Felix A. Westerhaus

Heterogenized cobalt oxide catalysts for nitroarene reduction by pyrolysis of molecularly defined complexes

Molecularly well-defined homogeneous catalysts are known for a wide variety of chemical transformations. The effect of small changes in molecular structure can be studied in detail and used to optimize many processes. However, many industrial processes require heterogeneous catalysts because of their stability, ease of separation and recyclability, but these are more difficult to control on a molecular level. Here, we describe the conversion of homogeneous cobalt complexes into heterogeneous cobalt oxide catalysts via immobilization and pyrolysis on activated carbon. The catalysts thus produced are useful for the industrially important reduction of nitroarenes to anilines. The ligand indirectly controls the selectivity and activity of the recyclable catalyst and catalyst optimization can be performed at the level of the solution-phase precursor before conversion into the active heterogeneous catalyst. Pyrolysis of defined nitrogen-ligated cobalt acetate complexes onto a commercial carbon support transforms the complexes into heterogeneous Co3O4 materials. These reusable non-noble-metal catalysts are highly selective for the industrially important hydrogenation of structurally diverse and functionalized nitroarenes to anilines.

General and Selective Iron-Catalyzed Transfer Hydrogenation of Nitroarenes without Base

The first well-defined iron-based catalyst system for the reduction of nitroarenes to anilines has been developed applying formic acid as reducing agent. A broad range of substrates including other reducible functional groups were converted to the corresponding anilines in good to excellent yields at mild conditions. Notably, the process constitutes a rare example of base-free transfer hydrogenations.

A molecularly defined iron-catalyst for the selective hydrogenation of α,β-unsaturated aldehydes.

A selective iron-based catalyst system for the hydrogenation of α,β-unsaturated aldehydes to allylic alcohols is presented. Applying the defined iron-tetraphos complex [FeF(L)][BF4] (L = P(PhPPh2)3) in the presence of trifluoroacetic acid a broad range of aldehydes are reduced in high yields using low catalyst loadings (0.05-1 mol %). Excellent chemoselectivity for the reduction of aldehydes in the presence of other reducible moieties, for example, ketones, olefins, esters, etc. is achieved. Based on the in situ detected hydride species [FeH(H2)(L)](+) a catalytic cycle is proposed that is supported by computational calculations.

General and selective reductive amination of carbonyl compounds using a core–shell structured Co3O4/NGr@C catalyst

The application of heterogenized non-noble metal-based catalysts in selective catalytic hydrogenation processes is still challenging. In this respect, the preparation of a well-defined cobalt-based catalyst was investigated by immobilization of the corresponding cobalt(II)-phenanthroline-chelate on Vulcan XC72R carbon powder. The formed core–shell structured cobalt/cobalt oxide nanocomposites are encapsulated by nitrogen-enriched graphene layers. This promising cheap heterogeneous catalyst allows for an efficient domino reductive amination of carbonyl compounds with nitroarenes.

A convenient and general ruthenium-catalyzed transfer hydrogenation of nitro- and azobenzenes.

An easily accessible in situ catalyst composed of [{RuCl(2)(p-cymene)}(2)] and terpyridine has been developed for the selective transfer hydrogenation of aromatic nitro and azo compounds. The procedure is general and the selectivity of the catalyst has been demonstrated by applying a series of structurally diverse nitro and azo compounds (see scheme).

Phosphine–Imidazolyl Ligands for the Efficient Ruthenium‐Catalyzed Hydrogenation of Carboxylic Esters

The synthesis of phosphine-imidazolyl ligands 1 and 2 in good yields is presented. In combination with [{Ru(benzene)Cl(2)}(2)], ligands 1 c and 1 e formed efficient catalyst systems for the selective hydrogenation of various carboxylic esters into their corresponding primary alcohols. Furthermore, the structures of four ruthenium complexes with ligands 1 b, 1 c, 1 d, and 1 e were determined by X-ray crystallography, which showed the formation of different coordination modes depending on the ligand structure.

Ruthenium Catalysts for Hydrogenation of Aromatic and Aliphatic Esters: Make Use of Bidentate Carbene Ligands

Committed carbenes: The convenient application of bidentate carbene ligands is described for the hydrogenation of carboxylic acid esters. The ligand precursors are easily synthesized through the dimerization of N-substituted imidazoles with diiodomethane. The catalyst is generated in situ and exhibits good activity and functional group tolerance for the hydrogenation of aromatic and aliphatic carboxylic acid esters.