Textbook Question
Would you expect the following to produce an equal or unequal mixture of stereoisomers?
(c)
10. Addition Reactions
Hydroboration
4:43 minutesProblem 71b
Textbook Question
Predict the product of each of the following hydroboration–oxidation or oxymercuration–reduction reactions used in the modern synthetic organic chemistry literature (modified to use reagents we are used to seeing).
(b) A two-step hydroboration–oxidation was used to prepare a silanediol peptidomimetic as a serine protease inhibitor (Org. Lett. 2012, 14, 4422–4425).
Verified step by step guidance1
Step 1: Recognize the reaction type. This is a hydroboration–oxidation reaction, which involves the addition of water across a double bond in an anti-Markovnikov fashion. The reagents used are borane (BH₃) followed by hydrogen peroxide (H₂O₂) in a basic solution (NaOH).
Step 2: Identify the alkene in the molecule. The double bond in the cyclic structure is the site of the reaction. The silicon atom and phenyl groups (Ph) are not directly involved in the reaction but are part of the molecule's framework.
Step 3: Predict the regioselectivity. Hydroboration–oxidation adds the hydroxyl group (-OH) to the less substituted carbon of the double bond, following anti-Markovnikov selectivity. The hydrogen (H) is added to the more substituted carbon.
Step 4: Consider stereochemistry. Hydroboration–oxidation proceeds with syn addition, meaning the hydrogen and hydroxyl group are added to the same face of the double bond. This stereochemical outcome should be reflected in the product.
Step 5: Draw the product. Replace the double bond with a single bond, and add the hydroxyl group (-OH) to the less substituted carbon and a hydrogen atom to the more substituted carbon. Ensure the stereochemistry is consistent with syn addition.
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Key Concepts
Here are the essential concepts you must grasp in order to answer the question correctly.
Hydroboration-Oxidation
Hydroboration-oxidation is a two-step reaction process used to convert alkenes into alcohols. In the first step, an alkene reacts with borane (BH3) to form an organoborane intermediate, which is then oxidized in the second step using hydrogen peroxide (H2O2) and a base, typically sodium hydroxide (NaOH). This reaction is notable for its anti-Markovnikov selectivity, meaning that the hydroxyl group is added to the less substituted carbon of the alkene.
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Oxymercuration-Reduction
Oxymercuration-reduction is another method for converting alkenes to alcohols, involving the addition of mercuric acetate (Hg(OAc)2) to the alkene, followed by reduction with sodium borohydride (NaBH4). This reaction proceeds through a mercurinium ion intermediate, allowing for Markovnikov addition of the hydroxyl group. It is particularly useful for synthesizing alcohols with high regioselectivity and minimal rearrangement.
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Regioselectivity
Regioselectivity refers to the preference of a chemical reaction to yield one structural isomer over others when multiple possibilities exist. In the context of hydroboration-oxidation and oxymercuration-reduction, understanding regioselectivity is crucial for predicting the major product formed from an alkene. The reactions favor specific pathways that lead to the formation of products with distinct functional groups positioned at specific carbon atoms, which is essential for designing synthetic routes in organic chemistry.
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