Hydration of Alkenes
Oxymercuration of Alkenes
The addition of a hydroxyl group to an alkene can be accomplished in several ways:
- Acid-catalyzed hydration producing an alcohol with Markovnikov orientation
- Hydroboration and subsequent oxidation yielding an alcohol with anti-Markovnikov orientation
Non-catalytic, direct hydration of alkenes requires very drastic conditions, lacks regioselectivity and can give rise to side reactions. In order to achieve Markovnikov addition on a laboratory scale, an organometallic reaction,oxymercuration, is used.
This reaction proceeds in two steps with mercuric acetate (Hg(OAc)2) as the reagent. Mercuric acetate is at equilibrium with the mercuric acetate cation (HgOAc+) which reacts with the π bond of the alkene. Subsequently, the resulting three-membered ring of the mercurinium ion is opened by nucleophilic attack of water. These are the two steps of oxymercuration (addition of oxygen and mercury). Oxymercuration is followed by a reductive demercuration in which the mercury acetoxy group is substituted by hydrogen.
In the case of an unsymmetrically substituted double bond, the three-membered mercurinium ion resulting from an electrophilic reaction of the merucric acatate cation with the π bond shows unequal bond lengths between the two C-Hg bonds with the shorter bond connecting the metal with the lower-substituted carbon. The cause of this ring distortion is due to some extent to a reduced ring strain but predominantly to the higher stabilization of the partial positive charge at the higher-substituted carbon atom through hyperconjugation. The existence of a cyclic intermediate is undeniable because no rearrangement products are found in the subsequent ring opening by water. In addition, the reaction stereoselectively produces the trans-configurated mercury-substituted alcohol, which is formed by the back-side attack of water on the cyclic intermediate. The nucleophilic back-side attack preferably occurs at the partially positively charged carbon atom forming almost exclusively the Markovnikov product. If an intermediate carbenium ion would have been formed, rearrangement products certainly would have been observed thus confirming the described mechanism.
Oxymercuration is followed by in situ treatment with sodium borohydride (NaBH4) resulting in the reductive demercuration of the mercurated alcohol. The mechanism of this reaction is not yet completely established but the mercury acetoxy group is formally reduced to elemental mercury and substituted by hydrogen.
Advantages of oxymercuration:
- Almost exclusive Markovnikov addition
- Alkenes can be hydrated in high yields and under milder conditions than in an acid-catalyzed hydrations
- Rearrangement products are rarely found