Reagent Selection for Selective Reduction of Aldehyde Functionality

To determine the reagent(s) suitable for the given chemical transformation, we must analyze the structural changes occurring from the reactant to the product:

1. Analysis of Functional Groups

• Reactant: Contains four functional groups:

  1. Aldehyde group ($-\text{CHO}$)
  2. Carboxylic acid group ($-\text{COOH}$)
  3. Ester group ($-\text{COO-}{t}\text{-Bu}$)
  4. Epoxide (oxirane) ring

• Product:

  1. The aldehyde group ($-\text{CHO}$) is selectively reduced to a primary alcohol ($-\text{CH}_2\text{OH}$).
  2. The carboxylic acid ($-\text{COOH}$), ester ($-\text{COO-}{t}\text{-Bu}$), and epoxide functional groups remain intact/unreacted.

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2. Evaluation of Reagents

• Option (A): $\text{LiAlH}_4$ in $(\text{C}_2\text{H}_5)_2\text{O}$

  • $\text{LiAlH}_4$ is a strong, non-selective reducing agent.
  • It reduces aldehydes, carboxylic acids, and esters to alcohols, and opens epoxide rings to form alcohols.
  • Therefore, it cannot bring about the desired selective reduction of only the aldehyde group.
  • Incorrect.

• Option (B): $\text{BH}_3$ in $\text{THF}$

  • Borane ($\text{BH}_3\cdot\text{THF}$) preferentially and rapidly reduces carboxylic acid groups ($-\text{COOH}$) over aldehydes, esters, and epoxides due to the electrophilic nature of $\text{BH}_3$.
  • Thus, it would reduce the carboxylic acid group first.
  • Incorrect.

• Option (C): $\text{NaBH}_4$ in $\text{C}_2\text{H}_5\text{OH}$

  • Sodium borohydride ($\text{NaBH}_4$) is a mild nucleophilic reducing agent.
  • It selectively reduces aldehydes and ketones to alcohols.
  • Under standard conditions, $\text{NaBH}_4$ does not reduce carboxylic acids, esters, or unfunctionalized epoxides.
  • Thus, it selectively reduces the aldehyde group ($-\text{CHO}$) to a primary alcohol ($-\text{CH}_2\text{OH}$) without affecting the other functional groups.
  • Correct.

• Option (D): Raney $\text{Ni}/\text{H}_2$ in $\text{THF}$

  • Catalytic hydrogenation with Raney $\text{Ni}/\text{H}_2$ under mild conditions selectively reduces the aldehyde ($-\text{CHO}$) group to a primary alcohol ($-\text{CH}_2\text{OH}$).
  • Carboxylic acids, esters, and epoxide rings are generally resistant to reduction under these mild catalytic conditions.
  • Correct.

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Conclusion

The reagents capable of executing the given selective reduction are (C) and (D).