In light of your answer to Assessment 8.73, predict the product of the following oxymercuration–reduction reactions, each of which results in a single product. [Don't worry about the absolute stereochemistry, though these reactions are also stereoselective.] (b) This reaction sequence was used in the synthesis of (+) -lineatin, a monoterpene aggregation pheromone from the female ambrosia beetle Trypodendron lineatum (Org. Lett. 2004, 6, 1449–1452).
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Identify the reaction type: The given reaction is an oxymercuration-reduction reaction, which is used to convert alkenes to alcohols without rearrangement.
Analyze the starting material: The structure contains a cyclobutene ring with a hydroxyl group and an ester group. The double bond is the site of reaction.
Understand the mechanism: In the first step, oxymercuration involves the addition of mercuric acetate (Hg(OAc)₂) and water across the double bond, forming a mercurinium ion intermediate.
Predict the regioselectivity: The mercurinium ion is attacked by water at the more substituted carbon, leading to Markovnikov addition of the hydroxyl group.
Complete the reaction: In the second step, sodium borohydride (NaBH₄) reduces the mercurinium ion, replacing the mercury with a hydrogen atom, resulting in the formation of the alcohol product.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Oxymercuration-Demercuration
Oxymercuration-demercuration is a two-step reaction used to convert alkenes into alcohols. The first step involves the addition of mercuric acetate (Hg(OAc)2) and water, forming a mercurinium ion intermediate. The second step uses sodium borohydride (NaBH4) to reduce the intermediate, replacing the mercury with a hydrogen atom, resulting in an alcohol with Markovnikov orientation.
Markovnikov's rule predicts the regiochemistry of electrophilic addition reactions, stating that the hydrogen atom will add to the less substituted carbon of the double bond, while the more substituted carbon receives the other group. In oxymercuration-demercuration, this rule ensures the alcohol forms at the more substituted carbon, providing regioselectivity in the product formation.
Stereoselectivity refers to the preference for the formation of one stereoisomer over another in a chemical reaction. Although oxymercuration-demercuration is stereoselective, the question specifies not to worry about absolute stereochemistry. This implies that the reaction will favor a specific stereochemical outcome, but the focus is on the regiochemistry of the product.