Synthesis of N-Heterocycles via Oxidant-Free Dehydrocyclization of Alcohols Using Heterogeneous Catalysts

Iron-catalyzed synthesis of substituted 3-arylquinolin-2(1H)-ones via an intramolecular dehydrogenative coupling of amido-alcohols

Here we report an iron-complex-catalyzed synthesis of various mono- and di-substituted quinolin-2(1H)-ones achieved via the intramolecular acceptorless dehydrogenative cyclization of amido-alcohols. This approach for the synthesis of N-heterocycles has provided access to underdescribed disubstituted quinolinones and represents an alternative to the well-known palladium-catalyzed coupling reactions.

Synthesis of N-heterocycles through alcohol dehydrogenative coupling

Nitrogen heterocycles are found in the structures of many biologically important compounds, as well as materials used in the synthesis of fine chemicals. Notably, ~59% of US Food and Drug Administration-approved small-molecule drugs contain nitrogen heterocycles. It is therefore meaningful to explore greener or more sustainable methods for their synthesis. The use of alcohols as reagents is attractive as they can be readily obtained from biomass derived natural resources. In the last two decades, alcohol dehydrogenative coupling reaction to synthesize various heterocycles were extensively explored which furnished hydrogen (H2) and water (H2O) as the two greener byproducts. In this protocol, we describe several efficient catalytic transformations to synthesize quinolines, 1,8-naphthyridines, quinoxalines, quinazolines, pyrimidines, benzimidazoles, pyrroles and pyridines, using alcohol as starting materials. We also describe the synthesis of several homogeneous iridium/ruthenium catalysts and heterogeneous cobalt/copper catalysts that can be used in these transformations. The reaction setup is simple; in a Schlenk/reaction tube with magnetic stir-bar, alcohol, corresponding coupling reagents (nucleophiles), catalyst, base and solvent (water or organic solvent such as toluene, dioxane or p-xylene) are added. The reaction mixture is refluxed at the specified temperature (110-150 °C)-either in air or under argon-to furnish these heterocycles. Synthesis of the catalysts takes 3-5 h and the coupling reactions take 4-5 h depending on the target product. The cobalt- and copper-based heterogeneous catalytic systems displayed an good catalyst recyclability.

Single-Atom Fe-Catalyzed Acceptorless Dehydrogenative Coupling to Quinolines

A single-atom iron catalyst was found to exhibit exceptional reactivity in acceptorless dehydrogenative coupling for quinoline synthesis, outperforming known homogeneous and nanocatalyst systems. Detailed characterizations, including aberration-corrected HAADF-STEM, XANES, and EXAFS, jointly confirmed the presence of atomically dispersed iron centers. Various functionalized quinolines were efficiently synthesized from different amino alcohols and a range of ketones or alcohols. The iron single-atom catalyst achieved a turnover number (TON) of up to 105, far exceeding the results of current homogeneous and nanocatalyst systems. Detailed mechanistic studies verified the significance of single-atom Fe sites in the dehydrogenation process.

Potent pincer-zinc catalyzed homogeneous α-alkylation and Friedländer quinoline synthesis reaction of secondary alcohols/ketones with primary alcohols

Herein, we describe an air- and moisture-stable, homogeneous zinc catalyst stabilised using an electron deficient N^N^N pincer-type ligand. This ternary, penta-coordinated neutral molecular catalyst [Zn(N^N^N)Cl2] selectively produces α-alkylated ketone derivatives (14 examples) through a one-pot acceptorless dehydrogenative coupling (ADC) reaction between secondary and primary alcohols using the borrowing hydrogen (BH) approach in good to excellent isolated yields (up to 93%). It is worth noting that this catalyst also provides an eco-friendly route for the synthesis of quinoline derivatives (30 examples) using 2-aminobenzyl alcohols as alkylating agents via successive dehydrogenative coupling and N-annulation reactions. This cost effective, easy to synthesize and environmentally benign catalyst shows excellent stability in catalytic cycles under open-air conditions, as evident from its high turnover number (∼104), and is activated by using a catalytic amount of base under milder conditions.

Cobalt nanoparticle-catalysed N-alkylation of amides with alcohols

A protocol for efficient N-alkylation of benzamides with alcohols in the presence of cobalt-nanocatalysts is described. Key to the success of this general methodology is the use of highly dispersed cobalt nanoparticles supported on carbon, which are obtained from the pyrolysis of cobalt(ii) acetate and o-phenylenediamine as a ligand at suitable temperatures. The catalytic material shows a broad substrate scope and good tolerance to functional groups. Apart from the synthesis of a variety of secondary amides (>45 products), the catalyst allows for the conversion of more challenging aliphatic alcohols and amides, including biobased and macromolecular amides. The practical applicability of the catalyst is underlined by the successful recycling and reusability.

Titanium-Catalyzed Selective N-Alkylation of Amines with Alcohols via Borrowing Hydrogen Methodology

The N-alkylation of amines with alcohols using earth-abundant and nonprecious metal catalysts has gained considerable attention in the pharmaceutical industry. We described titanium-catalyzed synthetic protocol for N-alkylation of amines with alcohols via borrowing hydrogen or hydrogen autotransfer reactions. The methodology enables the selective monoalkylation of various substituted (hetero)aromatic amines in good to excellent yields (up to 97% yield). The importance of the protocol was further demonstrated by the applicability of earth-abundant metal catalysis and the synthesis of 32 N-alkylated amines. The work allows the utilization of titanium-based catalysts for various reactions to expand the nature blueprint in catalysis.

Activation of Ethanol via Conjunction of a Photocatalyst and a HAT Reagent for the Synthesis of Benzimidazoles

The transformation of ethanol to value-added chemicals has tremendous potential. However, generally, harsh reaction conditions are needed for the functionalization of ethanol due to its high dehydrogenation energy. Herein, a metal-free photo-mediated activation of challenging ethanol and higher aliphatic alcohols for the synthesis of differently functionalized benzimidazoles under mild conditions is disclosed. The interplay of a photocatalyst and a HAT reagent facilitated the activation of aliphatic alcohols. A wide array of diamines with different functional groups were well tolerated, and the protocol was also extended to N-substituted diamines for the synthesis of industrially important benzimidazoles. A probable catalytic cycle was proposed based on various mechanistic studies.

Silver Supported Nanoparticles on [Mg4 Al-LDH] as an Efficient Catalyst for the α-Alkylation of Nitriles, Oxindoles and Other Carboxylic Acid Derivatives with Alcohols

An efficient heterogeneous silver-catalyzed α-alkylation of nitriles and oxindoles using alcohols via borrowing hydrogen strategy has been developed for the first time. The active nanostructured material, namely [Ag/Mg4 Al-LDH], composed by silver nanoparticles (3-4 nm average particle size) homogeneously stabilized onto a [Mg4 Al-LDH] support with suitable Brønsted basic properties, constitutes a stable catalyst for the sustainable building of novel C-C bonds from alcohols and C-nucleophiles. By applying this catalyst, a broad range of α-functionalized nitriles and oxindoles has been accessed with good to excellent isolated yields and without the addition of external bases. Moreover, the novel silver nanocatalyst has also demonstrated its successful application to the cyclization of N-[2-(hydroxymethyl)phenyl]-2-phenylacetamides to afford 3-arylquinolin-2(1H)-ones, through a one-pot dehydrogenation and intramolecular α-alkylation. Control experiments, kinetic studies, and characterization data of a variety of [Ag/LDH]-type materials confirmed the silver role in the dehydrogenation and hydrogenation steps, while [Mg4 Al-LDH] matrix is able to catalyze condensation. Interestingly, these studies suggest as key point for the successful activity of [Ag/Mg4 Al-LDH], in comparison with other [Ag/LDH]-type nanocatalysts, the suitable acid-base properties of this material.

A novel, efficient and magnetically recyclable Cu-Ni bimetallic alloy nanoparticle as a highly active bifunctional catalyst for Pd-free Sonogashira and C-N cross-coupling reactions: a combined theoretical and experimental study

In this study, a Fe3O4@SiO2@Cyt-Ni/Cu nanocomposite as a novel heterogeneous bimetallic catalyst was synthesized which exhibits efficient performance for the Sonogashira and C-N cross-coupling reactions. The characterization of the catalyst was studied by FT-IR, PXRD, VSM, EDX, TEM, FE-SEM and TGA analyses. The geometry optimization and relative energies of the designed bimetallic complexes were theoretically determined using density functional theory (DFT) calculation at the B3LYP/6-31G**/LANL2DZ level. The catalyst showed good activity in the coupling of various aryl halides with alkynes (Sonogashira reaction) as well as aryl halide with N-heterocycles and achieved coupling products with good to high yields for all of them in a short time. The high catalytic performance could be due to the synergistic effect between Ni and Cu, which causes the reaction to proceed more efficiently. This heterogeneous nanocatalyst could be easily recovered from the reaction mixture with an external magnet and reused for 7 consecutive runs with minimal loss of catalytic activity.

Metal-Free Construction of Multisubstituted Indolizines via Intramolecular Amination of Allylic Alcohols

An intramolecular amination of allylic alcohols is developed as an efficient and general access to biologically important multisubstituted indolizines and their variants. Two metal-free synthetic platforms including using aqueous hydrochloric acid solution as the solvent and p-toluenesulfonic acid as the catalyst have been established, enabling the divergent synthesis of these valuable compounds in high yields.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Chemodivergent 1,3-Dipolar Cycloadditions of C,N-Cyclic Azomethine Imines with γ-Sulfonamido-α,β-Unsaturated Ketones to Synthesize Tricyclic Dinitrogen-Fused Heterocycles

1,3-Dipolar cycloadditions of C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones were developed to synthesize tricyclic dinitrogen-fused heterocycles. Highly functionalized tricyclic tetrahydroisoquinolines were readily obtained in good to high yields in the [3 + 2]-cycloaddition reaction of N-Bz-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones under mild reaction conditions. Moreover, DABCO-catalyzed cycloaddition of N-Ts-protected C,N-cyclic azomethine imines with γ-NHTs-α,β-unsaturated ketones followed by cleavage of the tosyl group is a convenient route to synthesize tetrahydropyrazolo [5,1-a]isoquinolines in good yields with excellent diastereoselectivities.

N, S Co-Coordinated Zinc Single-Atom Catalysts for N-Alkylation of Aromatic Amines with Alcohols: The Role of S-Doping in the Reaction

S-doping emerged as a promising approach to further improve the catalytic performance of carbon-based materials for organic synthesis. Herein, a facile and gram-scale strategy was developed using zeolitic imidazole frameworks (ZIFs) as a precursor for the fabrication of the ZIF-derived N, S co-doped carbon-supported zinc single-atom catalyst (CNS@Zn₁-AA) via the pyrolysis of S-doped ZIF-8, which was modified by aniline, ammonia and thiourea and prepared by one-pot ball milling at room temperature. This catalyst, in which Zn is dispersed as the single atom, displays superior activity in N-alkylation via the hydrogen-borrowing strategy (120 °C, turnover frequency (TOF) up to 8.4 h^-1). S-doping significantly enhanced the catalytic activity of CNS@Zn₁-AA, as it increased the specific surface area and defects of this material and simultaneously increased the electron density of Zn sites in this catalyst. Furthermore, this catalyst had excellent stability and recyclability, and no obvious loss in activity after eight runs.

DDQ/Fe(NO3)3-Catalyzed Aerobic Synthesis of 3-Acyl Indoles and an In Silico Study for the Binding Affinity of N-Tosyl-3-acyl Indoles toward RdRp against SARS-CoV-2

In the present study, we herein report a DDQ-catalyzed new protocol for the synthesis of substituted 3-acylindoles. Being a potential system for virtual hydrogen storage, introduction of catalytic DDQ in combination with Fe(NO₃)₃·9H₂O and molecular oxygen as co-catalysts offers a regioselective oxo-functionalization of C-3 alkyl-/aryllidine indolines even with scale-up investigations. Intermediate isolation, their spectroscopic characterization, and the density functional theory calculations indicate that the method involves dehydrogenative allylic hydroxylation and 1,3-functional group isomerization/aromatization followed by terminal oxidation to afford 3-acylindoles quantitatively with very high regioselectivity. This method is very general for a large number of substrates with varieties of functional groups tolerance emerging high-yield outcome. Moreover, molecular docking studies were performed for some selected ligands with an RNA-dependent RNA polymerase complex (RdRp complex) of SARS-CoV-2 to illustrate the binding potential of those ligands. The docking results revealed that few of the ligands possess the potential to inhibit the RdRp of SARS-Cov-2 with binding energies (-6.7 to -8.19 kcal/mol), which are comparably higher with respect to the reported binding energies of the conventional re-purposed drugs such as Remdesivir, Ribavirin, and so forth (-4 to -7 kcal/mol).

Synthesis and characterization of thiophene-derived palladium(ii) complex immobilized on FSM-16 and its application in the novel synthesis of 7-(aryl)-7,12-dihydro-6H-indeno[1,2,4]triazolo[1,5-a]pyrimidine-6-one derivatives

The present study aims at synthesizing a palladium complex with a thiophene-carboimine ligand, supported on FSM-16 as a mesoporous silica support. Firstly, the prepared FSM-16 was modified using 3-aminopropyl group. The imine bond was subsequently formed by condensation of FSM-16-propyl amine with thiophene-2-carbaldehyde. Finally, the imine/thiophene-FSM-16 reacted with PdCl2 to form PdCl2-imine/thiophene-FSM-16. The structural and physicochemical properties of the prepared nanocomposite were characterized using FT-IR, TEM, XRD, FE-SEM, EDS, BET, and TGA analyses. PdCl2-imine/thiophene-FSM-16 exhibited efficient catalytic activity in the synthesis of indeno-1,2,4-triazolo[1,5-a]pyrimidine derivatives via a new three-component reaction between indan-1,3-dione, aromatic aldehydes and 3-amino-1H-1,2,4-triazole in water as the green solvent. Significantly, the heterogeneous catalyst can be easily separated from the reaction mixture and reused in another reaction.

Recent Advances in Carbon-Based Iron Catalysts for Organic Synthesis

Carbon-based iron catalysts combining the advantages of iron and carbon material are efficient and sustainable catalysts for green organic synthesis. The present review summarizes the recent examples of carbon-based iron catalysts for organic reactions, including reduction, oxidation, tandem and other reactions. In addition, the introduction strategies of iron into carbon materials and the structure and activity relationship (SAR) between these catalysts and organic reactions are also highlighted. Moreover, the challenges and opportunities of organic synthesis over carbon-based iron catalysts have also been addressed. This review will stimulate more systematic and in-depth investigations on carbon-based iron catalysts for exploring sustainable organic chemistry.

Efficient iron single-atom catalysts for selective ammoxidation of alcohols to nitriles

Zeolitic imidazolate frameworks derived Fe₁-N-C catalysts with isolated single iron atoms have been synthesized and applied for selective ammoxidation reactions. For the preparation of the different Fe-based materials, benzylamine as an additive proved to be essential to tune the morphology and size of ZIFs resulting in uniform and smaller particles, which allow stable atomically dispersed Fe-N₄ active sites. The optimal catalyst Fe₁-N-C achieves an efficient synthesis of various aryl, heterocyclic, allylic, and aliphatic nitriles from alcohols in water under very mild conditions. With its chemoselectivity, recyclability, high efficiency under mild conditions this new system complements the toolbox of catalysts for nitrile synthesis, which are important intermediates with many applications in life sciences and industry.

Amine-functionalized MCM-41 as an efficient catalyst for the synthesis of sulfur/dinitrogen-fused heterocycles via [3+3] cycloaddition in water

The sulfur/dinitrogen-containing heterocycle is an important chemical motif and is found widely in natural products, drug molecules, catalysts and functional materials.

Exploring the regioselectivity of the cyanoalkylation of 3-aza-1,5-dienes: photoinduced synthesis of 3-cyanoalkyl-4-pyrrolin-2-ones

Regioselective cyanoalkylalkenylation of 3-aza-1,5-dienes with oxime esters induced by visible light.

ZIF-Derived Metal/N-Doped Porous Carbon Nanocomposites: Efficient Catalysts for Organic Transformations

Recently, zeolitic imidazolate framework (ZIF)-derived metal/N-doped porous carbon nanocomposites (M@NCs) have emerged as a class of appealing heterogeneous catalysts applied in organic synthesis, and the striking features mainly involve low-cost...

Fe–FeOx nanoparticles encapsulated in N-doped carbon material: a facile catalyst for selective synthesis of quinazolines from alcohols in water

A Fe–FeOx@NC catalyst with N-doped carbon encapsulated Fe–FeOx nanoparticles has excellent performance in the synthesis of quinazolines.

A heterogeneous cobalt catalyst for C–C bond formation by a borrowing hydrogen strategy

Co–MgO/TiO2 exhibited high activity for α-alkylation of ketones with alcohols through a borrowing hydrogen strategy without the addition of bases which were utilized in reported heterogeneous catalytic systems.

Catalytic oxidative dehydrogenation of N-heterocycles with nitrogen/phosphorus co-doped porous carbon materials

A metal-free oxidative dehydrogenation of N-heterocycles utilizing a nitrogen/phosphorus co-doped porous carbon (NPCH) catalyst is reported. The optimal material is robust against traditional poisoning agents and shows high antioxidant resistance. It exhibits good catalytic performance for the synthesis of various quinoline, indole, isoquinoline, and quinoxaline ‘on-water’ under air atmosphere. The active sites in the NPCH catalyst are proposed to be phosphorus and nitrogen centers within the porous carbon network.

Green oxidations made easy. Metal-free dehydrogenation of N-heterocycles are possible in using N,P-co-doped porous carbon materials “on” water using air.