Alkene oxymercuration-demercuration
Abstract
Oxymercuration-demercuration is a two-step reaction that converts an alkene into a Markovnikov alcohol via an electrophilic addition mechanism, using mercuric acetate Hg(OAc)2 followed by NaBH4 reduction. It is the method of choice for Markovnikov hydration of alkenes when carbocation rearrangements must be avoided.
Keywords
Alkene, Alcohol, Markovnikov
Category
Alkene
Mechanism
First, Hg(OAc)2 dissociates partially in solution, generating an electrophilic monoacetate HgOAc+ species. The alkene attacks the electrophilic mercury center, forming a cyclic mercurinium ion (analogous to the bromonium ion in Alkene halogenation). Water acts as nucleophile and attacks the more substituted carbon of the mercurinium ion, i.e. the carbon that stabilizes the most the positive charge of the mercurinium ion, in a SN2-like fashion from the back side, leading to an organomercury intermediate. Finally, NaBH4 is added to the reaction medium and reduces the C-Hg bond, replacing mercury with hydrogen.
It is important to understand that in acid-catalyzed hydration, a free carbocation is formed, which can undergo hydride or alkyl shifts. However, in oxymercuration, no free carbocation exists, the positive charge is constrained within the bridged mercurinium ion. Therefore, water attacks before any rearrangement can take place.
