Magnetic nanocatalysts: Synthesis and application in

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Magnetic nanocatalysts: Synthesis and application in multicomponent reactions Meng-Nan Chen, Li-Ping Mo, Zhen-Shui Cui and Zhan-Hui Zhang The application of the magnetic nanocatalysts is a rapidly growing field for the development of sustainable and green processes. Magnetic separation not only avoids the need for catalyst filtration or centrifugation after completion of the reaction, but also provides practical techniques for recovering these catalysts. Multicomponent reactions are recognized as very powerful tools in synthetic organic and medicinal chemistry for the synthesis of the complex products in a single step from simple starting materials. The combination of magnetic nanocatalysts and multicomponent reactions will become an emerging strategic research area and is an ideal blend for the development of sustainable methods in green synthetic chemistry. This review focuses on the synthesis and application of magnetic nanocatalysts as novel task-specific catalysts for multicomponent reactions in recent years. Addresses National Demonstration Center for Experimental Chemistry Education, College of Chemistry and Material Science, Hebei Normal University, Shijiazhuang 050024, PR China Corresponding author: Zhang, Zhan-Hui ([email protected])

Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37 Edited by Dr. Rajender Varma This review comes from a themed issue on Nanocatalysis https://doi.org/10.1016/j.cogsc.2018.08.009 2452-2236/© 2018 Elsevier B.V. All rights reserved.

Introduction From the viewpoint of green and sustainable chemistry, the design of efficient and economical chemical processes using heterogeneous catalysts to prepare fine chemical and pharmaceutical products through multicomponent reactions (MCRs) has recently gained significant interest from academia and industries. However, the main limitation associated with heterogeneous catalysts is their reduced catalytic activity. The best solution to this problem is to keep the size of the particles in the heterogeneous catalyst as small as possible. Nanostructured catalysts represent a new frontier between homogeneous and heterogeneous catalysis and are referred to as “quasi homogeneous” (or soluble heterogeneous) catalysts because of their nano nature and high surface area [1e3]. However, for such www.sciencedirect.com

suspensions of particles with less than 100 nm, the conventional separation techniques such as filtration and centrifugation, become tedious and hamper the complete separation of the catalyst. To meet the unmet challenge, magnetic nanocatalysts (MNCs) have emerged as attractive candidates because of their unique separable features by magnetic forces [4e17]. MNCs in organic reactions have many advantages, i) magnetic nanoparticles (MNPs) are accessible from inexpensive and non-toxic materials; ii) the stability of catalyst linkages allows the use of more environmentally friendly solvents than homogeneous catalysis; iii) simple separation using an external magnet avoids use of extra chemicals and additional filtration or centrifugation step during the separation process; iv) the fabrication of MNCs is generally simple, scalable, safe, cost-effective, and controllable; v) catalyst leaching is usually lower than other material-supported catalysis. Meanwhile, multicomponent reactions (MCRs), in which three or more reactants react together in a onepot process to form a new product with essentially all of the atoms of the starting materials incorporated without intermediary purification, are becoming important tools for the rapid and efficient construction of structurally diverse and complex molecules. MCRs offer significant advantages over conventional lineartype synthesis since they have all features that contribute to an ideal synthesis, such as high atom economy and bond-forming efficiency, waste prevention, quick and simple implementation, solvents, reagents, time and energy saving as well as avoidance of complex purification procedure [18e23]. Under such inspiration, research on the MNCscatalyzed multicomponent reactions has made significant advances in recent years. This mini-review focuses on the application of different types of MNCs in the promotion of the MCRs and covers literature published in the last three years.

Bare MNPs The most commonly employed magnetic metals are Fe, Co, and Ni. MNPs are readily accessible by different synthetic methodologies, including using coprecipitation, sol-gel techniques, microemulsion, laser pyrolysis, thermal decomposition, hydrothermal synthesis, sonolysis, microwave irradiation, and biological synthesis. Many types of magnetic nanoparticles can be Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

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synthesized, including pure metals (Fe, Co, and Ni), alloys (CoPt3, FePt, and FeCo), iron oxides (FeO, Fe2O3, and Fe3O4) and ferrites (MFe2O4, M = Ni, Co, Mn, Cu, or Zn). They can be used directly as catalysts or as supports for further modification or functionalization with the catalytic species [24]. Among all the MNPs, the most commonly employed bare MNPs are Fe3O4, MnFe2O4, and CoFe2O4 due to their simple preparation process with high saturation magnetization, lower toxicity, affordability and stability under harsh conditions. In 2015, Hedayati and co-workers reported a facile and economical method for the preparation of 4-aryl-octahydroquinazolin-2,5-dione derivatives via three component reaction of an aldehyde, dimedone and urea (or thiourea) using unmodified nano Fe3O4 as an efficient catalyst for the first time in aqueous ethanol under reflux conditions. A wide range of aromatic aldehydes afforded the corresponding products in good yields. Mention must be made here that aromatic aldehydes

with electron-withdrawing groups gave higher yields than those with electron-donating groups (Scheme 1(1)) [25]. This catalyst was also employed as a catalyst for preparation of tetrahydro-2,4-dioxo-1H-ben-zo[b] [1,5]diazepine -3-yl-2-methylpropanamides via one-pot, three-component reaction of aromatic diamines, Meldrum’s acid, and isocyanides in CH2Cl2 at room temperature. The products were obtained in short time in excellent yields. Further, the recovered catalyst was reused successfully in six subsequent reactions without significant loss of catalytic activity (Scheme 1-(2)) [26]. Soleimani-Amiri et al. have reported the synthesis of chromene derivatives by employing magnetically recoverable Fe3O4 nanoparticles. The reaction involves three components coupling of 4-hydroxycumarin, isothiocyanates, and isocyanides in water and proceeds in short time with high yields (Scheme 1-(3)) [27]. The four-component domino reaction of amines, dialkyl acetylenedicarboxylates, and formaldehyde in the presence of Fe3O4 in EtOH at room temperature for the onepot synthesis of functionalized dihydro-2-oxopyrroles

Scheme 1

Current Opinion in Green and Sustainable Chemistry

Multicomponent reactions catalyzed by bare MNPs. Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

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Magnetic nanocatalyst in multicomponent reactions Chen et al.

was reported by Nickraftar et al. All reactions proceeded with good to excellent yields in short time and the catalyst could be used up to six times without significant loss of its activity (Scheme 1-(4)) [28]. Also, magnetically recoverable CuFe2O4 was also prepared and used for the synthesis of chromeno[4,3-b]pyrrol-4(1H)-one derivatives in aqueous media. The reaction of a variety of glyoxal derivatives, amine and 4-aminocoumarin were carried out in the presence of this catalyst and almost in all cases, products were obtained with very good yields (Scheme 1-(5)) [29]. Our group prepared magnetic CuFeO2 by the solegel process and sequential

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annealing. It was found CuFeO2 was an efficient catalyst for the one-pot, three-component reaction of 2aminopyridines, aldehydes, and alkynes to prepare an array of imidazo[1,2-a]pyridines using citric acide dimethylurea melt as a green solvent. The catalytic system can be successfully reused six times with keeping its catalytic performance (Scheme 1-(6)) [30].

Magnetic sulfonic acid catalysts Sulfonic acid is one of the most prevalent acid catalysts widely used in various organic transformations. However, this homogeneous acid requires special treatment in

Scheme 2


Multicomponent reactions catalyzed by magnetic sulfonic acid catalysts. www.sciencedirect.com

Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

30 Nanocatalysis

a neutral form and involves energy-inefficient separation processes for catalyst and product, leading to the formation of large amounts of sulfate waste. Although there are many reports on the preparation and applications of immobilization of sulfonic acid, magnetic solid sulfonic acid catalysts opened up a new path to show an amazing and efficient system to simplify catalyst recovery for various organic synthesis reactions because of their high catalytic activity, ease of separation, minimization of environmental waste and clean reaction products. Recently, Salunkhe and co-workers reported the synthesis of sulfuric acid-functionalized silica-coated magnetic nanoparticles (MgFe2O4@SiO2eSO3H). Firstly, MgFe2O4 was prepared using a simple chemical co-precipitation method of iron(II) and magnesium(II) ions. Next, a modified surface of MgFe2O4 particles was performed by the capping of silica onto a magnesium ferrite surface followed by the hydrolysis of TEOS using ammonia to provide reaction sites for functionalization. The MgFe2O4@eSiO2 served as the support for the functionalization of the sulfuric group by adding chlorosulfonic acid in chloroform. This catalyst was found to be an efficient and magnetically recoverable catalyst for a straightforward synthesis of benzoxazinones and benzthioxazinones through the one-pot multicomponent reaction of b-naphthol, urea or thiourea and aldehydes under microwave irradiation (Scheme 2-(1)) [31]. TiO2-coated magnetite nanoparticle-supported sulfonic acid (Fe3O4eTiO2eSO3H) is synthesized by immobilizing eSO3H groups on the surface of nano-Fe3O4e TiO2. This catalyst exhibited a high catalytic activity for the synthesis of heterocyclic compounds such as 1,8dioxodecahydroacridine derivatives in multicomponent reaction (Scheme 2-(2)) [32]. In 2015, a sulfonated magnetic cellulose-based nanocomposite (Fe3O4@cellulose-OSO3H) was prepared and it was used as a green nanocatalyst for the synthesis of a-aminonitriles by a one-pot three-component condensation reaction of aldehydes or ketones, amines and trimethylsilylcyanide in ethanol at room temperature (Scheme 2-(3)) [33]. Aromatic sulfonates were introduced into Fe3O4 nanoparticles by Ghorbani-Vaghei and co-workers via four major steps. The resulting 7-aminonaphthalene-1,3disulfonic acid-functionalized magnetic Fe3O4 nanoparticles were used as magnetic powerful solid acid catalyst in a straightforward convergent synthesis of 3hydroxy-2-pyrrolidinones via the one-pot reaction of aromatic aldehydes, arylamines and acetylenedicarboxylate in EtOH at room temperature. A wide range of aromatic aldehydes and aromatic amines were examined. Results indicated that in all cases products were formed in good to excellent yields and the catalyst was Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

reused for the six cycles without any notable loss of its activity (Scheme 2-(4)) [34]. A similar immobilized S-sulfonic acid was reported by Moradi and co-workers. The grafting of the 3-(trimethoxysilyl)-1-propanethiol onto Fe3O4@SiO2 was followed by treatment with chlorosulfonic acid in anhydrous CH2Cl2 to obtained Fe3O4@SiO2@(CH2)3SeSO3H. The supported catalyst was active for the synthesis of pyrido[2,3-d]pyrimidines via threecomponent reaction of aryl aldehyde, malononitrile and 2,4-diamino-6-hydroxypyrimidine at 100 C under neat conditions (Scheme 2-(5)) [35]. A new magnetic sulfonic acid catalyst was obtained through the reaction of silanol groups, on the surface of silica coated Fe3O4 magnetic nanoparticles, with (3chloropropyl)triethoxysilane followed by hexamethylenetetramine and chlorosulfonic acid. It was applied to the synthesis of pyranopyrazole compounds via fourcomponent reaction of aldehyde, malononitrile, dimethyl acetylenedicarboxylate and hydrazine hydrate at room temperature under solvent free condition (Scheme 2-(6)) [36]. The sulfanilic acid supported on magnetic Fe3O4 was designed and synthesized via the reaction of sulfanilic acid with the oxirane-functionalized magnetic nanoparticles. This MNP-immobilized sulfanilic acid was shown to be an efficient catalyst for three-component reaction of curcumin, barbituric acids and aldehydes. A class of curcumin-based pyrano[2,3-d]pyrimidine derivatives were obtained in good to excellent yields. The catalyst was recycled up to 5 times without considerable loss in catalytic efficiency (Scheme 2-(7)) [37].

MNPs supported organocatalysts In 2018, Afradi and co-workers prepared recyclable magnetically modified vitamin B3 (Fe3O4@Niacin) and it was used to promote four-component condensation reaction between aldehydes, ketones, malononitrile, and ammonium acetate for synthesis of 2-amino-3cyanopyridine derivatives under microwave irradiation in water (Scheme 3-(1)) [38]. Magnetic Fe3O4 supported imidazole salt was easily prepared and exhibited high catalytic performances for the three-component reaction of arylaldehydes, malononitrile and dimedone. The catalyst was found to be reusable for five times without a noticeable loss its activity. The catalytic system was also successfully extended to the three-component reaction of arylaldehydes, malononitrile and other active methylene compounds. Some pyran annulated heterocyclic systems were obtained in high to excellent yield (Scheme 3-(2)) [39]. www.sciencedirect.com


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Scheme 3


Multicomponent reactions catalyzed by MNPs supported organocatalysts.

MNPs supported metal catalysts MNP-supported metal catalysts have been widely used to promote different organic reactions. In most cases, the metal species can be loaded onto the magnetic support by co-precipitation during MNP synthesis or through post-modification of the MNP functions. The use of these MNCs can solve the separation and recycling problems encountered in many catalytic reactions. Most importantly, these catalysts not only have high catalytic activity but also have high chemical stability. In 2015, Nemati et al. successfully developed a Cu2O supported on nano-CuFe2O4 by reduction of CuCl2 in alkaline media in the presence of hydroxylamine hydrochloride. This catalyst was able to catalyze threecomponent coupling reaction of aldehydes, amines, and alkynes (A3 coupling) under solvent-free conditions at 90 C to produce tertiary and quaternary carboncontaining propargylic amines (Scheme 4-(1)) [40]. In 2017, Kamali and Shirini reported a magnetically recoverable Fe3O4@SiO2eZrCl2 catalyst for the synthesis of tetrahydrobenzimidazo[2,1-b]quinazolin1(2H)-one derivatives via the three-component reaction of an aldehyde, 2-aminobenzimidazole and 5,5dimethyl-1,3-cyclohexanedione under solvent-free conditions. Furthermore, this catalytic system could also be applied to three-component reaction of phthalhydrazide, dimedone and aldehyde to give 2H-indazolo [2,1-b]phthalazine-triones in good to high yields (Scheme 4-(2)) [41]. Due to the presence of both hydroxyl and anionic sulfate groups in carrageenans, ƛ-carrageenan was used as www.sciencedirect.com

magnetic nanoparticle surface coating agent by Keshavarzipour et al., Zn2þ ions were immobilized on the surface of the coated magnetite to generate a novel heterogeneous and efficient Lewis acid catalyst. This catalyst was applied to promote the reaction of aromatic aldehydes, enolizable aldehydes and aniline derivatives for synthesis of biologically important quinoline derivatives without using any toxic solvent or co-catalyst (Scheme 4-(3)) [42]. Sadjadi et al. prepared magnetic Fe2O3 NPs-supported Agþ containing (1-methyl-3-(3-trimethoxysilylpropyl) imidazolium chloride ionic liquid (IL) catalyst by immobilizing a ILeAg complex on the surface of MNPs and demonstrated their use as heterogeneous catalysts for promoting ultrasonic-assisted synthesis of propargylamines from reaction of amine, phenylacetylene and aldehyde as well as the synthesis of benzo[b]furans through reaction of amine, phenylacetylene and salicylaldehydes (Scheme 4-(4)) [43]. Shojaei and co-workers immobilized CuI complexes with N-heterocyclic carbene ligands on the modified surface of MNPs with cellulose to generate magnetic cellulose supported N-heterocyclic carbeneecopper complex. It was applied as a highly active catalyst in one-pot three-component reaction of sulfonyl azides, secondary amines and triethylamine to afford N-sulfonylformamidines. Copper catalyzed oxidative transformation of CeN bond of triethylamine is a key step to give desired products. In contrast with the good reactivity of the conventional secondary amines, aromatic amines and NH containing heteroaromatics had no activity in these reactions (Scheme 4-(5)) [44]. Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

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Scheme 4


Multicomponent reactions catalyzed by MNPs supported metal catalysts.

A novel magnetically recoverable functionalized magnetic nanoparticle supported-Cu complex are reported by our research group. The catalyst was prepared via a three-step process. At first, NiFe2O4 NPs were synthesized by a chemical precipitation technique in the basic condition. Then, the anchoring L-glutamic acid on the surface of NiFe2O4 NPs was completed in MeOH, followed by the addition of Cu(OAc)2 and NaBH4 under alkaline conditions to obtain the immobilized copper complex NiFe2O4-glutamate-Cu. Studies revealed that this magnetic catalyst is a new and highly efficient green catalyst for the synthesis of a wide variety of 1,4disubstituted-1,2,3-triazoles via one-pot three-component click reactions of sodium azide, non-activated terminal alkynes, and different azide precursors such as epoxides, benzyl chloride, and aryl boronic acids at room temperature in water (Scheme 5) [45]. This magnetically recycled catalyst could be used for at least ten successive runs without significant loss of its activity. Multi-walled carbon nanotube was supported on the surface of magnetite nanoparticles CoFe2O4 NPs, to introduce hydrophilic groups (such as COOH and OH) to help adsorb the active metal such as Cu. The synthesized magnetic CoFe2O4/CNTeCu was used as an Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

efficient catalyst for the promotion of three-component reactions of 2-aminopyridines, aldehydes and nitromethane. The reactions proceeded in PEG 400 under aerobic conditions at 80 C and the corresponding 3nitro-2-arylimidazo[1,2-a]pyridines were obtained in good to excellent yields (Scheme 6) [46]. The amount of Cu leaching was found to be 0.60 ppm after eighth cycle. Graphene oxide (GO) is a remarkable support for encapsulation of MNPs because of its two-dimensional plate-like structure and large specific surface area. GO possesses rich oxygen containing functionalities such as carboxyl, epoxide, hydroxyl, which makes GO as a fascinating support to provide covalent attachment of various specific groups [47e51]. Our group synthesized magnetically separable GO supported molybdenum (Fe3O4/GO-Mo) catalyst (Scheme 7). The catalytic performance of Fe3O4/GO-Mo was tested in one-pot, three-component reaction of isatins, malononitrile and anilinolactones in deep eutectic solvents (DESs) [52] based on choline chloride and urea. The magnetic nanocatalyst exhibits high catalytic activity for synthesis of spiro-oxindole dihydropyridines and was simply separated and reused for eight cycles without significant loss in performance [53]. www.sciencedirect.com


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Scheme 5


Preparation and application of NiFe2O4-glutamate-Cu in synthesis of 1,2,3-triazoles.

Magnetic metal-organic frameworks (MOFs) In the last ten years, metal-organic frameworks (MOFs) composed of metal ions and polyfunctional organic ligands are a new class of porous crystalline materials that exhibit great potential in various fields due to their high porosity, large surface area and adjustable pore size [54]. Development of magnetic MOF by combination of the aforementioned characteristics of MOFs with magnetic nanoparticles has significant superiorities in organic reactions. Our group prepared magnetic MOF-5 (NiFe2O4@MOF-5) via a three-step process using NiFe2O4 as a magnetic core, a zinc ion as a connector and terephthalic acid (H2BDC) as a linker (Scheme 8). This magnetically separable catalyst exhibited high catalytic activity for the synthesis of a variety of 2-substituted alkyl and aryl(indolyl)kojic acid derivatives via a onepot, three-component reaction of aldehyde, indole, and kojic acid under solvent-free conditions. It was www.sciencedirect.com

found that the magnetically recoverable catalyst NiFe2O4@MOF-5 can be reused without significant loss of its activity [55]. A magnetically supported IRMOF-3 as a novel heterogeneous catalyst was designed, synthesized and fully characterized, and it was used for the four-component (pseudo seven-component) reaction of propane-1,3diamine, dimethyl but-2-ynedioate, aromatic amines and formaldehyde to give bis-N-aryl-3aminodihydropyrrol-2-one-4-carboxylates in high yields (Scheme 9) [56].

Conclusions and outlook Different types of magnetic nanocatalysts have developed to effectively catalyze multi-component reaction for the synthesis of some structural diversity of compounds. Most reactions could be carried out under mild Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37

34 Nanocatalysis

Scheme 6


Preparation and application of CoFe2O4/CNT-Cu in synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines.

conditions and the product is obtained in high yield in short reaction times. Moreover, these catalysts are recyclable and can be reused in successive trials without significantly reducing their catalytic activity. Although remarkable progress has been made in terms of diversity

of the reactions using magnetic nanocatalysts, there are several limitations that still need to be addressed in the future research in this context. 1) The long-term stability of magnetic catalysts and the loss of metals in the catalyst under harsh conditions need to be improved so

Scheme 7


Preparation and application of Fe3O4/GO-Mo in synthesis of spiro-oxindole dihydropyridine derivatives. Current Opinion in Green and Sustainable Chemistry 2019, 15:27–37



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Scheme 8


Preparation and application of NiFe2O4@MOF-5 in synthesis of 2-substituted (indolyl)kojic acid derivatives. Scheme 9


Synthesis of substituted bis-dihydropyrrol-2-ones in the presence of CoFe2O4@SiO2@IRMOF-3.

as to meet the requirements of multi kilogram scale synthesis toward industrial production; 2) In order to prevent aggregation and to achieve better grafting of catalyst species on pre-synthesized MNPs, it is important to select suitable ligands with stable or coated/ encapsulated materials. 3) Most of the synthetic approaches of functionalized magnetic nanocatalysts involving multistep reactions are tedious and expensive. The development of more general, facile and easily scaled synthesis strategy is still highly desirable. 4) The effective synthesis of magnetic catalysts for the immobilized chiral ligands or enzymes and their application in the preparation of chiral compounds in multicomponent asymmetric reactions will cause widespread concern. 5) The continued need for further and fully research on magnetically recoverable multifunctional catalyst in a wide range of multicomponent reactions to develop more efficient and green chemical processes.

Conflict of interest statement

References Papers of particular interest, published within the period of review, have been highlighted as: * of special interest * * of outstanding interest 1. *

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Cheng T, Zhang D, Li H, Liu G: Magnetically recoverable nanoparticles as efficient catalysts for organic transformations in aqueous medium. Green Chem 2014, 16: 3401–3427.

Nothing declared.

Acknowledgments Authors acknowledge funding from the Science and Technology Research Foundation of Hebei Normal University (L2018Z06) and the National Natural Science Foundation of China (21272053). www.sciencedirect.com


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