Question

a. Draw the structure and configuration of the product expected of the reaction between 1-bromo-1-hexyne and diethylborane, \(\left(\mathrm{C}_{2} \mathrm{H}_{5}\right)_{2} \mathrm{BH}\). b. When the product is treated with sodium methoxide, \(\mathrm{NaOCH}_{3}\), then with propanoic acid, trans-3-octene is formed. Show the steps involved in forming this trans-alkene.

Short answer

Trans-3-octene is the result of reacting vinyl bromide with sodium methoxide and propanoic acid.

Step-by-step solution

Reaction Setup

1-Bromo-1-hexyne reacts with diethylborane, \( (\mathrm{C}_2\mathrm{H}_5)_2\mathrm{BH} \), where B attaches to the terminal carbon of the alkyne, leading to hydroboration.

Hydroboration

Diethylborane adds across the triple bond of 1-bromo-1-hexyne in an anti-Markovnikov fashion. This step forms a vinyl borane where the boron is attached to the terminal end of the alkyne.

Formation of Vinyl Bromide

The reaction forms a vinyl bromide following the hydroboration step. The molecule now has a C-Br bond and a B(C2H5)2 group attached to adjacent carbons of the double bond.

Reaction with Sodium Methoxide

Sodium methoxide \( \mathrm{NaOCH}_3 \) will remove the diethylborane group from the vinyl bromide, replacing it with a methoxide group.

Reaction with Propanoic Acid

The intermediate product from the previous step then reacts with propanoic acid, leading to the removal of the methoxy group and regeneration of a more stable alkene bond.

Trans-3-Octene Formation

After the treatment with sodium methoxide followed by propanoic acid, the reaction stabilizes as \( \text{trans-3-octene} \), a trans-alkene due to elimination that occurs in a trans manner.

Key concepts

Hydroboration

Hydroboration is a crucial reaction in organic chemistry that facilitates the addition of boron across carbon-carbon multiple bonds, such as alkenes and alkynes. When 1-bromo-1-hexyne reacts with diethylborane, hydroboration occurs as diethylborane,

• adds to the triple bond of the alkyne in an orderly fashion, and
• connects with the terminal carbon, initiating the transformation.

This transformation occurs in an anti-Markovnikov manner, meaning the boron atom attaches to the less substituted carbon atom in the alkyne. This ultimately leads to the formation of a vinyl borane, where boron is now bonded to the terminal carbon atom.

Vinyl Borane

The formation of vinyl borane is a direct consequence of the hydroboration process. Upon the addition of boron,

• a structure emerges where the carbon-to-boron bond is formed at the extremity of the former alkyne,
• creating what is known as a vinyl borane.

The importance of vinyl boranes lies in their reactivity, as they serve as excellent intermediates in the synthesis of various organic compounds. Here, the vinyl borane possesses a characteristic C-B bond at what was the terminal position of the 1-bromo-1-hexyne.

Sodium Methoxide

In this reaction sequence, sodium methoxide \( \mathrm{NaOCH}_3 \) plays a significant role as a nucleophile.

• First, it reacts with the vinyl borane, effectively replacing the boron group with a methoxide group.
• Nucleophilic substitution takes place, where the vinyl bromide formed from the hydroboration step loses the diethylborane and gains a new methoxide group.

This step is essential for the further transformation of the compound leading to the eventual formation of trans-3-octene.

Trans-3-Octene

The synthesis of trans-3-octene is a two-step process following the initial hydroboration. The vinyl bromide product treated with sodium methoxide undergoes a further reaction with propanoic acid.

• This subsequent reaction leads to the removal of the methoxy group, favoring the generation of a stable alkene form.
• The double bond formed in this process is in a trans configuration, thus resulting in the creation of trans-3-octene.

This trans arrangement provides the molecule with stability and lower energy due to the trans configuration of substituents around the alkene bond.

Anti-Markovnikov Addition

Anti-Markovnikov addition refers to the process where the less substituted carbon atom in a double or triple bond becomes bonded to the addend.

• This concept is exemplified in the reaction of 1-bromo-1-hexyne with diethylborane.
• Unlike Markovnikov's addition, where the more substituted carbon would form the bond, anti-Markovnikov addition places the new group on the less hindered carbon.

This principle allows the reaction to produce compounds like vinyl borane, steering the formation of certain products that are not accessible through traditional Markovnikov methodologies.