Coupling Of Amines Research Articles

Confined synthesis of conjugated microporous polymers for selective photocatalytic oxidation of amines

Photocatalytic oxidative coupling of amines is considered a mild, efficient, and sustainable strategy for the synthesis of imines. As a versatile organic semiconductor, conjugated microporous polymers (CMPs) are attractive in photocatalysis areas due to the diversity of their polymeric monomers. Herein, we report that in addition to the design of monomers, size-confined polymerization is also a feasible strategy to modulate the structure and photocatalysis properties of CMPs. We adopted dibromopyrazine as polymeric units to prepare pyrazine-involved hollow spherical CMPs (H-PyB) using a template method and successfully performed size-confined polymerization of hollow samples by resizing the templates. Interestingly, the small confinement space induced the formation of CMPs with better conjugate extensibility, resulting in enhanced conductivity, narrowed bandgaps, improved photoelectric performance, etc. As a result, small-sized H-PyB CMPs had superior activity for the photocatalytic oxidation of amines. Particularly, the smallest H-PyB CMPs that we designed in the present work exhibited excellent performance for the photocatalytic coupling oxidation of amines. When using benzylamine as a model substrate, the yield of the corresponding imine reached ∼ 113 mmol·g−1·h−1, accompanied by almost 100 % selectivity. Furthermore, the as-designed confined samples exhibited stable photocatalytic activity as well as good applicability for oxidative coupling of different amines. This work not merely reports a kind of CMP photocatalysts with excellent performance for the imine coupling oxidation but also proposes an alternative strategy for constructing high-performance organic photocatalysts by size-confined synthesis.

Development of Molybdenum oxide Promoted CeO2−SiO2 Mixed‐oxide Catalyst for Efficient Catalytic Oxidation of Benzylamine to N‐Benzylidenebenzylamine

AbstractThis study explores a rational synthesis of a molybdenum‐promoted Ce−Si mixed oxide catalyst (MoO3/CeO2−SiO2) to regulate synergies between MoO3 and SiO2 dopant on CeO2, achieving high selectivity in solvent‐free catalytic oxidative coupling of amines to imines using molecular O2 as the oxidant. Comparative efficiency tests were conducted with bare CeO2, CeO2−SiO2, and MoO3/CeO2 catalysts. Characterization techniques, including XRD, Raman spectroscopy, N2‐adsorption‐desorption analysis, FTIR, NH3−TPD, TEM, and XPS, were employed to assess textural properties, acidic features, promoter and dopant dispersion in CeO2 lattice, and oxygen defects. Mo/Ce−Si catalyst exhibited superior acidic sites and a higher concentration of Ce3+ ions (Iu/ITotal), indicating increased oxygen vacancies. This catalyst demonstrated exceptional performance in the oxidative coupling of benzylamine, providing higher conversion and selectivity to the corresponding imine. Remarkably, the Mo/Ce−Si catalyst maintained consistent performance over five recycling runs. The catalyst also proved effective for the selective oxidative coupling of various amine substrates with diverse steric and electronic properties. Experimental results confirmed a plausible reaction mechanism within the concise catalyst design.

Novel chromatic reaction based on the redox reaction between tertiary aromatic amines and gold(Ⅲ) or cerium(Ⅳ)

Abstract Herein, a novel chromatic reaction based on the oxidative self-coupling of tertiary aromatic amines and driven by a redox reaction with gold(III) or cerium(IV) is developed. The dimers resulting from the oxidation of tertiary aromatic amines with gold(III) or cerium(IV) show colors depending on their chemical structures. Substituting a hydrogen atom on the aromatic ring with a methyl or methoxy group induces a bathochromic shift in the color developed via oxidation. This redox reaction is highly selective for gold(III) and cerium(IV) because of their high redox potentials and complexation abilities with tertiary aromatic amines, thus suggesting the development of a selective analytical method for gold(III) and cerium(IV). Cyclic voltammograms and ultraviolet–visible light absorption spectra confirm that the oxidative coupling of tertiary aromatic amines develops color. The addition of reductants, such as l-ascorbic acid, degrades the color of the resultant colored species, thus indicating that the quinoid structure of the colored species caused color development. Thus, the developed redox reaction of tertiary aromatic amines with gold(III) or cerium(IV) provides a new chromogenic platform for determining gold(III), cerium(IV), and ascorbic acid.

Cu@CuOx/WO3 with photo-regulated singlet oxygen and oxygen adatoms generation for selective photocatalytic aromatic amines to imines

Photocatalysts can absorb light and activate molecular O2 under mild conditions, but the generation of unsuitable reactive oxygen species often limits their use in synthesizing fine chemicals. To address this issue, we disperse 1 wt% copper on tungsten trioxide (WO3) support to create an efficient catalyst for selective oxidative coupling of aromatic amines to imines under sunlight irradiation at room temperature. Copper consists of a metallic copper core and an oxide shell. Experimental and density functional theory calculations have confirmed that Cu2O is the primary activation site. Under λ < 475 nm, the light excites electrons of the valence bands in Cu2O and WO3, which activate O2 to superoxide radical •O2−. Then rapidly transforms into oxygen adatoms (•O) and oxygen anion radicals (•O−) species on the surface of Cu2O. Simultaneously, it is captured by holes in the WO3 valence band to generate singlet oxygen (1O2). •O bind to 1O2 promoting the coupling reaction of amines. When λ > 475 nm, intense light absorption due to the localized surface plasmon resonance excites numerous electrons in Cu to promote the oxidative coupling with the adsorbed O2. This study presents a promising approach towards the design of high-performance photocatalysts for solar energy conversion and environmentally-friendly oxidative organic synthesis.

Manganese-Catalyzed Mono-N-Methylation of Aliphatic Primary Amines without the Requirement of External High-Hydrogen Pressure.

The synthesis of mono-N-methylated aliphatic primary amines has traditionally been challenging, requiring noble metal catalysts and high-pressure H2 for achieving satisfactory yields and selectivity. Herein, we developed an approach for the selective coupling of methanol and aliphatic primary amines, without high-pressure hydrogen, using a manganese-based catalyst. Remarkably, up to 98 % yields with broad substrate scope were achieved at low catalyst loadings. Notably, due to the weak base-catalyzed alcoholysis of formamide intermediates, our novel protocol not only obviates the addition of high-pressure H2 but also prevents side secondary N-methylation, supported by control experiments and density functional theory calculations.

Manganese‐Catalyzed Mono‐N‐Methylation of Aliphatic Primary Amines without the Requirement of External High‐Hydrogen Pressure

AbstractThe synthesis of mono‐N‐methylated aliphatic primary amines has traditionally been challenging, requiring noble metal catalysts and high‐pressure H2 for achieving satisfactory yields and selectivity. Herein, we developed an approach for the selective coupling of methanol and aliphatic primary amines, without high‐pressure hydrogen, using a manganese‐based catalyst. Remarkably, up to 98 % yields with broad substrate scope were achieved at low catalyst loadings. Notably, due to the weak base‐catalyzed alcoholysis of formamide intermediates, our novel protocol not only obviates the addition of high‐pressure H2 but also prevents side secondary N‐methylation, supported by control experiments and density functional theory calculations.

Carbazoquinocin Analogues as Small Molecule Biomimetic Organocatalysts in Dehydrogenative Coupling of Amines.

A new series of carbazole-cored biomimetic ortho-quinone catalysts structurally resembling carbazoquinocin alkaloids have been introduced to promote tunable, metal cocatalyst-free, organocatalytic, dehydrogenative amine oxidation under aerobic conditions. Differently substituted benzyl amines were tolerated under optimized conditions to provide imines in excellent yields. Further efficacy of the catalyst was demonstrated by synthesizing cross-coupled imines efficiently. Control experiments and in-depth DFT studies disclosed a covalent transamination pathway as a plausible mechanism for this newly developed catalytic system.

Cu-Catalyzed Coupling of Aliphatic Amines with Alkylboronic Esters.

We report a Cu-catalyzed oxidative coupling of aliphatic amines with benzylic and aliphatic boronic esters to give high value alkyl amines, products found widely in applications from medicinal chemistry to materials science. This operationally simple reaction, which can be performed on gram scale, runs under mild conditions and exhibits broad functional group tolerance. The terminal oxidant of the reaction is O2 from the air, avoiding the need for additional chemical oxidants. Investigation into the reaction mechanism suggests that the boronic ester is activated by an aminyl radical, formed through oxidation of the amine by the Cu catalyst, to give a key alkyl radical intermediate. To demonstrate its utility and potential for late-stage functionalization, we showcase the method as the final step in the total synthesis of a TRPV1 antagonist.

Oxidative Coupling of Amines Using a 4-Nitrophenyl­acethylene-Functionilzed Cu2O Crystal

Oxidative Coupling of Amines Using a 4-Nitrophenyl­acethylene-Functionilzed Cu2O Crystal

Thermal conductive nylon 6 composites using hybrid fillers to construct a three‐dimensional thermal conductive network

AbstractThe rapid progress in miniaturization and integration of semiconductor power devices has made heat dissipation a critical concern in the development of high‐performance semiconductor devices, thereby increasing the demands for the heat transfer efficiency of polymer composites. To address this issue, an efficient three‐dimensional (3D) heat conduction network structure is constructed in the polymer matrix by leveraging the mutual reaction of amino and epoxy groups on the surface of the filler treated by coupling during the melt mixing process, this approach leads to an enhancement in the thermal conductivity of the composites. First, the inert surfaces of graphene nanosheets (EX‐G) and carbon fibers (CF) are coated with polydopamine (PDA) to form active sites. Subsequently, the graphene (EX‐G), alumina (Al2O3), and carbon fiber (CF) are treated with an amine coupling agent (KH550) and epoxy coupling agent (KH560), respectively. Notably, at a filler content of 25 wt%, the thermal conductivity (TC) of the composites increases by approximately 282% compared to pure nylon 6. This research contributes novel insights into the field of thermally conductive polymer composites.Highlights The surface of the EX‐G and CF covered by PDA generates active sites. Fillers reduce the interface thermal resistance through chemical bonds. Fillers surface reaction builds a 3D thermal conductive network. 3D network structure can significantly improve the TC of composites. Only 25 wt% of hybrid fillers in a 282% increased TC of composites.

Polyoxometalate-dependent Photocatalytic Activity of Radical-doped Perylenediimide-based Hybrid Materials.

Inorganic-organic hybrid materials are a kind of multiduty materials with high crystallinity and definite structures, built from functional inorganic and organic components with highly tunable photochemical properties. Perylenediimides (PDIs) are a kind of strong visible light-absorbing organic dyes with π-electron-deficient planes and photochemical properties depending on their micro-environment, which provides a platform for designing tunable and efficient hybrid photocatalytic materials. Herein, four radical-doped PDI-based crystalline hybrid materials, Cl4-PDI⋅SiW12O40 (1), Cl4-PDI⋅SiMo12O40 (2), Cl4-PDI⋅PW12O40 (3), and Cl4-PDI⋅PMo12O40 (4), were attained by slow diffusion of polyoxometalates (POMs) into acidified Cl4-PDI solutions. The obtained PDI-based crystalline hybrid materials not only exhibited prominent photochromism, but also possessed reactive organic radicals under ambient conditions. Furthermore, all hybrid materials could be easily photoreduced to their radical anions (Cl4-PDI⋅-), and then underwent a second photoexcitation to form energetic excited state radical anions (Cl4-PDI⋅-*). However, experiments and theoretical calculations demonstrated that the formed energetic Cl4-PDI⋅-* showed unusual POM-dependent photocatalytic efficiencies toward the oxidative coupling of amines and the iodoperfluoroalkylation of alkenes; higher photocatalytic efficiencies were found for hybrid materials 1 (anion: SiW12O40 4-) and 2 (anion: SiMo12O40 4-) compared to 3 (anion: PW12O40 3-) and 4 (anion: PMo12O40 3-). The photocatalytic efficiencies of these hybrid materials are mainly controlled by the energy differences between the SOMO-2 level of Cl4-PDI⋅-* and the LUMO level of the POMs. The structure-photocatalytic activity relationships established in present work provide new research directions to both the photocatalysis and hybrid material fields, and will promote the integration of these areas to explore new materials with interesting properties.

Selective oxidative coupling of amines through light‐activated bismuth halide perovskites

AbstractBismuth halide perovskites have emerged as promising alternatives for sustainable and efficient photocatalysts for light‐induced organic transformations. Caesium bismuth halide perovskites (Cs3Bi2(ClxBr1‐x)9) with different ratios of chloride and bromide were synthesised using a one‐step solution process. These perovskites were then employed as photocatalysts for the photocatalytic oxidative coupling of benzylamine (BZA) to produce N‐benzylidenebenzylamine (BZI) under ultraviolet‐visible light radiation at ambient conditions. The incorporation of bromide ions into the chloride‐based perovskite resulted in a noticeable reduction in the bandgap, leading to an enhancement in the photocatalytic activity. Conversely, the addition of chloride ions within the bromide‐based perovskite was found to be beneficial for the material's photostability while simultaneously maintaining its high photocatalytic activity. The optimal compromise between the stability and photocatalytic activity for the BZI production was achieved using Cs3Bi2(Cl0.8Br0.2)9, being attained a total BZA conversion and the maximum BZI production with 100 % of selectivity after 6 hours of irradiation. The efficiency of this photocatalyst was ascribed to the highest current density observed during the photocurrent experiments, which improves the charge carrier mobility of the resulting material.

Structural Conjugation Tuning in Covalent Organic Frameworks Boosts Charge Transfer and Photocatalysis Performances.

Structural conjugation greatly affects the optical and electronic properties of the COF photocatalyst. Herein, we show that 2D hydrazone COFs with either π-extended biphenyl (BPh-COF) or acetylene (AC-COF) frameworks demonstrated distinct charge transfer and photocatalytic performances. The two COFs show good crystallinity and decent porosity as their frameworks are enforced by intra/interlayers hydrogen bonding. However, computational and experimental data reveal that AC-COF managed broader visible-light absorption and narrower optical bandgaps and performed efficient photoinduced charge separation and transfer in comparison with BPh-COF, meaning that the ethynyl skeleton with enhanced planarity better improves the π-conjugation of the whole structure. As a result, AC-COF exhibited an ideal bandgap for rapid oxidative coupling of amines under visible-light irradiation. Furthermore, taking advantage of its better charge transfer properties, AC-COF demonstrated considerable enhanced product conversion and notable functional tolerance for metallaphotocatalytic C-O cross-coupling of a wide range of both aryl bromides and chlorides with alcohols. More importantly, besides being recoverable, AC-COF showcased the previously inaccessible etherification of dihaloarene. This report shows a facile approach for manipulating the structure-activity relationship and paves the way for the development of a COF photocatalyst for solar-to-chemical energy conversion.

Open Air Direct Oxidative Coupling of Alcohols and Amines to Imines Catalyzed by Ruthenium Nanoclusters Supported on a Mesoporous Carbon (CMK-8) in/on Water

Open Air Direct Oxidative Coupling of Alcohols and Amines to Imines Catalyzed by Ruthenium Nanoclusters Supported on a Mesoporous Carbon (CMK-8) in/on Water

A Mechanistic Insight on CuI‐Catalyzed Synthesis of Oxazolidinones through a Four‐Component Reaction

AbstractIn this contribution, we describe a straightforward and efficient synthesis of oxazolidin‐2‐ones through a Cu(I)‐catalyzed four‐component coupling of amines, aldehydes, terminal alkynes, and CO2 via carboxylative cyclization of propargylamine intermediates. This strategy enables the selective synthesis of a variety of (Z)‐5‐alkylidene‐oxazolidin‐2‐ones substituted in positions 3, 4 and 5 of the heterocyclic core with a variety of functional groups, well‐tolerated in terms of scope. A mechanistic study was carried out monitoring the process in solution by different techniques (operando FTIR, multi‐nuclear NMR), with the aim of identifying the intermediates, both organic compounds and Cu(I) coordination complexes, which showed the plausible multi‐metallic nature of the catalytic intermediates (identified by X‐ray diffraction analyses on monocrystal).

Formation of Sterically Congested C−N Bonds by Electrochemical Reductive Coupling of Amines and α‐Bromocarboxamides

AbstractNucleophilic substitution of α‐bromocarboxamide with amines have been demonstrated to synthesize sterically congested C−N bonds. Electrochemical reductive coupling is an efficient route to access hindered amino acid derivatives possessing a quaternary carbon center with excellent yields under mild reaction conditions. Cyclic voltammetry analyses indicated the cathodic reduction of α‐bromocarboxamides in the presence of amines as nucleophiles. The control experiment using the N‐methyl derivative of α‐bromocarboxamide suggested that the aziridinone is the most probable intermediate in this method.

Access to Functionalized Materials by Metal-Free Ring-Opening Metathesis Polymerization of Active Esters and Divergent Postpolymerization Modification.

Metal-free ring-opening metathesis polymerization (MF-ROMP) is an emerging polymerization strategy that provides access to ROMP materials by using organic initiators and photoredox catalysts. Unlike metal-mediated ROMP, MF-ROMP is not highly tolerant toward functionalized monomers. Herein, we report that pentafluorophenyl esters are polymerizable under MF-ROMP conditions to produce homopolymers, statistical copolymers, and block copolymers. Amine coupling agents were then used to install a range of functional groups via acyl substitution including alkynes, amino acid derivatives, fluorophores, and redox active moieties. Overall, these findings provide a framework to prepare functionalized ROMP polymers without the risk of metal contamination.

Broad Spectrum Enantioselective Amide Bond Synthetase from Streptoalloteichus hindustanus.

The synthesis of amide bonds is one of the most frequently performed reactions in pharmaceutical synthesis, but the requirement for stoichiometric quantities of coupling agents and activated substrates in established methods has prompted interest in biocatalytic alternatives. Amide Bond Synthetases (ABSs) actively catalyze both the ATP-dependent adenylation of carboxylic acid substrates and their subsequent amidation using an amine nucleophile, both within the active site of the enzyme, enabling the use of only a small excess of the amine partner. We have assessed the ability of an ABS from Streptoalloteichus hindustanus (ShABS) to couple a range of carboxylic acid substrates and amines to form amine products. ShABS displayed superior activity to a previously studied ABS, McbA, and a remarkable complementary substrate specificity that included the enantioselective formation of a library of amides from racemic acid and amine coupling partners. The X-ray crystallographic structure of ShABS has permitted mutational mapping of the carboxylic acid and amine binding sites, revealing key roles for L207 and F246 in determining the enantioselectivity of the enzyme with respect to chiral acid and amine substrates. ShABS was applied to the synthesis of pharmaceutical amides, including ilepcimide, lazabemide, trimethobenzamide, and cinepazide, the last with 99% conversion and 95% isolated yield. These findings provide a blueprint for enabling a contemporary pharmaceutical synthesis of one of the most significant classes of small molecule drugs using biocatalysis.

Cobalt-catalyzed synthesis of amides from alkenes and amines promoted by light.

Catalytic methods to couple alkene and amine feedstocks are valuable in synthetic chemistry. The direct carbonylative coupling of alkenes and amines holds promise as a perfectly atom-economical approach to amide synthesis, but general methods remain underdeveloped. Herein, we report an alkene hydroaminocarbonylation catalyzed by unmodified, inexpensive cobalt carbonyl under mild conditions and low pressure promoted by light. Silane addition after the reaction enables sequential cobalt-catalyzed amide reduction, constituting a formal alkene hydroaminomethylation. These methods exhibit exceptional scope across both alkene and amine components with high chemo- and regioselectivity and proceed efficiently even in the absence of solvent. The formation of a hydridocobalt through photodissociation of a carbonyl ligand is proposed to enable catalytic activity under mild conditions, which addresses a long-standing challenge in catalysis.

Tailoring flavin-based photosensitizers for efficient photooxidative coupling of benzylic amines.

Photooxidative coupling of benzylic amines using naturally abundant O2 as an oxidant under visible light irradiation is an alternative green approach to synthesis imines and is of both fundamental and practical significance. We investigated the photophysical properties of flavin (FL) that is a naturally available sensitizer and its derivatives, i.e. 9-bromoflavin (MB-FL), 7,8-dibromoflavin (DB-FL) and 10-phenylflavin (Ph-FL), as well as the performance of these FL-based sensitizers (FLPSs) in the photooxidative coupling of benzylic amines to imines combining experimental and theoretical efforts. We showed that chemical functionalization with Br and phenyl effectively improves the photophysical properties of these FLPSs, in terms of absorption in the visible light range, singlet oxygen quantum yields, triplet lifetime, etc. Apart from nearly quantitative selectivity for the production of imines, the performance of DB-FL is superior to those of other FLPSs, and it is among the best photocatalysts for imine synthesis. Specifically, 0.5 mol% DB-FL is capable of converting 91% of 0.2 mmol benzylamine and more than 80% of 0.2 mmol fluorobenzylic amine derivatives into their corresponding imines in 5 h batch runs. Mechanistic investigation finely explained the observed photophysical properties of FLPSs and highlighted the dominant role of electron transfer in FLPS sensitized coupling of benzylic amines to imines. This work not only helps to understand the pathways for photocatalysis with FLPSs but also paves the way for the design of novel and efficient PSs to promote organic synthesis.