Homogeneous and heterogeneous catalytic reduction of amides and related compounds using molecular hydrogen

ChemSpaX: exploration of chemical space by automated functionalization of molecular scaffold

Exploration of the local chemical space of molecular scaffolds by post-functionalization (PF) is a promising route to discover novel molecules with desired structure and function. PF with rationally chosen substituents based on known electronic and steric properties is a commonly used experimental and computational strategy in screening, design and optimization of catalytic scaffolds. Automated generation of reasonably accurate geometric representations of post-functionalized molecular scaffolds is highly desirable for data-driven applications. However, automated PF of transition metal (TM) complexes remains challenging. In this work a Python-based workflow, ChemSpaX, that is aimed at automating the PF of a given molecular scaffold with special emphasis on TM complexes, is introduced. In three representative applications of ChemSpaX by comparing with DFT and DFT-B calculations, we show that the generated structures have a reasonable quality for use in computational screening applications. Furthermore, we show that ChemSpaX generated geometries can be used in machine learning applications to accurately predict DFT computed HOMO-LUMO gaps for transition metal complexes. ChemSpaX is open-source and aims to bolster and democratize the efforts of the scientific community towards data-driven chemical discovery.

Homogeneous Catalysis for Sustainable Energy: Hydrogen and Methanol Economies, Fuels from Biomass, and Related Topics

As the world pledges to significantly cut carbon emissions, the demand for sustainable and clean energy has now become more important than ever. This includes both production and storage of energy carriers, a majority of which involve catalytic reactions. This article reviews recent developments of homogeneous catalysts in emerging applications of sustainable energy. The most important focus has been on hydrogen storage as several efficient homogeneous catalysts have been reported recently for (de)hydrogenative transformations promising to the hydrogen economy. Another direction that has been extensively covered in this review is that of the methanol economy. Homogeneous catalysts investigated for the production of methanol from CO2, CO, and HCOOH have been discussed in detail. Moreover, catalytic processes for the production of conventional fuels (higher alkanes such as diesel, wax) from biomass or lower alkanes have also been discussed. A section has also been dedicated to the production of ethylene glycol from CO and H2 using homogeneous catalysts. Well-defined transition metal complexes, in particular, pincer complexes, have been discussed in more detail due to their high activity and well-studied mechanisms.

Hydrogenative Depolymerization of Polyurethanes Catalyzed by a Manganese Pincer Complex

Chemical recycling, in particular hydrogenative depolymerization, offers a promising way to utilize plastic waste. This report covers the manganese-catalyzed hydrogenation of polyurethane materials to the corresponding monomeric units. The key to success is a Mn pincer complex as a potent hydrogenation catalyst in combination with elevated temperatures (up to 200 °C) and appropriate solvents to ensure sufficient solubility of the polymers. A wide range of polyurethane samples of varying polyol and isocyanate compositions, some of which feature significant amounts of urea functionalities, are depolymerized, releasing polyetherols and diaminotoluene (TDA) in yields of up to 89 % and 76 %, respectively.

Nano Ag/AgCl wires-photocatalyzed hydrogen production and transfer hydrogenation of Knoevenagel-type products

An investigation of the relationship between the morphology of Ag/AgCl nanostructured composites with their catalytic performance has been reported.

Additive-free selective methylation of secondary amines with formic acid over a Pd/In2O3 catalyst

Formic acid is the sole carbon and hydrogen source in the additive-free catalytic methylation of amines.

Hydroborative reduction of amides to amines mediated by La(CH2C6H4NMe2-o)3

La(CH2C6H4NMe2-o)3/HBpin is an efficient catalytic system for the deoxygenative reduction of primary, secondary and tertiary amides to amines.

Computational Studies on the Mechanisms for Deaminative Amide Hydrogenation by Homogeneous Bifunctional Catalysts

The deaminative hydrogenation of amides is one of the most convenient pathways for the synthesis of amines and alcohols. The ideal source of reducing equivalents for this reaction is molecular hydrogen, though, in practice, this approach requires high pressures and temperatures, with many catalysts achieving only small turnover numbers and frequencies. Nonetheless, during the last ten years, this field has made major advances towards larger turnovers under milder conditions thanks to the development of bifunctional catalysts. These systems promote the heterolytic cleavage of hydrogen into proton and hydride by combining a basic ligand with an acidic metal centre. The present review focuses on the computational study of the reaction mechanism underlying bifunctional catalysis. This review is structured around the fundamental steps of this mechanism, namely the C=O and C–N hydrogenation of the amide, the C–N protonolysis of the hemiaminal, the C=O hydrogenation of the aldehyde, and the competition between hydrogen activation and catalyst deactivation. In line with the complexity of the mechanism, we also provide a perspective on the use of microkinetic models. Both Noyori- and Milstein-type catalysts are discussed and compared.

A Dual Anchoring Strategy for the Directed Evolution of Improved Artificial Transfer Hydrogenases Based on Carbonic Anhydrase

Artificial metalloenzymes result from anchoring a metal cofactor within a host protein. Such hybrid catalysts combine the selectivity and specificity of enzymes with the versatility of (abiotic) transition metals to catalyze new-to-nature reactions in an evolvable scaffold. With the aim of improving the localization of an arylsulfonamide-bearing iridium-pianostool catalyst within human carbonic anhydrase II (hCAII) for the enantioselective reduction of prochiral imines, we introduced a covalent linkage between the host and the guest. Herein, we show that a judiciously positioned cysteine residue reacts with a p-nitropicolinamide ligand bound to iridium to afford an additional sulfonamide covalent linkage. Three rounds of directed evolution, performed on the dually anchored cofactor, led to improved activity and selectivity for the enantioselective reduction of harmaline (up to 97% ee (R) and >350 turnovers on a preparative scale). To evaluate the substrate scope, the best hits of each generation were tested with eight substrates. X-ray analysis, carried out at various stages of the evolutionary trajectory, was used to scrutinize (i) the nature of the covalent linkage between the cofactor and the host as well as (ii) the remodeling of the substrate-binding pocket.

Facile Synthesis of Alkylidene Phthalides by Rhodium-Catalyzed Domino C-H Acylation/Annulation of Benzamides with Aliphatic Carboxylic Acids

The Rh-catalyzed ortho-C(sp² )-H functionalization of 8-aminoquinoline-derived benzamides with aliphatic acyl fluorides generated in situ from the corresponding acids has been developed. This reaction initiated with 8-aminoquinoline-directed ortho-C(sp² )-H acylation, which was accompanied by subsequent intramolecular nucleophilic acyl substitution of amide group to produce alkylidene phthalides This approach exhibits high stereo-selectivity for Z-isomer products, and tolerates a variety of functional groups as well as aliphatic carboxylic acids with diverse structural scaffolds.

Hydrogenation of Secondary Amides using Phosphane Oxide and Frustrated Lewis Pair Catalysis

The metal‐free catalytic hydrogenation of secondary carboxylic acid amides is developed. The reduction is realized by two new catalytic reactions. First, the amide is converted into the imidoyl chloride by triphosgene (CO(OCCl₃)₂) using novel phosphorus(V) catalysts. Second, the in situ generated imidoyl chlorides are hydrogenated in high yields by an FLP‐catalyst. Mechanistic and quantum mechanical calculations support an autoinduced catalytic cycle for the hydrogenation with chloride acting as unusual Lewis base for FLP‐mediated H₂‐activation.

Metal-free, Oxidative α-Coupling of Aldehydes with Amine Nucleophiles for the Preparation of Congested C(sp3)-N Bonds

This article discloses the direct α-amination of α-branched aldehydes applying nitrogen-based nucleophiles. Under organocatalyzed, oxidative conditions α-branched aldehydes are umpoled to their electrophilic synthons and, subsequently, displaced by a variety of nucleophilic amines to form tetrasubstituted tertiary centers. A similar strategy has been previously employed to form congested C-C, C-O, and C-S bonds; however, unsatisfactory results were received when extending the methodology to include C-N bonds. Initially, intramolecular α-amination reactions were undertaken to foster dihydroquinoxaline-type products. A solvent exchange to the polar, aprotic solvent, MeNO₂, proved critical to facilitate intermolecular α-C-N bond formation with a wide range of amine coupling partners (N-heterocycles, N,N-diaryl amines, and anilines). Application of the solvent exchange to the enantioselective S_N2-DKR manifold provided distinct regimes leading to refinement in yield and enantioselectivity.

Depolymerization of Technical-Grade Polyamide 66 and Polyurethane Materials through Hydrogenation

Chemical recycling provides a promising solution to utilize plastic waste. Here, a catalytic hydrogenative depolymerization of polyamide 66 (PA 66) and polyurethane (PU) was developed. The system employed Ru pincer complexes at high temperature (200 °C) in THF solution, and even technical-grade polymers could be hydrogenated with satisfactory yields under these conditions. A comparison of the system with some known heterogeneous catalysts as well as catalyst poisoning tests supported the homogeneity of the system. These results demonstrate the potential of chemical recycling to regain building blocks for polymers and will be interesting for the further development of polymer hydrogenation.

Asymmetric Deoxygenative Alkynylation of Tertiary Amides Enabled by Iridium/Copper Bimetallic Relay Catalysis

A variety of inert tertiary amides have been successfully transformed into synthetically important chiral propargylamines in high yields with good to excellent enantioselectivities via a relayed sequence of Ir catalyzed partial reduction and Cu/GARPHOS catalyzed asymmetric alkynylation with terminal alkynes. The reaction was readily extended to some drug molecules and the transformations of representative products have been demonstrated, thus attesting the practical utilities and the robust nature of the protocol.

Catalytic Reductive Alcohol Etherifications with Carbonyl-Based Compounds or CO2 and Related Transformations for the Synthesis of Ether Derivatives

Ether derivatives have myriad applications in several areas of chemical industry and academia. Hence, the development of more effective and sustainable protocols for their production is highly desired. Among the different methodologies reported for ether synthesis, catalytic reductive alcohol etherifications with carbonyl-based moieties (aldehydes/ketones and carboxylic acid derivatives) have emerged in the last years as a potential tool. These processes constitute appealing routes for the selective production of both symmetrical and asymmetrical ethers (including O-heterocycles) with an increased molecular complexity. Likewise, ester-to-ether catalytic reductions and hydrogenative alcohol etherifications with CO2 to dialkoxymethanes and other acetals, albeit in less extent, have undergone important advances, too. In this Review, an update of the recent progresses in the area of catalytic reductive alcohol etherifications using carbonyl-based compounds and CO2 have been described with a special focus on organic synthetic applications and catalyst design. Complementarily, recent progress made in catalytic acetal/ketal-to-ether or ester-to-ether reductions and other related transformations have been also summarized.

Reduction of Amides to Amines with Pinacolborane Catalyzed by Heterogeneous Lanthanum Catalyst La(CH2C6H4NMe2-o)3@SBA-15

Hydroboration of amides is a useful synthetic strategy to access the corresponding amines. In this contribution, it was found that the supported lanthanum benzyl material La(CH₂C₆H₄NMe₂-o)₃@SBA-15 was highly active for the hydroboration of primary, secondary, and tertiary amides to amines with pinacolborane. These reactions selectively produced target amines and showed good tolerance for functional groups such as -NO₂, -halogen, and -CN, as well as heteroatoms such as S and O. This reduction procedure exhibited the recyclable and reusable property of heterogeneous catalysts and was applicable to gram-scale synthesis. The reaction mechanisms were proposed based on some control experiments and the previous literature. This is the first example of hydroborative reduction of amides to amines mediated by heterogeneous catalysts.

Indirect reduction of CO2 and recycling of polymers by manganese-catalyzed transfer hydrogenation of amides, carbamates, urea derivatives, and polyurethanes

The reduction of polar bonds, in particular carbonyl groups, is of fundamental importance in organic chemistry and biology. Herein, we report a manganese pincer complex as a versatile catalyst for the transfer hydrogenation of amides, carbamates, urea derivatives, and even polyurethanes leading to the corresponding alcohols, amines, and methanol as products. Since these compound classes can be prepared using CO2 as a C1 building block the reported reaction represents an approach to the indirect reduction of CO2. Notably, these are the first examples on the reduction of carbamates and urea derivatives as well as on the C-N bond cleavage in amides by transfer hydrogenation. The general applicability of this methodology is highlighted by the successful reduction of 12 urea derivatives, 26 carbamates and 11 amides. The corresponding amines, alcohols and methanol were obtained in good to excellent yields up to 97%. Furthermore, polyurethanes were successfully converted which represents a viable strategy towards a circular economy. Based on control experiments and the observed intermediates a feasible mechanism is proposed.

Merging Electron Transfer with 1,2-Metalate Rearrangement: Deoxygenative Arylation of Aromatic Amides with Arylboronic Esters

Amides are essentially inert carboxyl derivatives in many types of chemical transformations. In particular, deoxygenative C-C bond formation of amides to synthetically important amines is a long-standing challenge for synthetic chemists due to the inertness of the resonance-stabilized amide C=O bond. Herein, it is disclosed that by merging electron-transfer-induced activation with 1,2-metalate rearrangement, a wide range of aromatic amides react smoothly with arylboron reagents, affording a series of biologically relevant diarylmethylamines as deoxygenative C-C bond cross-coupling products. With its simplicity and versatility, this reaction shows great promise in the synthesis of amines from amides, which may open up new avenues in retrosynthetic planning and find widespread use in academia and industry.

Identifying the preferential pathways of CO2 capture and hydrogenation to methanol over an Mn(I)-PNP catalyst: a computational study

CO2 hydrogenation to CH3OH is a crucial conversion for several purposes. Density functional theory (DFT) studies have been performed to explore the mechanistic pathways of newly reported CO2 capture and hydrogenation to methanol. The present study describes the multistep transformation of CO2 to methanol. In this case we have introduced 2-amino-1-propanol to capture CO2 and hydrogenation of the CO2 captured product (oxazolidinone) in the presence of an active Mn(i)-PNP based catalyst. All the plausible pathways for oxazolidinone hydrogenation to methanol have been explored in detail. Here, hydride and proton transfer steps are very important for oxazolidinone hydrogenation, whereas heterolytic H2 cleavage is the most important step for the regeneration of the catalyst. Our detailed study shows that C-N bond hydrogenation followed by C-O and C[double bond, length as m-dash]O bond hydrogenations or C-O bond hydrogenation followed by C-N and C[double bond, length as m-dash]O bond hydrogenations are the most favourable pathways for oxazolidinone hydrogenation to methanol with a total reaction free energy barrier of 36.9 kcal mol-1 for both the pathways in the presence of a Mn(i)-PNP catalyst.

Catalytic Hydrogenation of Polyurethanes to Base Chemicals: From Model Systems to Commercial and End-of-Life Polyurethane Materials

Polyurethane (PU) is a highly valued polymer prepared from diisocyanates and polyols, and it is used in everyday products, such as shoe soles, mattresses, and insulation materials, but also for the construction of sophisticated parts of medical devices, wind turbine blades, aircrafts, and spacecrafts, to name a few. As PU is most commonly used as a thermoset polymer composed of cross-linked structures, its recycling is complicated and inefficient, leading to increasing PU waste accumulating every year. Catalytic hydrogenation represents an atom-efficient means for the deconstruction of polyurethanes, but so far the identification of an efficient catalyst for the disassembly of real-life and end-of-life PU samples has not been demonstrated. In this work, we reveal that a commercially available catalyst, Ir- iPrMACHO, under 30 bar H2 and 150-180 °C, is a general catalyst for the effective hydrogenation of the four cornerstones of PU: flexible solid, flexible foamed, rigid solid, and rigid foamed, leading to the isolation of aromatic amines and a polyol fraction. For the first time, a variety of commercial PU materials, including examples of foams, inline skating wheels, shoe soles, and insulation materials, has been deconstructed into the two fractions. Most desirable, our reaction conditions include the use of isopropyl alcohol as a representative of a green solvent. It is speculated that a partial glycolysis at the surface of the PU particles is taking place in this solvent and reaction temperatures in the presence of catalytic amounts of base. As such a more efficient hydrogenation of the solubilized PU fragments in isopropyl alcohol becomes possible. As the isolated anilines are precursors to the original isocyanate building blocks, and methods for their conversion are well-known, the work reported in this paper provides a realistic indication of a potential circular plastic economy solution for PU. Preliminary experiments were also undertaken applying Mn- iPrMACHO for the deconstruction of a commercial flexible PU foam. Although successful, more forcing conditions were required than those when applying Ir- iPrMACHO.

3-(tert-Butyl)-N-(4-methoxybenzyl)-1-methyl-1H-pyrazol-5-amine

We reported an efficient one-pot two-step synthesis of 3-(tert-butyl)-N-(4-methoxybenzyl)-1-methyl-1H-pyrazol-5-amine 3 in good yield by a solvent-free condensation/reduction reaction sequence starting from 3-(tert-butyl)-1-methyl-1H-pyrazol-5-amine 1 and p-methoxybenzaldehyde 2. The one-pot reductive amination proceeded by the formation in situ of the N-(5-pyrazolyl)imine 4 as key synthetic intermediate of other valuable pyrazole derivatives. This methodology is distinguished by its operational easiness, short reaction time, isolation and purification of the aldimine intermediate is not required. The structure of the synthesized N-heterocyclic amine 3 was fully characterized by FTIR-ATR, 1D and 2D NMR experiments, EIMS, and elemental analysis.

General Synthesis of Secondary Alkylamines by Reductive Alkylation of Nitriles by Aldehydes and Ketones

The development of C-N bond formation reactions is highly desirable due to their importance in biology and chemistry. Recent progress in 3d metal catalysis is indicative of unique selectivity patterns that may permit solving challenges of chemical synthesis. We report here on a catalytic C-N bond formation reaction-the reductive alkylation of nitriles. Aldehydes or ketones and nitriles, all abundantly available and low-cost starting materials, undergo a reductive coupling to form secondary alkylamines and inexpensive hydrogen is used as the reducing agent. The reaction has a very broad scope and many functional groups, including hydrogenation-sensitive examples, are tolerated. We developed a novel cobalt catalyst, which is nanostructured, reusable, and easy to handle. The key seems the earth-abundant metal in combination with a porous support material, N-doped SiC, synthesized from acrylonitrile and a commercially available polycarbosilane.

The cooperative role of innocent ligand in N-heterocyclic carbene manganese catalyzed carbon dioxide hydrogenation

The concept of Lewis acidic π* cooperation was proposed for innocent CO ligand in NHC–Mn catalyzed CO2 hydrogenation by systematic DFT studies.