Abstract
We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols. Compared with most previous reports for this transformation, which use high temperatures (typically, 100–200 °C) and H2 pressures (10–100 bar), this system proceeds under extremely mild conditions (RT, 5–10 bar of H2). The hydrogenation is catalyzed by well-defined ruthenium-PNNH pincer complexes (0.5 mol %) with potential dual modes of metal–ligand cooperation. An unusual Ru-amidate complex was formed and crystallographically characterized. Mechanistic investigations indicate that the room-temperature hydrogenation proceeds predominantly via the Ru–N amido/amine metal–ligand cooperation.
Highlights
We report a room-temperature protocol for the hydrogenation of various amides to produce amines and alcohols
Amide hydrogenation can proceed either via C−N bond cleavage or by C−O bond cleavage (Scheme 1).5a,b We are interested in the C−N bond cleavage pathway, which is challenging because of the water losing tendency of the hemiaminal intermediate to form imine
Hydrogenation and dehydrogenative formation of the amide bond has been instrumental in the recent development of promising liquid organic hydrogen carrier (LOHC) systems based on alcohols and amines.[7]
Summary
Temperature amide hydrogenation to amines and alcohols catalyzed by the Ru-PNNH complexes at low catalyst loading (0.5 mol %) under low H2 pressures (5−10 bar). Changing the N-substitution of the catalyst from t-Bu to benzyl group (Ru-2), the catalytic activity increased even further, and complete conversion of the amide to the amine and alcohol was observed after 20 h (entry 2). It is likely that the acidic nature of the N-H proton of PNNH ligand assists in lowering the energy requirement of this rate-determining step, allowing the hydrogenation to proceed even at RT via the amido pathway This can explain the higher catalytic activity of Ru-2 compared with Ru1, where the Bn substitution of the N donor atom makes the ligand N-H of Ru-2 less basic as compared with Ru-1 with t-Bu group in the N atom.
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