Ultra-small cobalt nanoparticles from molecularly-defined Co-salen complexes for catalytic synthesis of amines

Reductive Amination of Aldehyde and Ketone with Ammonia and H2 by an In Situ-Generated Cobalt Catalyst under Mild Conditions

Herein, we present the simplest approach for the synthesis of primary amines via reductive amination using H2 as a reductant and aqueous ammonia as a nitrogen source, catalyzed by amorphous Co particles. The highly active Co particles were prepared in situ by simply mixing commercially available CoCl2 and NaBH4/NaHBEt3 without any ligand or support. This reaction system features mild conditions (80 °C, 1-10 bar), high selectivity (99%), a wide substrate scope, simple operation, and easy separation of the catalyst. The successful large-scale application of this reaction in the production of primary amines suggests its potential industrial interest.

Employing Ammonia for the Synthesis of Primary Amines: Recent Achievements over Heterogeneous Catalysts

Primary amines represent highly privileged chemicals for synthesis of polymers, pharmaceuticals, agrochemicals, coatings, etc. Consequently, the development of efficient and green methodologies for the production of primary amines are of great importance in chemical industry. Owing to the advantages of low cost and ease in availability, ammonia is considered as a feasible nitrogen source for synthesis of N-containing compounds. Thus, the efficient transformation of ammonia into primary amines has received much attention. In this review, the commonly applied synthetic routes to produce primary amines from ammonia were summarized, including the reductive amination of carbonyl compounds, the hydrogen transfer amination of alcohols, the hydroamination of olefins and the arylation with ammonia, in which the catalytic performance of the recent heterogeneous catalysts is discussed. Additionally, various strategies to modulate the surface properties of catalysts are outlined in conjunction with the analysis of reaction mechanism. Particularly, the amination of the biomass-derived substrates is highlighted, which could provide competitive advantages in chemical industry and stimulate the development of sustainable catalysis in the future. Ultimately, perspectives into the challenges and opportunities for synthesis of primary amines with ammonia as N-resource are discussed.

Tweaking Photo CO2 Reduction by Altering Lewis Acidic Sites in Metalated-Porous Organic Polymer for Adjustable H2 /CO Ratio in Syngas Production

Herein, we have specifically designed two metalated porous organic polymers (Zn-POP and Co-POP) for syngas (CO+H2 ) production from gaseous CO2 . The variable H2 /CO ratio of syngas with the highest efficiency was produced in water medium (without an organic hole scavenger and photosensitizer) by utilizing the basic principle of Lewis acid/base chemistry. Also, we observed the formation of entirely different major products during photocatalytic CO2 reduction and water splitting with the help of the two catalysts, where CO (145.65 μmol g-1  h-1 ) and H2 (434.7 μmol g-1  h-1 ) production were preferentially obtained over Co-POP & Zn-POP, respectively. The higher electron density/better Lewis basic nature of Co-POP was investigated further using XPS, XANES, and NH3 -TPD studies, which considerably improve CO2 activation capacity. Moreover, the structure-activity relationship was confirmed via in situ DRIFTS and DFT studies, which demonstrated the formation of COOH* intermediate along with the thermodynamic feasibility of CO2 reduction over Co-POP while water splitting occurred preferentially over Zn-POP.

In-situ reconstruction of CoBOx enables formation of Co for synthesis of benzylamine through reductive amination

Cobalt (Co) as a substitute of noble-metal catalysts shows high catalytic capability for production of the widely used primary amines through the reductive amination. However, the synthesis of Co catalysts usually involves the introduction of organic compounds and the high-temperature pyrolysis, which is complicated and difficult for large-scale applications. Herein, we demonstrated a facile and efficient strategy for the preparation of Co catalysts through the in situ reconstruction of cobalt borate (CoBO_x) during the reductive amination, delivering a high catalytic activity for production of benzylamine from benzaldehyde and ammonia. Initially, CoBO_x was transformed into Co(OH)₂ through the interaction with ammonia and subsequently reduced to Co nanoparticles by H₂ under the reaction environments. The in situ generated Co catalysts exhibited a satisfactory activity and selectivity to the target product, which overmatched the commonly used Co/C, Pt or Raney Ni catalysts. We anticipate that such an in situ reconstruction of CoBO_x by reactants during the reaction could provide a new approach for the design and optimization of catalysts to produce primary amines.

Ambient-Temperature Reductive Amination of 5-Hydroxymethylfurfural Over Al2 O3 -Supported Carbon-Doped Nickel Catalyst

An efficient catalytic system for the conversion of 5-hydroxymethylfurfural (HMF) into N-containing compounds over low-cost non-noble-metal catalysts is preferable, but it is challenging to reach high conversion and selectivity under mild conditions. Herein, an Al₂ O₃ -supported carbon-doped Ni catalyst was obtained via the direct pyrolysis-reduction of a mixture of Ni₃ (BTC)₂  ⋅ 12H₂ O and Al₂ O₃ , generating stable Ni⁰ species due to the presence of carbon residue. A high yield of 96 % was observed in the reductive amination of HMF into 5-hydroxymethyl furfurylamine (HMFA) with ammonia and hydrogen at ambient temperature. The catalyst was recyclable and could be applied to the ambient-temperature synthesis of HMF-based secondary/tertiary amines and other biomass-derived amines from the carbonyl compounds. The significant performance was attributable to the synergistic effect of Ni⁰ species and acidic property of the support Al₂ O₃ , which promoted the selective ammonolysis of the imine intermediate while inhibiting the potential side reaction of over-hydrogenation.

Synergistic Effects of Earth-Abundant Metal-Metal Oxide Enable Reductive Amination of Carbonyls at 50 °C

Reductive amination of carbonyls to primary amines is of importance to the synthesis of fine chemicals; however, this reaction with heterogeneous catalysts containing earth-abundant metals under mild conditions remains scarce. Here, we show that the nickel catalyst with mixed oxidation states enables such synthesis of primary amines under low temperature (50 °C) and H₂ pressure (0.9 MPa). The catalyst shows activity in both water and toluene. The high activity likely results from the formation of small (ca. 4.6 nm) partially oxidized nickel nanoparticles (NPs) homogeneously anchored onto the silica and their synergistic effect. Detailed characterizations indicate stabilization of NPs through strong metal support interaction via electron donation from the metal to support. We identify that the support endowed with an amphoteric nature shows better performance. This strategy of making small metal-metal oxide NPs will open an avenue toward the rational development of efficient catalysts that would allow for other organic transformations under mild reaction conditions.

Facile preparation of ultrafine Pd nanoparticles anchored on covalent triazine frameworks catalysts for efficient N-alkylation

The fabrication of stable and efficient catalysts for green and economic catalytic transformation is significant. Here, highly stable covalent triazine frameworks (CTF-1) were used as the supporting material for anchoring ultrafine Pd nanoparticles (NPs) via a facile impregnation process and a one-pot calcination-reduction strategy. The widespread dispersion of ultrafine Pd NPs was a result of the abundant high nitrogen-content triazine groups of CTF-1 that endowed the catalyst Pd@CTF-1 with high catalytic activity. The catalytic performance of Pd@CTF-1 was demonstrated by the one-pot N-alkylation of benzaldehyde with aniline (or nitrobenzene) under mild reaction conditions, and Pd@CTF-1 exhibited a wide range of general applicability for N-alkylation reactions. The reaction mechanism for the N-alkylation reaction was also studied in detail. In addition, the Pd@CTF-1 catalyst exhibited high thermal and chemical stability, maintaining good catalytic efficiency after multiple reaction cycles. This study provides new insights for the fabrication of organic supporting materials with highly dispersed active catalytic sites that can lead to excellent catalytic performance for efficient, economical, and green reactions.

The mechanism of direct reductive amination of aldehyde and amine with formic acid catalyzed by boron trifluoride complexes: insights from a DFT study

A volcano diagram of BF3 catalytic species and their activities was proposed for the DRA of aldehyde and amine with formic acid.

Highly active heterogeneous hydrogenation catalysts prepared from cobalt complexes and rice husk waste

Highly active heterogeneous catalysts for the hydrogenation of nitro compounds were made by pyrolysis of rice husk waste impregnated with cobalt complexes followed by base-treatment. The catalysts show high selectivity and broad substrate scope.

Direct reductive amination of ketones with ammonium salt catalysed by Cp*Ir(III) complexes bearing an amidato ligand

A series of half-sandwich Ir(III) complexes 1-6 bearing an amidato bidentate ligand were conveniently synthesized and applied to the catalytic Leuckart-Wallach reaction to produce racemic α-chiral primary amines. With 0.1 mol% of complex 1, a broad range of ketones, including aryl ketones, dialkyl ketones, cyclic ketones, α-keto acids, α-keto esters and diketones, could be transformed to their corresponding primary amines with moderate to excellent yields (40%-95%). Asymmetric transformation was also attempted with chiral Ir complexes 3-6, and 16% ee of the desired primary amine was obtained. Despite the unsatisfactory enantio-control achieved so far, the current exploration might stimulate more efforts towards the discovery of better chiral catalysts for this challenging but important transformation.

Cobalt-catalyzed cross-coupling reactions of aryl- and alkylaluminum derivatives with (hetero)aryl and alkyl bromides

A simple cobalt complex, such as Co(phen)Cl₂, turned out to be a highly efficient and cheap precatalyst for a host of cross-coupling reactions involving aromatic and aliphatic organoaluminum reagents with aryl, heteroaryl and alkyl bromides. New C(sp²)-C(sp²) and C(sp²)-C(sp³) bonds were formed in good to excellent yields and with high chemoselectivity, under mild reaction conditions.

Bifunctional Catalyst Derived from Sulfur-Doped VMoOx Nanolayer Shelled Co Nanosheets for Efficient Water Splitting

A novel sulfur-doped vanadium-molybdenum oxide nanolayer shelling over two-dimensional cobalt nanosheets (2D Co@S-VMoO_x NSs) was synthesized via a facile approach. The formation of such a unique 2D core@shell structure together with unusual sulfur doping effect increased the electrochemically active surface area and provided excellent electric conductivity, thereby boosting the activities for hydrogen evolution reaction (HER) and oxygen evolution reaction (OER). As a result, only low overpotentials of 73 and 274 mV were required to achieve a current response of 10 mA cm^-2 toward HER and OER, respectively. Using the 2D Co@S-VMoO_x NSs on nickel foam as both cathode and anode electrode, the fabricated electrolyzer showed superior performance with a small cell voltage of 1.55 V at 10 mA cm^-2 and excellent stability. These results suggested that the 2D Co@S-VMoO_x NSs material might be a potential bifunctional catalyst for green hydrogen production via electrochemical water splitting.

Earth-abundant Metal-catalyzed Reductive Amination: Recent Advances and Prospect for Future Catalysis

Nitrogen-containing compounds, as an important class of chemicals, have been used widely in pharmaceuticals, materials synthesis. Transition metal-catalyzed reductive amination of an aldehyde or a ketone with ammonia or an amine has been proved to be an efficient and practical method for the preparation of nitrogen-containing compounds in academia and industry for a century. Given the above, several effective methods using transition metals have been developed in recent years. Noble transition metals like Pd, Pt, and Au-based catalysts have been predominately used in reductive amination. Because of their high prices, strict official regulations of residues in pharmaceuticals, and deleterious effects on the biological system, their industrial applications are severely hampered. With the increasing sustainable and environmental problems, the Earth-abundant transition metals including Ti, Fe, Co, Ni, and Zr have also been investigated for the reductive amination reaction and showed great potential to the advancement of sustainable and cost-effective reductive amination processes. This critical review will mainly summarize the work using Earth-abundant metals. The effects of different transition metals used in catalytic reduction amination were discussed and compared, and some suggestions were given. The last section highlights the catalytic activities of bi- and tri-metallic catalysts. Indeed, this latter family is very promising and simultaneously benefits from increased stability, and selectivity, compared to monometallic NPs, due to synergistic substrate activation. Few comprehensive reviews focusing on Earth-abundant transition metals catalyst has been published since 1948, although several authors reported some summaries dealing with one or the other part of this aspect. It is hoped that this critical review will inspire researchers to develop new efficient and selective earth-abundant metal catalysts for highly, environmentally sustainable reductive amination methods, as well as improve the pharmaceutical industry and related chemical synthesis company traditional method with the utilization of the green method widely.

Difunctionalization of Alkynones by Base-Mediated Reaction with α,α-Dithioketones

A novel 1,2-difunctionalization of alkynones via an umpolung strategy for the synthesis of tetrasubstituted olefins has been developed. This procedure is realized by a formal C-C σ-bond cleavage reaction of cyclic α,α-dithioketones and subsequent deprotection. Notable features of this approach include excellent yields, mild reaction conditions, a broad substrate scope, and operational simplicity.

Co-Catalyzed Synthesis of Primary Amines via Reductive Amination employing Hydrogen under very mild Conditions

Nanostructured and reusable 3d-metal catalysts that operate with high activity and selectivity in important chemical reactions are highly desirable. Here, a cobalt catalyst was developed for the synthesis of primary amines via reductive amination employing hydrogen as the reducing agent and easy-to-handle ammonia, dissolved in water, as the nitrogen source. The catalyst operates under very mild conditions (1.5 mol% catalyst loading, 50 °C and 10 bar H₂ pressure) and outperforms commercially available noble metal catalysts (Pd, Pt, Ru, Rh, Ir). A broad scope and a very good functional group tolerance were observed. The key for the high activity seemed to be the used support: an N-doped amorphous carbon material with small and turbostratically disordered graphitic domains, which is microporous with a bimodal size distribution and with basic NH functionalities in the pores.

Sonochemical synthesis of Co3O4 nanoparticles deposited on GO sheets and their potential application as a nanofiller in MMMs for O2/N2 separation

In this study we report an environmentally friendly, facile and straightforward sonochemical synthetic strategy for a Co3O4/GO nanocomposite using N,N'-bis(salicylidene)ethylenediaminocobalt(ii) as a precursor and graphene oxide sheets as an immobilization support for Co3O4 nanoparticles. The synthesis was facilitated by physical and chemical effects of cavitation bubbles. The synthesized nanocomposite was thoroughly characterized for its composition and morphology using Fourier transform infrared spectroscopy (FTIR), Energy dispersive X-ray spectroscopy (EDS), Scanning electron microscopy (SEM), UV-visible, Raman and X-ray diffraction spectroscopy (XRD), etc. The results show Co3O4 nanoparticles of 10 nm (SD 3 nm) were prepared on well exfoliated sheets of GO. The applicability of the synthesized Co3O4/GO nanocomposite was optimized as a nanofiller for mixed matrix membranes (MMMs) comprised of poly(2-acrylamido-2-methyl-1-propanesulfonic acid) and polyvinyl chloride. The affinity of the prepared MMMs was evaluated for the separation of O2/N2 gases by varying the concentration of nanofiller, i.e. 0.03%, 0.04%, 0.05% and 0.075% (w/v). The results display high separation performance for O2/N2 gases with excellent permeance (N2 167 GPU and O2 432 GPU at 1 bar) and O2/N2 selectivity of 2.58, when the MMMs were loaded with 0.05% (w/v) of Co3O4/GO nanocomposite.

Air Stable Iridium Catalysts for Direct Reductive Amination of Ketones

Half-sandwich iridium complexes bearing bidentate urea-phosphorus ligands were found to catalyze the direct reductive amination of aromatic and aliphatic ketones under mild conditions at 0.5 mol % loading with high selectivity towards primary amines. One of the complexes was found to be active in both the Leuckart-Wallach (NH₄ CO₂ H) type reaction as well as in the hydrogenative (H₂ /NH₄ AcO) reductive amination. The protocol with ammonium formate does not require an inert atmosphere, dry solvents, as well as additives and in contrast to previous reports takes place in hexafluoroisopropanol (HFIP) instead of methanol. Applying NH₄ CO₂ D or D₂ resulted in a high degree of deuterium incorporation into the primary amine α-position.

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.

Co(III)-Salen immobilized cellulose nanocrystals for efficient catalytic CO2 fixation into cyclic carbonates under mild conditions

Searching for green, recyclable and highly efficient catalyst for the synthesis of cyclic carbonates from CO₂ is of great importance because it is profitable for reducing the greenhouse effects and meets the principles of green chemistry. Herein, a series of cellulose nanocrystals, either the pristine or modified ones (TEMPO oxidized and Co(III)salen immobilized), were explored as catalysts for cycloaddition of epoxides and carbon dioxide. The impact of surface properties on the performance of the as-made catalysts was investigated. Co(III)-salen grafted cellulose nanocrystals was proven to be the most effective catalyst in this study, which could afford excellent yield up to 99 % after 24 h even under low CO₂ pressures of 0.1 MPa. They can be easily recovered and reused for at least 4 times, demonstrating their excellent stability. We found that the surface functional groups such as enriched sulfate or carboxylic groups could also account for the enhanced catalytic activity. This work highlights the applications of green and sustainable nanoparticles in a cycloaddition reaction and offers a sustainable solution in industrial catalysis related to CO₂ conversions.

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. Many of these reduction steps require the presence of a catalyst to activate the reducing agent. The reductive amination is impressive with regard to the product scope since primary, secondary, and tertiary alkyl amines are accessible and hydrogen is the most attractive reducing agent, especially if large-scale product formation is an issue, since hydrogen is inexpensive and abundantly available. Alkyl amines are intensively produced and use fine and bulk chemicals. They are key functional groups in many pharmaceuticals, agro chemicals, or materials. In this review, we summarize the work published on reductive amination employing hydrogen as the reducing agent. No comprehensive review focusing on this subject has been published since 1948, albeit many interesting summaries dealing with one or the other aspect of reductive amination have appeared. Impressive progress in using catalysts based on earth-abundant metals, especially nanostructured heterogeneous catalysts, has been made during the early development of the field and in recent years.

Catalytic reductive aminations using molecular hydrogen for synthesis of different kinds of amines

Catalytic reductive aminations using molecular hydrogen represent an essential and widely used methodology for the synthesis of different kinds of amines.