Textbook Question
Suggest an alkene to undergo hydroboration–oxidation (1. BH3 2. NaOH, H2O2) to give exclusively the alcohols shown. Pay close attention to the relative (but not absolute) stereochemical outcome.
(b)
10. Addition Reactions
Hydroboration
5:40 minutesProblem 71c
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).
(c) A similar sequence was featured in the synthesis of muricadienin, a proposed precursor in the biosynthesis of solamin (Org. Lett. 2014, 16, 5886–5889).
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 BH₃ (borane) followed by NaOH and H₂O₂ (hydrogen peroxide).
Step 2: Identify the alkene in the molecule. The structure contains a terminal alkyne and a conjugated diene. Focus on the terminal alkyne, as hydroboration–oxidation typically targets the triple bond in this case.
Step 3: Predict the regioselectivity. Hydroboration–oxidation adds water across the triple bond in an anti-Markovnikov fashion, meaning the hydroxyl group (-OH) will attach to the less substituted carbon of the alkyne.
Step 4: Consider the stereochemistry. Hydroboration–oxidation of alkynes typically results in the formation of an enol intermediate, which tautomerizes to a ketone. The final product will be a ketone at the terminal carbon.
Step 5: Draw the product. The terminal alkyne will be converted into a ketone, while the rest of the molecule remains unchanged. Ensure that no other functional groups are affected by the reaction conditions.
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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 that converts alkenes into alcohols. In the first step, borane (BH3) adds across the double bond of the alkene, forming an organoborane intermediate. This is followed by oxidation with hydrogen peroxide (H2O2) in the presence of a base, which replaces the boron atom with a hydroxyl group, yielding an alcohol. This reaction is notable for its anti-Markovnikov selectivity.
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Oxymercuration-Reduction
Oxymercuration-reduction is another method for converting alkenes to alcohols. It involves the addition of mercuric acetate (Hg(OAc)2) to the alkene, forming a mercurinium ion intermediate. Water then attacks this intermediate, leading to the formation of an alcohol after reduction with sodium borohydride (NaBH4). This reaction also follows Markovnikov's rule, providing a way to selectively add functional groups to alkenes.
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Stereochemistry of Reactions
Understanding the stereochemistry of hydroboration-oxidation and oxymercuration-reduction is crucial for predicting the products of these reactions. Hydroboration leads to syn-addition, where both substituents add to the same side of the double bond, while oxymercuration typically results in anti-addition. This difference in stereochemistry can affect the final product's configuration, which is important in synthetic organic chemistry, especially when synthesizing complex molecules.
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