Table of contents

1. A Review of General Chemistry5h 5m
2. Molecular Representations1h 14m
3. Acids and Bases2h 46m
4. Alkanes and Cycloalkanes4h 17m
5. Chirality3h 39m
6. Thermodynamics and Kinetics1h 22m
7. Substitution Reactions1h 48m
8. Elimination Reactions2h 30m
9. Alkenes and Alkynes2h 9m
10. Addition Reactions3h 19m
11. Radical Reactions1h 58m
12. Alcohols, Ethers, Epoxides and Thiols2h 42m
13. Alcohols and Carbonyl Compounds2h 17m
14. Synthetic Techniques1h 26m
15. Analytical Techniques:IR, NMR, Mass Spect7h 3m
16. Conjugated Systems6h 13m
17. Ultraviolet Spectroscopy51m
18. Aromaticity2h 34m
19. Reactions of Aromatics: EAS and Beyond5h 1m
20. Phenols55m
21. Aldehydes and Ketones: Nucleophilic Addition4h 56m
22. Carboxylic Acid Derivatives: NAS2h 51m
23. The Chemistry of Thioesters, Phophate Ester and Phosphate Anhydrides1h 10m
24. Enolate Chemistry: Reactions at the Alpha-Carbon1h 53m
25. Condensation Chemistry2h 9m
26. Amines1h 43m
27. Heterocycles2h 0m
28. Carbohydrates5h 53m
29. Amino Acids4h 20m
30. Peptides and Proteins2h 42m
31. Catalysis in Organic Reactions1h 30m
32. Lipids 2h 50m
33. The Organic Chemistry of Metabolic Pathways2h 52m
34. Nucleic Acids1h 32m
35. Transition Metals6h 14m
36. Synthetic Polymers1h 49m

10. Addition Reactions

Hydroboration

Organic Chemistry

10. Addition Reactions

Hydroboration

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4:15 minutes

Problem 16b

Textbook Question

When 1,2-dimethylcyclopentene undergoes hydroboration–oxidation, one diastereomer of the product predominates. Show why this addition is stereospecific, and predict the stereochemistry of the major product.

Verified step by step guidance

Understand the reaction: Hydroboration–oxidation is a two-step reaction. In the first step, borane (BH₃ or a derivative) adds to the alkene in a syn addition, meaning both the boron and hydrogen add to the same face of the double bond. In the second step, oxidation with hydrogen peroxide (H₂O₂) and hydroxide (OH⁻) replaces the boron with a hydroxyl group (-OH). This reaction follows anti-Markovnikov regioselectivity, meaning the -OH group ends up on the less substituted carbon of the double bond.

Analyze the structure of 1,2-dimethylcyclopentene: The double bond in 1,2-dimethylcyclopentene is between two carbons in a cyclopentane ring, with methyl groups attached to the 1 and 2 positions. The steric hindrance caused by the methyl groups will influence the approach of the borane reagent.

Determine the stereospecificity of the hydroboration step: The borane adds to the double bond in a syn fashion, meaning both the boron and hydrogen add to the same face of the cyclopentene ring. The steric hindrance from the methyl groups will favor one face of the double bond for the addition, leading to a specific diastereomer.

Predict the stereochemistry of the oxidation step: During the oxidation step, the boron atom is replaced by a hydroxyl group (-OH) without altering the stereochemistry established in the hydroboration step. This means the -OH group and the hydrogen added in the first step will remain on the same face of the ring.

Conclude the stereochemistry of the major product: The major product will have the hydroxyl group and the hydrogen added to the same face of the cyclopentane ring, resulting in a specific diastereomer. The stereochemistry is determined by the steric hindrance of the methyl groups, which directs the addition to the less hindered face of the double bond.

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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 in a syn addition, resulting in the formation of an organoborane intermediate. The second step involves oxidation with hydrogen peroxide (H2O2) and a base, converting the boron to a hydroxyl group. This process is stereospecific, meaning it leads to a specific stereochemical outcome based on the geometry of the starting alkene.

Stereospecificity

Stereospecificity refers to a reaction where the stereochemistry of the reactant determines the stereochemistry of the product. In the case of hydroboration-oxidation, the syn addition of borane to the alkene ensures that both substituents are added to the same side of the double bond. This results in a specific diastereomer being formed, as the spatial arrangement of the substituents in the product is directly influenced by the configuration of the starting material.

Diastereomers

Diastereomers are stereoisomers that are not mirror images of each other and have different physical properties. In the context of the hydroboration-oxidation of 1,2-dimethylcyclopentene, the reaction can yield multiple diastereomers due to the different spatial arrangements of the substituents. The predominance of one diastereomer over another can be predicted based on steric factors and the mechanism of the reaction, which favors the formation of the more stable product.

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Related Practice

Textbook Question

Disiamylborane adds only once to alkynes by virtue of its two bulky secondary isoamyl groups. Disiamylborane is prepared by the reaction of BH₃·THF with an alkene.

a. Draw the structural formulas of the reagents and the products in the preparation of disiamylborane.

b. Explain why the reaction in part (a) goes only as far as the dialkylborane. Why is Sia₃B not formed?

Textbook Question

In the hydroboration of 1-methylcyclopentene shown in Solved Problem 8-3, the reagents are achiral, and the products are chiral. The product is a racemic mixture of trans-2-methylcyclopentanol, but only one enantiomer is shown. Show how the other enantiomer is formed.

Textbook Question

Show how you would synthesize each compound using methylenecyclohexane as your starting material.

(c)

Textbook Question

When (Z)-3-methylhex-3-ene undergoes hydroboration–oxidation, two isomeric products are formed. Give their structures, and label each asymmetric carbon atom as (R) or (S). What is the relationship between these isomers?

Textbook Question

An inexperienced graduate student treated dec-5-ene with borane in THF, placed the flask in a refrigerator, and left for a party. When he returned from the party, he discovered that the refrigerator was broken and that it had gotten quite warm inside. Although all the THF had evaporated from the flask, he treated the residue with basic hydrogen peroxide. To his surprise, he recovered a fair yield of decan-1-ol. Use a mechanism to show how this reaction might have occurred. (Hint: The addition of BH₃ is reversible.)

Textbook Question

The bulky borane 9-BBN was developed to enhance the selectivity of hydroboration. In this example, 9-BBN adds to the less hindered carbon with 99.3% regioselectivity, compared with only 57% for diborane.

a. Show the two organic products generated when the trialkylborane is oxidized with H₂O₂/NaOH.

Textbook Question

Draw the organic products you would expect to isolate from the following reactions (after hydrolysis).

(m)

Textbook Question

b. 9-BBN is synthesized by adding BH₃ across a symmetric, cyclic diene. What is the structure of the diene?

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