Kathrin Junge

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.

Zinc-Catalyzed Reduction of Amides: Unprecedented Selectivity and Functional Group Tolerance

A novel zinc-catalyzed reduction of tertiary amides was developed. This system shows remarkable chemoselectivity and substrate scope tolerating ester, ether, nitro, cyano, azo, and keto substituents.

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 convenient and general iron-catalyzed reduction of amides to amines.

Amines constitute an important class of compounds in chemistry and biology. They are widely used in the pharmaceutical industry for crop protection and natural product synthesis, as well as for the preparation of advanced materials. Among the different procedures for their synthesis, the reduction of amides is one of the most fundamental methods. In general, reactive alkali metal hydrides or boron hydrides are used for such reduction processes. However, their air and moisture sensitivty, their low functional group tolerance, and tedious purification procedures are drawbacks. Interestingly, Cole-Hamilton and co-workers showed that catalytic hydrogenation of amides using molecular hydrogen constitutes a highly attractive access to amines, but the vigorous reaction conditions, that is, high pressures and elevated temperature, limit this methodology. During the last decade, metal-catalyzed hydrosilylations of amides have received considerable interest. Although various catalyst systems including Rh, Ru, Pt, Pd, Ir, Os, Re, Mn, Mo, In, and Ti have proven to be effective for this reduction, the development of cost-efficient and environmentally benign catalysts for this transformation is still desirable. In addition, most of the known catalyst systems either require expensive silanes or have limited functional group tolerance. During our recent studies on iron-catalyzed dehydrations of primary amides in the presence of hydrosilanes, we isolated the corresponding secondary amine as a by-product. Thus, our attention focused on this side-reaction of reducing carboxamides to amines. The abundant availability of iron makes it a highly attractive candidate for catalysis and a “cheap metal for noble tasks”. 7] Herein we report the first general iron-catalyzed hydrosilylation of amides to generate amines. Initially, the reaction of N,N-dimethylbenzamide (1 a) with PhSiH3 in toluene was investigated as a model system to identify and optimize critical reaction parameters (Table 1). As expected the reaction did not occur in the absence of any catalyst (Table 1, entry 1). In contrast, when using 2 mol% of cheap [Fe3(CO)12], an excellent yield (97%) of N,N-dimethylbenzylamine (2a) was obtained (Table 1, entry 3). Other iron sources, such as [Fe2(CO)9], Fe(OAc)2, [Fe(acac)2], and [Fe(acac)3] are also reactive but resulted in lower product yields (Table 1, entries 2 and 4–9). Next, we investigated the influence of different silanes on the reaction. To our delight the reduction also took place in the presence of inexpensive polymethylhydrosiloxane (PMHS) to give the desired product in 93 % yield (Table 1, entry 16). Advantageously, this silane is also easily separated from the reaction mixture. Additional studies revealed that the reduction proceeded best in the presence of an excess of four equivalents of Si H (Table 1, entry 17). Among the different solvents tested, toluene and di-n-butyl ether gave the best results (Table 1, entries 17 and 19). With respect to the mechanism, we propose that the ironcatalyzed reduction of tertiary amides proceeds somewhat similarly to the ruthenium-catalyzed procedure reported by Nagashima and co-workers. Reaction of the hydrosilane with the iron precursor should yield an activated species, Table 1: Iron-catalyzed reduction of N,N-dimethylbenzamide.

Selective Reduction of Carboxylic Acid Derivatives by Catalytic Hydrosilylation

In the last decade, an increasing number of useful catalytic reductions of carboxylic acid derivatives with hydrosilanes have been developed. Notably, the combination of an appropriate silane and catalyst enables unprecedented chemoselectivity that is not possible with traditional organometallic hydrides or hydrogenation catalysts. For example, amides and esters can be reduced preferentially in the presence of ketones or even aldehydes. We believe that catalytic hydrosilylations will be used more often in the future in challenging organic syntheses, as the reaction procedures are straightforward, and the reactivity of the silane can be fine-tuned. So far, the synthetic potential of these processes has clearly been underestimated. They even hold promise for industrial applications, as inexpensive and readily available silanes, such as polymethylhydrosiloxane, offer useful possibilities on a larger scale.

Cooperative Transition‐Metal and Chiral Brønsted Acid Catalysis: Enantioselective Hydrogenation of Imines To Form Amines

Significance: The authors report a protocol for the enantioselective hydrogenation of various ketimines in the presence of a chiral Brønsted catalyst and a well-defined nonchiral iron catalyst. This work demonstrates that enantioselective reduction reactions with hydrogen can be performed without employing precious-metal catalysts and chiral ligands yielding products with high yields and enantioselectivities. Comment: NMR spectroscopic studies revealed the formation of complex 1 when a 1:1 mixture of TRIP and the Knölker iron complex (cat. 1) were mixed. Upon addition of a ketimine to the reaction mixture, the formation of complex 2 was observed. These results suggest that a cooperative catalytic system is operative for this transformation. Fe OC CO H R1 R2 N R3 (S)-TRIP (1 mol%), cat. 1 (5 mol%)

Hydrogenation of Esters to Alcohols with a Well‐Defined Iron Complex

We present the first base-free Fe-catalyzed ester reduction applying molecular hydrogen. Without any additives, a variety of carboxylic acid esters and lactones were hydrogenated with high efficiency. Computations reveal an outer-sphere mechanism involving simultaneous hydrogen transfer from the iron center and the ligand. This assumption is supported by NMR experiments.

Iron-catalyzed selective reduction of nitroarenes to anilines using organosilanes

The iron-catalyzed reduction of aromatic nitro compounds to the corresponding anilines applying organosilanes is reported. In the presence of FeX(2)-R(3)P catalysts a series of nitroarenes is selectively reduced tolerating a wide range of functional groups.

A General Catalytic Methylation of Amines Using Carbon Dioxide

Carbon dioxide is the most abundant carbon source responsible for the generation of all organic compounds in nature. Its use as an inexpensive and nontoxic C1 feedstock is of increasing interest for the production of value-added chemicals. Owing to its high stability, well designed activation of CO2 and a thermodynamic driving force are required for efficient transformations. In this respect, in recent years important developments in the conversion of carbon dioxide into formates, methanol (methoxides), and methane have been reported. For example, under hydrosilylation conditions CO2 is reduced to silyl formates in the presence of catalytic amounts of organic bases, Ru complexes, or Cu complexes; its reduction to silyl methoxides can be catalyzed by N-heterocyclic carbenes; and, when catalyzed by Zr/B(C6F5)3, frustrated Lewis pairs/B(C6F5)3, or Ir-pincer complexes, it can even be reduced to methane. Notably, using hydrogen, the reduction of CO2 to formic acid derivatives can be catalyzed by Rh, Ir, Ru, and Fe complexes. More recently, its reduction to methanol has been achieved using Ru-pincer complexes and multi-catalyst cascade catalysis. Though there exists numerous reactions between amines and CO2, to the best of our knowledge there is only one example known that describes the synthetically interesting methylation of amines by CO2. More specifically, Vaska and co-workers reported the formation of methylamine as a minor product using Ru or Os complexes. However, it was later suspected that an alkyl group exchange was (partially) responsible for the methylamine product. Hence, it remains that no general catalytic methylation reactions using carbon dioxide is known to date. Instead, activated methyl compounds, such as methyl iodide, dimethyl sulfate, MeOTf, diazomethane, and reductive amination systems (HCHO/ reductant), are often used for this purpose. However, the toxicity of most of these reagents and/or the limited substrate scope attract organic chemists to extend this research area. In this regard, it is interesting to note that the use of dimethyl carbonate or methanol as eco-friendly alternatives has been more recently reported as well. Herein, we describe for the first time a single Ru complex that is able to convert carbon dioxide and amines into various kinds of N-methylated products. Our initial design was motivated by previous reports on the dehydration of primary amides using silanes and the hydrosilylation of carboxylic acid derivatives by us and other groups. Hence, we started to investigate the reaction of carbon dioxide and N-methylaniline (1a) in the presence of silanes as a model system (Table 1). To identify active catalysts around 15 different metal precursors including Ru, Rh, Cu and Fe complexes and 12 phosphine and nitrogen ligands were tested using phenyl silane as reductant (Figure S1 and Table S2). As shown in Table 1, commercially available [RuCl2(dmso)4] (dmso = dimethylsulfoxide) proved to be the best catalyst precursor, giving dimethylaniline in 70% yield (Table 1, entry 3); no reaction occurred without the catalyst. To our delight, using nBuPAd2 (4 mol%; Ad = adamantyl) improved the yields up to 92 % (Table 1, entry 5; see also the Supporting Information, Figure S1). Using other types of metal complexes, hydrosilanes, or solvents led to much lower reactivity (2–63 % yield; Table 1, entries 6–9). However, when using highly polar acetonitrile as the solvent, the best reactivity was obtained (98 % yield; Table 1, entry 10). Methylation reactions of nitrogen compounds are of major importance in biology, for example, in epigenetics, embryonic development, and some cancer growth. Therefore, the methylation of different types of amines was studied in

Selective Catalytic Hydrogenations of Nitriles, Ketones, and Aldehydes by Well-Defined Manganese Pincer Complexes

Hydrogenations constitute fundamental processes in organic chemistry and allow for atom-efficient and clean functional group transformations. In fact, the selective reduction of nitriles, ketones, and aldehydes with molecular hydrogen permits access to a green synthesis of valuable amines and alcohols. Despite more than a century of developments in homogeneous and heterogeneous catalysis, efforts toward the creation of new useful and broadly applicable catalyst systems are ongoing. Recently, Earth-abundant metals have attracted significant interest in this area. In the present study, we describe for the first time specific molecular-defined manganese complexes that allow for the hydrogenation of various polar functional groups. Under optimal conditions, we achieve good functional group tolerance, and industrially important substrates, e.g., for the flavor and fragrance industry, are selectively reduced.