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2.5.6 Reductive Amination of C=O Bonds

DOI: 10.1055/sos-SD-202-00095

Xu, L.; Wu, X.; Xiao, J.Science of Synthesis: Stereoselective Synthesis, (20112287.

The imino C=N bond is generally formed by reacting a carbonyl group with an amine. Therefore, direct reductive amination (DRA), i.e. reduction of an imine while it is formed in the presence of a reducing reagent without isolation, is very appealing. Indeed, direct reductive amination has been a cornerstone transformation in chemical synthesis.[‌1‌,‌2‌,‌6‌] Direct reductive amination is not, however, without problems. The carbonyl group itself is reducible, giving rise to an issue of chemoselectivity. Further, the reaction of the carbonyl with the amine results in an equilibrium that usually disfavors the product imine unless water is removed. Still further complication arises from the various intermediates that may appear, such as hemiaminals, aminals, enamines, and iminium ions, with each of them being reducible. Consequently, there have been only a few catalysts that allow efficient asymmetric direct reductive amination (ADRA).[‌3‌,‌4‌,‌6‌,‌8‌,‌101‌‌103‌]