Magnetically recoverable catalysts for the preparation of pyridine derivatives: an overview

Abstract

Magnetically recoverable nano-catalysts can be readily separated from the reaction medium using an external magnet. In recent years, chemistry researchers have employed them as catalysts in chemical reactions. The high surface area, simple preparation, and modification are among their major advantages. Pyridine derivatives are an important category of heterocyclic compounds, which show a wide range of excellent biological activities, including IKK-β inhibitors, anti-microbial agents, A2A adenosine receptor antagonists, inhibitors of HIV-1 integrase, anti-tumor, anti-inflammatory, and anti-Parkinsonism. Recently, the catalytic activity of magnetic nanoparticles was investigated in multicomponent reactions in the synthesis of pyridine derivatives, which is discussed in this review.

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Ghodsi Mohammadi Ziarani

Ghodsi Mohammadi Ziarani was born in Iran in 1964. She received her BSc degree in Chemistry from the Teacher Training University, Tehran, Iran, in 1987, her M.Sc. degree in Organic Chemistry from the Teacher Training University, Tehran, Iran, under the supervision of Professor Jafar Asgarin and Professor Mohammad Ali Bigdeli in 1991 and her PhD degree in asymmetric synthesis (Biotransformation) from Laval University, Quebec, Canada under the supervision of Professor Chenevert, in 2000. She is a Full Professor of Organic Chemistry in the chemistry department of Alzahra University. Her research interests include organic synthesis, heterocyclic synthesis, asymmetric synthesis, natural product synthesis, synthetic methodology, and applications of nano-heterogeneous catalysts in multicomponent reactions.

Zohreh Kheilkordi

Zohreh Kheilkordi was born in Ramsar/Mazandaran, Iran, in 1990. She received her BSc in Chemistry from Mazandaran University, Babolsar in 2012, and her M.Sc. in Organic Chemistry from Yazd University, under the supervision of Dr Mohammad Ali Amrollahi, in 2014. She received her PhD degree in organic chemistry from Alzahra University, Tehran, Iran, under the supervision of Prof. Ghodsi Mohammadi Ziarani, in 2019. She is currently a postdoctoral researcher in Organic Chemistry at Alzahra University under the supervision of Prof. Ghodsi Mohammadi Ziarani.

Fatemeh Mohajer

Fatemeh Mohajer was born in Tehran, Iran, and she received her BSc in Applied Chemistry from Bu-Ali Sina University and M.Sc degree in Organic Chemistry from Azad University in Karaj. She is a PhD student under the supervision of Prof. Ghodsi Mohammadi Ziarani at Alzahra University in Tehran, Iran.

Alireza Badiei

Alireza Badiei was born in Iran in 1965. He received his BSc and MSc degrees in Chemistry and Inorganic Chemistry from the Teacher Training University (Kharazmi), Tehran, Iran, in 1988 and 1991, respectively, and his PhD degree in the synthesis and modification of nanoporous materials from Laval University, Quebec, Canada, in 2000. He is currently a full Professor in the Chemistry faculty of Tehran University. His research interests include nanoporous materials synthesis, modification of nanoporous materials, and application of organic–inorganic hybrid materials in various fields such as catalysis, adsorption, separation, and sensors.

Rafael Luque

Rafael Luque, Full Professor from Departamento de Quimica Organica at UCO, Spain as well as Director of the Scientific Center for Molecular Design and Synthesis of Innovative compounds for Medicine at RUDN University, Russia, Distinguished Chair Professor at Xi'an Jiaotong University and DSFP Fellow at King Saud University, Saudi Arabia is an internationally recognized leader and mentor in the areas of (nano)materials science and Green Chemistry/Sustainability (h-index = 83, >34 000 citations to own work, 2018, 2019 and 2020 Highly Cited Researcher-Clarivate Analytics).

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1. Introduction

In recent decades, nanotechnology has attracted much attention in various fields.^1,2 One of the most influential families of nanomaterials is magnetic nanoparticles, which have been extensively employed in different sciences, including drug delivery,³ illness recognition,⁴ water desalination,⁵ ambiance scrubbing,⁶ and chemical catalysis.⁷ Recently, magnetic nano-catalysts have attracted the consideration of many researchers due to their high activity, selectivity, availability, large surface area, low toxicity, excellent reusability, and easy separation.^8,9 Magnetic nanoparticles (MNPs) have high surface-to-volume ratios, and can be functionalized with inorganic and organic compounds.^10–15 The magnetic nano-catalysts can be separated by external magnetic fields.¹⁶ Fe₃O₄ nanoparticles can be coated with organic and inorganic materials, including silica,¹⁷ surfactants,¹⁸ polymers,^17,19 cellulose,²⁰ carbon,²¹ chitosan,²² as well as prepared with a core–shell structure. The coating layer on magnetic nanoparticles can be prevented from aggregation or oxidation and their stability can be increased.

Heterocyclic compounds have high biological and pharmaceutical activities. Among them, pyridine derivatives are important heterocyclic compounds, which attracted the attention of scientists. Pharmaceutical molecules and natural products can be based on heterocyclic compounds such as pyridine derivatives,²³ which have biological activities, such as inhibitors of HIV-1 integrase, A2A adenosine receptor antagonists, IKK-β inhibitors, anti-microbial, anti-tumor, analgesic, anti-inflammatory, and antipyretic agents.²⁴ In continuation our research work,^25–29 this contribution will be aimed to discuss the synthesis of magnetic nano-catalysts as well as their applications in the synthesis of pyridine derivatives.

2. The synthesis of pyridine derivatives by diverse magnetic catalysts

2.1. Basic magnetic catalyst

The core–shell structure of Fe₃O₄@KCC-1-npr-NH₂ 6 as an effective basic magnetic catalyst was prepared and employed in the synthesis of tetrahydro di-pyrazolopyridines by Azizi, and his co-workers. Core–shell Fe₃O₄@KCC-1 4 was prepared by adding cetyl trimethyl ammonium bromide (CTAB) 2 and tetraethylorthosilicate (TEOS) 3. Then, Fe₃O₄@KCC-1 4 was functionalized with 3-aminopropyl)triethoxysilane 5 to produce Fe₃O₄@KCC-1-npr-NH₂ 6 with excellent basic properties. Details for the preparation of Fe₃O₄@KCC-1-npr-NH₂ 6 are shown in Scheme 1. Various characterization techniques, including FT-IR, SEM, TEM, BET, and XRD, confirmed the structure of Fe₃O₄@KCC-1-npr-NH₂ 6 as magnetic nano-catalyst.³⁰

Scheme 1 Synthesis of Fe₃O₄@KCC-1-npr-NH₂ 6.

Fe₃O₄@KCC-1-nPr-NH₂ 6 was employed in the tetra-component reaction of ethyl acetoacetate 7, hydrazine hydrate 8, ammonium acetate 10, and various aromatic aldehydes 9 in ethanol under reflux condition for the synthesis of tetrahydrodipyrazolo pyridine 11 in excellent yields, short reaction times. According to obtained results, different substituents including electron-donating or electron-withdrawing groups on the aromatic ring, did not affect the product yields. All products were obtained in high purity and excellent yields. Also, the anticancer activity of tetrahydrodipyrazolo pyridine derivatives 11 was studied that some of these compounds showed good cytotoxic activity toward types of cancer cell (Scheme 2).³⁰

Scheme 2 Synthesis of tetrahydrodipyrazolopyridine 11.

Fe₃O₄ MNPs 1 were also synthesized according to the literature,³¹ and then coated by TEOS to yield Fe₃O₄@SiO₂ MNPs 4,³² which were modified by 3-aminoropropyl-trimethoxysilane (APTS) 5 to provide Fe₃O₄@SiO₂-pr-NH₂ MNPs 6, followed by mixing with a solution of N,N-dimethylaniline 12, and formaldehyde 13 in DMF, and then refluxed for 24 h to provide poly N,N-dimethylaniline-formaldehyde supported on silica-coated Fe₃O₄ MNPs (PDMAF-MNPs) 14 (Scheme 3).³³

Scheme 3 Synthesis of poly N,N-dimethylaniline-formaldehyde supported on silica-coated Fe₃O₄ MNPs (PDMAF-MNPs) 14.

PDMAF-MNPs was investigated in the multicomponent reaction of aldehydes 9, malononitrile 16, ammonium acetate 10, and various ketones 15 under reflux condition in EtOH to obtain 2-amino-3-cyanopyridines 17 in high yields. It was demonstrated that the electron-donating groups results in low reaction yields and long reaction time (Scheme 4).³³

Scheme 4 Synthesis of 2-amino-3-cyanopyridines 17.

In another example, iron oxide 1 was prepared and reacted with tetraethylorthosilicate (TEOS) 3 to provide Fe₃O₄@SiO₂ 4,³⁴ which was treated with 3-chloropropyltriethoxysilane 18 to give Fe₃O₄@SiO₂@Pr-Cl 19, followed by the reaction with the ligand bearing morpholine tags 20 to obtain the nano-magnetic catalyst 21 (Scheme 5).³⁵

Scheme 5 Synthesis of magnetic nanoparticles with morpholine tags 21.

The nano-magnetic catalyst 21 was examined in the multicomponent reaction of benzaldehydes 9, acetophenone derivatives 22, malononitrile 16, and ammonium acetate 10 under the solvent-free condition in 80 °C for the preparation of 2-amino-4,6-diphenylnicotinonitriles 23 (Scheme 6).³⁵

Scheme 6 Synthesis of 2-amino-4,6-diphenylnicotinonitriles 23.

Nano-magnetic Fe₃O₄–Si–(CH₂)₃–N=CH–Ph–OMe MNPs 29 was prepared by the reaction of Fe·Cl₃·6H₂O 24, FeCl₂·4H₂O 25, and NH₄OH 26 in H₂O under N₂ atmosphere to prepare Fe₃O₄ MNPs 1, which was functionalized with aminopropyl silane 5 to provide Fe₃O₄–Si–[CH₂]₃–NH₂ 27, followed by modification with 4-methoxy benzaldehyde 28 under reflux conditions in ethanol for 24 h (Scheme 7).³⁶

Scheme 7 Synthesis of Fe₃O₄–Si–(CH₂)₃–N=CH–Ph–OMe MNPs 29.

Fe₃O₄–Si–(CH₂)₃–N=CH–Ph–OMe MNPs 29 was used in the synthesis of 2-amino-3-cyanopyridines 23 via the multicomponent reaction of various aromatic aldehydes 9, 2-acetylnaphthalene 31, or deoxybenzoin 31, malononitrile 16, and ammonium acetate 10 under solvent-free conditions at 120 °C for 40–70 min in good to high yield in short times (Scheme 8).³⁶

Scheme 8 Synthesis of 2-amino-3-cyanopyridines 23.

2.2. Acidic magnetic catalysts

Fe₃O₄@Co^II (macrocyclic Schiff base ligand) 34 was synthesized as an efficient and recoverable catalyst for the synthesis of thiopyridine. Macrocyclic Schiff base ligand 32 was obtained via reaction of 2,2′-(1,4-diazepane-1,4-diyl)-di-aniline 30 and 2,3-dihydroxybenzaldehyde 31 in ethanol under reflux for 24 hours. Then, a mixture of FeCl₃·6H₂O 24, FeCl₂·4H₂O 25, and NH₄OH 26 was stirred in H₂O under N₂ gas at 100 °C to give Fe₃O₄ 1, which was treated with macrocyclic Schiff base ligand (III) 32 to give Fe₃O₄-supported macrocyclic Schiff base ligand (III) 33, followed by the reaction with Co(Cl)₂·6H₂O EtOH under reflux for 24 hours to obtain Fe₃O₄@macrocyclic Schiff base ligand 34 (Scheme 9).³⁷

Scheme 9 Synthesis of Fe₃O₄@Co^II (macrocyclic Schiff base ligand) 34.

Fe₃O₄@macrocyclic Schiff base ligand 34 was employed in the synthesis of 2-amino-4-aryl-6-(phenylsulfanyl)pyridine-3,5-dicarbonitrile derivatives 35 via three-component reaction of aldehyde derivatives 9, malononitrile 16, thiophenol 36 under solvent-free conditions (Scheme 10). The catalytic activity of Fe₃O₄@Co^II (macrocyclic Schiff base ligand) 34 was separately compared to that of Fe₃O₄, macrocyclic Schiff base ligand, Fe₃O₄@macrocyclic Schiff base ligand 33. It was demonstrated that Fe₃O₄@Co^II 34 showed the best results.³⁷

Scheme 10 Synthesis of 2-amino-4-aryl-6-(phenylsulfanyl)pyridine-3,5-dicarbonitrile derivatives 35.

4-Aroyl-3-methyl-1,6-diaryl-1H-pyrazolo[3,4-b] pyridine-5-carbonitrile derivatives 40 were synthesized via one-pot, the four-component reaction of 1-aryl-3-methyl-1H-pyrazol-5-(4H) one 39, 3-aryl-3-oxopropanenitriles 37, arylglyoxals 38, and ammonium acetate 10 in the presence of metal oxide silica based-metal bifunctional LDH (layered double hydroxide) as a magnetic nano-catalyst in EtOH/H₂O (1 : 1) under the reflux conditions (Scheme 11). In addition, pyrazolo[3,4-b] pyridines 40 have biological and pharmacological activity.³⁸

Scheme 11 Synthesis of pyrazolo[3,4-b] pyridines 40.

CoFe₂O₄@SiO₂–SO₃H 44 was synthesized as a reusable nano-catalyst by Hosseinzadeh et al. Initially, CoFe₂O₄ magnetic nanoparticles 42 were prepared according to previous works.³⁹ Then, it was modified with tetraethylorthosilicate to provide CoFe₂O₄@SiO₂ 43,^40. which was dispersed in dry CH₂Cl₂, and ClSO₃H to give CoFe₂O₄@SiO₂–SO₃H 44 (Scheme 12).⁴¹

Scheme 12 Synthesis of CoFe₂O₄@Silica MNPs 44.

CoFe₂O₄@Silica MNPs 44 was used in the multicomponent reaction of aldehydes 9, acetophenone 22, malononitrile 16, and ammonium acetate 10 in solvent-free conditions under MW irradiation to provide 2-amino-4,6-diarylnicotinonitrile derivatives 23 in good yields (Scheme 13).⁴¹

Scheme 13 Synthesis of 2-amino-4,6-diarylnicotinonitrile derivatives 23.

Forouzandehdel and co-workers synthesized a novel, recyclable nano-catalyst Fe₃O₄@GO_TfOH/Ag/St-PEG-AcA 45, which was employed in the synthesis of 2,4,6-tri-arylpyridine derivatives 46 by the reaction of aldehyde derivatives 9, acetophenone 22, and ammonium acetate 10 in H₂O at room temperature (Scheme 14).⁴²

Scheme 14 Synthesis of 2,4,6-triarylpyridine derivatives 46.

Fe₃O₄@SiO₂@Pr-SO₃H 48 was employed as heterogeneous acidic catalyst in the multicomponent reaction of 1,3-indandione 47, aromatic aldehydes 9, acetophenone or propiophenone 22, and ammonium acetate 10 under solvent-free conditions at 80 °C to obtain indeno[1,2-b]pyridines 49 (Scheme 15).⁴³

Scheme 15 Synthesis of indeno[1,2-b]pyridines 49.

Hosseinzadeh and et al. synthesized 2,6-diaryl-substituted pyridine derivatives 23 via tetra component reaction of aldehyde derivatives 9, acetophenone 22, malononitrile 16, and ammonium acetate 10 in the presence of CoFe₂O₄@SiO₂–SO₃H 50 under microwave irradiation and solvent-free conditions (Scheme 16).⁴⁴

Scheme 16 Synthesis of 2,6-diaryl-substituted pyridine derivatives 23.

Halloysite nanotubes CuFe₂O₄@HNTs 53 was synthesized by the reaction of Halloysite nanotubes HNTs 51 was added to Fe(NO₃)₃·9H₂O and 0.14 g (0.58 mmol) of Cu(NO₃)₂·3H₂O in distilled water and stirred at room temperature for 1 h, and then the solution of NaOH was added dropwise to it for 10 min at 25 °C, followed by stirring for 2 h at 90 °C to give CuFe₂O₄@HNTs 52, which was separated by an external magnet, and washed four times with distilled water, dried for 4 h, and calcinated at 500 °C for 5 h to yield extra pure CuFe₂O₄@HNTs 53 (Scheme 17).⁴⁵

Scheme 17 Synthesis of CuFe₂O₄@HNTs 53.

The catalytic activity of CuFe₂O₄@HNTs 53 was tested in the synthesis of pyrazolopyridine derivatives 55 via the multicomponent reaction of ethyl acetoacetate 7, hydrazine hydrate 54, benzaldehyde 9, and ammonium acetate 10 in EtOH at room temperature for 20 min (Scheme 18).⁴⁵

Scheme 18 Synthesis of pyrazolopyridine derivatives 55.

Maleki and co-workers also synthesized Fe₂O₃@Fe₃O₄@Co₃O₄ 56 as catalyst to provide polysubstituted pyridines 57 through the pseudo-four-component reaction of aldehyde derivatives 9, malononitrile 16, and ammonium acetate 10 under solvent-free conditions at 110 °C (Scheme 19).⁴⁶

Scheme 19 The synthesis of polysubstituted pyridines 57.

In 2019, Mohammadi and co-workers also prepared 2-amino-3-cyanopyridine 23 via multicomponent reaction of aromatic aldehydes 9, acetophenone derivatives 22, malononitrile 16, and ammonium acetate 10, in the presence of SrFe₁₂O₁₉ as magnetic catalyst under solvent-free conditions at 100 °C. The spectrophotometric properties of 2-amino-4,6-diphenylnicotinonitrile 23 as organo-ligand and several metal ions such as Ag⁺, Cd²⁺, Co²⁺, Cr³⁺, Cu²⁺, Fe³⁺, Hg²⁺, Mn²⁺, Ni²⁺, Pb²⁺, and Zn²⁺ in CH₃CN solution at 25 °C was also investigated. According to the results, 2-amino-4,6-diphenylnicotinonitrile 23 exhibited a good complexation as organo-ligand with Hg²⁺ (Scheme 20).⁴⁷

Scheme 20 Synthesis of 2-amino-3-cyanopyridine 23.

Fe₃O₄-supported Schiff-base copper(II) complexes 58 were reported by Mahmoudi-GomYek et al. Ligand 32 was synthesized via the reaction of 2,2′-[piperazine-1,4-diylbis-(methylene)]dianiline 30 and 2-hydroxy-3-methoxy benzaldehyde 31. The reaction of FeCl₃·6H₂O 24, FeCl₂·4H₂O 25 and NH₄OH in H₂O under N₂ atmosphere provided Fe₃O₄ MNPs 1, which were functionalized by 3-chloropropyl(trimethoxy)silane (CPTMS) 18 to give Fe₃O₄@Si-PrCl 19. The reaction of compound 32 with Fe₃O₄@Si-PrCl 19 gave the compound 57, which reacted with Cu(NO₃)₂·9H₂O to yield Fe₃O₄-supported Schiff-base copper(II) complex 58 (Scheme 21).⁴⁸

Scheme 21 Synthesis of Fe₃O₄-supported Schiff-base copper(II) complex 58.

Fe₃O₄@SPNC 58 was used as catalyst in the synthesis of pyrano[2,3-b]pyridine-3-carboxamide derivatives 61 via the three-component reaction of aldehydes 9, 2-isocyanoacetamide 59, and 3-cyano-6-hydroxy-4-methyl-pyridin-2(1H)-one 60 under solvent-free conditions at 80 °C (Scheme 22).⁴⁸

Scheme 22 Synthesis of pyrano[2,3-b]pyridine-3-carboxamide derivatives 61.

Similar Cu complexes on magnetic nanomaterials were also synthesized from Fe₃O₄@CPTMS MNPs 19 (ref. 49 and ⁵⁰) according to the literature. The reaction of Fe₃O₄@CPTMS MNPs 19, acetylacetone 62 and sodium hydride in toluene at 80 °C under nitrogen atmosphere gave Fe₃O₄@SiO₂-n-Pr-acac MNPs 63, which was reacted with 2-aminobenzenethiol 64 in EtOH under reflux condition and nitrogen atmosphere to provide Fe₃O₄@SiO₂-acac-2ATP 65, followed by reacting with Cu(NO₃)₂·9H₂O in ethanol under reflux and nitrogen gas for 12 h to obtain Fe₃O₄@SiO₂-acac-2ATP-Cu(II) 66 (Scheme 23).⁵¹

Scheme 23 Synthesis of Fe₃O₄@SiO₂-acac-2ATP-Cu(II) MNPs 66.

Fe₃O₄@SiO₂-acac-2ATP-Cu(II) MNPs 66 was then employed as catalyst in the three-component reaction of aldehydes 9, malononitrile 16, and 3-cyano-6-hydroxy-4-methyl pyridine-2(1H)-one 67 under solvent-free conditions at 80 °C for the synthesis of 4H-pyrano[2,3-b]pyridine-3,6-dicarbonitrile derivatives 68 by Azarifar and co-works (Scheme 24).⁵¹

Scheme 24 Synthesis of 4H-pyrano[2,3-b]pyridine-3,6-dicarbonitrile derivatives 68.

Gajaganti and his co-workers utilised nano-Fe₃O₄ as a catalyst in the synthesis of 2,4,6-tri-arylpyridines 71 via a three-component reaction of acetophenone derivatives 22, methyl arenes 70, and ammonium acetate 10 (Scheme 25).⁵²

Scheme 25 Synthesis of 2,4,6-tri-arylpyridines 71.

Similar Fe₃O₄ multi-walled carbon nanotubes (MWCNTs) were prepared and employed as catalyst in the three-component reaction of ketones 72, different cinnamaldehyde 73, and ammonium acetate 10 to synthesize the functionalized pyridines 74 (Scheme 26).⁵³

Scheme 26 Synthesis of functionalized pyridines 74.

The eggshell powder was coated on the surface of magnetic nano-Fe₃O₄ 1, to give nano-Fe₃O₄@eggshell 75, which was treated with ClSO₃H to yield nano-magnetic acid catalyst Fe₃O₄@Ca(HSO₄)₂ 76. In this process, CaCO₃ from the eggshell was converted to Ca(HSO₄)₂ through reaction with ClSO₃H (Scheme 27).⁵⁴

Scheme 27 Synthesis of Fe₃O₄@Ca(HSO₄)₂ 76.

Nano-Fe₃O₄@Ca(HSO₄)₂ 76 was subsequently utilised in the synthesis of 2-amino-3-cyanopyridines 23 via four-component reaction of different benzaldehydes 9, acetophenone 22, ammonium acetate 10, and malononitrile 16 under solvent-free conditions at 90 °C for 5–15 min (Scheme 28).⁵⁴

Scheme 28 Synthesis of 2-amino-3cyanopyridines 23.

2.3. Ionic liquid-based magnetic nanomaterials

Fe₃O₄@O₂PO₂(CH₂)₂NH₂ MNPs 78 was prepared according to the reported method.^34,55 After dispersion in the ultrasonic bath, it was reacted with CF₃CO₂H to prepare Fe₃O₄@O₂PO₂(CH₂)₂NH₃ CF₃CO₂ 79 (Scheme 29).⁵⁶

Scheme 29 Synthesis of Fe₃O₄@O₂PO₂(CH₂)₂NH₃⁺ CF₃CO₂⁻ 79.

Fe₃O₄@O₂PO₂(CH₂)₂NH₃⁺ CF₃CO₂⁻ 79 was employed in the multicomponent reaction between various acetyl pyridines 80, aryl aldehydes 9, and ammonium acetate 10 under solvent-free reaction conditions at 120 °C to synthesize terpyridines 81 (Scheme 30).⁵⁷

Scheme 30 Synthesis of terpyridines 81.

CuI/Fe₃O₄ NPs@Biimidazole IL-KCC-1 86 was prepared by Azizi et al. in 2020. Firstly, 1-methyl-3-(oxiran2-ylmethyl)-1H-imidazol-3-ium chloride 83 and sodium methoxide were added to the prepared KCC-1 82 in dimethylformamide (DMF), and stirred for 60 min under a nitrogen atmosphere at 60 °C. Methanol and DMF were subsequently evaporated under vacuum to obtain 1-methyl-3-(oxiran-2-yl-methyl)-1H-imidazolium chloride (ILCl-g-KCC-1) 84.⁵⁸ Then, solid potassium hydroxide was added to ILCl-g-KCC-1 84 to yield IL-KCC-1 85 by replacing chloride ions with hydroxide ions. Fe₃O₄ NPs were subsequently doped on the substrate of IL-KCC-1 84 and treated with CuI/MeOH to obtain CuI/Fe₃O₄ NPs@Biimidazole IL-KCC-1 86 (Scheme 31).

Scheme 31 Synthesis of CuI/Fe₃O₄ NPs@Biimidazole IL-KCC-1 86.

CuI/Fe₃O₄ NPs@IL-KCC-1 86 was investigated in the three-component reaction of 2-aminopyridine 87, aldehydes 9, phenylacetylene 88, and CTAB in H₂O under reflux condition to obtaib imidazo[1,2-a]pyridines 89 in high yields (Scheme 32).⁵⁹

Scheme 32 Synthesis of imidazo[1,2-a]pyridines 89.

Shojaei et al. was studied the catalytic activity of guanidinium hydrogen sulfate on Fe₃O₄ nanoparticles 91 in the pseudo-four-component reactions of aryl aldehydes 9 with 3-amino-1-phenyl-2-pyrazolin-5-one 90 to give spiro[pyrazole-pyrazolo[3,4-b]pyridine]-dione derivatives 92 under mild conditions (Scheme 33).⁶⁰

Scheme 33 Synthesis of spiro [pyrazole-pyrazolo[3,4-b]pyridine]-dione derivatives 92.

2.4. Bifunctional magnetic catalysts

In 2019, Edrisi et al. synthesized g-C₃N₄ 94 according to the reported method.⁶¹ g-C₃N₄ 94 was functionalized with Fe₃O₄ nanoparticles⁶² to give Fe₃O₄@g-C₃N₄ 95. Finally, Fe₃O₄@g-C₃N₄–SO₃H 96 was washed with methanol and ethyl acetate and afterward dried under vacuum at 60 °C (Scheme 34).⁶³

Scheme 34 Synthesis of Fe₃O₄@g–C₃N₄–SO₃H 96.

Fe₃O₄@g-C₃N₄–SO₃H 96 was then utilized in the synthesis of pyridine derivatives 98 via the one-pot multicomponent reaction of different aldehydes 9, various ketones 97, ammonium acetate 10, and malononitrile 16 in H₂O under ultrasonic irradiation (Scheme 35).⁶³

Scheme 35 Synthesis of pyridine derivatives 98.

Torabi and et al. prepared Ligand 101 via the reaction of 1H-benzo[d]imidazol-2-amine 100 and compound 99 under solvent-free conditions. Fe₃O₄ was then functionalized with tetraethyl orthosilicate (TEOS) in toluene under reflux conditions to give Fe₃O₄@SiO₂ 4, which was reacted with ligand 101 to yield Fe₃O₄@SiO₂@(CH₂)₃-urea-benzimidazole 102, followed by the reaction with chlorosulfuric acid in dichloromethane to obtain Fe₃O₄@SiO₂@(CH₂)₃-urea-benzimidazole sulfonic acid 103 (Scheme 36).⁶⁴

Scheme 36 Synthesis of Fe₃O₄@SiO₂@(CH₂)₃-urea-benzimidazole sulfonic acid 103.

Fe₃O₄@SiO₂@(CH₂)₃-urea-benzimidazole sulfonic acid 103 was employed in the synthesis of 2-amino-3-cyano pyridines 23 through the multicomponent reaction of benzaldehyde 9, malononitrile 16, methyl isopropyl ketone 31, and ammonium acetate 10 under solvent-free conditions at 70 °C (Scheme 37).⁶⁴

Scheme 37 Synthesis of 2-amino-3-cyano pyridines 23.

Initially, according to previous works,⁶⁵ Fe₃O₄@SiO₂@Pr-Cl 19 was prepared and dispersed in dry DMF, and then reacted with ciprofloxacin 104 to give Fe₃O₄@SiO₂@Pr-ciprofloxacin 105 (Scheme 38).⁶⁶

Scheme 38 Synthesis of Fe₃O₄@SiO₂@Pr-ciprofloxacin 105.

Fe₃O₄@SiO₂@Pr-Cip 105 was then investigated in the synthesis of imidazo[1,2-a]pyridines 107 through the three-component reaction of various benzaldehyde 9, 2-aminopyridine 87, and cyclohexyl isocyanide 106 (Scheme 39).⁶⁶

Scheme 39 Synthesis of the imidazo[1,2-a]pyridines 107.

Mohammadi et al. synthesized Fe₂O₃ nanoparticles 1 according to a previously reported method.⁶⁷ Calcination of Fe₂O₃ provided γ-Fe₂O₃ 108, which was convered to γ-Fe₂O₃@SiO₂ MNPs 109 by the reaction with tetraethyl orthosilicate (TEOS) 3, followed by the functionalization with γ-aminobutyric acid 110 to yield γ-Fe₂O₃@SiO₂-aminobutyric acid nanoparticles 111. Then, it was dispersed in chloroform and reacted with chlorosulfonic acid to provide γ-Fe₂O₃@SiO₂ γ-aminobutyric acid-SO₃H 112 (Scheme 40).⁶⁸

Scheme 40 γ-Fe₂O₃@SiO₂ γ-aminobutyric acid-SO₃H 112.

γ-Fe₂O₃@SiO₂@4-(sulfoamino)butanoic acid-SO₃H 112 was utilized in the synthesis of 5-(aryl)-5H-spiro[diindeno[1,2-b:2′,1′-e]pyridine-11,30-indoline]-2′,10,12-trione derivatives 115 through the pseudo four-component reaction of 1,3-indandione 47, isatins 113 with various aromatic amines 114 (Scheme 41).⁶⁸

Scheme 41 Synthesis of 5-(aryl)-5H-spiro[diindeno[1,2-b:2′,1′-e] pyridine-11,30-indoline]-2′,10,12-trione derivatives 115.

Fe₃O₄@Si-Pr-Cl 19 was reacted with chitosan and acetic acid solutions to provide chitosan-coated MNPs 116, which were modified with 2-formylpyridine 117 to give compound 118, followed by the reaction with manganese chloride to provide manganese Schiff-base complex Fe₃O₄@CSBMn 119 (Scheme 42).^69,70

Scheme 42 Synthesis of Fe₃O₄@CSBMn 119.

Fe₃O₄@CSBMn 119 was employed in the synthesis of 3-iminoaryl-imidazo[1,2-a]pyridine (IAIP) derivatives 122 through the three-component reaction of aryl halide derivatives 120, trimethylsilyl cyanide 121, and 2-aminopyridine 89 (Scheme 43). According to the results, the aldehydes with an electron-withdrawing group provided higher yields in comparison with electron-donating groups.⁷⁰

Scheme 43 Synthesis of 3-iminoaryl-imidazo[1,2-a]pyridine (IAIP) derivatives 122.

Multi-walled carbon nanotubes systems MWCNTs-COOH 123 (ref. 71) were synthesized according to the literature. A mixture of FeCl₃·6H₂O and FeCl₂·4H₂O was added to MWCNTs-COOH 123 in distilled water and stirred at 50 °C to give the magnetic multi-walled carbon nanotubes (MMWCNTs) 124, which were subsequently reacted with 1-ethyl-3-(3-dimethyl aminopropyl) carbodiimide hydrochloride (EDC·HCl) and N-hydroxysuccinimide (NHS) to obtain MMWCNTs-D-NH₂ 125 followed by reaction with 1,4-butanesultone 126 to yield MMWCNTs-D–(CH₂)₄–SO₃H 127 (Scheme 44).⁷²

Scheme 44 Synthesis of MMWCNTs-D–(CH₂)₄–SO₃H 127.

MMWCNTs-D–(CH₂)₄–SO₃H 127 was employed in the synthesis of dihydro-1H-Indeno[1,2-b] Pyridines 128 by the reaction of various aldehydes 9, 1,3-indandione 47, ethyl acetoacetate 7, and ammonium acetate 10 (Scheme 45).⁷²

Scheme 45 Synthesis of dihydro-1H-indeno[1,2-b] Pyridines 128.

3. Conclusions

Due to the high importance of magnetic nano-catalysts, featuring non-toxic nature, high surface area, simple preparation, easy surface modification, and simple separation, such systems have relevant applications in organic synthesis and catalysis. In this contribution, the synthesis methods of magnetic nano-catalysts have been disclosed in view of their applications in the synthesis of pyridine derivatives. According to most studies, these catalysts have excellent activities to target products, also featuring high reusability with the possibility to be recycled several times without reducing their catalytic activities.

Conflicts of interest

The authors declare no conflict of interest.

Acknowledgements

We are grateful for the Research Council support of Alzahra University. R. Luque gratefully acknowledges MINECO for funding under project PID2019-109953GB-I00. This paper has been supported by RUDN University Strategic Academic Leadership Program (R. Luque).

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