Question

Consider the following compound.

The compound undergoes reaction with two equivalents of diisobutyl aluminium hydride followed by acid hydrolysis to provide P as the major product. The structure of P is:

• A. Structure with both functional groups reduced to $-\text{CHO}$.
• B. Structure with ester reduced to $-\text{CHO}$ and nitrile remaining intact as $-\text{CN}$.
• C. Structure with ester reduced to $-\text{CH}_2\text{OH}$ and nitrile remaining intact as $-\text{CN}$.
• D. Structure with ester reduced to $-\text{CH}_2\text{OH}$ and nitrile reduced to $-\text{CH}_2\text{NH}_2$.

Hint:

At low temperatures (e.g., $-78\ ^\circ\text{C}$), DIBAL-H is the premier reagent for stopping reductions of both esters and nitriles at the aldehyde stage.

Answer 1

The Correct Option is A

Step 1: Understanding the Question:
The question asks for the major product P obtained when a cyclohexene derivative containing both an ester group ($-\text{CO}_2\text{Et}$) and a nitrile group ($-\text{CN}$) is treated with two equivalents of diisobutylaluminium hydride (DIBAL-H), followed by acid hydrolysis.

Step 2: Key Formula or Approach:
DIBAL-H is a electrophilic, sterically hindered reducing agent that is commonly used at low temperatures to selectively reduce:
1. Esters (R-COOR') to aldehydes (R-CHO).
2. Nitriles (R-CN) to imines, which upon subsequent acid hydrolysis yield aldehydes (R-CHO).

Step 3: Detailed Explanation:

• The starting material is ethyl 4-cyanocyclohex-3-ene-1-carboxylate.

• It possesses two distinct functional groups susceptible to reduction: an ester group ($-\text{CO}_2\text{Et}$) and a nitrile group ($-\text{CN}$).

• The researcher adds exactly two equivalents of DIBAL-H at a low temperature.

• One equivalent of DIBAL-H reacts with the ester group, forming a hemiacetal intermediate that is stable at low temperatures, preventing further reduction to an alcohol.

• The second equivalent of DIBAL-H coordinates to the nitrogen of the nitrile group and reduces it to an imine intermediate.

• Upon subsequent acid hydrolysis ($\text{H}_3\text{O}^+$), the hemiacetal intermediate collapses to release ethanol and form an aldehyde group ($-\text{CHO}$).

• At the same time, the imine intermediate undergoes complete hydrolysis to release ammonia and yield an aldehyde group ($-\text{CHO}$).

• Thus, both functional groups are selectively converted into aldehyde groups ($-\text{CHO}$), while the carbon-carbon double bond of the cyclohexene ring remains completely unaffected.


Step 4: Final Answer:
Therefore, the correct structure of P is the dialdehyde shown in Option A.