酰胺到胺:LiAlH₄ 还原

Amide reduction with lithium aluminum hydride proceeds through nucleophilic acyl substitution. A hydride ion attacks the carbonyl carbon, forming a tetrahedral intermediate. The carbonyl oxygen then bonds with aluminum hydride, and an aluminate anion leaves, generating an iminium ion. A second hydride attacks the iminium ion to produce the amine product.

Primary amides reduce to primary amines, secondary amides yield secondary amines, and tertiary amides produce tertiary amines. The reduction pattern preserves the nitrogen substitution level from the starting amide. This one-to-one correspondence makes predicting reduction products straightforward for all amide types.

Two equivalents of lithium aluminum hydride are needed because each equivalent provides hydride ions for the reduction. The first hydride attacks the carbonyl carbon to form the tetrahedral intermediate, while the second hydride attacks the iminium ion intermediate to complete the conversion to amine.

The amide carbonyl group is converted into a methylene group during reduction. This transformation replaces the carbon-oxygen double bond with a carbon-hydrogen bond, fundamentally changing the functional group from a carbonyl to a hydrocarbon unit in the final amine product.

Lactams are cyclic amides formed when the nitrogen and carbonyl carbon are part of a ring structure. Lactams undergo reduction with lithium aluminum hydride to form cyclic amines, maintaining the ring structure while converting the carbonyl to a methylene group, similar to acyclic amide reduction.

Aluminum hydride acts as a Lewis acid in the reduction mechanism. After the hydride ion attacks the carbonyl carbon, the aluminum hydride coordinates with the carbonyl oxygen to form an oxygen-aluminum bond, facilitating the rearrangement of electron pairs and expulsion of the aluminate anion leaving group.

The iminium ion intermediate is formed after the aluminate anion leaves the tetrahedral intermediate. This positively charged carbon-nitrogen double bond species is then attacked by a second hydride ion to yield the final amine product, representing a critical step in completing the reduction sequence.