14.20: Esters to Alcohols: Hydride Reductions
Esters are reduced to primary alcohols when treated with a strong reducing agent like lithium aluminum hydride. The reaction requires two equivalents of the reducing agent and proceeds via an aldehyde intermediate.
Lithium aluminum hydride is a source of hydride ions and functions as a nucleophile. The mechanism proceeds in three steps. Firstly, the nucleophilic hydride ion attacks the carbonyl carbon of the ester to form a tetrahedral intermediate. Subsequently, the carbonyl group re-forms, and the alkoxide ion departs as the leaving group, generating an aldehyde. Lastly, a second nucleophilic attack by the hydride ion at the carbonyl carbon of the aldehyde yields an alkoxide ion, which gives a primary alcohol as the final product upon protonation.
However, in case the desired product is an aldehyde, it is possible to stop the reaction at the aldehyde intermediate by using a milder reducing agent like diisobutylaluminum hydride or lithium tri(t-butoxy) aluminum hydride, at a lower temperature.