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

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. This journal is © The Royal Society of Chemistry.

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.

The synthesis of pyridine derivatives by diverse magnetic catalysts

Basic magnetic catalyst

The core–shell structure of Fe3O4@KCC-1-npr-NH26 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-NH26 with excellent basic properties. Details for the preparation of Fe3O4@KCC-1-npr-NH26 are shown in Scheme 1. Various characterization techniques, including FT-IR, SEM, TEM, BET, and XRD, confirmed the structure of Fe3O4@KCC-1-npr-NH26 as magnetic nano-catalyst.[30]

Scheme 1

Synthesis of Fe3O4@KCC-1-npr-NH26.

Fe3O4@KCC-1-nPr-NH26 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@SiO24,[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–NCH–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–NH227, 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–NCH–Ph–OMe MNPs 29.

Fe3O4–Si–(CH2)3–NCH–Ph–OMe MNPs 29 was used in the synthesis of 2-amino-3-cyanopyridines 23via 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.

Acidic magnetic catalysts

Fe3O4@CoII (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 Fe3O41, 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@CoII (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 35via three-component reaction of aldehyde derivatives 9, malononitrile 16, thiophenol 36 under solvent-free conditions (Scheme 10). The catalytic activity of Fe3O4@CoII (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@CoII34 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@SiO243,[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@GOTfOH/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 23via 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 55via 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@Co3O456 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 23via 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 61via 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 71via 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-Fe3O41, to give nano-Fe3O4@eggshell 75, which was treated with ClSO3H to yield nano-magnetic acid catalyst Fe3O4@Ca(HSO4)276. 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)276.

Nano-Fe3O4@Ca(HSO4)276 was subsequently utilised in the synthesis of 2-amino-3-cyanopyridines 23via 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.

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 CF3CO279 (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.

Bifunctional magnetic catalysts

In 2019, Edrisi et al. synthesized g-C3N494 according to the reported method.[61] g-C3N494 was functionalized with Fe3O4 nanoparticles[62] to give Fe3O4@g-C3N495. 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 98via 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 101via 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@SiO24, 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-Fe2O3108, 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-NH2125 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.

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.