A Catalytic Version of the Knorr Pyrrole Synthesis Permits Access to Pyrroles and Pyridines.

Aromatic -heterocycles, such as pyrroles and pyridines, are important natural products and bulk and fine chemicals with numerous applications as active ingredients of pharmaceuticals and agrochemicals, as catalysts, and in materials sciences. We report here a catalytic version of the Knorr pyrrole synthesis in which simple and diversely available starting materials, such as 1,2-amino alcohols or 1,3-amino alcohols and keto esters, undergo a dehydrogenative coupling to form pyrroles and pyridines, respectively. Our reaction forms hydrogen as a collectible (and usable) byproduct and is mediated by a well-defined Mn catalyst.

A Selective Iron(I) Hydrogenation Catalyst.

Iron is the most abundant transition metal of the Earth's crust, and the understanding of its function in key technologies, such as catalysis, is highly important. We report here on an iron(I) hydrogenation catalyst. Our catalyst activates hydrogen via heterolytic bond cleavage, forms a monohydride, and hydrogenates polar double bonds via a bimetallic pathway (potassium-assisted hydride transfer).

Rational design of N-heterocyclic compound classes via regenerative cyclization of diamines.

The discovery of reactions is a central topic in chemistry and especially interesting if access to compound classes, which have not yet been synthesized, is permitted. N-Heterocyclic compounds are very important due to their numerous applications in life and material science. We introduce here a consecutive three-component reaction, classes of N-heterocyclic compounds, and the associated synthesis concept (regenerative cyclisation).

Transition-Metal-Catalyzed Reductive Amination Employing Hydrogen.

The reductive amination, the reaction of an aldehyde or a ketone with ammonia or an amine in the presence of a reducing agent and often a catalyst, is an important amine synthesis and has been intensively investigated in academia and industry for a century. Besides aldehydes, ketones, or amines, starting materials have been used that can be converted into an aldehyde or ketone (for instance, carboxylic acids or organic carbonate or nitriles) or into an amine (for instance, a nitro compound) in the presence of the same reducing agent and catalyst. Mechanistically, the reaction starts with a condensation step during which the carbonyl compound reacts with ammonia or an amine, forming the corresponding imine followed by the reduction of the imine to the alkyl amine product.

Chromium-Catalyzed Alkylation of Amines by Alcohols.

The alkylation of amines by alcohols is a broadly applicable, sustainable, and selective method for the synthesis of alkyl amines, which are important bulk and fine chemicals, pharmaceuticals, and agrochemicals. We show that Cr complexes can catalyze this C-N bond formation reaction. We synthesized and isolated 35 examples of alkylated amines, including 13 previously undisclosed products, and the use of amino alcohols as alkylating agents was demonstrated.

Manganese-Catalyzed β-Methylation of Alcohols by Methanol.

We report an earth-abundant-metal-catalyzed double and single methylation of alcohols. A manganese catalyst, which operates at low catalyst loadings and short reaction times, mediates these reactions efficiently. A broad scope of primary and secondary alcohols, including purely aliphatic examples, and 1,2-aminoalcohols can be methylated.

Mechanistic Studies of Hydride Transfer to Imines from a Highly Active and Chemoselective Manganate Catalyst.

We introduce a highly active and chemoselective manganese catalyst for the hydrogenation of imines. The catalyst has a large scope, can reduce aldimines and ketimines, and tolerates a variety of functional groups, among them hydrogenation sensitive examples such as an olefin, a ketone, nitriles, nitro groups, and an aryl iodo substituent or a benzyl ether. We could investigate the transfer step between imines and the hydride complex in detail.

3d-Metal Catalyzed N- and C-Alkylation Reactions via Borrowing Hydrogen or Hydrogen Autotransfer.

The conservation of our element resources is a fundamental challenge of mankind. The development of alcohol refunctionalization reactions is a possible fossil carbon conservation strategy since alcohols can be obtained from indigestible and abundantly available biomass. The conservation of our rare noble metals, frequently used in key technologies such as catalysis, might be feasible by replacing them with highly abundant metals.

Manganese-Catalyzed Dehydrogenative Alkylation or α-Olefination of Alkyl-Substituted N-Heteroarenes with Alcohols.

Catalysis with earth-abundant transition metals is an option to help save our rare noble-metal resources and is especially interesting when novel reactivity or selectivity patterns are observed. We report here on a novel reaction, namely the dehydrogenative alkylation or α-olefination of alkyl-substituted N-heteroarenes with alcohols. Manganese complexes developed in our laboratory catalyze the reaction with high efficiency whereas iron and cobalt complexes stabilized by the same ligands are essentially inactive.

Manganese-Catalyzed Sustainable Synthesis of Pyrroles from Alcohols and Amino Alcohols.

The development of reactions that convert alcohols into important chemical compounds saves our fossil carbon resources as alcohols can be obtained from indigestible biomass such as lignocellulose. The conservation of our rare noble metals is of similar importance, and their replacement by abundantly available transition metals, such as Mn, Fe, or Co (base or nonprecious metals), in key technologies such as catalysis is a promising option. Herein, we report on the first base-metal-catalyzed synthesis of pyrroles from alcohols and amino alcohols.

Cobalt-Catalyzed Alkylation of Secondary Alcohols with Primary Alcohols via Borrowing Hydrogen/Hydrogen Autotransfer.

Alcohols are promising sustainable starting materials because they can be obtained from abundant and indigestible biomass. The substitution of expensive noble metals in catalysis by earth abundant 3d metals, such as Mn, Fe, or Co, (nonprecious or base metals) is a related key concept with respect to sustainability. Here, we report on the first cobalt-catalyzed alkylation of secondary alcohols with primary alcohols.

Synthesis of meta-Functionalized Pyridines by Selective Dehydrogenative Heterocondensation of β- and γ-Amino Alcohols.

New reactions that convert alcohols into important classes of compounds are becoming increasingly important as their development contributes to the conservation of our fossil carbon feedstock and the reduction of CO emissions. Two key catalytic alcohol conversion concepts are borrowing hydrogen or hydrogen autotransfer and acceptorless dehydrogenative condensation. Herein, we combined both concepts to synthesize meta-functionalized pyridines.

Highly Active and Selective Manganese C=O Bond Hydrogenation Catalysts: The Importance of the Multidentate Ligand, the Ancillary Ligands, and the Oxidation State.

The replacement of expensive noble metals by earth-abundant transition metals is a central topic in catalysis. Herein, we introduce a highly active and selective homogeneous manganese-based C=O bond hydrogenation catalyst. Our catalyst has a broad substrate scope, it is able to hydrogenate aryl-alkyl, diaryl, dialkyl, and cycloalkyl ketones as well as aldehydes.

Cobalt-Catalyzed Alkylation of Aromatic Amines by Alcohols.

The implementation of inexpensive, Earth-abundant metals in typical noble-metal-mediated chemistry is a major goal in homogeneous catalysis. A sustainable or green reaction that has received a lot of attention in recent years and is preferentially catalyzed by Ir or Ru complexes is the alkylation of amines by alcohols. It is based on the borrowing hydrogen or hydrogen autotransfer concept.

New iridium catalysts for the selective alkylation of amines by alcohols under mild conditions and for the synthesis of quinolines by acceptor-less dehydrogenative condensation.

A novel family of iridium catalysts stabilised by P,N-ligands have been introduced. The ligands are based on imidazo[1,5-b]pyridazin-7-amines and can be synthesised with a broad variety of substitution patterns. The catalysts were synthesised quantitatively from the protonated ligands and a commercially available iridium precursor.

The synthesis of benzimidazoles and quinoxalines from aromatic diamines and alcohols by iridium-catalyzed acceptorless dehydrogenative alkylation.

Benzimidazoles and quinoxalines are important N-heteroaromatics with many applications in pharmaceutical and chemical industry. Here, the synthesis of both classes of compounds starting from aromatic diamines and alcohols (benzimidazoles) or diols (quinoxalines) is reported. The reactions proceed through acceptorless dehydrogenative condensation steps.

Ultrasound-assisted design of metal nanocomposites.

A one-step method was developed to produce metal nanocomposites from metal alloys under ultrasound irradiation. Systematic investigation of ultrasound effects on various metal particles reveals cavitation-induced recrystallization and oxidation of metals as main factors in the process. The fact that different metals react in dramatically different fashion towards ultrasound irradiation was exploited for the formation of nanoscale composites.