Reductive amination

Reductive amination is a chemical reaction which involves the conversion of a carbonyl group to an amine. The carbonyl group is most commonly a ketone or an aldehyde.

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Reaction mechanism

In organic synthesis

In this organic reaction, the amine first reacts with the carbonyl group, in a reversible manner, to form the imine accompanied by the loss of one molecule of water by alkylimino-de-oxo-bisubstitution. The equilibrium between aldehyde/ketone and imine can be shifted toward imine formation by removal of the formed water through physical or chemical means. This intermediate imine can then be isolated and reduced with a suitable reducing agent (e.g. sodium borohydride). This is indirect reductive amination.

However, it is also possible to carry out the same reaction all in one pot, with the imine formation and reduction occurring concurrently. This is known as direct reductive amination and is carried out with reducing agents that are more reactive toward imines than ketones, such as sodium cyanoborohydride (NaBH₃CN) or sodium triacetoxyborohydride (NaBH(OCOCH₃)₃).

This reaction is related to the Eschweiler-Clarke reaction in which amines are methylated to tertiary amines, the Leuckart-Wallach reaction with formic acid and to other amine alkylation methods as the Mannich reaction and the Petasis reaction.

Scope

In industry tertiary amines such as triethylamine and Hunig's base, are formed directly from ketones with a gasmixture of ammonia and hydrogen and a suitable catalyst .

The reaction depicted below is an example of an asymmetric reductive amination of an aryl ketone and an aniline.^[1][2]

The formation of only one enantiomer from the racemic reactant is attributed to dynamic kinetic resolution because both enantiomers in the racemic imine intermediate interconvert through their common enamine.

Biochemistry

A step in the biosynthesis of many α-amino acids is the reductive amination of an α-ketoacid. The process is catalyzed by pyridoxal phosphate and the ammonia source is glutamate. The initial step entails formation of an imine, but the hydride equivalents are supplied by a reduced pyridine to give an aldimine, which hydrolyzes to the amine.^[3] The sequence from keto-acid to amino acid can be summarized as follows:

HO₂CC(O)R → HO₂CC(=NCH-X)R → HO₂CCH(N=CH-X)R → HO₂CCH(NH₂)R

References

1. ^ Catalytic Asymmetric Reductive Amination of Aldehydes via Dynamic Kinetic Resolution Sebastian Hoffmann, Marcello Nicoletti, and Benjamin List J. Am. Chem. Soc. 2006, 128(40), 13074 - 13075. (doi:10.1021/ja065404r)
2. ^ In this reaction the reducing agent is an Hantzsch ester and the chiral catalyst a hydrogen phosphate of the BINAP family. Molecular sieves remove formed water. The enantiomeric ratio is 99:1.
3. ^ Nelson, D. L.; Cox, M. M. "Lehninger, Principles of Biochemistry" 3rd Ed. Worth Publishing: New York, 2000. ISNB 1-57259-153-6.

See also

• Forster-Decker method