In 2018, Milstein and coworkers introduced a protocol for the hydrogenation of
amides to amines. In this seminal work, a lutidine-based PNP pincer ligand turned
out to be the best candidate for this transformation as depicted in Scheme 13.
Addition of B(C 6 F 5 ) 3 as Lewis acid increased the reactivity of the system drastically
involving coordination on the oxygen of the amide group and the hemiaminal
intermediate [28].
An interesting pathway for reductive amination was reported by Sortais and
coworkers in 2018. After condensation of aldehyde and amine, the so-formed
imine was hydrogenated to the corresponding amine as shown in Scheme 14. A
broad variety of aromatic and aliphatic in situ formed imines were successfully
reduced by employing Mn5 as pre-catalyst. This procedure showed high functional
group tolerance, whereas esters, amides, nitriles, and ketones were not hydrogenated. However, this concept was limited to in situ formed aldimines [29].
Scheme 12 Hydrogenation of amides yielding alcohols and amines catalyzed by Mn13
Scheme 13 Lewis acid-assisted hydrogenation of amides to amines mediated by Mn14
Scheme 14 Hydrogenation of in situ formed aldimines to amines catalyzed by Mn5
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S. Weber and K. Kirchner