14.20 : Esters to Alcohols: Hydride Reductions

The reduction of esters with strong reducing agents like lithium aluminum hydride proceeds through a nucleophilic acyl substitution to form primary alcohols.

Notably, two alcohols are formed from an ester, one derived from the acyl group and the other from the alkoxy group. Therefore, the reaction requires two equivalents of the reducing agent, which acts as a source of hydride ions.

The mechanism begins with a nucleophilic attack by the hydride ion at the carbonyl carbon, forming a tetrahedral intermediate.

Next, the carbonyl group is reconstructed with the departure of the alkoxide ion, giving an aldehyde.

Subsequently, a second equivalent of the hydride ion attacks the aldehyde, generating an alkoxide intermediate.

Lastly, protonation of the alkoxide gives a primary alcohol as the final product.

A milder reducing agent like lithium tri(tert-butoxy) aluminum hydride selectively reduces esters to aldehydes.

This reaction is performed at a low temperature, about −78 °C, to prevent further reduction to alcohol.