Chapter 12, Problem 12.53P

Interpretation Introduction

(a)

Interpretation:

The appropriate alkene or alkyne from which the given compound can be produced is to be determined along with the necessary reagents and special reaction conditions.

Concept introduction:

Oxidation of alkenes and alkynes is carried out by the hydroboration-oxidation reaction. Alkenes are oxidized to alcohol while alkynes are oxidized to the corresponding carbonyl compounds. Terminal alkynes are oxidized to the corresponding aldehyde, and internal alkynes are oxidized to the corresponding ketone by a sterically hindered borane like disiamylborane ${{\text{(C}}_{\text{5}}{\text{H}}_{\text{11}}\text{)}}_{\text{2}}\text{BH}$. Disiamylborane is added only once to an alkyne because the bulky alkyl groups in it prevents the addition of a second molecule. An enol is formed initially on oxidation by basic ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, which tautomerizes to the more stable keto form.

The initial addition of borane is a syn, anti-Markovnikov addition via a four-membered cyclic transition state. Boron adds to the less substituted (less hindered) carbon because it is less electronegative than hydrogen. This results in an anti-Markovnikov addition of a water molecule to the double bond.

Answer to Problem 12.53P

The appropriate alkene used to synthesize the given compound is

The necessary reagents and special reaction conditions for the synthesis are $\text{1}{\text{. BH}}_{\text{3}}\text{, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Explanation of Solution

The structure of the given compound is

Alcohol with a specific stereochemistry is to be synthesized from an alkene. A reaction involving a carbocation needs to be avoided to prevent unwanted carbocation rearrangements. Also, the OH group must be added to a less substituted carbon, i.e., an anti-Markovnikov addition is needed. Therefore, the reaction needs to be carried out using hydroboration-oxidation.

The appropriate alkene for the synthesis of the given compound would be

The given compound is synthesized by using the above alkene via hydroboration-oxidation reaction. So the necessary reagents for the reaction are $\text{1}{\text{. BH}}_{\text{3}}\text{, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

In the first step, an electrophilic addition of borane across the double bond of the alkene takes place either from above or below the plane of the alkene. So a mixture of enantiomers is obtained after oxidation of the adduct by basic ${\text{H}}_{\text{2}}\text{O}$.

Conclusion

The alkene necessary for the synthesis of the given compound and the reagents and special conditions for the reaction are determined on the basis of the structure of the given compound.

Interpretation Introduction

(b)

Interpretation:

The appropriate alkene or alkyne from which the given compound can be produced is to be determined along with the necessary reagents and special reaction conditions.

Concept introduction:

Oxidation of alkenes and alkynes is carried out by the hydroboration-oxidation reaction. Alkenes are oxidized to alcohol while alkynes are oxidized to the corresponding carbonyl compounds. Terminal alkynes are oxidized to the corresponding aldehyde, and internal alkynes are oxidized to the corresponding ketone by a sterically hindered borane like disiamylborane ${{\text{(C}}_{\text{5}}{\text{H}}_{\text{11}}\text{)}}_{\text{2}}\text{BH}$. Disiamylborane is added only once to an alkyne because the bulky alkyl groups in it prevents the addition of a second molecule. An enol is formed initially on oxidation by basic ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, which tautomerizes to the more stable keto form.

The initial addition of borane is a syn, anti-Markovnikov addition via a four-membered cyclic transition state. Boron adds to the less substituted (less hindered) carbon because it is less electronegative than hydrogen. This results in an anti-Markovnikov addition of a water molecule to the double bond.

Answer to Problem 12.53P

The appropriate alkyne used to synthesize the given compound is

The necessary reagents and special reaction conditions to synthesize the given compound are $\text{1}\text{. disiamylborane, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Explanation of Solution

The given compound is

It is a ketone, so the starting compound must be an alkyne. A hydroboration-oxidation reaction can convert an alkyne into a ketone. Since only one molecule of borane is to be added, a bulky reagent like disiamylborane is more appropriate than borane. Also, the dialkylborane part must add to the less hindered carbon of the triple bond. Therefore, the triple bond must be between the carbon bonded to oxygen and the carbon close to the bulky tertiary carbon. Therefore, the alkyne that can be used is

A sterically hindered dialkylborane, like disiamylborane ${\left({\text{C}}_{\text{5}}{\text{H}}_{\text{11}}\right)}_{\text{2}}\text{BH}$, needs to be used instead of simple borane to prevent a second hydroboration. The bulky ${{\text{(C}}_{\text{5}}{\text{H}}_{\text{11}}\text{)}}_{\text{2}}\text{B}$ group will add to the less hindered carbon attached to the secondary carbon while hydrogen will add to the more hindered carbon attached to the tertiary carbon. Subsequent oxidation with ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\text{, NaOH}$ will form an enol. The enol will then tautomerize to the more stable ketone, the intended product.

Thus the specific reagents and reaction conditions are $\text{1}\text{. disiamylborane, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Conclusion

The alkene necessary for the synthesis of the given compound and the reagents and special conditions for the reaction are determined on the basis of the structure of the given compound.

Interpretation Introduction

(c)

Interpretation:

The appropriate alkene or alkyne from which the given compound can be produced is to be determined along with the necessary reagents and special reaction conditions.

Concept introduction:

Oxidation of alkenes and alkynes is carried out by the hydroboration-oxidation reaction. Alkenes are oxidized to alcohol while alkynes are oxidized to the corresponding carbonyl compounds. Terminal alkynes are oxidized to the corresponding aldehyde, and internal alkynes are oxidized to the corresponding ketone by a sterically hindered borane like disiamylborane ${{\text{(C}}_{\text{5}}{\text{H}}_{\text{11}}\text{)}}_{\text{2}}\text{BH}$. Disiamylborane is added only once to an alkyne because the bulky alkyl groups in it prevents the addition of a second molecule. An enol is formed initially on oxidation by basic ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, which tautomerizes to the more stable keto form.

The initial addition of borane is a syn, anti-Markovnikov addition via a four-membered cyclic transition state. Boron adds to the less substituted (less hindered) carbon because it is less electronegative than hydrogen. This results in an anti-Markovnikov addition of a water molecule to the double bond.

Answer to Problem 12.53P

The appropriate alkyne used to synthesize the given compound is

The necessary reagents and special reaction conditions to synthesize the given compound are $\text{1}\text{. disiamylborane, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Explanation of Solution

The given compound is

It is an aldehyde, so it can be prepared from a terminal alkyne by hydroboration-oxidation. In the hydroboration reaction, boron is added to the terminal carbon. So the appropriate alkyne for the synthesis of the given compound is

The alkyne is treated with the bulky disiamylborane to prevent the addition of a second molecule and formation of a mixture of products. Subsequent treatment of the adduct by ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\text{, NaOH}$ oxidizes it to a terminal enol, which tautomerizes to a more stable aldehyde.

Thus, the necessary reagents and special reaction conditions for the synthesis are $\text{1}\text{. disiamylborane, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Conclusion

The alkene necessary for the synthesis of the given compound and the reagents and special conditions for the reaction are determined on the basis of the structure of the given compound.

Interpretation Introduction

(d)

Interpretation:

The appropriate alkene or alkyne from which the given compound can be produced is to be determined along with the necessary reagents and special reaction conditions.

Concept introduction:

Oxidation of alkenes and alkynes is carried out by the hydroboration-oxidation reaction. Alkenes are oxidized to alcohol while alkynes are oxidized to the corresponding carbonyl compounds. Terminal alkynes are oxidized to the corresponding aldehyde, and internal alkynes are oxidized to the corresponding ketone by a sterically hindered borane like disiamylborane ${{\text{(C}}_{\text{5}}{\text{H}}_{\text{11}}\text{)}}_{\text{2}}\text{BH}$. Disiamylborane is added only once to an alkyne because the bulky alkyl groups in it prevents the addition of a second molecule. An enol is formed initially on oxidation by basic ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}$, which tautomerizes to the more stable keto form.

The initial addition of borane is a syn, anti-Markovnikov addition via a four-membered cyclic transition state. Boron adds to the less substituted (less hindered) carbon because it is less electronegative than hydrogen. This results in an anti-Markovnikov addition of a water molecule to the double bond.

Answer to Problem 12.53P

The appropriate alkene used to synthesize the given compound is

The necessary reagents and special reaction conditions for the synthesis are $\text{1}{\text{. BH}}_{\text{3}}\text{, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Explanation of Solution

Since the product is an alcohol, an alkene with a methylene substituent on a cyclopentane ring would be appropriate as the starting compound.

Treating this alkene with borane in THF will add ${\text{BH}}_{\text{2}}$ to the less substituted terminal carbon of the substituent while hydrogen will add to the ring carbon. Subsequent treatment with ${\text{H}}_{\text{2}}{\text{O}}_{\text{2}}\text{, NaOH}$ will oxidize the adduct to the alcohol.

Thus, the necessary reagents for the reaction are $\text{1}{\text{. BH}}_{\text{3}}\text{, THF}$ and $\text{2}{\text{. NaOH, H}}_{\text{2}}{\text{O}}_{\text{2}}{\text{, H}}_{\text{2}}\text{O}$.

Conclusion

The alkene necessary for the synthesis of the given compound and the reagents and special conditions for the reaction are determined on the basis of the structure of the given compound.