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DOI: 10.1055/sos-SD-202-00095

Xu, L.; Wu, X.; Xiao, J.Science of Synthesis: Stereoselective Synthesis, (20112288.

Direct reductive amination with molecular hydrogen by heterogeneous transition-metal catalysts such as platinum, palladium, nickel, and ruthenium has a long tradition,[‌104‌] and several amines have been prepared commercially by this method.[‌105‌] For asymmetric direct reductive amination, however, good enantioselectivities have been achieved only since 2000.[‌8‌,‌101‌‌103‌] The first example is seen in the synthesis of (S)-metolachlor in 78% ee with the iridiumXyliphos (1) catalyst (Scheme 2).[‌106‌] Further development shows that rhodium catalysts with chiral diphosphine ligands enable the asymmetric direct reductive amination of α-keto acids with benzylamine, affording good yields and excellent enantiomeric excesses in some cases;[‌107‌] an iridium catalyst with chiral ligand (S,S)-2 is effective in the asymmetric direct reductive amination of aromatic ketones with p-anisidine, furnishing >99% yield and good to excellent enantiomeric excesses.[‌108‌] The asymmetric direct reductive amination of aliphatic ketones via hydrogenation has led to moderate to excellent stereoselectivities with catalysts, such as [(R)-2,2-bis(diphenylphosphino)-1,1-binaphthyl]palladium(II),[‌109‌] and the heterogeneous Raney nickel, palladium on carbon, and platinum on carbon aided by a Lewis acid such as ytterbium(III) acetate[‌110‌] or titanium(IV) isopropoxide,[‌111‌‌113‌] where the amine substrate is chiral. Other enantioselective methods have appeared more recently.