10. Addition Reactions

Which is more highly regioselective: reaction of an alkene with BH₃ or with 9-BBN?

Predict the major products of the following reactions, and give the structures of any intermediates. Include stereochemistry where appropriate.

(c)

Which of the following is the major organic product formed when 1-hexyne undergoes hydroboration–oxidation (using ${\mathrm{BH}}_3$ followed by $H{2}_O/\mathrm{OH}/H{2}_O{2}_{}$)?

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.

Which of the following alcohols can be synthesized selectively by hydroboration-oxidation of an alkene?

Unlike hydroboration–oxidation, the addition of H₂O catalyzed by H₃O⁺ is not stereospecific. Thinking carefully about the mechanism of the reaction, give two reasons why.

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)

Which of the following best describes the first step in the hydroboration mechanism when an alkene reacts with $B_{2}H6$ (diborane) in the presence of tetrahydrofuran (THF)?

Predict the major products of the following reactions.

(c) 2-methylpent-2-ene + BH₃⋅THF

(d) the product from part (c) + H₂O₂/OH⁻

How can the following compounds be synthesized, starting with a hydrocarbon that has the same number of carbons as the desired product?

b. CH₃CH₂CH₂CH₂OH

When (E)-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? What is the relationship between the products formed from (Z)-3-methylhex-3-ene and those formed from (E)-3-methylhex-3-ene?

Show how you would synthesize the following alcohol from appropriate alkene.

(b)

We have studied a number of pericyclic reactions previously. Draw the mechanism of the steps shown. The section number where this material was first studied is given for your review.

(c)

Limonene is one of the compounds that give lemons their tangy odor. Show the structures of the products expected when limonene reacts with an excess of each of these reagents.

a. borane in tetrahydrofuran, followed by basic hydrogen peroxide

Explain why, in hydroboration–oxidation, HO⁻ and HOOH cannot be added until after the hydroboration reaction is over.