Question

Identify the suitable reagent for the following conversion: $Ph-C(=O)-OCH_3$ $\longrightarrow$ $Ph-CHO$

• A. (i) $AlH(iBu)_{2}$ (ii) $H_{2}O$
• B. (i) $NaBH_{4}$, (ii) $H^{+}H_{2}O$
• C. $H_{2}$ $Pd-BaSO_{4}$
• D. (i) $LiAlH_{4}$, (ii) $H^{+}$$H_{2}O$

Hint:

Use DIBAL-H for selective reduction of esters to aldehydes; LiAlH\textsubscript{4} over-reduces to alcohols.

Answer 1

The Correct Option is A

The transformation is from an ester ($Ph-COOCH_3$) to an aldehyde ($Ph-CHO$). This selective reduction requires a mild and bulky reducing agent that reduces esters only to aldehydes without further reducing to alcohols. 

- Option (1): Diisobutylaluminium hydride (DIBAL-H or $AlH(iBu)_{2}$) at low temperatures (–78°C), followed by hydrolysis, reduces esters to aldehydes. This is the correct reagent.
- Option (2): $NaBH_{4}$ is too mild to reduce esters.
- Option (3): $H_{2}$ $Pd-BaSO_{4}$ is used for Rosenmund reduction (acid chlorides to aldehydes), not esters.
- Option (4): $LiAlH_{4}$ is strong and will reduce esters all the way to primary alcohols.

Thus, only DIBAL-H allows selective conversion of ester to aldehyde.