Nanolayered Cobalt–Molybdenum Sulfides as Highly Chemo- and Regioselective Catalysts for the Hydrogenation of Quinoline Derivatives

Copper-Catalyzed Transfer Hydrogenation of N-Heteroaromatics with an Oxazaborolidine Complex

The first-time use of the oxazaborolidine complex in transfer hydrogenation was accomplished. It was prepared without difficulty from cheap materials: ethanolamine and BH₃·THF. A general and efficient method for copper-catalyzed transfer hydrogenation of a variety of N-heteroaromatics with an oxazaborolidine-BH₃ complex under mild reaction conditions afforded the corresponding hydrogenated products in up to 96% yield. Mechanistic studies indicate that the hydrogen source originated from water and borane that coordinate with the nitrogen atom of oxazaborolidine. Accordingly, a plausible mechanism for this reaction was proposed. This method was successfully used in the key step synthesis of natural products (±)-angustureine and (±)-galipinine in three steps.

Nanolayered cobalt-molybdenum sulphides (Co-Mo-S) catalyse borrowing hydrogen C-S bond formation reactions of thiols or H2S with alcohols

Nanolayered cobalt-molybdenum sulphide (Co-Mo-S) materials have been established as excellent catalysts for C-S bond construction. These catalysts allow for the preparation of a broad range of thioethers in good to excellent yields from structurally diverse thiols and readily available primary as well as secondary alcohols. Chemoselectivity in the presence of sensitive groups such as double bonds, nitriles, carboxylic esters and halogens has been demonstrated. It is also shown that the reaction takes place through a hydrogen-autotransfer (borrowing hydrogen) mechanism that involves Co-Mo-S-mediated dehydrogenation and hydrogenation reactions. A novel catalytic protocol based on the thioetherification of alcohols with hydrogen sulphide (H2S) to furnish symmetrical thioethers has also been developed using these earth-abundant metal-based sulphide catalysts.

Cobalt-bridged secondary building units in a titanium metal-organic framework catalyze cascade reduction of N-heteroarenes

We report here a novel Ti3-BPDC metal-organic framework (MOF) constructed from biphenyl-4,4'-dicarboxylate (BPDC) linkers and Ti3(OH)2 secondary building units (SBUs) with permanent porosity and large 1D channels. Ti-OH groups from neighboring SBUs point toward each other with an O-O distance of 2 Å, and upon deprotonation, act as the first bidentate SBU-based ligands to support CoII-hydride species for effective cascade reduction of N-heteroarenes (such as pyridines and quinolines) via sequential dearomative hydroboration and hydrogenation, affording piperidine and 1,2,3,4-tetrahydroquinoline derivatives with excellent activity (turnover number ∼ 1980) and chemoselectivity.

A robust iron catalyst for the selective hydrogenation of substituted (iso)quinolones

By applying N-doped carbon modified iron-based catalysts, the controlled hydrogenation of N-heteroarenes, especially (iso)quinolones, is achieved. Crucial for activity is the catalyst preparation by pyrolysis of a carbon-impregnated composite, obtained from iron(ii) acetate and N-aryliminopyridines. As demonstrated by TEM, XRD, XPS and Raman spectroscopy, the synthesized material is composed of Fe(0), Fe3C and FeN x in a N-doped carbon matrix. The decent catalytic activity of this robust and easily recyclable Fe-material allowed for the selective hydrogenation of various (iso)quinoline derivatives, even in the presence of reducible functional groups, such as nitriles, halogens, esters and amides. For a proof-of-concept, this nanostructured catalyst was implemented in the multistep synthesis of natural products and pharmaceutical lead compounds as well as modification of photoluminescent materials. As such this methodology constitutes the first heterogeneous iron-catalyzed hydrogenation of substituted (iso)quinolones with synthetic importance.