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.
Reported herein is ag eneral and efficient dualdeoxygenative coupling of primary alcohols with 2-arylethanols catalyzedb yawell-defined Mn/PNP pincer complex. This reaction is the first example of the catalytic dualdeoxygenation of alcohols using an on-noble-metal catalyst. Both deoxygenative homocoupling of 2-arylethanols (17 examples) and their deoxygenative cross-coupling with other primary alcohols (20 examples) proceeded smoothly to form the corresponding alkenes by ad ehydrogenation and deformylation reaction sequence. Scheme 1. Catalytic tandem dual-deoxygenative coupling of alcohols according to Obora et al. (2011) and Johnson et al. (2018).
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