Oxymercuration of Alkenes to form Ethers using Hg(OAc)2
Description: When an alkene is treated with an alcohol in the presence of mercuric acetate [Hg(OAc)2] it will add to the more substituted position of the alkene (Markovnikoff) to give an ether. The mercury can then be removed using NaBH4.
Notes: The alcohol is generally used as the solvent here.
Notes: Note that in the second reaction, no rearrangement occurs (this would have happened for the acid-catalyzed reaction).
The last example is an intramolecular reaction – often gives students a hard time.
The byproducts in all cases are NaOAc, HOAc, BH3 and Hg(s)
Mechanism: The alkene attacks Hg(OAc)2, displacing acetate and forming a mercuronium ion (Step 1, arrows A and B). Next, the more substituted carbon is attacked by the solvent alcohol (Step 2, arrows C and D). The resulting protonated ether is then deprotonated by acetate (Step 3, arrows E and F). Removal of mercury is done with NaBH4. The mechanism of this step is not generally considered to be “important” for Org 1/ Org 2. However it is shown separately below.
Mechanism for removal of mercury:
Notes: Attack of alcohol (Step 2) occurs trans to the mercury, although that is not depicted here. Removal of mercury proceeds through formation of a free radical, so the reaction is not stereoselective.