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 conversion involves transforming an ester ($Ph-COOCH_3$) into an aldehyde ($Ph-CHO$). This specific reduction necessitates a gentle yet sterically hindered reducing agent that achieves ester-to-aldehyde reduction without proceeding further to alcohols.

- Option (1): Diisobutylaluminium hydride (DIBAL-H or $AlH(iBu)_{2}$), when used at low temperatures (–78°C) and followed by hydrolysis, effectively reduces esters to aldehydes. This reagent is appropriate for the task.
- Option (2): Sodium borohydride ($NaBH_{4}$) lacks sufficient reactivity to reduce esters.
- Option (3): Hydrogen gas with palladium on barium sulfate ($H_{2}$ $Pd-BaSO_{4}$) is employed for the Rosenmund reduction, which converts acid chlorides to aldehydes, not esters.
- Option (4): Lithium aluminum hydride ($LiAlH_{4}$) is a potent reducing agent that will reduce esters completely to primary alcohols.

Therefore, DIBAL-H is the sole option that facilitates the selective transformation of an ester to an aldehyde.