rather than water, then ethers are obtained via an alkoxymercuration :

• The only difference here is a change in the nucleophile from H₂O to ROH

• What other methods are there for hydrating alkenes ? 
• Which species in this reaction is/are organometallic species ? 
• Why aren't rearrangements a problem with this process ? 
• Why does the water attack the more highly substituted carbon ? 

• Addition reactions of alkenes (Ch 6)
• Hydration of Alkynes (Ch 9)

MECHANISM FOR REACTION OF ALKENES WITH Hg(OAc)2 / H2O
Step 1: The C=C p electrons act as the nucleophile with the electrophilic Hg and loss of an acetate ion as a leaving group, forming the mercurinium ion.
Step 2: Water functions as a nucleophile and attacks one of the carbons substituted with mercury resulting in cleavage of the C-Hg bond.
Step 3: The acetate ion functions as a base deprotonating the oxonium ion to give the alcohol. This completes the oxymercuration part of the reaction.
Step 4:(mechanism not shown) The hydride reduces the Hg off, creating a C-H bond while breaking the C-Hg bond. This is the demercuration part of the process.