Recent Advances in Aerobic Oxidation of Alcohols and Amines to Imines

Linker Engineering of High-Nuclearity {V12@P8W48}-Based Metal-Organic Frameworks for Green-Light-Driven Oxidative Coupling of Amines

The development of long-wavelength visible-light-responsive and reusable photocatalysts for organic transformation is of significant interest. Herein, we report the design and synthesis of high-nuclearity {V12@P8W48}-based metal-organic frameworks, POMOF1 and POMOF2, as heterogeneous photocatalysts for long-wavelength light-triggered oxidation. Linker engineering, by tuning from visible-light-inactive triazole (L1) to a photosensitive anthraquinone-derived ligand (L2), not only leads to the generation of porous 1D open channels within POMOF2 but also imparts a strong peak absorption centered at 500 nm. Moreover, the integration of {V12@P8W48} and Cu2+ ions together with L2 into POMOF2 enables the continued and broad absorption ranging from the ultraviolet to near-infrared region. Consequently, POMOF2 exhibited excellent activity in the green-light-driven oxidative coupling of benzylamines, affording a series of imines with high conversions of up to 99% under mild conditions. In contrast, POMOF1 could barely promote the reaction under the same conditions, further confirming the advantage of linker modulation. POMOF2 is stable and can be reused for three cycles with little loss of catalytic activity and structural integrity. This work highlights the potential of linker engineering as an efficient approach for designing long-wavelength photocatalysts, which can further push forward photoredox catalysis.

Hierarchical Co9S8/CdS Hollow Nanocages for Efficient and Robust Cooperative Coupling Photoredox Synthesis

The elaborate development of photocatalysts that can maximize the utility of solar energy with simultaneous photocatalytic H2 evolution and high-quality chemicals production holds great promise in the field of selective photoredox synthesis. Herein, the in situ growth of CdS nanowires on the outside surface of Co9S8 hollow nanocages is reported to create a hierarchical CdS/Co9S8 hollow photocatalyst for efficient photochemical coupling of amines to produce imines and H2. This ingenious hierarchical photocatalyst has a unique hollow structure and dense interfacial contact with a type-I band orientation, which facilitates the interfacial transfer of charge carriers. The photocatalytic activity of this hierarchical hollow catalyst is significantly enhanced in comparison to CdS and Co9S8, and distinctively exhibits excellent recycling stability. Mechanistic studies have shown that the carbon-centered radical Ph(•CH)NH2 is essential for the reaction. Additionally, the CdS/Co9S8 hollow hybrid also exhibits outstanding performance in coupling various primary amines to the corresponding imines, highlighting the broad applicability of this photocatalyst for C-N coupling reactions. This work offers a new strategy for the efficient design of heterostructured hollow semiconductor-based photocatalysts for the cooperative coupling of clean fuel production and fine chemical synthesis.

Engineering Atomic Sites and Proton Transfer Microenvironments for Bioinspired Photocatalytic Alcohol-Amine Coupling

Achieving a precise understanding and accurate design of heterogeneous catalysts based on bioinspired principles is challenging yet crucial to digging out optimal materials for artificial catalysis. Here, an ADH-mimicking dual-site photocatalyst (YCuCdS) is developed, and demonstrates the powerful effects of atomic site configuration and proton transfer environments on alcohol-amine coupling. Mechanism studies reveal that the alcohol substrate is effectively dehydrogenated at the Y sites, forming the carbonyl intermediates that rapidly experience condensation with the amine. Meanwhile, the released hydrogen species (Hads) migrate from adjacent Cu sites to active S atoms, promoting H2 production and hindering the over-hydrogenation of imine. As a result, a high imine yield of 92% is achieved, along with an H2 production rate of 7400 µmol g-1 h-1. This work showcases an effective strategy for the design of heterogeneous catalysts with bioinspiration.

Chromium catalyzed acceptorless dehydrogenative (cross)coupling of primary amines to secondary imines

We present here acceptorless dehydrogenative coupling of primary amines to form secondary aldimines catalyzed by a complex of Earth-abundant chromium. The reaction is promoted by Cr(DAFO)(CO)4 (DAFO = 4,5-diazafluorene-9-one) without using any additives, base or oxidant generating NH3 and H2 as sole by-products. Dehydrogenative cross-coupling of primary amines with aniline derivatives to unsymmetrical secondary imines was also achieved with good to excellent yields. A probable mechanism is proposed based on the stoichiometric investigation and computational studies.

Structural and Interfacial Characterization of a Photocatalytic Titanium MOF-Phosphate Glass Composite

Metal-organic framework (MOF) composites are proposed as solutions to the mechanical instability of pure MOF materials. Here, we present a new compositional series of recently discovered MOF-crystalline inorganic glass composites. In this case, formed by the combination of a photocatalytic titanium MOF (MIL-125-NH2) and a phosphate-based glass (20%Na2O-10%Na2SO4-70%P2O5). This new family of composites has been synthesized and characterized using powder X-ray diffraction, thermal gravimetric analysis, differential scanning calorimetry, scanning electron microscopy, and X-ray total scattering. Through analysis of the pair distribution function extracted from X-ray total scattering data, the atom-atom interactions at the MOF-glass interface are described. Nitrogen and carbon dioxide isotherms demonstrate good surface area values despite the pelletization and mixing of the MOF with a dense inorganic glass. The catalytic activity of these materials was investigated in the photooxidation of amines to imines, showing the retention of the photocatalytic effectiveness of the parent pristine MOF.

Photocatalytic Oxidative Coupling of Benzyl Alcohol and Benzylamine for Imine Synthesis Using Immobilized Cs3Bi2Br9 Perovskite

The oxidative cross-coupling of benzyl alcohol (BA) and benzylamine (BZA) is employed for the production of the corresponding imine, N-benzylidenebenzylamine (BZI), under visible light irradiation (light-emitting diodes (LE with λmax = 417 nm) and mild reaction conditions. The cesium bismuth halide perovskites (Cs2Bi3Br9, CBB) are synthesized by a one-step solution process as a sustainable alternative for the widely used Pb-halide perovskites. The CBB photocatalyst is immobilized on a polyethylene terephthalate (PET) structure designed explicitly for three-dimensional (3D) printing to operate in both batch and continuous modes to overcome the need for a final catalyst separation step. The complete conversion of BZA and BA is achieved after 1 h, yielding 70% of BZI in basic medium operating in batch mode. Comparable results are found between the suspended and immobilized catalysts for imine production. Additionally, continuous production of BZI is successfully achieved using immobilized CBB, with a maximum yield of 0.35 mm of BZI after a 2 h reaction. The supported CBB perovskites demonstrate high stability after multiple uses. Finally, a comprehensive photocatalytic pathway for cross-coupling BZA with BA is proposed.

Cu-UiO-66 Catalyzed Synthesis of Imines via Acceptorless Dehydrogenative Coupling of Alcohols and Amines

Herein, the Cu-UiO-66 catalyst was developed for acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines. The Cu-UiO-66 catalyst was synthesized by installing Cu2+ onto Zr-oxo clusters in UiO-66, and the catalyst efficiently catalyzes the ADC reaction under mild and environmentally friendly conditions with excellent selectivity. Mechanistic studies reveal that the O2⋅- radicals and porosity of formed in Cu-UiO-66 participate cooperatively during the catalytic cycle. Meanwhile, the only by-product of the system is environmentally benign water. Cycling tests and hot filtration tests showed that the Cu-UiO-66 catalyst exhibited excellent stability and catalytic activity during the reaction. Importantly, the Cu-UiO-66 catalyst might provide a promising strategy for the ADC reaction between alcohols and amines to produce imines.

Aerobic Reconstruction of Amines to Amides: A C-N/C-C Bond Cleavage Approach

Herein, an aerobic reconstruction of amines to amides via C(sp3)-N bond and C(sp2)-C(sp3) bond cleavage is described. This method features a metal-free reaction, insensitivity to oxygen or moisture, and ambient air as the terminal oxidant. Preliminary mechanistic studies suggest that the reaction pathway of amine oxidation, followed by imine exchange and Beckmann rearrangement, is involved.

Porous Bimetallic Ti-MOFs for Photocatalytic Oxidation of Amines in Air

A family of microporous titanium-containing metal-organic frameworks (denoted as M2Ti-CPCDC, M = Mn, Co, Ni) has been synthesized by using a bimetallic [M2Ti(μ3-O)(COO)6] cluster and a tritopic carbazole-based organic ligand H3CPCDC. M2Ti-CPCDC are stable and display permanent porosity for N2 and CO2 uptake, ranking among the most porous titanium-based metal-organic frameworks. M2Ti-CPCDC crystals exhibit n-type semiconductor behavior. Further catalytic studies demonstrate that all M2Ti-CPCDC materials are applicable for triggering photo-oxidative reactions of amines in air. More specifically, amines with electron-donating groups afford the aldehydes as the main products, while amines bearing electron-withdrawing groups give rise to imines as the main product. Among them, Mn2Ti-CPCDC exhibit the best photocatalytic activity, with conversion of benzylamine up to 99% and selectivity of 99%. Mn2Ti-CPCDC could be recycled in at least three runs while retaining crystallinity and catalytic activity. The reaction mechanism indicates that photoinduced hole (h+), superoxide radical anion (O2·-), and singlet oxygen (1O2) are the main active species involved in the photo-oxidation process.

Electrochemical Dimerization of o-Aminophenols and Hydrogen Borrowing-like Cascade to Synthesize N-Monoalkylated Aminophenoxazinones via Paired Electrolysis

A novel electrocatalytic dimerization of o-aminphenols and a hydrogen borrowing-like cascade for synthesizing N-monoalkylated aminophenoxazinones have been developed. This electrocatalytic reaction uses a constant current mode in an undivided cell and is free of metal catalysis, open to the air, and eco-friendly. In particular, this protocol exhibits a wide substrate range and provides versatile N-monoalkylated aminophenoxazinones in medium to good yields. The results of our mechanistic research reveal that this protocol involves a cascade of electrochemical cyclocondensation of o-aminphenols and the hydrogen transfer process via paired electrolysis.

Photoswitchable Imines Drive Dynamic Covalent Systems to Nonequilibrium Steady States

Coupling a photochemical reaction to a thermal exchange process can drive the latter to a nonequilibrium steady state (NESS) under photoirradiation. Typically, systems use separate motifs for photoresponse and equilibrium-related processes. Here, we show that photoswitchable imines can fulfill both roles simultaneously, autonomously driving a dynamic covalent system into a NESS under continuous light irradiation. We demonstrate this using transimination reactions, where E-to-Z photoisomerism generates a more kinetically labile species. At the NESS, energy is stored both in the metastable Z-isomer of the imine and in the system's nonequilibrium constitution; when the light is switched off, this stored energy is released as the system reverts to its equilibrium state. The system operates autonomously under continuous light irradiation and exhibits characteristics of a light-driven information ratchet. This is enabled by the dual-role of the imine linkage as both the photochromic and dynamic covalent bond. This work highlights the ability and application of these imines to drive systems to NESSs, thus offering a novel approach in the field of systems chemistry.

Transient-State Self-Bipolarized Organic Frameworks of Single Aromatic Units for Natural Sunlight-Driven Photosynthesis of H2O2

Constructing π-conjugated polymer structures through covalent bonds dominates the design of organic framework photocatalysts, which significantly depends on the selection of multiple donor-acceptor building blocks to narrow the optical gap and increase the lifetimes of charge carriers. In this work, self-bipolarized organic frameworks of single aromatic units are demonstrated as novel broad-spectrum-responsive photocatalysts for H2O2 production. The preparation of such photocatalysts is only to fix the aromatic units (such as 1,3,5-triphenylbenzene) with alkane linkers in 3D space. Self-bipolarized aromatic units can drive the H2O2 production from H2O and O2 under natural sunlight, wide pH ranges (3.0-10.0) and natural water sources. Moreover, it can be extended to catalyze the oxidative coupling of amines. Experimental and theoretical investigation demonstrate that such a strategy obeys the mechanism of through-space π-conjugation, where the closely face-to-face overlapped aromatic rings permit the electron and energy transfer through the large-area delocalization of the electron cloud under visible light irradiation. This work introduces a novel design concept for the development of organic photocatalysts, which will break the restriction of conventional through-band π-conjugation structure and will open a new way in the synthesis of organic photocatalysts.

Hollow structured CdS@ZnIn2S4 Z-scheme heterojunction for bifunctional photocatalytic hydrogen evolution and selective benzylamine oxidation

Photocatalytic hydrogen evolution (PHE) is frequently constrained by inadequate light utilization and the rapid combination rate of the photogenerated electron-hole pairs. Additionally, conventional PHE processes are often facilitated by the addition of sacrificial reagents to consume photo-induced holes, which makes this approach economically unfavorable. Herein, we designed a spatially separated bifunctional cocatalyst decorated Z-scheme heterojunction of hollow structured CdS (HCdS) @ZnIn2S4 (ZIS), which was prepared by a sacrificial hard template method followed by photo-deposition. Consequently, PdOx@HCdS@ZIS@Pt exhibited efficient PHE (86.38 mmol·g-1·h-1) and benzylamine (BA) oxidation coupling (164.75 mmol·g-1·h-1) with high selectivity (97.34 %). The unique hollow core-shelled morphology and bifunctional cocatalyst loading in this work hold great potential for the design and synthesis of bifunctional Z-scheme photocatalysts.

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.

Insights into interfacial mechanisms: CsPbBr3 nanocrystals as sustainable photocatalysts for primary amine oxidation

CsPbBr3 nanocrystals (NCs) employed as a photocatalyst resulted in efficient benzylamine oxidation under oxygen atmosphere. Improved reaction yields stem from favorable -NH2 functional group interactions on the NC surface, while additional interactions with -OMe or -SMe functional groups post-product formation result in lower yields. These insights into interfacial interactions and mechanistic aspects advance sustainable chemical transformations through cost-effective and recyclable CsPbBr3 NC-catalyzed primary amine oxidation.

Efficient conversion of propane in a microchannel reactor at ambient conditions

The oxidative dehydrogenation of propane, primarily sourced from shale gas, holds promise in meeting the surging global demand for propylene. However, this process necessitates high operating temperatures, which amplifies safety concerns in its application due to the use of mixed propane and oxygen. Moreover, these elevated temperatures may heighten the risk of overoxidation, leading to carbon dioxide formation. Here we introduce a microchannel reaction system designed for the oxidative dehydrogenation of propane within an aqueous environment, enabling highly selective and active propylene production at room temperature and ambient pressure with mitigated safety risks. A propylene selectivity of over 92% and production rate of 19.57 mmol mCu-2 h-1 are simultaneously achieved. This exceptional performance stems from the in situ creation of a highly active, oxygen-containing Cu catalytic surface for propane activation, and the enhanced propane transfer via an enlarged gas-liquid interfacial area and a reduced diffusion path by establishing a gas-liquid Taylor flow using a custom-made T-junction microdevice. This microchannel reaction system offers an appealing approach to accelerate gas-liquid-solid reactions limited by the solubility of gaseous reactant.

Luminescence and Excited-State Reactivity in a Heteroleptic Tricyanido Fe(III) Complex

Harnessing sunlight via photosensitizing molecules is key for novel optical applications and solar-to-chemical energy conversion. Exploiting abundant metals such as iron is attractive but becomes challenging due to typically fast nonradiative relaxation processes. In this work, we report on the luminescence and excited-state reactivity of the heteroleptic [FeIII(pzTp)(CN)3]- complex (pzTp = tetrakis(pyrazolyl)borate), which incorporates a σ-donating trispyrazolyl chelate ligand and three monodentate σ-donating and π-accepting cyanide ligands. Contrary to the nonemissive [Fe(CN)6]3-, a broad emission band centered at 600 nm at room temperature has been recorded for the heteroleptic analogue attributed to the radiative deactivation from a 2LMCT excited state with a luminescence quantum yield of 0.02% and a lifetime of 80 ps in chloroform at room temperature. Bimolecular reactivity of the 2LMCT excited state was successfully applied to different alcohol photo-oxidation, identifying a cyanide-H bonding as a key reaction intermediate. Finally, this research demonstrated the exciting potential of [Fe(pzTp)(CN)3]- as a photo-oxidant, paving the way for further exploration and development of emissive Fe-based photosensitizers competent for photochemical transformations.

Green imine synthesis from amines using transition metal and micellar catalysis

Imines are a versatile class of chemicals with applications in pharmaceuticals and as synthetic intermediates. While imines are conventionally synthesized via aldehyde-amine condensation, their direct preparation from amines can avoid the need for the independent preparation of the aldehyde coupling partner and associated constraints with regard to aldehyde storage and purification. The direct preparation of imines from amines typically utilizes transition metal catalysis and is often well-aligned with green chemistry principles. This review provides a comprehensive overview of transition metal catalysed imine synthesis, with a particular focus on the copper-catalyzed oxidative coupling of amines. The emerging application of micellar catalysis for imine synthesis is also surveyed due to its potential to avoid the use of hazardous solvents and intensify these reactions through reduced catalyst loadings and locally increased reactant concentrations. Future directions relating to the confluence of these two areas are proposed towards the more sustainable preparation of imines.

Efficient one-pot syntheses of secondary amines from nitro aromatics and benzyl alcohols over Pd/NiTi-LDH under visible light

Solar energy-induced cascade/tandem reactions in one-pot are sustainable and green. Herein, the Pd/NiTi-LDH nanocomposite, with Pd nanoparticles (NPs) (∼3-6 nm) deposited on NiTi-LDH nanosheets, was obtained and was applied in the reaction between nitro aromatics and alcohols to synthesize secondary amines under visible light. The superior performance observed over the as-obtained Pd/NiTi-LDH nanocomposite for this reaction can be attributed to a successful merging of Pd-based hydrogenation and LDH-based photocatalysis, in which consecutive light-induced hydrogenation of nitro compounds to amines, dehydrogenation of alcohols to aldehydes, condensation between the in situ formed aldehydes and amines to imines and the hydrogenation of final imines to generate the desired secondary amines were realized in one pot over Pd/NiTi-LDH under visible light. This work shows an effective and green strategy in the synthesis of secondary amines. This study also demonstrates the high potential of using metal/LDH nanocomposites for light-initiated organic syntheses.

A porous Anderson-type polyoxometalate-based metal-organic framework as a multifunctional platform for selective oxidative coupling with amines

Incorporating catalytic units into a crystalline porous matrix represents a facile way to build high-efficiency heterogeneous catalysts, and by rational design of the porous skeleton with appropriate building blocks the catalytic performance can be significantly enhanced for a series of organic transformations owing to the synergistic effect from the multicomponent and confined porous microenvironment around catalytically active sites. Herein, we demonstrate that the design and synthesis of a porous polyoxometalate-based metal-organic framework YL2(H2O)2[CrMo6O18(PET)2]·4H2O (POMOF-1) constructed from Anderson-type [CrMo6O18(PET)2] (PET = pentaerythritol), which can be employed as a multifunctional platform for synthesis of N-containing compounds via selective oxidative coupling with amines. POMOF-1 features microporous 1D channels defined by Y3+ and L, with [CrMo6O18(PET)2] arranged orderly between adjacent Lvia electrostatic interactions. Upon using POMOF-1 as a catalyst and H2O2 as an oxidant, a variety of amines could be effectively converted to value-added amides, imines and azobenzenes via the oxidative cross-coupling with alcohols or homo-coupling. In particular, POMOF-1 showed dramatically improved activity for the N-formylation reaction owing to the synergistic and confinement effect, with the yield of amides up to 95% and 4 times higher than that of homogeneous [CrMo6O18(PET)2]. Meanwhile, the oxidative homo-coupling of arylmethylamines and arylamines can be facilely tuned by adjustment of the amount of oxidant, solvent and additive, affording imines and azobenzenes in high selectivity and yield, respectively. POMOF-1 is robust and can be reused for 5 cycles with little loss of catalytic activity and structural integrity. The work demonstrates that the combination of catalytically active POMs with crystalline porous MOFs holds great potential to build robust and recyclable heterogeneous systems with enhanced activity and selectivity for multifunctional catalysis.

Construction of Functionalized α-Imino Ketones via Pd-Catalyzed C-H Addition to Nitriles/Aerobic Oxidation Sequences

Pd(II)-catalyzed addition of sp2 C-H to nitrile/aerobic oxidation sequences for the preparation of functionalized α-imino ketones is described in which readily available heteroarenes and O-acyl cyanohydrins were employed. Various functionalized targeted molecules can be prepared in good yields with high atom and step economy. Moreover, a broad substrate scope and the ready manipulation and availability of the reaction partners enable this protocol to be appealing to explore the chemical space of the construction of functionalized α-imino ketones with high efficiency.

Ce4+/Ce3+ Redox Effect-Promoted CdS/CeO2 Heterojunction Photocatalyst for the Atom Economic Synthesis of Imines under Visible Light

The employment of stoichiometric alcohols and amines for imine synthesis under mild and green reaction conditions is still a challenge in the field. In this work, based on our research foundation in the thermocatalytic synthesis of imines over ceria, a CdS/CeO2 heterojunction photocatalyst was constructed and successfully realized the atom-economic synthesis of imines under visible light without additives at room temperature. Mechanistic experiments and corresponding characterizations indicated that the CdS/CeO2 heterojunction can improve the separation efficiency of photogenerated carriers, which can be further enhanced by the Ce4+/Ce3+ redox pair by rapidly combining photogenerated e-. The in situ-reduced Ce3+ can better activate O2 to form Ce-O-O·, which, together with h+, efficiently accelerates alcohol oxidation, which is the rate-determined step for the synthesis of imines via oxidative coupling reaction of alcohol and amine. In addition, our photocatalyst exhibited fairly decent reusability and substrate universality. This work solves problems of using base additives and excess amine or alcohol in the reported photocatalytic systems and provides new insight for designing CeO2-based photocatalytic oxidation catalysts.

Thermodynamic Evaluations of Amines as Hydrides or Two Hydrogen Ions Reductants and Imines as Protons or Two Hydrogen Ions Acceptors, as Well as Their Application in Hydrogenation Reactions

Since the hydrogenation of imines (X) and the dehydrogenation of amines (XH2) generally involve the two hydrogen ions (H- + H+) transfer, the thermodynamic abilities of various amines releasing hydrides or two hydrogen ions as well as various imines accepting protons or two hydrogen ions are important and characteristic physical parameters. In this work, the pKa values of 84 protonated imines (XH+) in acetonitrile were predicted. Combining Gibbs free energy changes of amines releasing hydrides in acetonitrile from our previous work with the pKa(XH+) values, the Gibbs free energy changes of amines releasing two hydrogen ions and imines accepting two hydrogen ions were derived using Hess's law by constructing thermochemical cycles, and the thermodynamic evaluations of amines as hydrides or two hydrogen ions reductants and imines as protons or two hydrogen ions acceptors are well compared and discussed. Eventually, the practical application of thermodynamic data for amines and imines on hydrogenation feasibility, mechanism, and possible elementary steps was shown and discussed in this paper from the point of thermodynamics.

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.

Microwave-Assisted Synthesis of E-Aldimines, N-Heterocycles, and H2 by Dehydrogenative Coupling of Benzyl Alcohol and Aniline Derivatives Using CoCl2 as a Catalyst

The acceptorless dehydrogenative coupling (ADC) between alcohols and amines to produce imines has been achieved mostly by employing precious-metal-based complexes or complexes of earth-abundant metal ions with sensitive and complicated ligand systems as catalysts mostly under harsh reaction conditions. Methodologies using readily available earth-abundant metal salts as catalysts without the requirement of ligand, oxidant, or any external additives are not explored. We report an unprecedented microwave-assisted CoCl₂-catalyzed acceptorless dehydrogenative coupling of benzyl alcohol and amine for the synthesis of E-aldimines, N-heterocycles, and H₂ under mild condition, without any complicated exogenous ligand template, oxidant, or other additives. This environmentally benign methodology exhibits broad substrate scope (43 including 7 new products) with fair functional-group tolerance on the aniline ring. Detection of metal-associated intermediate by gas chromatography (GC) and HRMS, H₂ detection by GC, and kinetic isotope effect reveal the mechanism of this CoCl_2-catalyzed reaction to be via ADC. Furthermore, kinetic experiments and Hammett analysis with variation in the nature of substituents over the aniline ring reveal the insight into the reaction mechanism with different substituents.

A benzimidazole scaffold as a promising inhibitor against SARS-CoV-2

The manuscript reports the green-chemical synthesis of a new diindole-substituted benzimidazole compound, B1 through a straightforward route in coupling between indolyl-3-carboxaldehyde and o-phenylenediamine in water medium under the aerobic condition at 75 ºC. The single crystal X-ray structural analysis of B1 suggests that the disubstituted benzimidazole compound crystallizes in a monoclinic system and the indole groups exist in a perpendicular fashion with respect to benzimidazole moiety. The SARS-CoV-2 screening activity has been studied against 1 × 10e4 VeroE6 cells in a dose-dependent manner following Hoechst 33342 and nucleocapsid staining activity with respect to remdesivir. The compound exhibits 92.4% cell viability for 30 h and 35.1% inhibition against VeroE6 cells at non-cytotoxic concentration. Molecular docking studies predict high binding propensities of B1 with the main protease (M^pro) and non-structural (nsp2 and nsp7-nsp8) proteins of SARS-CoV-2 through a number of non-covalent interactions. Molecular dynamics (MD) simulation analysis for 100 ns confirms the formation of stable conformations of B1-docked proteins with significant changes of binding energy, attributing the potential inhibition properties of the synthetic benzimidazole scaffold against SARS-CoV-2. Communicated by Ramaswamy H. Sarma.

Interfacial Reaction-Directed Green Synthesis of CeO2-MnO2 Catalysts for Imine Production through Oxidative Coupling of Alcohols and Amines

Direct oxidative coupling of alcohols with amines over cheap but efficient catalysts is a promising choice for imine formation. In this study, porous CeO₂-MnO₂ binary oxides were prepared via an interfacial reaction between Ce₂(SO₄)₃ and KMnO₄ at room temperature without any additives. The as-prepared porous CeO₂-MnO₂ catalyst has a higher fraction of Ce³⁺, Mn³⁺, and Mn⁴⁺ and contains larger surface area and more oxygen vacancies. During the oxidative coupling reaction of alcohol with amine to imine, the as-obtained CeO₂-MnO₂ catalyst is motivated by the above encouraging characteristics and exhibits superior catalytic activity (98% conversion and 97% selectivity) and can also work effectively under a wide scope of temperatures and substrates. The in-depth in situ DRIFTS and density functional theory (DFT) results demonstrate that there is a strong interaction between CeO₂ and MnO₂ in the CeO₂-MnO₂ catalyst, exhibiting especially a positive synergistic effect in the direct coupling of alcohol and amine reaction.

Electrochemically Enabled Direct C3-Formylation of Imidazopyridines with Me3 N as a Carbonyl Source

A metal-free and oxidant-free electrochemically enabled strategy for C-3 formylation of imidazopyridines using trimethylamine as a one-carbon source has been established. This conversion has high functional group compatibility under mild conditions and ensures late-stage functionalization of pharmaceutical molecules. Furthermore, unexpected hexafluoroisopropoxylation products have been observed in some cases. Mechanistic studies using cyclic voltammetry and control experiments reveal the key intermediate of the formylation process.

Switching the Photoreactions of Ir(III) Diamine Complexes between C-N Coupling and Dehydrogenation under Visible Light Irradiation

The selective photoreactions under mild conditions play an important role in synthetic chemistry. Herein, efficient and mild protocols for switching the photoreactions of Ir(III)-diamine complexes between the interligand C-N coupling and dehydrogenation are developed in the presence of O₂ in EtOH solution. The photoreactions of achiral diamine complexes rac-[Ir(L)₂(dm)](PF₆) (L is 2-phenylquinoline or 2-(2,4-difluorophenyl)quinoline, dm is 1,2-ethylenediamine, 1,2-diaminopropane, 2-methyl-1,2-diamino-propane, or N,N'-dimethyl-1,2-ethylenediamine) are competitive in the oxidative C-N coupling and dehydrogenation at room temperature, which can be switched into the interligand C-N coupling reaction at 60 °C, affording hexadentate complexes in good to excellent yields, or the dehydrogenative reaction in the presence of a catalytic amount of TEMPO as an additive, affording imine complexes. Mechanism studies reveal that ¹O₂ is the major reactive oxygen species, and metal aminyl is the key intermediate in the formation of the oxidative C-N coupling and imine products in the photoreaction processes. These will provide a new and practical protocol for the synthesis of multidentate and imine ligands in situ via the postcoordinated strategy under mild conditions.

Innovative green oxidation of amines to imines under atmospheric oxygen

In recent years, the development of environmentally benign molecular construction methods has been of great importance, and especially, resource recycling, high atomic efficiency, and low environmental impact are in high demand. From this point of view, attention has also been focused on the development of one-pot synthesis of pharmaceuticals and functional molecules. Imines are excellent synthetic intermediates of these useful molecules, and the environmentally friendly oxidative synthesis of imines from amines has been energetically developed using oxygen (or air), which is abundantly available on the Earth, as an oxidant. This review focuses on the latest innovative and green oxidation systems of amines to imines under atmospheric oxygen, and their application to one-pot/eco-friendly and sustainable synthesis of pharmaceuticals and functional molecules. In particular, catalytic systems that activate molecular oxygen are categorized and described in detail as transition metal catalytic systems, photoirradiated catalytic systems, and organocatalytic systems.

Tipping Gold Nanobipyramids with Titania for the Use of Plasmonic Hotspots to Drive Amine Coupling

Designing plasmonic photocatalysts with spatially controlled catalytic sites is an effective strategy to boost the sunlight-driven chemical transformation efficiency through plasmonic enhancement. Herein, we describe a facile method for the synthesis of TiO₂-tipped Au nanobipyramids (NBPs) to give (Au NBP)/t-TiO₂ nanodumbbells. The surfactant cetyltrimethylammonium bromide concentration is the key factor in the construction of this type of unique nanostructure. The photocatalytic aerobic oxidative coupling of amines using the plasmonic photocatalysts with the dumbbell-like and core@shell structures indicates that the TiO₂-tipped ends for the photo-reduction and the exposed adjacent Au surface for the photo-oxidation on (Au NBP)/t-TiO₂ can significantly improve the photocatalytic activity. The underlying mechanism of the photocatalytic oxidative coupling of benzylamine over (Au NBP)/t-TiO₂ has been thoroughly investigated. Both experimental and simulation results for (Au NBP)/t-TiO₂ and (Au nanorod)/t-TiO₂ confirm the important effect of the plasmonic hotspots on the enhancement of the photocatalytic activity.

Selective desaturation of amides: a direct approach to enamides

C(sp3)-H bond desaturation has been an attractive strategy in organic synthesis. Enamides are important structural fragments in pharmaceuticals and versatile synthons in organic synthesis. However, the dehydrogenation of amides usually occurs on the acyl side benefitting from enolate chemistry like the desaturation of ketones and esters. Herein, we demonstrate an Fe-assisted regioselective oxidative desaturation of amides, which provides an efficient approach to enamides and β-halogenated enamides.

In silico anti-SARS-CoV-2 activities of five-membered heterocycle-substituted benzimidazoles

The manuscript deals with cost-effective synthesis, structural characterization and in silico SARS-CoV-2 screening activity of 5-membered heterocycle-substituted benzimidazole derivatives, 1-((1H-pyrrol-2-yl)methyl)-2-(1H-pyrrol-2-yl)-1H-benzo[d]imidazole (L1), 2-(furan-2-yl)-1-(furan-2-ylmethyl)-1H-benzo[d]imidazole (L2), 2-(thiophen-2-yl)-1-(thiophen-2-ylmethyl)-1H-benzo[d]imidazole (L3). The benzimidazole compounds were synthesized through a green-synthetic approach by coupling of 5-membered heterocyclic-carboxaldehyde and o-phenylenediamine in water under an aerobic condition. The compounds were characterized by various spectroscopic methods and X-ray structural analysis. The suitable single-crystals of the methyl derivative of L3 were grown as L3' which crystallized in a monoclinic system and the thiophene groups co-existed in a nearly a perpendicular orientation. Further, in silico anti-SARS-CoV-2 proficiency of the synthetic derivatives is evaluated against main protease (M^pro) and non-structural proteins (nsp2 and nsp7) of SARS-CoV-2. Molecular docking and molecular dynamics analysis of the ligands (L1-L3) against M^pro and nsp2 and nsp7 for 50 ns reveal that L3 turns out to be the superlative antiviral candidate against M^pro, nsp2 and nsp7 of SARS-CoV-2 as evident from the binding score and stability of the ligand-docked complexes with considerable binding energy changes.

Periodically nanoporous hydrogen-bonded organic frameworks for high performance photocatalysis

The development of highly catalytic hydrogen-bonded organic frameworks (HOFs) is of great importance, but remains challenging. Herein, we demonstrate the fabrication of a periodically nanoporous HOF for high performance photocatalysis. Compared with the conventional microporous HOFs, the nanoporous HOF architecture has a larger number of free carboxyl groups on the surface and presents greatly improved photoelectrochemical properties. It exhibits high catalytic activity for the photo-oxidative coupling of amines under mild conditions such as air atmosphere and room temperature and without any co-catalysts, sacrificial reagents or photosensitizers. The relationship between the structure, properties and catalytic performance of the nanoporous HOF was studied by experimental and theoretical investigations. It shows that such a HOF structure facilitates reactant adsorption and O₂ dissociation, thus promoting the oxidative coupling reaction. This work provides a new way for improving the catalytic performance of a single HOF.

An S-scheme α-Fe2O3/Cu2O photocatalyst for an enhanced primary amine oxidative coupling reaction under visible light

Photoredox catalysis under visible light has been recorded as a potential and reassuring recipe for organic synthesis. More and more heterojunction catalysts have appeared in people's fields of vision, especially those with S-scheme heterojunction structures. The S-scheme heterojunction structure of a photocatalyst can remarkably improve its photocatalytic efficiency. Here, an S-scheme α-Fe₂O₃/Cu₂O heterojunction photocatalyst was designed and fabricated for the primary amine oxidative coupling reaction. On account of that, the S-scheme structure effectively separated the photogenerated electron-hole pairs and enhanced the photoredox ability of the photocatalytic system. The α-Fe₂O₃/Cu₂O composite could also enhance the reactivity to a large extent. This work will provide new insight into the design of photocatalysts with a more reasonable structure and higher performance.

Bioinspired o-Naphthoquinone-Catalyzed Aerobic Oxidation of Alcohols to Aldehydes and Ketones

A biomimetic alcohol dehydrogenase (ADH)-like oxidation protocol was developed using an ortho-naphthoquinone catalyst in the presence of a catalytic amount of base. The developed organocatalytic aerobic oxidation protocol proceeds through the intramolecular 1,5-hydrogen atom transfer of naphthalene alkoxide intermediates, a mechanistically distinctive feature from the previous alcohol dehydrogenase mimics that require metals in the active form of catalysts. The ADH-like aerobic oxidation protocol should provide green alternatives to the existing stoichiometric and metal-catalyzed alcohol oxidation reactions.

Thermal Hydroquinone Oxidation on Co/N-doped Carbon Proceeds by a Band-Mediated Electrochemical Mechanism

Molecular metal complexes catalyze aerobic oxidation reactions via redox cycling at the metal center to effect sequential activation of O₂ and the substrate. Metal surfaces can catalyze the same transformations by coupling independent half-reactions for oxygen reduction and substrate oxidation mediated via the exchange of band-electrons. Metal- and nitrogen-doped carbons (MNCs) are promising catalysts for aerobic oxidation that consist of molecule-like active sites embedded in conductive carbon hosts. Owing to their combined molecular and metallic features, it remains unclear whether they catalyze aerobic oxidation via the sequential redox cycling pathways of molecules or band-mediated pathways of metals. Herein, we simultaneously track the potential of the catalyst and the rate of turnover of aerobic hydroquinone oxidation on a cobalt-based MNC catalyst in contact with a carbon electrode. By comparing operando measurements of rate and potential with the current-voltage behavior of each constituent half-reaction under identical conditions, we show that these molecular materials can display the band-mediated reaction mechanisms of extended metallic solids. We show that the action of these band-mediated mechanisms explains the fractional reaction orders in both oxygen and hydroquinone, the time evolution of catalyst potential and rate, and the dependence of rate on the overall reaction free energy. Selective poisoning experiments suggest that oxygen reduction proceeds at cobalt sites, whereas hydroquinone oxidation proceeds at native carbon-oxide defects on the MNC catalyst. These findings highlight that molecule-like active sites can take advantage of band-mediated mechanisms when coupled to conductive hosts.

Visible-Light-Driven Oxidation of Amines to Imines in Air Catalyzed by Polyoxometalate-Tris(bipyridine)ruthenium Hybrid Compounds

The development of visible-light photocatalysts for the selective oxidative coupling of amines to imines is an area of great interest. Herein, four hybrid compounds based on polyoxometalate anions and tris(bipyridine)ruthenium cations, Ru(bpy)₃[M₆O₁₉] (M = Mo, W) 1-2, [Ru(bpy)₃]₂[Mo₈O₂₆] 3, [Ru(bpy)₃]₂[W₁₀O₃₂] 4, are prepared and characterized by X-ray diffraction (single-crystal and powder), elemental analysis, energy-dispersive X-ray spectroscopy (EDS) analysis, infrared (IR) spectroscopy, and solid diffuse reflective spectroscopy. Single-crystal structural analysis indicates that polyoxometalate anions and tris(bipyridine)ruthenium cations interact with each other through extensive hydrogen bonds in these compounds. These hybrid species with strong visible-light-harvesting abilities and suitable photocatalytic energy potentials show excellent photocatalytic activity and selectivity for the oxidation of amines to imines at room temperature in air as an oxidant. Among them, compound 1 with the [Mo₆O₁₉]^2- anion has the highest catalytic activity, which can swiftly convert >99.0% of benzylamine into N-benzylidenebenzylamine with a selectivity of 98.0% in 25 min illumination by a 10 W 445 nm light-emitting diode (LED). Its turnover frequency reaches 392 h^-1, which is not only better than the homogeneous catalyst [Ru(bpy)₃]Cl₂ but also much superior to those achieved over most of reported heterogeneous catalysts. Moreover, it shows a wide generality for various aromatic amines, accompanied by the advantages of good recyclability and stability. The photocatalytic oxidation mechanism of amines to the corresponding imines over polyoxometalate-based hybrid compounds was fully investigated.

Controlled Assembly of Ru-Containing Polyoxometalates for Photocatalytic Activity of the Primary Amine Coupling Reaction

Three Ru-induced structural interconversion polyoxometalates (POMs), Na₁₃H₅[Ru₄(H₂O)₂(Cl)₂(WO₂)₄(AsW₉O₃₃)₄]·43H₂O (1), K₅Na₉H₈[Ru₂(WO₂)₄(AsW₉O₃₃)₄]·50H₂O (2), and KNa₁₃H₁₄[(WO₂)₄(AsW₉O₃₃)₄]·34H₂O (3), were successfully synthesized and thoroughly characterized. Interconversion of structures was accomplished by changing the number of active sites for compounds 1-3. All three compounds contain one {As₄W₄₀O₁₄₀} unit, showing similar structural characteristics except for the active center number (Ru). Interestingly, compound 1 [turnover number (TON)= 486; turnover frequency (TOF)= 20 h^-1] showed highly efficient photocatalysis in achieving oxidative coupling of primary amines. Compound 2 (TON = 406, TOF = 17 h^-1) was also found to promote the oxidative coupling with relatively poor efficiency; however, compound 3 (TON = 178; TOF = 7.4 h^-1) had no obvious contribution to the coupling reaction system, and a chain of evidence indicates that the catalytic performances are strongly dependent on element contents of active sites. Furthermore, the Ru-containing POM-based photocatalysts are conveniently recyclable and reusable during the photocatalytic processes. This study demonstrates the possibility of tuning the catalytic efficiency and stability of POM-based photocatalysts by well designing and controlling their structures. The possible reaction mechanism for the photocatalysis synthesis of imine product is also proposed based on experimental studies.

Merging of the photocatalyst decatungstate and naphthalene diimide in a hybrid structure for the oxidative coupling of amines

Designing and developing novel hybrid materials for the effective photoconversion of organic substrates is of great importance. Crystalline hybrid heterostructures, as an attractive class of material, are composed of semiconducting organic and inorganic components with fast-responsive charge-separated properties and thus they are promising photocatalysts. Naphthalene diimides (NDIs) and decatungstate (W₁₀O₃₂^4-) are two versatile semiconductor components that have been utilized as building blocks for the construction of functional materials for various applications. In this context, we demonstrated that the combination of an electron-deficient NDI derivative with W₁₀O₃₂^4- resulted in an organic-inorganic hybrid compound, namely Zn₂(DPNDI)(W₁₀O₃₂)(DMA)₆ (DPNDI = N,N'-di-(4-pyridyl)-1,4,5,8-naphthalene diimide) (1). Because of consecutive photo-induced electron transfer processes among the components, this hybrid compound exhibits fast-responsive reversible photochromic properties, and it efficiently photocatalytically oxidizes amines to imines under mild conditions with high yields and an excellent substrate application range.