Hydrogenation Of Imines Research Articles

Thermo-processable chitosan-based plastic substitute with self-adaptiveness and closed-loop recyclability

The increasing environmental burden generated by disposable plastic wastes makes the development of sustainable substitute materials an emergent task. As one of the most abundant bioresources, chitosan (CS) has been considered as a potential candidate for plastic substitution. Conventionally, CS-based materials are fabricated through a solution-processing procedure due to the high crystallinity of CS. Herein, we designed a CS-based material via integrating CS into the network of polyimine (PI), which shows thermomechanical processability similar to plastics. Strong interactions were achieved through dynamic imine bond and hydrogen bond and thus formed a thermo-processable dynamic composite network. These CS-based plastic substitutes exhibit exceptional mechanical performances, excellent thermal/chemical stability, and a series of self-adaptiveness, including re-healing, reprocessing and multi-layer laminating. Notably, CPs can be easily degraded and 100% recycled for the production of next-generation materials. This work provides an alternative route to produce green and sustainable biomass materials as a plastic substitute.

Mechanical, microstructural, and rheological characterization of gelatin-dialdehyde starch hydrogels constructed by dual dynamic crosslinking

Novel gelatin/dialdehyde starch (GDS) hydrogels based on gelatin were prepared using dialdehyde starch as a macromolecular crosslinking agent. Fourier transform infrared spectroscopy, Raman spectroscopy, and X-ray diffraction showed that the dynamic hydrogel network was formed by covalent imine bonds and hydrogen bonds between dialdehyde starch and gelatin. The synergistic physical and chemical cross-linking endowed the hydrogels with excellent mechanical properties (maximum compressive stress was 0.55 MPa), fatigue resistance (they maintained their shape after ten compression cycles), toughness (54.6 kN/m2), and self-recoverability of the polymeric network. In the rheological tests, all the storage modulus were higher than loss modulus, indicating that they are elastic gels and not viscous sols. The polymer network of GDS hydrogels could recover rapidly when subjected to oscillating shear strain. In addition, with increasing dialdehyde starch concentration, the hydrogels had higher swelling stability and a denser microstructure. Given these excellent properties, the hydrogels have potential application prospects as cell culture scaffolds and carriers of bioactive substance.

Anchored Palladium Complex-Generated Clusters on Zirconia: Efficiency in Reductive N-Alkylation of Amines with Carbonyl Compounds under Hydrogen Atmosphere.

Carbon-nitrogen bond formation is an important method on both laboratory and industrial scales because it realizes the production of valuable pharmaceuticals, agrochemicals, and fine chemicals. Direct reductive N-alkylation of amines with carbonyl compounds via intermediary imine compounds, especially under catalytic hydrogenation conditions, is one of the most convenient, economical, and environmentally friendly methods for this process. Here we report a novel palladium species on zirconia having specific activity towards hydrogenation of imines but other carbonyl groups remaining intact. The present catalytic property offers a practical synthetic method of functionalized secondary amines by reductive N-alkylation under mild conditions with high atom-efficiency. Mechanistic studies revealed that the catalytically active species is the palladium cluster, which is generated in situ from molecular palladium complexes on the support by exposure to atmospheric hydrogen. These fundamental findings are expected to progress in developing novel cluster catalysts for chemical processes directed towards a sustainable society.

Rheology and texture analysis of gelatin/dialdehyde starch hydrogel carriers for curcumin controlled release

A hydrogel carrier was prepared by synergistic cross-linking of dynamic imine bonds and hydrogen bonds between gelatin and dialdehyde starch (DS). The dynamic gelation process and rheological and mechanical properties of hydrogels with different concentrations of DS were studied by dynamic rheology and texture analysis. By changing the content of DS, the gelation kinetics of the hydrogel were adjusted to determine the different gelation times and ensure good injectability. Hydrogels have a stable and firm network structure, and the network can regenerate rapidly after dissociation and collapse. The hydrogel had self-recovery and fatigue resistance, and its hardness, elasticity, deformation capacity, compressive strength and modulus increased with increasing DS content. The hydrogel had good pH-dependent controlled-release ability for curcumin. Injectable hydrogels with biocompatibility, tunable mechanical properties and self-healing properties were used as drug or nutrient delivery carriers for more controlled placement and release.

A Theoretical Study on the Borane-Catalyzed Reductive Amination of Aniline and Benzaldehyde with Dihydrogen: The Origins of Chemoselectivity.

Density functional theory calculations are used in this study to investigate the product selectivity and mechanism of borane-catalyzed reductive aldehyde amination by a H2 reducing agent. Knowing that different boranes yield different products, two typical boranes, (B(2,6-Cl2C6H3)(p-HC6F4)2 and B(C6F5)3), are studied. Of the seven possible pathways of B(2,6-Cl2C6H3)(p-HC6F4)2-catalyzed aldehyde amination analyzed herein, four are favorable. Three of the four favorable pathways involve imine intermediates, and the fourth is a Lewis acid-base synergistic pathway that involves amine-alcohol condensation. As for the B(C6F5)3 catalyst, it forms a highly stable Lewis adduct with aniline, which impedes the hydrogenation of imine. Therefore, the product of B(C6F5)3-catalyzed reductive amination of benzaldehyde and aniline is an imine. The linear relationship between the charge on the boron atom in the Lewis acid and the relative energies of the Lewis adduct and H2 splitting transition state indicates that this parameter determines product selectivity. Indeed, when the natural charge on boron is larger than 1, an amine is produced, whereas when the charge is less than 1, an imine is produced. Hence, the selectivity of products can be controlled by adjusting the natural charge of the boron atom in the Lewis acid catalyst.

Development of spirocyclic phosphoramidite-based hybrid diphosphorus ligands for enantioselective iridium-catalyzed hydrogenation of imines.

Unsymmetrical hybrid chiral diphosphorus ligands bearing a spirocyclic phosphoramidite scaffold have been developed and successfully applied in the iridium-catalyzed asymmetric hydrogenation of imines. With this newly developed chiral iridium catalytic system, a wide range of imines including sterically hindered ones could be hydrogenated to give the corresponding optically active amines in high yields (up to >99%) and with excellent enantioselectivities (up to >99% ee). The utility of this hydrogenation has been demonstrated by the preparation of the chiral fungicide (S)-benalaxyl.

Parahydrogen-induced polarization study of imine hydrogenations mediated by a metal-free catalyst.

Parahydrogen-induced polarization is a nuclear spin hyperpolarization technique that can provide strongly enhanced NMR signals for catalytic hydrogenation reaction products and intermediates. Among other matters, this can be employed to study the mechanisms of the corresponding chemical transformations. Commonly, noble metal complexes are used for reactions with parahydrogen. Herein, we present a PHIP study of metal-free imine hydrogenations catalyzed by the ansa-aminoborane catalyst QCAT. We discuss the reaction mechanism by showing the pairwise nature of the initial hydrogen activation step that leads to the formation of the negative net nuclear spin polarization of N-H hydrogen in the QCAT-H2 intermediate, enabling the further transfer of parahydrogen-originating protons to the imine substrate with the accumulation of hyperpolarized amine products. Parahydrogen-induced polarization also demonstrates the reversibility of the catalytic cycle.

Versatile Levulinic Acid-Derived Dynamic Covalent Thermosets Enabled by in Situ Generated Imine and Multiple Hydrogen Bonds

Versatile Levulinic Acid-Derived Dynamic Covalent Thermosets Enabled by in Situ Generated Imine and Multiple Hydrogen Bonds

Carboxamide substituted tetramethylcyclopentadiene - synthesis, characterisation and its iridium(III) complex catalysed reduction of imines.

The novel dimeric iodo-iridium(III) complex, [Ir(Cp*CONMe2)I2]2, (Cp*CONMe2 = η5-N,N-2,3,4,5-hexamethylcyclopenta-2,4-diene carboxamide) bearing an amide moiety within the tetramethylcyclopentadiene ring, has been synthesised and characterised. The ligand Cp*CONMe2 is synthesised as two regioisomers, however the 2-substituted isomer exists as two distinguishable conformers due to restricted rotation about the amide carbonyl carbon and the ring carbon. The relative acidities of Cp*CONMe2 and Cp* are compared by their relative rates of H/D exchange. The iridium complex of N,N-2,3,4,5-hexamethylcyclopenta-2-4-diene carboxamide [IrCp*CONMe2] and (R,R)-1,2-diphenyl-N'-tosylethane-1,2-diamine ((R,R)-TsDPEN) has been evaluated in the transfer hydrogenation of imines under acidic conditions - a 5 : 2 molar ratio of formic acid : triethylamine as the hydride source for the transfer hydrogenation of 1-methyl-3,4-dihydroisoquinoline (DHIQ) and its 6,7-dimethoxy derivative in acetonitrile. A decreasing enantiomeric excess with reaction progress is attributed to different kinetic orders for formation of the two product amine enantiomers. The pseudo zero-order formation of the R-amine may be due to a pre-steady-state formation of the less stable form of the diastereomeric catalyst. By contrast, both enantiomeric amines from 1-fluorinated methyl DHIQs as substrates for reduction are formed by pseudo first-order processes.

Efficient visible light initiated one-pot syntheses of secondary amines from nitro aromatics and benzyl alcohols over Pd@NH2-UiO-66(Zr)

Pd@NH2-UiO-66(Zr), with small-sized Pd nanoparticles encapsulated inside the cavities of NH2-UiO-66(Zr), was obtained via a double-solvent impregnation followed by a photoreduction process, which shows superior activity for the visible light initiated syntheses of secondary amines from nitro compounds and alcohols, via a sequential photocatalytic hydrogenation of nitro compounds/dehydrogenation of alcohols, condensation of amines and aldehydes to imines, and the hydrogenation of imines. Simultaneous consumption of photogenerated electrons/holes in the photocatalytic hydrogenation of nitro compounds to amines and dehydrogenation of alcohols to aldehydes promotes the whole reaction. Due to the confinement effect of the cavity and the small-sized Pd nanoparticles, Pd@NH2-UiO-66(Zr) shows significantly superior performance as compared with Pd/NH2-UiO-66(Zr), in which larger Pd nanoparticles are deposited on the surface. This study provides an efficient and green strategy for the production of secondary amines and highlights the great potential of using M/MOFs nanocomposites as multifunctional catalysts for light induced one-pot tandem reactions.

Probing Catalyst Function - Electronic Modulation of Chiral Polyborate Anionic Catalysts.

Boroxinate complexes of VAPOL and VANOL are a chiral anionic platform that can serve as a versatile staging arena for asymmetric catalysis. The structural underpinning of the platform is a chiral polyborate core that covalently links together alcohols (or phenols) and vaulted biaryl ligands. The polyborate platform is assembled in situ by the substrate of the reaction, and thus a multiplex of chiral catalysts can be rapidly assembled from various alcohols (or phenols) and bis-phenol ligands for screening of catalyst activity. In the present study, variations in the steric and electronic properties of the phenol/alcohol component of the boroxinate catalyst are probed to reveal their effects on the asymmetric induction in the catalytic asymmetric aziridination reaction. A Hammett study is consistent with a mechanism in which the two substrates are hydrogen-bonded to the boroxinate core in the enantiogenic step. The results of the Hammett study are supported by a computational study in which it is found that the H-O distance of the protonated imine hydrogen bonded to the anionic boroxinate core decreases with an increase in the electron releasing ability of the phenol unit incorporated into the boroxinate. The results are not consistent with a mechanism in which the boroxinate catalyst functions as a Lewis acid and activates the imine by a Lewis acid/Lewis base interaction.

Heterometallic Mg−Ba Hydride Clusters in Hydrogenation Catalysis

AbstractReaction of a MgN“2/BaN”2 mixture (N“=N(SiMe3)2) with PhSiH3 gave three unique heterometallic Mg/Ba hydride clusters: Mg5Ba4H11N”7 ⋅ (benzene)2 (1), Mg4Ba7H13N“9 ⋅ (toluene)2 (2) and Mg7Ba12H26N”12 (3). Product formation is controlled by the Mg/Ba ratio and temperature. Crystal structures are described. While 3 is fully insoluble, clusters 1 and 2 retain their structures in aromatic solvents. DFT calculations and AIM analyses indicate highly ionic bonding with Mg−H and Ba−H bond paths. Also unusual H−⋅⋅⋅H− bond paths are observed. Catalytic hydrogenation with MgN“2, BaN”2 and the mixture MgN“2/BaN”2 has been studied. Whereas MgN“2 is only active in imine hydrogenation, alkene and alkyne hydrogenation needs the presence of Ba. The catalytic activity of the MgN”2/BaN“2 mixture lies in general between that of its individual components and strong cooperative effects are not evident.

One-pot amination of aldehydes and ketones over heterogeneous catalysts for production of secondary amines

ABSTRACT This review summarizes the recent studies on the synthesis of secondary amines by one-pot amination of aldehydes and ketones over heterogeneous catalysts. Amines are widely applied as the key intermediates in chemical industry for the synthesis of various commodities such as agrochemicals, drugs, detergents, lubricants, food-additives and polymers. Direct catalytic reductive amination of carbonyl compounds was considered which generally includes two steps: (i) formation of imines by interactions of aldehydes or ketones with amines, and (ii) subsequent hydrogenation of imines. Synthesis of secondary amines from carbonyl compounds and amines generated in situ under reaction conditions from their progenitors, e.g., respectively, alcohols or nitro-compounds, is also discussed in detail. Recent progress in application of hydrogen sources alternative to gaseous H2, such as formic acid, NaBH4, CO and water, favored development of metal-free catalysts including solid acid catalysts. The review addresses the scope of the amination reaction with aldehydes/ketones and nitro/amine compounds of different structure, the effect of the solvent, reaction conditions and catalyst properties. In addition, catalyst regeneration and reuse, kinetic regularities and kinetic modeling with an emphasis on the continuous mode of one-pot amination have been systematically summarized and discussed. It is suggested that the future work should focus on revealing the role of the catalytically active sites addressing their acid–base properties and the correlation between catalyst properties and the reaction performance, elucidating kinetic parameters and designing feasible reactor system for further industrial implementation.

Uncovering the Activity of Alkaline Earth Metal Hydrogenation Catalysis Through Molecular Volcano Plots

Recent advances in alkaline earth (Ae) metal hydrogenation catalysis have broadened the spectrum of potential catalysts to include candidates from the main group, providing a sustainable alternative to the commonly used transition metals. Although Ae-amides have already been demonstrated to catalyze hydrogenation of imines and alkenes, a lucid understanding of how different metal/ligand combinations influence the catalytic activity is yet to be established. In this article, we use linear scaling relationships and molecular volcano plots to assess the potential of the Ae metal-based catalysts for the hydrogenation of alkenes. By analyzing combinations of eight metals (mono-, bi-, tri-, and tetravalent) and seven ligands, we delineate the impact of metal-ligand interplay on the hydrogenation activity. Our findings highlight that the catalytic activity is majorly determined by the charge and the size of the metal ions. While bivalent Ae metal cations delicately regulate the binding and the release of the reactants and the products, respectively, providing the right balance for this reaction, ligands play only a minor role in determining their catalytic activity. We show how volcano plots can be utilized for the rapid screening of prospective Ae catalysts to establish a guideline to achieve maximum activity in facilitating the hydrogenation process.

A novel polymeric hybrid sensory smart material for the prompt recognition of mercury ions in water

An acrylamide-xanthene hybrid polymeric probe was synthesized for the selective and sensitive detection of mercury ions (Hg2+) in 100% water. The non-polymeric form of probe displayed extraordinary selectivity above other additional metal cations via both a visual color change and fluorescence change with exceptional reversibility and responds rapidly to changes in the environment. The 1H NMR titration study revealed the binding of an Hg2+ ion on the imine hydrogen of the probe by the opening of the spiro-lactam ring, which instigates an instant color change. Reversible behavior of the polymeric probe using an aqueous solution of EDTA was confirmed by both absorption and fluorescence studies. The prepared polymeric probe detects Hg2+ in 100% water and is a promising material for the mercury recognition in real-time.

1-{(E)-[4-(4-Hy-droxy-phen-yl)butan-2-yl-idene]amino}-3-phenyl-thio-urea: crystal structure, Hirshfeld surface analysis and computational study.

The title thio-urea derivative, C17H19N3OS, adopts a U-shaped conformation with the dihedral angle between the terminal aromatic rings being 73.64 (5)°. The major twist in the mol-ecule occurs about the ethane bond with the Ci-Ce-Ce-Cb torsion angle being -78.12 (18)°; i = imine, e = ethane and b = benzene. The configuration about the imine bond is E, the N-bound H atoms lie on opposite sides of the mol-ecule and an intra-molecular amine-N-H⋯N(imine) hydrogen bond is evident. In the mol-ecular packing, hydroxyl-O-H⋯S(thione) and amine-N-H⋯O hydrogen bonding feature within a linear, supra-molecular chain. The chains are connected into a layer in the ab plane by a combination of methyl-ene-C-H⋯S(thione), methyl-ene-C-H⋯O(hydrox-yl), methyl-C-H⋯π(phen-yl) and phenyl-C-H⋯π(hy-droxy-benzene) inter-actions. The layers stack without directional inter-actions between them. The analysis of the calculated Hirshfeld surface highlights the presence of weak methyl-C-H⋯O(hydrox-yl) and H⋯H inter-actions in the inter-layer region. Computational chemistry indicates that dispersion energy is the major contributor to the overall stabilization of the mol-ecular packing.

Ruthenacycles and Iridacycles as Transfer Hydrogenation Catalysts.

In this review, we describe the synthesis and use in hydrogen transfer reactions of ruthenacycles and iridacycles. The review limits itself to metallacycles where a ligand is bound in bidentate fashion to either ruthenium or iridium via a carbon–metal sigma bond, as well as a dative bond from a heteroatom or an N-heterocyclic carbene. Pincer complexes fall outside the scope. Described are applications in (asymmetric) transfer hydrogenation of aldehydes, ketones, and imines, as well as reductive aminations. Oxidation reactions, i.e., classical Oppenauer oxidation, which is the reverse of transfer hydrogenation, as well as dehydrogenations and oxidations with oxygen, are described. Racemizations of alcohols and secondary amines are also catalyzed by ruthenacycles and iridacycles.

Reductive Amination of 5-Hydroxymethylfurfural by the Hydrogenation of Intermediate Imines in the Presence of a Pt/Al2O3 Catalyst in a Flow Reactor

Reductive Amination of 5-Hydroxymethylfurfural by the Hydrogenation of Intermediate Imines in the Presence of a Pt/Al2O3 Catalyst in a Flow Reactor

LiAlH4‐catalyzed Imine Hydrogenation with Dihydrogen: New DFT Mechanistic Insights

AbstractRecently, it was found that the widely used stoichiometric reducing reagent LiAlH4 can also be used as a useful catalyst for imine hydrogenation with H2 under relatively mild conditions. In this work, extensive state‐of‐the‐art DFT calculations are conducted to explore the detailed catalytic mechanism. In sharp contrast to the recent proposal involving heterolytic H2‐activation over Al−N amide bonds after multiple imine insertion, our new mechanism highlights the dual role of the Lewis‐acidic lithium cation in frustrated‐Lewis‐pair‐like activation of both imine substrate and H2 when combined with the aluminum‐bound hydride and amide ligands, respectively. Ionic Li−N amide bonds are about 13 kcal/mol more reactive than Al−N amide bonds for H2 activation, thus providing a useful structural feature for rational design of active hydrogenation catalysts.

Cationic Aluminium Complexes as Catalysts for Imine Hydrogenation

Strongly Lewis acidic cationic aluminium complexes, stabilized by β–diketiminate (BDI) ligands and free of Lewis bases, have been prepared as their B(C6F5)4 − salts and were investigated for catalytic activity in imine hydrogenation. The backbone (R1) and N (R2) substituents on the R1,R2BDI ligand (R1,R2BDI=HC[C(R1)N(R2)]2) influence sterics and Lewis acidity. Ligand bulk increases along the row Me,DIPPBDI<Me,DIPePBDI≈tBu,DIPPBDI<tBu,DIPePBDI; DIPP=2,6‐C(H)Me2‐phenyl, DIPeP=2,6‐C(H)Et2‐phenyl. The Gutmann‐Beckett test showed acceptor numbers of: (tBu,DIPPBDI)AlMe+ 85.6, (tBu,DIPePBDI)AlMe+ 85.9, (Me,DIPPBDI)AlMe+ 89.7, (Me,DIPePBDI)AlMe+ 90.8, (Me,DIPPBDI)AlH+ 95.3. Steric and electronic factors need to be balanced for catalytic activity in imine hydrogenation. Open, highly Lewis acidic, cations strongly coordinate imine rendering it inactive as a Frustrated Lewis Pair (FLP). The bulkiest cations do not coordinate imine but its combination is also not an active catalyst. The cation (tBu,DIPPBDI)AlMe+ shows the best catalytic activity for various imines and is also an active catalyst for the Tishchenko reaction of benzaldehyde to benzylbenzoate. DFT calculations on the mechanism of imine hydrogenation catalysed by cationic Al complexes reveal two interconnected catalytic cycles operating in concert. Hydrogen is activated either by FLP reactivity of an Al⋅⋅⋅imine couple or, after formation of significant quantities of amine, by reaction with an Al⋅⋅⋅amine couple. The latter autocatalytic Al⋅⋅⋅amine cycle is energetically favoured.