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Chapter 19: Amines Back To Chapter

19.18: Preparation of Amines: Reductive Amination of Aldehydes and Ketones

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Preparation of Amines: Reductive Amination of Aldehydes and Ketones

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Recall that carbonyl compounds and primary amines react to form imines, which can be further reduced to secondary amines without isolation.

This one-pot conversion of carbonyl compounds to amines using another amine and a reducing agent is called reductive amination.

The reaction introduces a C–H and a C–N bond in place of a C=O bond.

Reductive amination uses catalytic hydrogenation or a selective hydride reducing agent like NaBH₃CN.

The imine formed from condensing a carbonyl compound and a primary amine is protonated at the nitrogen center.

Next, NaBH₃CN transfers the hydride ion to the imine carbon, reducing the imine to an amine.

NaBH₃CN is a NaBH₄ derivative, but unlike the cyano compound, NaBH₄ is not suitable for reductive amination as it reduces the carbonyl precursor, inhibiting imine formation.

The electron-withdrawing cyano group stabilizes the negative charge, making the hydride more selective in reducing the protonated imine instead of the carbonyl group.

Depending on the amine precursor, reductive amination can give primary, secondary and tertiary amines.

Using reductive amination, an antidepressant drug, Fluoxetine, is synthesized from an aldehyde and a primary amine.

19.18: Preparation of Amines: Reductive Amination of Aldehydes and Ketones

Carbonyl compounds and primary amines undergo reductive amination first to produce imines, followed by secondary amines in the same reaction mixture, using selective reducing agents like sodium cyanoborohydride or sodium triacetoxyborohydride. Reductive amination produces different degrees of substitution of amines depending on the starting amine substrate.

Reductive amination using sodium cyanoborohydride as the reducing agent is called the Borch reaction. Sodium cyanoborohydride is a mild reductant that works under mildly acidic conditions.

Reductive amination has several applications, such as synthesizing the central nervous stimulant amphetamine and biosynthesis of the amino acid proline.

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Preparation Of Amines Reductive Amination Aldehydes Ketones Carbonyl Compounds Primary Amines Imines Secondary Amines Selective Reducing Agents Sodium Cyanoborohydride Sodium Triacetoxyborohydride Degrees Of Substitution Borch Reaction Mildly Acidic Conditions Central Nervous Stimulant Amphetamine Biosynthesis Amino Acid Proline

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