Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed
hydrogenation reactions have attracted broad
interest since the first report in 2016. Among the reported catalytic
systems, Mn catalysts supported by tridentate PNP- and NNP-pincer
ligands have most commonly been used. For example, a number of PNP-Mn
pincer catalysts have been reported for the hydrogenation of aldehydes,
aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn
pincer catalysts have been shown to be active in the hydrogenation
of less-reactive substrates such as amides, carbonates, carbamates,
and urea derivations. These observations indicated that Mn catalysts
supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation
reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed
hydrogenation reactions has yet to be confirmed. Herein, we investigated
the origin and applicability of this ligand effect. A combination
of experimental and theoretical investigations showed that NNP-pincer
ligands on the Mn complexes were more electron-rich and less sterically
hindered than their PNP counterparts, leading to higher reactivity
in a series of Mn-catalyzed hydrogenation reactions. Inspired by the
ligand effect on Mn-catalyzed hydrogenations, we developed the first
Mn-catalyzed hydrogenation of N-heterocycles. Specifically,
NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding
PNP-Mn pincer catalysts afforded low reactivity under the same conditions.
This verified that such a ligand effect is generally applicable in
hydrogenation reactions of both carbonyl and noncarbonyl substrates
based on Mn catalysis.