8.3b Oxymercuration Demercuration
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|Two Groups Added|| || |
|Reagents Added|| 1. Hg(OAc)2, H2O|
|Other Characteristics||Intermediate is a 3-membered ring (mercurinium ion).|
Oxymercuration-demercuration (a.k.a. oxymercuration-reduction) results in the Markovnikov addition of a hydrogen (less substituted side) and a hydroxyl group (more substituted side) across an alkene forming an alcohol. The reaction exhibits anti stereospecificity and is not subject to rearrangements as the intermediate is not a carbocation but a mercurinium ion instead. The reaction occurs in two steps with the first involving the addition of mercuric acetate and water (1. Hg(OAc)2, H2O). The resulting intermediate is then reduced with sodium borohydride (2. NaBH4).
Undergraduate students are typically only responsible for the mechanism of oxymercuration involving the first set of reagents, mercuric acetate and water (1. Hg(OAc)2, H2O) as the mechanism for the demercuration (reduction) step is not precisely known. Mechanisms have been proposed for this reduction but there is no accepted consensus at this time.
Oxymercuration - 1. Hg(OAc)2, H2O
Step 1: Mercuric acetate ionizes to some extent and the pi electrons of the alkene attack a mercury cation which attacks one the of the alkene carbons back forming a three-membered ring with Hg referred to as a mercurinium ion.
While the mercurinium ion is more stable than a carbocation it is still a rather high energy intermediate, and this first step is the rate determining step (slow step) of the reaction. Also, the mercurinium ion intermediate does not undergo rearrangements such as are possible with carbocations.
Step 2: Ring-Opening of the Mercurinium Ion
Water carries out back-side attack (as in SN2) on the more substituted carbon of the mercurinium ion resulting in the bond to the Hg in the mercurinium ion breaking (analogous to loss of a leaving group in SN2 even though it is still attached to the adjacent carbon atom) and the opening of the 3-membered ring.
The back-side attack results in inversion of configuration at the carbon attacked helping to explain the nature of the anti stereospecificity.
Step 3: Proton Transfer
Acetate or water deprotonates the oxonium ion intermediate producing an alcohol.
Dermercuration (Reduction) - 2. NaBH4