Synthesis Of Imines Research Articles

Unexpected but prominent imines formation in Beckmann rearrangement of (spiro)pyranocoumarin oximes

This paper presents the successful implementation of Beckmann rearrangement on a series of (spiro)pyranocoumarin oximes in polyphosphoric acid (PPA) medium. The method demonstrated excellent proficiency in the synthesis of unexpected imines as major products and amides as minor products. The labeled 15NH2OH·HCl control experiment yielded oxime and rearrangement products that incorporated the 15N label. Furthermore, we have proposed a possible rearrangement mechanism. NMR spectroscopy data (correlation HMBC and 15N NMR) confirmed the structure of the synthesized products. These findings provide a new and efficient approach for constructing complex imines with diverse applications in synthetic organic chemistry.

Selective aerobic coupling of amines to imines using solar spectrum-responsive flower-like Nen-graphene quantum dots (GQDs) decorated with 2, 4-dinitrophenylhydrazine (PH) as a photocatalyst

Indeed, the development of ecologically benign molecular fabrication methods for highly efficient graphene quantum dots-based photocatalysts is of great significant. Graphene quantum dots-based photocatalysts have promising applications in various field, including environmental remediation, energy conversion, and splitting of water. However, ensuring resource reusability and minimizing the environmental impact are crucial considerations in the development. From this perspective, attention has also been paid to the creation of easy to make solar light harvesting graphene quantum dots-based photocatalysts for synthesising pharmaceuticals and functional imines compounds. Imines are excellent significant building blocks in pharmaceutical chemistry and excellent examples of these valuable compounds' synthetic intermediates, and the environmentally friendly oxidative synthesis of imines from amines. Therefore, herein, we designed a facile and efficient condensation route to synthesize the Nen-GQDs@PH photocatalyst. This route involves coupling of 2,4-dinitrophenylhydrazine (PH) with nitrogen-enriched graphene quantum dots (Nen-GQDs). The Nen-GQDs@PH as photocatalyst functions in a highly selective and efficient manner, leading to high amines conversion efficiency to imines (95%). Our results highlight a novel and environmentally safe approach for generating highly selective imines from various types of amines, setting a new benchmark in the current research field.

Cerium Oxide Nanoparticles Confined in Doped Mesoporous Carbons: A Strategy to Produce Catalysts for Imine Synthesis.

A group of (doped N or P) carbons were synthesized using soluble starch as a carbon precursor. Further, ceria nanoparticles (NPs) were confined into these (doped) carbon materials. The obtained solids were characterized by various techniques such as N2 physisorption, XRD, TEM, SEM, XPS, and XAS. These materials were used as catalysts for the oxidative coupling between benzyl alcohol and aniline as the model reaction. Ceria immobilized on mesoporous-doped carbon shows higher activity than the other materials, benchmark catalysts, and most of the previously reported catalysts. The control of the ceria NP size, the presence of Ce3+ cations, and an increment in the disorder in the ceria NP structure caused by a support-ceria interaction could increase the number of oxygen vacancies and improve its catalytic performance. CN-meso/CeO2 was also used as the catalyst for a rich scope of substrates, such as substituted aromatic alcohols, linear alcohols, and different types of amines. The influence of various reaction parameters (substrate content, reaction temperature, and catalyst content) on the activity of this catalyst was also checked.

Pd‐Catalyzed Switchable Access to Imines and Amines from Secondary Alcohols

AbstractThe use of Pd(OAc)2 as catalyst for the challenging synthesis of imines through acceptorless alcohol dehydrogenation using secondary alcohols and amines is reported. The reaction requires low catalyst loading, shows high selectivity for the formation of the imine without the need of adding any base or additive, and can be scaled up. Interestingly, when the catalytic reaction is performed in a closed system, in the presence of base, the reaction selectively yields the corresponding amine. To highlight the practical utility of the synthesis of imines, a family of structurally important scaffold, an indole, were synthesized.

Deaminative Addition of Alkylpyridinium Salt to Aldehyde.

Here we show that a primary amine can engage in the nucleophilic addition to an aldehyde to synthesize an alcohol following preactivation of the amine. The enabling reagent for this radical-polar crossover process is CrCl2. This reaction is selective for aldehydes and compatible with numerous functional groups, which are not tolerated under classical Grignard-type conditions. Complementary to the well-established imine synthesis, this deaminative alcohol synthesis can broadly expand the chemical space constructed by aldehydes and amines.

Steering the Au−FexCo1Oy interface for efficient imine synthesis at low temperature via oxidative coupling reaction

Low-temperature imine formation through oxidative coupling of alcohols with amines under base-free conditions is very important for fine-chemical synthesis. Supported-Au catalysts show superior performances toward selective oxidation reactions but are scarcely reported for the above application. In this work, the finely-dispersed Au nanoparticles (ca. 2.4 nm) loaded on magnetic FexCo1Oy oxides were developed as novel, efficient and reusable heterogeneous catalyst. Varying the Fe/Co molar ratio allowed tuning the Au−FexCo1Oy interface and further boosting the selective oxidation performance of Au. The optimum Au/Fe0.25Co1Oy catalyst showed >99% yield of benzylideneaniline and 20.1 molimine molAu−1 h−1 productivity using the model reaction at 30 °C under air atmosphere without alkaline additives. This was the best result compared to benchmark Au-based catalysts in the literature. In addition, this catalyst exhibited superior performances towards gram-scaled synthesis and diverse substrates. XRD, N2-physisorption, TEM, H2-TPR, O2-TPD, EPR, pyridine-adsorbed FT-IR, and kinetic investigations were used to study this Au catalytic system. It was found that the synergy between Fe2O3 and Co3O4 can induce formation of abundantly mobile and reactive oxygen species (i.e., O2− and O− related to oxygen vacancies) and electron-rich metallic Au species on the surface of mesoporous FexCo1Oy oxides. These interfacial features were disclosed to be vital to activating molecular O2 and O H bond in an alcohol. Besides, it was demonstrated that the surface Lewis acidic sites associated with coordinatively unsaturated Co3+ ions can help facilitate the tandem oxidation-condensation reactions.

Simultaneous activation of aldehydes and methylamine liberation by 4,4ʹ-trimethylenedipiperidine in the solid-state synthesis of N-methyl imines

Simultaneous activation of aldehydes and methylamine liberation by 4,4ʹ-trimethylenedipiperidine in the solid-state synthesis of N-methyl imines

Autocatalytic Green Synthesis of Imines in Traceless Medium

AbstractFor the first time, condensations of amines with carbonyl compounds have been performed in supercritical carbon dioxide (sc‐CO2). The reactions with aldehydes or active ketones proceed at moderate temperatures (35–55 °C) without use of external catalysts. The process is autocatalytic: it is accelerated by the carbonic acid generated in situ by interaction between the released water and the CO2 medium. The imine products were obtained in high yields in a crystalline form and did not require further purification. The one‐pot transfer hydrogenation and [4+2] cycloaddition reactions performed in the CO2 medium have uncovered attractive prospects for facile green synthesis of more complex and valuable compounds from the generated in situ imine products.

A Comprehensive Review on Cu‐Catalysed Aerobic Oxidation of Amines to Imines

AbstractImines are frequently used as valuable intermediates in organic transformation and pharmaceutical synthesis. Conventional imine synthesis includes Lewis acid catalysts, dehydrating agents, and unstable aldehydes. Developing eco‐friendly molecular assembly techniques has become increasingly important in recent years. In particular, there is a significant need for methods that recycle resources and have high atomic efficiency with minimal environmental impact. Oxidative synthesis of imines from amines has been investigated in presence of molecular oxygen (or air) which is readily accessible on Earth. The closely linked catalysts have been classified into metals, bio‐inspired, and MOF catalysts. Highly active, affordable, and adaptable copper based catalysts are given special attention here, some critical elements that influence reaction processes and catalytic activity are also displayed in the present study. The modern methods along with mechanistic routes for converting primary amines to imines using copper catalytic systems and ambient oxygen have been discussed in detail.

Plasmon-enhanced photocatalytic oxidative coupling of amines in the air using a delicate Ag nanowire@NH2-UiO-66 core-shell nanostructures

MOF-based core-shell structures with high surface area, abundant active sites, and broad absorption bands are viable alternatives to traditional single-component photocatalysts. In this report, we describe the design and construction of delicate Ag nanowires@NH2-UiO-66 with a core-shell structure for use as photocatalysts in imine synthesis under light. The optimized composites exhibited 80% imine production, which was higher than both MOF and Ag NWs. The significant improvement in photocatalytic activity under light may be attributed to the plasmonic effect of silver nanowires and their core-shell structure, which promotes the separation of electron-hole pairs. Moreover, the photocatalytic activity of the core-shell nanostructure may provide valuable insight into the design and construction of MOF-based composite photocatalysts for oxidative coupling of amines.

Physico- and Electrochemical Properties of First-Row Transition-Metal-Substituted Sandwich Polyoxometalates.

The physico- and electrochemical behaviors of a series of [WZn3(H2O)2(ZnW9O34)2]12- (Zn-WZn3) and its first-row transition-metal-substituted analogues [WZn(TM)2(H2O)2(ZnW9O34)2]12- (Zn-WZn(TM)2; TM = MnII, CoII, FeIII, NiII and CuII) are reported. Various spectroscopic studies, including Fourier transform infrared (FTIR) spectroscopy, UV-visible spectroscopy, electrospray ionization (ESI)-mass spectrometry, and Raman spectroscopy, show similar spectral patterns in all sandwich polyoxometalates (POMs) because of their isostructural geometry and constancy of the overall negative charge (-12). However, the electronic properties highly depend on the transition metals at the "sandwich core" and correlate well with the density functional theory (DFT) study. Further, depending on the substituted TM atoms, there is a decrease in the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) band-gap energy in these transition-metal-substituted POM (TMSP) complexes wrt Zn-WZn3, as confirmed by diffuse reflectance spectroscopy and DFT study. Cyclic voltammetry reveals that the electrochemistry of these sandwich POMs (Zn-WZn3 and TMSPs) is highly dependent on the pH of the solution. Moreover, the dioxygen binding/activation studies of these polyoxometalates show that Zn-WZn3 and Zn-WZnFe2 have better efficiency toward dioxygen binding, as confirmed by FTIR spectroscopy, Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and thermogravimetric analysis (TGA), which is also reflected in their catalytic activity toward imine synthesis.

Green, General and Low‐cost Synthesis of Porous Organic Polymers in Sub‐kilogram Scale for Catalysis and CO2 Capture

AbstractPorous organic polymers (POPs) with high porosity and tunable functionalities have been widely studied for use in gas separation, catalysis, energy conversion and energy storage. However, the high cost of organic monomers, and the use of toxic solvents and high temperatures during synthesis pose obstacles for large‐scale production. Herein, we report the synthesis of imine and aminal‐linked POPs using inexpensive diamine and dialdehyde monomers in green solvents. Theoretical calculations and control experiments show that using meta‐diamines is crucial for forming aminal linkages and branching porous networks from [2+2] polycondensation reactions. The method demonstrates good generality in that 6 POPs were successfully synthesized from different monomers. Additionally, we scaled up the synthesis in ethanol at room temperature, resulting in the production of POPs in sub‐kilogram quantities at a relatively low cost. Proof‐of‐concept studies demonstrate that the POPs can be used as high‐performance sorbents for CO2 separation and as porous substrates for efficient heterogeneous catalysis. This method provides an environmentally friendly and cost‐effective approach for large‐scale synthesis of various POPs.

Green, General and Low-cost Synthesis of Porous Organic Polymers in Sub-kilogram Scale for Catalysis and CO2 Capture.

Porous organic polymers (POPs) with high porosity and tunable functionalities have been widely studied for use in gas separation, catalysis, energy conversion and energy storage. However, the high cost of organic monomers, and the use of toxic solvents and high temperatures during synthesis pose obstacles for large-scale production. Herein, we report the synthesis of imine and aminal-linked POPs using inexpensive diamine and dialdehyde monomers in green solvents. Theoretical calculations and control experiments show that using meta-diamines is crucial for forming aminal linkages and branching porous networks from [2+2] polycondensation reactions. The method demonstrates good generality in that 6 POPs were successfully synthesized from different monomers. Additionally, we scaled up the synthesis in ethanol at room temperature, resulting in the production of POPs in sub-kilogram quantities at a relatively low cost. Proof-of-concept studies demonstrate that the POPs can be used as high-performance sorbents for CO2 separation and as porous substrates for efficient heterogeneous catalysis. This method provides an environmentally friendly and cost-effective approach for large-scale synthesis of various POPs.

I2‐Mediated Oxidation of Amines in Water toward Imines and Amides

AbstractA sustainable oxidation reaction of amines in water is established for the synthesis of imines and amides employing environmentally benign molecular iodine as the sole oxidant. The features of the present reaction also include no use of transition metals, operational simplicity and gram‐scale synthesis. It provides a facile access to imine and amide derivatives including bioactive molecules from readily accessible amine precursors.

Ring Closure Reactions for the Synthesis of Cyclic Imines: An Analysis from the 12 Principles of Green Chemistry and Circular Chemistry

Abstract: Compounds containing a C=N moiety, namely imines, have been widely used for industrial purposes due to their various biological activities. Cyclic imines are an essential class of nitrogen-based heterocycles and valuable scaffolds for designing and obtaining new biologically active compounds. However, the proposal and implementation of synthetic methods for this heterocyclic system are mainly conditioned by different structural and stereoelectronic considerations. Therefore, it can be complex and sometimes limited to a selected group of heterocyclic compounds. The following review paper is structured to search and collect different synthesis methods of cyclic imines and identify the main progress currently achieved. It addresses this topic using structural considerations, physical properties, and reactivity. The synthesis methods described below have implemented strategies based on cyclo-condensation reactions, radical cyclizations, electrocyclic closures, and carbon-carbon coupling by metal-organic catalysis. These methods have significantly contributed significantly to organic chemistry knowledge. In addition, an analysis of such synthesis methods from applying the principles of green and circular chemistry is presented, evaluating the potential application of these methods, limitations, perspectives, and their impact on the environment.

No Transition Metals Required - Oxygen Promoted Synthesis of Imines from Primary Alcohols and Amines under Ambient Conditions.

The synthesis of imines denotes a cornerstone in organic chemistry. The use of alcohols as renewable substituents for carbonyl-functionality represents an attractive opportunity. Consequently, carbonyl moieties can be in situ generated from alcohols upon transition-metal catalysis under inert atmosphere. Alternatively, bases can be utilized under aerobic conditions. In this context, we report the synthesis of imines from benzyl alcohols and anilines, promoted by KOt Bu under aerobic conditions at room temperature, in the absence of any transition-metal catalyst. A detailed investigation of the radical mechanism of the underlying reaction is presented. This reveals a complex reaction network fully supporting the experimental findings.

Synthesis and spectroscopic properties of iminosilanes

The synthesis of imines (from ethanal, valeraldehyd, pinacolon, aceton, propiophenon, isobutylmethylketon and isopropylmethylketon) via reactions with n-propylamine, n-hexylamine and aniline and the trimethylsilylation of these compounds are reported. The silylation with trimethylsilyltriflate and an auxiliary base (triethylamine) is compared to the deprotonation of the imines by n-butyllithium with subsequent reaction with chlorosilanes. Both procedures yield different products, which are analyzed by NMR and Raman spectroscopy. One reaction of the silylated compounds with CO2 is also reported.

Corrigendum to “Controllable synthesis of novel nanoporous manganese oxide catalysts for the direct synthesis of imines from alcohols and amines” [Chin. J. Chem. Eng. 27(10) (2019) 2438–2446

Corrigendum to “Controllable synthesis of novel nanoporous manganese oxide catalysts for the direct synthesis of imines from alcohols and amines” [Chin. J. Chem. Eng. 27(10) (2019) 2438–2446

Cobalt-Doped g-C3N4 Nanosheets for One-Pot Synthesis of Imines under Mild Conditions

The traditional synthesis methods of imines are limited by expensive catalysts, harsh reaction conditions, and low atomic efficiency. The one-pot synthesis of imines using nitrobenzene and benzaldehyde is a promising reaction path. Transition metal catalytic transfer hydrogenation offers advantages in terms of safety. It has attracted the attention of industrial production and academia as an alternative to conventional hydrogenation processes. Herein, we report an environmentally friendly Co/g–C3N4 nanocatalyst that uses inexpensive cobalt as the fundamental source of feedstock; formic acid is applied as the hydrogen donor, and the yield of the synthesis method can reach 96% under mild conditions. Besides, the catalyst proved to be stable and efficient despite five successive cycles of testing. Moreover, the excellent catalytic activity was demonstrated to be derived from the Co–Nx site based on the mechanistic analysis and characterization. For this work, a pathway will be developed to produce transfer hydrogenation of nitro compounds.

Efficient synthesis of imines using carbon nitride as photocatalyst

Heterogeneous photocatalytic systems can provide green organic synthesis at cost-effective processes. In the present work, an efficient, environment-friendly and sustainable heterogeneous photocatalyst, the metal-free exfoliated carbon nitride (GCN-T), is used in the oxidative self-coupling of benzylamine (BZA) to produce N-benzylidenebenzylamine (BZI) within the visible light range of irradiation (LEDs with λmax = 417 nm) under mild reaction conditions. The issue of selectivity is successively addressed with complete conversion of BZA achieved upon 45 min of reaction, under irradiation, for 97 % selectivity towards BZI production. Electron paramagnetic resonance (EPR) spectroscopy is used to investigate the main reactive species involved in the photocatalytic reaction. This analysis revealed that superoxide and singlet oxygen radicals played the leading role in the amine conversion in the heterogeneous photocatalytic mechanism. A reaction pathway is proposed to include all identified species generated during the oxidative self-coupling of BZA under visible light irradiation. The results described here add to the portfolio of customisable heterogeneous photocatalytic solutions towards efficient solar synthesis.