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2.5.6.2.1 Asymmetric Direct Reductive Amination of Ketones with Ammonium Formate

DOI: 10.1055/sos-SD-202-00095

Xu, L.; Wu, X.; Xiao, J.Science of Synthesis: Stereoselective Synthesis, (20112293.

The reduction of C=O compounds to amines using ammonium formate or formamide is known as the LeuckartWallach reaction.[‌144‌,‌145‌] There are a few studies that have demonstrated the effect of metal catalysis on the reaction.[‌146‌‌148‌] In particular, a ruthenium catalyst with chiral diphosphine ligand 76 shows good activity, chemoselectivity, and enantioselectivity in the asymmetric direct reductive amination of aryl ketones, with the best enantiomeric excesses being obtained with 510 equivalents of ammonium formate in ammonia/methanol (1525%) at temperatures between 60 and 85°C (Scheme 45).[‌131‌] N-Formylamines are the main product of the asymmetric direct reductive amination, hydrolysis of which releases the free amines (e.g., 77). The addition of ammonia enhances the enantioselectivity but decreases the reaction rate, and aqueous ammonia increases the yield of the alcohol byproduct. However, the reduction leads to low enantiomeric excesses for aliphatic ketones, such as octan-2-one (24% ee) and 2-methylcyclohexanone (64% ee).

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