Abstract
Recovery and reusability of catalysts is an important aspect in modern catalysis research especially in organic synthesis. Compared to the conventional separation, magnetic separation has emerged as a robust, highly efficient, easy and rapid separation technique for products and catalysts. Cobalt ferrite nanoparticles are a well-known material, recognized as CoFe2O4 MNPs, and can be used as both catalyst and a versatile support for functionalization of metals, organocatalysts. In recent times, catalysis research has clearly experienced a renaissance in the area of utility of cobalt ferrite (CoFe2O4 MNPs) nanocatalysts based on their ability for recovery and reusability; the activity of these CoFe2O4 MNPs was investigated in a category of organic reactions. In this review, the fabrication, characterization, and application of cobalt ferrite (CoFe2O4 MNPs) nanocatalysts (CF-MNPs) in organic reactions have well summarized.
1 Introduction
1.1 Catalysis
Catalysis plays an increasingly crucial role in chemical processes, and it is widely used at the heart of innumerable chemical processes from scientific research in academic laboratories to an economical research in industrial laboratories [1]. The catalyst is very important in chemistry science especially in organic synthesis; it can greatly promote the chemical processes and reactions and effectively and sustainably synthesize products [2]. The development of new, sustainable chemical processes for the synthesis of natural products, drugs, and biologically active molecules is impossible without the use of catalysis. A catalyst ensures that a specific chemical reaction requires less energy (such as heat). In recent times, catalysts and catalytic reactions have attracted special attention with the aim of finding meaningful applications in the pharmaceutical and fine chemical industries [3]. Catalysis has possessed a key role in improving and developing organic synthesis especially in medical chemistry and has become one of the most economically and ecologically impacting technologies so far [2]. The modern era of organic synthesis is shifting towards the path of creative strategies which fundamentally underlines or focuses on the concept of green chemistry especially the use of sustainable and green catalysts [1].
1.2 Green and sustainable catalysis
The development of green, sustainable, and economical chemical processes is one of the major challenges in modern chemistry science especially in green chemistry [4]. Green chemistry, also called sustainable chemistry, is a philosophy of chemical research and engineering that encourages the design of products and processes that minimize the use and generation of hazardous substances [5], [6]. Green catalysis is a subchapter of green chemistry but probably the most important one [5], and one of the urgently indispensable challenges facing chemists now is the design and use of environmentally benign catalysts [7]. The concept of green chemistry, which makes catalysis science even more creative, has become an integral part of sustainability [5]. From the perspective of green chemistry, an ideal and sustainable catalyst must possess a series of distinctive advantages such as low preparation cost, high activity, efficiency and selectivity, high stability, efficient recovery, and good recyclability [7]. Accordingly, the search for designing and using environmentally benign, sustainable, and efficiently reusable alternative catalytic systems has become an important challenge in modern catalysis science [5].
1.3 Homogeneous, heterogeneous, and heterogenized catalysis
Catalysts can be divided into two types depending on the reaction phase that they occupy: homogeneous and heterogeneous. Homogeneous catalysis is a type of catalysis in which the catalyst operates in the same phase as the reactants, generally dissolved in the reaction medium; chemists are unanimous on this theory. In homogenous catalytic system, all catalytic sites are accessible because of its solubility in the reaction medium [4]; it is possible to tune the chemo-, region- and enantioselectivity of the catalyst [8]. Homogeneous catalysts have a series of other advantages such as high selectivities, better yield, high turnover numbers, and easy optimization of catalytic systems by modification of ligand and metals [4], [5], [9], [10]. But here a notable disadvantage is glaring; separating homogenous catalysts from the desired products or reaction medium is a difficult, tedious, and time-consuming task and needs a series of costly and specific techniques [11]. It is essential to remove the catalyst because metal contamination is highly regulated, especially in the drug and pharmaceutical industry [5]. Also, it is not compatible with the principles of green chemistry in modern catalysis science especially from an economic standpoint [11]. To address the separation problems in homogeneous catalysis, chemists focused on designing new, green, and effective heterogonous catalytic systems. Heterogeneous catalysis is a type of catalysis in which the catalyst is present in a different or separate phase; normally the catalyst is a solid and the reactants are gas or liquid. In recent times, heterogenization of active molecules on support materials (such as inorganic silica and organic polymers) has been introduced as an efficient strategy for the developing ecofriendly catalytic systems that facilitate the recovery and reusability of the catalysts [12], [13]. However, the activities and selectivities of heterogeneous catalysts are generally lower than those of their homogeneous counterparts, due to the lower dimensionality of the interaction between the components and the catalyst surface [2], [7]. This becomes the crucial factor that restricts it from developing well [2]. In this respect, organic chemists are looking for new heterogeneous catalysts that possess the advantages of both homogenous catalysts (high activity and selectivity, etc.) and heterogeneous catalysts (easy catalyst separation, long catalytic life, thermal stability, and recyclability) [1].
1.4 Nanocatalysis and magnetic nanoparticles
Nanotechnology is the science and technology of precisely manipulating the structure of matter at the molecular level, in particular things that are <100 nm in size. Nanotechnology is the key and valuable achievement of the 21st century in all sciences, especially in chemistry science. During the last decade, chemistry science has witnessed the birth of a new technology revolution and created huge developments in catalysis as “nanocatalysis”. Nanocatalysis is a hot research topic which involves the use of nanomaterials (an average size of 1–100 nm) as catalysts for a variety of catalysis applications [1]. In fact, the nanocatalysts open up a new and marvelous chapter in catalysis processes and create numerous opportunities for chemists in order to achieve their goals. Nanocatalysts are attractive alternatives to conventional catalysts, because when the size of the material (catalyst or support) is decreased to the nanometer scale, the surface area is significantly increased and the material (catalyst or support) can be equally dispersed in solution, to form a homogenous emulsion [1]. Significant increase in catalytic activity, selectivity, and stability of nanocatalysts can be manipulated by tailoring chemical and physical properties such as size, shape, composition, and morphology [14]. Accordingly, the particle size and support employed for the dispersion of catalyst are two key factors that affect the efficiency of nanocatalysis processes [1]. Nanocatalysts are separated and recovered through filtration or centrifugation strategies. However, in most cases, isolation and recovery of nanocatalysts from the reaction medium is either a very difficult, tedious, and time-consuming task or impossible because their size decreases to nanoscale dimensions [15].
To overcome such limitations, magnetic separation was introduced as a logical solution [16], [17]. In recent times, magnetic separation has received profound attention because magnetic nanocatalysts (due to their insoluble and paramagnetic nature) can be readily separated from the reaction medium by using an external magnet, without the need for filtration, centrifugation, or other tedious workup processes [10], [18], [19], [20], [21], [22]. Also, most magnetic nanocatalysts (magnetic nanoparticles as catalyst or support) can be reused many times while keeping their initial activity [23]. Furthermore, recent literature studies clearly have shown that magnetic nanoparticles (MNPs) possess several admirable advantages such as high surface area to bulk ratios, low toxicity, high activity, thermal stability, and the capability of surface modifications and easy dispersion [24], [25], [26], [27], [28]. These characteristics thus make them more sustainable and ideal catalysts or supports than conventional samples [29]. In fact, magnetic separable catalysts are a well-favored and fascinating strategy to bridge the split between heterogeneous and homogenous catalysis [24], [30]. In describing the MNPs, it can be also said that the magnetic separation is in fact an admirable and valuable victory in areas of catalysis science [23].
1.5 Cobalt ferrite nanoparticles
Spinel ferrites are also named as cubic ferrites; they are generally applied at microwave frequencies due to high electrical resistivity and low eddy current losses. The chemical formula of spinel ferrites is MFe2O4. In the early 21st century, the spinel ferrite nanoparticles (MFe2O4, in which M represents one or more bivalent transition metals such as Mn, Fe, Co, Ni, Cu, and Zn) have become an important and efficient tool in modern catalytic organic synthesis due to their unique electronic and magnetic properties (easily separate by external magnet), which are quite different from the conventional bulk materials [1], [11], [23], [31], [32].
In this category (spinel ferrites), cobalt spinel ferrite (CoFe2O4) is an inverse spinel ferrite with all or most Co+2 ions occupying octahedral sites (B sites) and the Fe+3 ions on both tetrahedral (A sites) and B sites [33], [34]. Cobalt spinal ferrites (CoFe2O4) have received profound attention due to their strong anisotropy, high coercivity, moderate saturation magnetization, and good mechanical and excellent chemical stabilities at higher temperature, which are significantly different from their bulk counterparts [34], [35], [36]. Owing to these features, cobalt ferrites have been widely used in sensors, recording devices, magnetic cards, solar cells, magnetic drug delivery, biomedical, catalysis, and biotechnology [37], [38], [39], [40]. For catalysis applications, CoFe2O4 nanoparticles are required to have a narrow size distribution and high magnetization values [41]. The saturation magnetization (Ms) of CoFe2O4 nanoparticles is less than that of the bulk, and Ms decreases with decrease in size. When the crystallite size is almost equal to single-domain size, the coercivity reaches its highest value.
Compared to CoFe2O4, Fe3O4 and γ-Fe2O3 are more popular and easily prepared and have been efficiently utilized in many organic reactions, but they have a series of drawbacks. For example, Fe3O4 is fairly reactive to acidic and oxidative environments, and γ-Fe2O3 is also not thermally stable [42]. On the contrary, CoFe2O4 nanoparticles have a remarkable chemical stability [43]. The features of CoFe2O4 nanoparticles are strongly dependent on a series of factors such as chemical composition and microstructural characteristics, where the particle size and shape might be controlled in the fabrication processes [42], [44]. Accordingly, fabrication of ferrite nanoparticles of controllable morphology with excellent magnetic performance is attracting intensive interest [45].
1.6 Preparation of CoFe2O4 nanoparticles
A number of preparation techniques have been explored to synthesize CoFe2O4 nanoparticles such as microemulsion [46], sol-gel techniques [47], hydrothermal synthesis [48], solvothermal method [49], co-precipitation [50], electrochemical method [51], and combustion methods [52–54]. Although in most of methods, cobalt ferrite nanoparticles are achieved in the desired or required sizes and microstructures, they are difficult to apply on larger scales due to their expensive and complicated procedures, high reaction temperatures, long reaction times, toxic reagents and by-products, and their potential harm to the environment [53]. Among these, co-precipitation and hydrothermal techniques are easy, versatile, and low-cost methods for the preparation of cobalt ferrite nanoparticles. Co-precipitation is a rapid, easy, and economical process that offers a series of advantages such as controlled crystallite size, high limpidness, no agglomeration of the particles, and stable particle surface along with homogeneity [53]. In hydrothermal processes, two factors are very important: (1) optimization of hydrothermal synthetic method and (2) the ion doping or surface modification of cobalt ferrite nanoparticles [45]. Ion doping can lead to structural disorder, change in grain size and lattice strain, and cation redistribution as well as affect the physical and chemical features of cobalt ferrite nanoparticles [45].
1.7 Modifications of CoFe2O4 nanoparticles
Spinal ferrites nanoparticles, in particular cobalt ferrites, are very sensitive to agglomeration because of their small inter-particle distances and high surface energy and also exhibit high chemical reactivity as well as strong magnetic dipole interactions [55]. Natural agglomeration of these particles into larger clusters also restricts the use of such particles in various applications [55]. To overcome this problem, modification of ferrite nanoparticles using suitable stabilizing ligands or coating materials (including small molecules, silica, polymers, carbon, ionic liquids, metal or metal oxide NPs, and their layer-by-layer combinations) has been proved to be the best solution to date [7]. The modification techniques provide reaction sites or active groups for covalently grafting the active catalytic units onto the coated magnetic or ferrite nanoparticles to fabricate magnetically recoverable nanocatalysts [7]. Silica is the most common material for coating the surfaces of ferrite nanoparticles; coating ferrite nanoparticles with silica avoids unfavorable contacts with the core, which both improves chemical stability and prevents particle aggregation [56], [57]. The silica shell can be easily modified with different surface functional groups via covalent bonds between organo-silane molecules and silica shell. Dopamine derivatives, triethoxysilyl-, phosphonic acids-functionalized molecules, and glutathione were widely used to stabilize and functionalize MNPs [7]. Triethoxysilyl-functionalized molecules such as commercially available NH2-, SH-, and Cl-terminatedcompounds and their further functionalized derivatives are another type of popular reagents for surface modification of MNPs [7]. The linkage of MNPs with these silane reagents is achieved by coupling between the hydroxyl group of MNPs and silane reagent [7]. Mesoporous carbon is another coating based on a main-group element that is used for MNPs. In this category, graphene oxide, carbon nanofibers, and carbon nanotubes (CNTs) have attracted special attention [58]. The carbon layer can efficiently protect the spheres from dissolving in acidic environments because its dense structure can inhibit penetration of hydrogen ions [15]. CNTs have rich surface functional groups (mainly hydroxyl and carboxyl), which made CNTs as a good support material for ferrite and metal oxide nanoparticles [59].
1.8 Characterizations of CoFe2O4 nanoparticles
The characterization of chemical and physical properties of ferrite nanoparticles including: the size of the particles, the shape of the particles, surface characteristics, the presence of any surface coating, and the presence of impurities is very important in catalysis science [19]. Therefore, a series of spectroscopic techniques have been used to characterize cobalt ferrite nanoparticles.
Electron microscopy (transmission electron microscopy (TEM) and scanning electron microscopy (SEM): Since the size of ferrite materials is in the nano range (1–100 nm), TEM and SEM spectroscopic techniques are very important to determine the shape, size, and morphologies of nanoparticles [19].
High-resolution transmission electron microscopy (HR-TEM): HR-TEM is a more advanced technique that allows the observation of a single nanoparticle (<1 nm) and determination of its structural morphology [19].
Dynamic light scattering (DLS): DLS is a key spectroscopic technique to determine the average size diameter of the nanoparticles.
X-ray diffraction (XRD): XRD is a good spectroscopic technique to characterize crystalline phases, orientation, structural properties, and atomic arrangement of ferrite nanoparticles.
Inductive coupled plasma-atomic emission spectroscopy (ICP-AES): ICP-AES is a suitable spectroscopic technique to identify and determine metal composition in the nanomaterial or leaching of metals in the final nanomaterial [19].
Fourier transform infrared spectroscopy (FT-IR): FT-IR is an efficient spectroscopic technique to identify functional groups in nanomaterials, in particular functionalized nanocatalysts.
X-ray photoelectron spectroscopy (XPS): XPS is an effective spectroscopic technique to confirm the nature, binding energy, and oxidation state of nanomaterials.
N2 adsorption-desorption isotherms: A suitable spectroscopic technique to identify the Brunauer-Emmett-Teller theory (BET) surface area, pore volume, and average pore diameter.
Vibrating sample magnetometer (VSM): VSM is an efficient spectroscopic technique to determine the magnetic property of ferrite nanoparticles or functionalized magnetic nanocatalysts.
Energy-dispersive X-ray spectroscopy (EDS): EDS is a key spectroscopic technique to determine the percentage of elements or components in nanomaterials especially in functionalized nanocatalysts.
Thermogravimetric analysis (TGA): TGA is an effective spectroscopic technique to study both the thermal stability of nanomaterials and the bond formation between ferrite nanoparticles and catalyst.
2 Cobalt ferrite nanocatalysts in organic synthesis
In recent times, COFe2O4 nanocatalysts or cobalt ferrite-supported nanocatalysts have been successfully used in organic synthesis for a category of valuable reactions.
2.1 Oxidation
Oxidation catalysis is an important domain of chemical research. Oxidation catalysis not only plays a key role in the modern chemical industry for the production of valuable intermediates such as alcohols, epoxides, aldehydes, ketones, and organic acids but also will contribute to the establishment of novel green and sustainable chemical processes [60].
2.1.1 Alcohol oxidation
Carbonyl compounds are important structural scaffolds in the synthesis of biologically and pharmaceutically active molecules, polymers, and various other industrially important fine chemicals [61], [62]. Selective oxidation of alcohols is the straightforward and most common route for the preparation of carbonyl compounds [63], [64]. In 2014, Ramazani and his coworkers [65] have described a fascinating protocol for the oxidation of alcohols to the corresponding carbonyl compounds using magnetically recoverable and reusable CoFe2O4 nanoparticles (CF-MNPs 1). The structure of as-fabricated CF-MNPs 1 was characterized by FT-IR, SEM, and XRD spectroscopic techniques. Alcohol oxidation reactions were performed under aqueous medium at room temperature in the presence of oxone (potassium hydrogen monopersulfate) as oxidant. Under the described conditions, a library of primary and secondary benzylic and aliphatic alcohols (1 mmol) was smoothly oxidized to the corresponding carbonyl products in good to excellent yields (Scheme 1). The CF-MNPs 1 was magnetically recovered and reused for six cycles without noticeable loss of catalytic activity.
CF-MNPs 1 catalyzed the oxidation of alcohols to the carbonyl compounds.
At the same time, Bhat and his research team immobilized nickel hydroxide complex on magnetic cobalt ferrite nanoparticles modified with 3-aminopropyltriethoxysilane (CoFe2O4@APTES@Ni(OH)2) (CF-MNPs 2) and successfully utilized this as a heterogeneous catalyst for the oxidation of alcohols to the carbonyl compounds. Synthesis steps of CF-MNPs 2 are designated in Scheme 2. As-prepared CF-MNPs 2 was characterized by FT-IR, atomic absorption spectrometry, XRD, SEM, TEM, energy-dispersive X-ray spectroscopy (EDX), and UV-vis spectroscopic techniques [66]. The best reaction conditions for the alcohol oxidation was found to be CF-MNPs 2 as the catalyst and H2O2 (30 wt% in water, 10.0 mmol) as the oxidant in acetonitrile at 80°C. This catalytic system was able to oxidize primary and secondary alcohols (1 mmol) efficiently (87%) to corresponding carbonyls in good yields (Scheme 3). Conversion and selectivity were determined by GC. The CF-MNPs 2 could be reused several times with only a slightly decrease in activity.
General route for the synthesis of nickel hydroxide/cobalt-ferrite (CF-MNPs 2).
CF-MNPs 2 catalyzed the oxidation of alcohols to the carbonyl compounds.
In another study, Dhar and coworkers reported facile preparation of magnetic cobalt ferrite nanoparticles (CF-MNPs 3) (by a hydrothermal process). As-prepared CF-MNPs 3 was well characterized by XRD, TEM, BET, and SEM spectroscopic techniques. TEM analysis showed formation of spherical particles in sizes 2–30 nm. The CF-MNPs 3 was used as magnetically recoverable catalyst for the oxidation of alcohols (0.5 mmol) to the corresponding aldehydes by periodic acid (1 mmol) as oxidant [67]. The reactions were performed in water at room temperature. A wide variety of benzylic and cyclic alcohols were oxidized to the target carbonyl products in high yields with tolerance of different functional groups (Scheme 4). The CF-MNPs 3 can be recovered and reused five times without significant decrease in reactivity.
Catalytic oxidation of alcohols by H5IO6 in the presence of CF-MNPs 3.
Very recently, Shaabani et al. reported the fabrication of Cr and Zn-substituted cobalt ferrite nanoparticles supported on guanidine-grafted graphene oxide nanosheets, Zn-CoFe2O4@-G-GO (CF-MNPs 4), and Zn-CoFe2O4@-G-GO (CF-MNPs 5) as magnetically recoverable catalysts for the oxidation of alcohols. As-prepared catalysts were well confirmed by a series of characterization techniques such as FT-IR spectroscopy, XRD, UV-Vis, TGA, TEM, SEM, EDX, and BET spectroscopic techniques [68]. Aerobic oxidation of alcohols (1 mmol) was carried out in o-xylene at 80°C. All results are listed in Table 1. The collected results in Table 1 revealed that both CF-MNPs 4 and CF-MNPs 5 (0.12 g) are efficient catalysts for the oxidation of alcohols to the corresponding carbonyl compounds. The catalysts could be reused five times without obvious decrease of the catalytic activity.
CF-MNPs 4 and CF-MNPs 5 catalyzed the oxidation of alcohols to the carbonyl compounds.
| Entry | Alcohol | CF-MNPs 4 | CF-MNPs 5 | ||
|---|---|---|---|---|---|
| Time (min) | Conversion (%) | Time (min) | Conversion (%) | ||
| 1 |  | 120 | 93 | 150 | 92 |
| 2 |  | 120 | 98 | 150 | 93 |
| 3 |  | 120 | 96 | 150 | 92 |
| 4 |  | 120 | 99 | 150 | 97 |
| 5 |  | 120 | 81 | 150 | 76 |
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the oxidation of alcohols were compared with the previously reported procedures in the literature (Table 2). As shown in Table 2, CF-MNPs catalysts provide a favorable catalytic performance for the oxidation of alcohols compared to other catalysts (in terms of product yield, reusability, reaction time, and reaction conditions).
Comparison of the activity of CF-MNPs catalysts with other reported catalysts in oxidation of alcohols.
| Entry | Catalyst | Examples | Recovery | Time | Yield (%) |
|---|---|---|---|---|---|
| 1 | CF-MNPs 1 | 19 | 6 | 15–240 min | 70–99 [65] |
| 2 | CF-MNPs 2 | 11 | 4 | 420 min | 97–99 [66] |
| 3 | CF-MNPs 3 | 11 | 6 | 50–90 min | 84–98 [67] |
| 4 | CF-MNPs 4 | 5 | 5 | 120 min | 81–99 [68] |
| 5 | CF-MNPs 5 | 5 | 5 | 150 min | 76–97 [68] |
| 6 | PW-MOF | 26 | 4 | 120–420 min | 14–98 [69] |
| 7 | Au/TiO2 | 6 | No | 24–72 h | 60–95 [70] |
| 8 | Fe(NO3)3·9H2O/ABNO | 26 | No | 2–12 h | 47–99 [71] |
| 9 | [RuH(CO)Cl(PPh3)3] | 41 | No | 18–24 h | 40–96 [72] |
2.1.2 Hydrocarbon oxidation
Oxidation of hydrocarbons with molecular oxygen is an important transformation in chemistry science especially in chemical industry [73], [74]. In 2009, Xia and his research team reported that magnetic CoFe2O4 nanocrystal (CF-MNPs 6) is an active heterogeneous catalyst for the aerobic oxidation of cyclohexane (37.3 mmol). The structure of CF-MNPs 6 was characterized by FT-IR, XRD, and TEM spectroscopic techniques. The TEM analysis of magnetic CoFe2O4 nanocrystal shows that the catalyst is formed of nanometer-sized particles (25–35 nm). After comprehensive experiments, aerobic oxidation under solvent-free conditions was considered as the standardized condition. As shown in Scheme 5, when pure CoFe2O4 was applied as catalyst (5 mg), 16.2% of cyclohexane conversion and 92.4% of selectivity for cyclohexanone and cyclohexanol were observed after 6 h. The catalyst can be readily recovered by an external magnet, and no significant loss of efficiency was observed when reused five successive times. CF-MNPs 6 was proved to be also efficient for oxidation of linear alkanes [75]. Collected results are listed in Table 3.
Oxidation of cyclohexane catalyzed by CF-MNPs 6.
Oxidation of linear alkanes catalyzed by CF-MNPs 6.
| No | Substrate | Conversion (%) | Selectivity (%) | ||
|---|---|---|---|---|---|
| 1 | Pentane | 3.1 | 2-Pentanone (56.3) | – | 3-Pentanone (43.7) |
| 2 | Hexane | 4.9 | 2-Hexanone (28.5) | – | 3-Hexanone (71.5) |
| 3 | Heptane | 5.7 | 2-Heptanone (62.4) | – | 4-Heptanone (37.6) |
| 4 | Octane | 9.1 | 2-Octanone (23.8) | 3-Octanone (70.0) | 4-Octanone (6.2) |
The selective oxidation of arenes to the corresponding aldehydes and ketones is one of the most important and useful transformations in organic synthesis, which has numerous applications in pharmaceutical, dyestuff, perfumery, and agrochemical industries [76], [77], [78]. In 2012, Kooti and Afshari reported the immobilization of a Schiff base molybdenum complex on silica-coated cobalt ferrite nanoparticles (CF-MNPs 7) as a novel heterogeneous catalyst for the oxidation of alkenes. Stepwise preparation of the CF-MNPs 7 catalyst is shown in Scheme 6. Silica-coated cobalt ferrite nanoparticles were fabricated and functionalized with Schiff base groups to yield immobilized bidentate ligands. The functionalized MNPs were then reacted with Mo(O2)2(acac)2, resulting in the novel immobilized molybdenum Schiff base catalyst [79]. The as-fabricated CF-MNPs 7 was characterized by FT-IR, SEM, TEM, TGA, XRD, and VSM spectroscopic techniques. The SEM and TEM images of Mo-salenSi@Si-CoFe2O4(CF-MNPs 7) show that the catalyst is formed of nanometer-sized particles (20–30 nm). The catalytic activity of CF-MNPs 7 (50 mg) was then tested in the oxidation of various alkenes (1 mmol) using 30% t-BuOOH as oxidant. The oxidation reactions are performed in 1,2-dichloroethane (5 ml) at 70°C. Collected results are listed in Table 4. As shown in Table 4, a series of alkenes and non-activated terminal olefin (1-octene) could be efficiently transformed to the corresponding products in admirable yields. The CF-MNPs 7 was also readily recovered and recycled (five times) with almost consistent activity.
Stepwise preparation of Schiff base molybdenum complex immobilized on silica-coated cobalt ferrite nanoparticles (CF-MNPs 7).
Oxidation of some alkenes by t-BuOOH catalyzed by CF-MNPs 7.
| No | Alkenes | Product | Conversion (%) | Selectivity (%) | Time (h) |
|---|---|---|---|---|---|
| 1 |  |  | 95 | 85 | 8 |
| 2 |  |  | 100 | 90 | 8 |
| 3 |  |  | 100 | 85 | 8 |
| 4 |  |  | 100 | 90 | 8 |
| 5 |  |  | 70 | 65 | 8 |
| 6 |  |  | 100 | 95 | 6 |
| 7 |  |  | 68 | 100 | 6 |
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the oxidation of hydrocarbons were compared with the previously reported procedures in the literature. Results are listed in Table 5.
2.2 Coupling reactions
Carbon-carbon and heteroatom (oxygen or sulfur) couplings are some of the most common and important reactions in organic synthetic chemistry [82], [83].
2.2.1 Carbon-carbon bond
Carbon-carbon cross-coupling is one of the most versatile reactions deployed in modern organic syntheses because it leads to the production of industrially, biologically, and pharmaceutically significant products [84]. Suzuki cross-coupling reaction, also called Suzuki-Miyaura reaction, is one type of carbon-carbon coupling of aryl halides with arylboronic acids to produce asymmetric biaryls [85]. In 2012, Phukan and his research team reported the fabrication of magnetically recoverable palladium catalyst through the direct addition of Pd nanoparticles during synthesis of cobalt ferrite nanoparticles. VSM, EDS, SEM, TEM, BET, and UV-vis spectroscopic techniques were utilized for the characterization of palladium catalyst (CF-MNPs 8) [86]. The catalytic activity of the CF-MNPs 8 (2 mg) was evaluated in the Suzuki coupling of various aryl bromides or iodides (0.5 mmol) with arylboronic acids (0.75 mmol) in refluxing ethanol and in the presence of Na2CO3 (1.25 mmol) as base. Various bases and solvents were tested to obtain the optimized conditions. As depicted in Scheme 7, symmetrical and unsymmetrical biaryl derivatives were prepared with reasonable yields. Also, Pd catalyst could be reused for four times without any noticeable loss of catalytic activity.
CF-MNPs 8 catalyzed Suzuki coupling of aryl halides with arylboronic acids.
2.2.2 Carbon-oxygen bond
Carbon-oxygen bond formation is arguably one of the most important coupling reactions in organic synthesis, because compounds containing C-O bonds (namely ethers) are significant solvents and synthetic important supplements for the production of fragrances, cosmetics, pharmaceuticals and dyestuffs [87], [88], [89]. In a nice publication, Matloubi-Moghaddam and coworkers found that the C-O coupling reaction between various types of aryl halides (1 mmol) and phenol derivatives (1.1 mmol) can be successfully catalyzed by cobalt ferrite magnetic nanoparticles (CF-MNPs 9). Unfortunately, the characterization of cobalt ferrite catalyst was not reported. Amongst tested bases and solvents, K2CO3 (1.2 mmol) and DMF were found to be the best conditions for further investigation [90]. As shown in Scheme 8, the C-O coupling reactions smoothly took place under the standardized conditions, and the ether products were obtained in venerable yields. It is noteworthy that aryl chlorides effectively participated in C-O coupling reactions, and the coupled products were afforded in high yields. Recycling of the CF-MNPs 9 was also checked in the coupling of bromobenzene and phenol, and only a slight loss of activity was observed after using eight times.
CF-MNPs 9 catalyzed C-O coupling reaction of various aryl halides and phenols.
2.2.3 Carbon-sulfur bond
The syntheses of organosulfur compounds (compounds containing carbon-sulfur bonds) have aroused significant interest in the last decade as a result of their presence in natural products and pharmacologically and biologically active molecules [13], [91], [92], [93], [94]. In this category (organosulfur compounds), sulfides occupy a very special place in organosulfur chemistry [13]. The C-S cross-coupling reaction of aryl halides with thiols is an efficient strategy for the preparation of sulfides [13]. In 2014, Zhou and his research team reported C-S coupling of 5-methyl-1,3,4-thiadiazol-2-thiol (0.2 mmol) with iodobenzene (0.24 mmol) catalyzed by CuI (0.01 mmol)/CF-MNPs 10 (0.02 mmol) system under K2CO3 (0.26 mmol) in DMF at 120°C [95]. The sulfide product was afforded in 83% yield after 12 h.
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the C-C and C-O coupling reactions were compared with the previously reported procedures in the literature. Results are listed in Table 6.
Comparison of the activity of CF-MNPs catalysts with other reported catalysts in C-C (entries 1–3) and C-O (entries 4–6) coupling reactions.
| Entry | Catalyst | Examples | Recovery | Time | Yield (%) |
|---|---|---|---|---|---|
| 1 | CF-MNPs 8 | 12 | 4 | 6–16 h | 70–92 [86] |
| 2 | Ni(dppb)Cl2 | 25 | No | 7 h | 35–82 [96] |
| 3 | Palladium | 10 | No | 1–24 h | 62–99 [97] |
| 4 | CF-MNPs 9 | 25 | 8 | 1–7.5 h | 80–96 [90] |
| 5 | CuI-TP-POP | 18 | 5 | 5–24 h | 55–93 [98] |
| 6 | Pd2dba3 | 13 | No | 20 h | 29–91 [99] |
2.3 Heterocycles synthesis
Heterocyclic chemistry is one of the most important and valuable branches in chemistry science [6]. Heterocycles are present in a broad library of drugs, most vitamins, many natural products, and biologically and industrially active molecules [100].
2.3.1 Five-membered heterocycles
2.3.1.1 Pyrroles
Compounds containing pyrrole ring system are an important class of biologically and pharmaceutically active molecules; pyrrole derivatives possess many pharmacological properties and can play important roles in biochemical processes [101], [102]. In 2014, Zhang and his research team designed a novel magnetically recoverable molybdenum catalyst via immobilization of molybdenum complex (Mo(acac)2) on the surface of silica-coated cobalt ferrite nanoparticles (CoFe2O4@SiO2) functionalized with 3-aminopropyltriethoxysilane under refluxing methanol conditions (Scheme 9). As-prepared CoFe2O4@SiO2-NH2-Mo(acac)2 (CF-MNPs 11) was well characterized by FT-IR, EDX, XRD, SEM, TEM, TGA, and VSM analysis [103]. The activity of CF-MNPs 11 (1 mol%) was evaluated in one-pot, four-component synthesis of functionalized pyrrole derivatives. As shown in Scheme 10, four-component reaction of aldehydes (1 mmol), amines (1 mmol), 1,3-dicarbonyl compounds (1 mmol), and nitromethane (0.5 ml) have been conducted under thermal solvent-free conditions, and a nice category of polysubstituted pyrrole products were afforded in moderate to excellent yields. The CF-MNPs 11 can be recycled five times with a negligible decrease in activity.
General route for the synthesis of magnetic CoFe2O4@SiO2 nanoparticle supported molybdenum (CF-MNPs 10).
CF-MNPs 10 catalyzed four-component and solvent-free synthesis of polysubstituted pyrrole derivatives.
In another report, Zhang and co-workers [104] have reported a new strategy to immobilize antimony (III) complex on surface of magnetic cobalt ferrite nanoparticles leading to a magnetically recoverable catalyst (CF-MNPs 12), which exhibits high catalytic efficiency in the synthesis of multisubstituted pyrroles. The general route involves the preparation of DABCO-functionalized silica-coated magnetic cobalt ferrite nanoparticles (CoFe2O4@SiO2-DABCO) upon reaction of DABCO ligand with magnetic CoFe2O4@SiO2 nanoparticles functionalized with 3-chloropropyltrimethoxysilane. Next, the reaction of CoFe2O4@SiO2-DABCO with antimony trichloride in refluxing acetone provided the final catalyst (CoFe2O4@SiO2-DABCO-Sb), as directed in Scheme 11. As-synthesized CF-MNPs 12 was subjected to characterize by several spectrum techniques such as FT-IR, EDX, XRD, SEM, and TEM spectroscopic techniques. Catalyst concentration and solvent type play a key role in the optimization of the reaction conditions. As shown in Scheme 12, CF-MNPs 12 (0.5 mol%) catalyzed the three-component reaction between amines (1 mmol), nitroolefins (1 mmol), and 1,3-dicarbonyl compounds (1 mmol) under refluxing ethanol, and a broad spectrum of N-protected functionalized pyrroles (48 examples) afforded in good to excellent yields. Reported results indicated the CF-MNPs 12 could be recycled for five runs without any loss of activity.
General route for the synthesis of magnetic CoFe2O4@SiO2 nanoparticle supported antimony (III) (CF-MNPs 12).
CF-MNPs 12 catalyzed three-component synthesis of multisubstituted pyrroles.
2.3.1.2 Imidazols and oxazoles
Imidazo[1,2-a]pyridines are an important class of N-heterocyclic compounds from the points of view of biological and pharmacological activities [105]. In this respect, a magnetically copper complex supported on magnetic CoFe2O4 CNT (CoFe2O4/CNT-C) was fabricated, by Zhang and coworkers [59], and evaluated as a recoverable heterogeneous catalyst (CF-MNPs 13) for the synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines via one-pot three-component reaction of 2-aminopyridines (1 mmol), aldehydes (1 mmol), and nitromethane (1.1 mmol) in thermal PEG 400 under aerobic conditions. As shown in Scheme 13, the addition of CuCl complex to a suspension of magnetic CoFe2O4 CNT in water led to the generation of the target catalyst (CF-MNPs 13). As-synthesized CF-MNPs 13 was characterized by several spectroscopic techniques such as FT-IR, VSM, XRD, SEM, TEM, and EDX. The reaction failed in the absence of the catalyst. Several solvents such as PEG 400, H2O, EtOH, H2O/EtOH, DMF, CH3CN, CH2Cl2, toluene, and MeOH were tested, and water was found to be the best solvent. Under the described conditions, a nice collection of 3-nitro-2-arylimidazo[1,2-a]pyridines were synthesized in high yields (Scheme 14). The recovered catalyst was able to be recycled at least eight times without loss of activity.
General route for the preparation of magnetic CoFe2O4 carbon nanotube supported Cu catalyst (CF-MNPs 13).
![Scheme 14: CF-MNPs 13 catalyzed three-component synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines.](/document/doi/10.1515/ntrev-2017-0138/asset/graphic/j_ntrev-2017-0138_scheme_014.jpg)
CF-MNPs 13 catalyzed three-component synthesis of 3-nitro-2-arylimidazo[1,2-a]pyridines.
Oxazoles and their derivatives have been recognized as key building blocks or synthetic intermediates for pharmaceutical and biological products [106], [107]. In a nice publication, Hajipour and Khorsandi reported the synthesis of benzimidazoles and benzoxazoles through intramolecular C-O and C-N cross coupling reactions catalyzed by magnetic cobalt ferrite nanoparticles (CF-MNPs 14). The structure of as-prepared cobalt ferrite (CF-MNPs 14) was analyzed by FT-IR, SEM, TEM, EDX, ICP, and XRD spectroscopic techniques [108]. To reach the standardized conditions, a number of solvents and bases were tested. It has been found that EtOH and K2CO3 (1.5 mmol) were the most efficient solvent and base, respectively. Under the standardized conditions, a library of benzimidazole and benzoxazole derivatives were prepared in moderate to excellent yields, as depicted in Schemes 15 and 16 . The CF-MNPs 14 could be easily recovered using an external magnet and reused several runs without significant loss of catalytic activity.
CF-MNPs 14 catalyzed synthesis of benzimidazoles.
CF-MNPs 14 catalyzed synthesis of benzoxazoles.
2.3.1.3 Thiazolidinones
Thiazolidinone derivatives have been known to possess a library of biological activities, including antifungal, antimicrobial, and inflammatory activities [109], [110]. Thiazolidinones are generally afforded via the three-component condensation between aldehydes with primary amines and mercaptoacetic acids. More recently, Safaei-Ghomi and his research group found that amine-functionalized silica-coated cobalt ferrite nanoparticles (CoFe2O4@SiO2/PrNH2) could be used as a highly efficient and magnetically recoverable catalyst (CF-MNPs 15) for the synthesis of 1,3-thiazolidin-4-ones. As shown in Scheme 17, CoFe2O4@SiO2/PrNH2 nanoparticles was prepared via the heterogenization of 3-aminopropyltriethoxysilane on silica-coated cobalt ferrite nanoparticles. FT-IR, SEM, and XRD spectroscopic techniques were applied for the characterization of as-prepared CF-MNPs 15 [111]. Only a trace amount of the product was observed in the absence of a catalyst. A series of catalysts and solvents were examined to obtain the standardized conditions. A wide array of 1,3-thiazolidin-4-one derivatives were obtained in reasonable yields by reacting aldehydes (2 mmol) with anilines (2 mmol) and thioglycolic acid (2 mmol) in refluxing toluene for suitable times (120–140 min), as given in Scheme 18. The recovered catalyst could be reused eight times with only a slight decrease in activity.
General route for the synthesis of CoFe2O4@SiO2/PrNH2 nanoparticles (CF-MNPs 15).
CF-MNPs 15 catalyzed three-component synthesis of 1,3-thiazolidin-4-one derivatives.
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the synthesis of pyrroles, imidazols, oxazoles, and thiazolidinones were compared with the previously reported procedures in the literature (Table 7). As shown in Table 7, CF-MNPs catalysts provide a favorable catalytic performance for the synthesis of pyrroles, imidazols, oxazoles, and thiazolidinones compared to other catalysts (in terms of product yield, reusability, reaction time, and reaction conditions).
Comparison of the activity of CF-MNPs catalysts with other reported catalysts in synthesis of pyrroles (entries 1–4), imidazols and oxazoles (entries 5–8), and thiazolidinones (entries 9–11).
| Entry | Catalyst | Examples | Recovery | Time | Yield (%) |
|---|---|---|---|---|---|
| 1 | CF-MNPs 11 | 43 | 5 | 0.5–48 h | 48–92 [103] |
| 2 | CF-MNPs 12 | 48 | 5 | 0.25–24 h | 47–92 [104] |
| 3 | Clay K10 | 15 | 3 | 5–8 h | 68–88 [112] |
| 4 | NiO NPs | 11 | 4 | 4 h | 28–86 [113] |
| 5 | CF-MNPs 13 | 20 | 8 | 3–6 h | 80–96 [59] |
| 6 | CF-MNPs 14 | 18 | 6 | 8–16 h | 22–82 [108] |
| 7 | CuI | 11 | No | 5 h | 34–75 [114] |
| 8 | [Pd(allyl)Cl]2 | 40 | No | 24 h | 46–88 [115] |
| 9 | CF-MNPs 15 | 15 | 8 | 2–2.4 h | 75–85 [111] |
| 10 | Bi(SCH2COOH)3 | 11 | No | 2 h | 70–90 [116] |
| 11 | FeNi3 | 9 | 3 | 1.5 h | 86–94 [117] |
2.3.2 Synthesis of six-membered heterocycles
2.3.2.1 Pyridines
Pyridines are one of the most important classes of nitrogenated heterocycles with promising applications in diverse areas, such as pharmaceuticals, dyes, agrochemical molecules, and luminescence materials [118], [119], [120]. In a nice publication, Zhang and his colleagues fabricated a novel organic inorganic hybrid heterogeneous catalyst by grafting chlorosulfuric acid on the surface of cobalt ferrite-graphene oxide that afforded magnetically recoverable graphene oxide-sulfonic acid (CoFe2O4-GO-SO3H) (CF-MNPs 16). Stepwise preparation of CF-MNPs 16 is designated in Scheme 19. First, graphene oxide (GO) was synthesized by oxidation of graphite powder according to a slightly modified Hummer’s method. The addition of chlorosulfonic acid into the CoFe2O4/GO suspension, formed from the reaction of graphene oxide with FeCl3, CoCl2 in the presence of ammonium hydroxide, in dichloromethane led to the preparation of CF-MNPs 16 [121]. As-fabricated CF-MNPs 16 was characterized by XRD, VSM, SEM, and TEM spectroscopic techniques. The activity of CF-MNPs 16 catalyst was then investigated in the synthesis of 3,6-di(pyridin-3-yl)-1H-pyrazolo[3,4-b]pyridine-5-carbonitriles. Poor and moderate yields were observed when the reaction was catalyzed by CoFe2O4, GO, and CoFe2O4-GO. Three-component reactions of 1-phenyl-3-(pyridine-3-yl)-1H-pyrazol-5-amine (1 mmol), 3-oxo-3-(pyridine-3yl)propanenitrile (1 mmol), and aldehydes (1 mmol) catalyzed by CF-MNPs 16 (50 mg) in thermal ChCl/glycerol under microwave irradiation led to the desired products with excellent yields in <15 min, as shown in Scheme 20. Recycling studies have indicated that the CF-MNPs 16 can be easily recovered and reused for eight cycles with sustained activity.
Stepwise preparation of magnetically separable graphene oxide anchored sulfonic acid (CF-MNPs 16).
![Scheme 20: CF-MNPs 16 catalyzed three-component synthesis of 3,6-di(pyridin-3-yl)-1H-pyrazolo[3,4-b]pyridine-5-carbonitriles.](/document/doi/10.1515/ntrev-2017-0138/asset/graphic/j_ntrev-2017-0138_scheme_020.jpg)
CF-MNPs 16 catalyzed three-component synthesis of 3,6-di(pyridin-3-yl)-1H-pyrazolo[3,4-b]pyridine-5-carbonitriles.
2.3.2.2 Pyrans
Pyran derivatives are an important class of oxygenated-heterocyclic compounds with category of biological and pharmacological activities like anti-viral, antibacterial, and anti-inflammatory activities [6], [122], [123]. A wide variety of 4-phenyl-4H-pyrano[3,2-h]quinolin-2-amine and 2-amino-4-phenyl-4H-pyrano[3,2-h] quinoline-3-carbonitrile derivatives was synthesized, by Sanasi and his research team, through three-component reaction of aromatic aldehydes (10 mmol), acetonitrile/malononitrile (10 mmol), and 8-hydoxyquinoline (10 mmol) catalyzed by magnetic nanocobalt ferrite (CF-MNPs 17) in EtOH under microwave irradiation (Scheme 21). The CF-MNPs 17 was prepared by sol-gel citrate precursor route and characterized by FT-IR, XRD, SEM, and TEM spectroscopic techniques [124]. Catalyst concentration was studied to reach the standardized conditions. The catalyst can be recovered with an external magnet and reused repeatedly (five runs without any significant loss of catalytic activity).
![Scheme 21: CF-MNPs 17 catalyzed the synthesis of 4-phenyl-4H-pyrano[3,2-h]quinolin-2-amine and 2-amino-4-phenyl-4H-pyrano[3,2-h] quinoline-3-carbonitrile derivatives.](/document/doi/10.1515/ntrev-2017-0138/asset/graphic/j_ntrev-2017-0138_scheme_021.jpg)
CF-MNPs 17 catalyzed the synthesis of 4-phenyl-4H-pyrano[3,2-h]quinolin-2-amine and 2-amino-4-phenyl-4H-pyrano[3,2-h] quinoline-3-carbonitrile derivatives.
In another report, Rajput and Kaur reported the synthesis of 2-amino-4-(phenyl)-5,6,7,8-tetrahydro-7, 7-dim ethyl-5-oxo-4H-pyrano-3-carbonitrile derivatives via cobalt ferrite nanoparticles (CF-MNPs 18)-catalyzed three-component reaction of aldehydes (2.5 mmol), 1,3-cyclic diketones (2.5 mmol), and active methylene compounds (malonitrile and ethyl acetoacetate) (2.5 mmol) under aqueous ethanol medium. As-prepared cobalt ferrite was characterized by XRD, VSM, and TEM spectroscopic techniques [125]. Poor yield was observed in the absence of a catalyst. Moderate yields were obtained when the reaction was catalyzed by Fe3O4 and ZnO. By the catalytic system, a diverse range of the desired products was prepared in good to excellent yields and short times (in <20 min), as shown in Scheme 22. The CF-MNPs 18 could be recycled 10 times by magnetic separation without any decrease in the catalytic activity.
CF-MNPs 18 catalyzed the synthesis of 2-amino-4-(phenyl)-5,6,7,8-tetrahydro-7, 7-dim ethyl-5-oxo-4H-pyrano-3-carbonitrile derivatives.
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the synthesis of pyridines and pyrans were compared with the previously reported procedures in the literature (Table 8). As shown in Table 8, CF-MNPs catalysts provide a favorable catalytic performance for the synthesis of pyridines and pyrans compared to other catalysts (in terms of product yield, reusability, reaction time, and reaction conditions).
Comparison of the activity of CF-MNPs catalysts with other reported catalysts in synthesis of pyridines (entries 1–3), and pyrans (entries 4–7).
| Entry | Catalyst | Examples | Recovery | Time | Yield (%) |
|---|---|---|---|---|---|
| 1 | CF-MNPs 16 | 15 | 8 | 10–15 min | 84–95 [121] |
| 2 | Chitosan | 8 | 5 | 10–30 min | 86–92 [126] |
| 3 | Fe3O(BPDC)3 | 9 | 5 | 420 min | 61–78 [127] |
| 4 | CF-MNPs 17 | 7 | 5 | 2 min | 86–92 [124] |
| 5 | CF-MNPs 18 | 28 | 10 | 3–20 min | 60–96 [125] |
| 6 | DABCO | 10 | No | 30–40 min | 82–94 [128] |
| 7 | Et3N | 10 | No | 43–110 min | 69–94 [129] |
2.4 Other reactions
2.4.1 Ring-opening reaction of epoxides
The addition of nucleophiles to epoxides is one of the most significant and valuable asymmetric organic transformations in organic synthesis, which proposes numerous benefits in the fields of chemical and pharmaceutical industry [130], [131]. The ring-opening reaction of epoxides with H2O2 is an efficient strategy for preparing β-hydroxy hydroperoxides (HHPs) (well known as compounds that possess antimalarial activity) [132], [133]. In 2013, Zhang and his research team found that magnetic CoFe2O4 nanoparticles-supported phosphomolybdate ([CoFe2O4@SiO2-PrNH2-PMo]) is an efficient, green, and recoverable heterogeneous catalyst (CF-MNPs 19) for ring-opening of various epoxides (1 mmol) with ethereal hydrogen peroxide (5 ml). As shown in Scheme 23, the CoFe2O4@SiO2-PrNH2-PMo catalyst was prepared by anchoring 3-aminopropyltrimethoxysilane on magnetic CoFe2O4 nanoparticles, followed the reaction with phosphomolybdic acid in dry tetrahydrofuran under ultrasonic conditions. The catalyst characterization was performed by a series of techniques such as FT-IR, SEM, TEM, EDS, XRD, VSM, and ICP-MS analysis. The TEM analysis confirmed this topic that CoFe2O4@SiO2-PrNH2-PMo NPs have a core-shell structure and good dispersity [134]. The catalytic activity of CF-MNPs 19 was evaluated in the ring-opening of epoxides with ethereal hydrogen peroxide (H2O2). In the absence of catalyst, only a trace amount of the product was observed even after 10 h. Under the standardized conditions (Scheme 24), the ring-opening reaction of epoxides with H2O2 was performed under mild conditions to give HHPs in respectable yields at ambient temperature [134]. The recovered catalyst could be reused at least eight times with only a slight decrease in activity.
Stepwise preparation of magnetic nanoparticles (CoFe2O4)-supported phosphomolybdate (CF-MNPs 19).
CF-MNPs 19 catalyzed the ring-opening reaction of epoxides with H2O2.
2.4.2 Knoevenagel condensation
Knoevenagel condensation is a well-known reaction in organic synthetic chemistry because the Knoevenagel products, namely, the α,β-unsaturated products or synthesized alkenes, play a key role in the synthesis of fine chemical, therapeutic drugs and heterocyclic compounds of biological importance [135], [136], [137]. In 2011, Phukan et al. have described a fascinating and efficient protocol for the Knoevenagel condensation using magnetically recoverable and reusable CoFe2O4 nanoparticles (CF-MNPs 20). The prepared CF-MNPs 20 was fully characterized by FT-IR, XRD, EDS, BET, TGA, TEM, VSM, and electron spin resonance spectroscopic techniques. The average size of the CF-MNPs 20 from the TEM analysis was found to be 40–50 nm which is consistent with the particle size obtained from XRD analysis [138]. The catalytic activity of CF-MNPs 20 was evaluated in Knoevenagel condensation of aldehydes (1 mmol) and ethylcyanoacetat (1 mmol). To identify the optimal conditions, the effect of catalyst loading and solvent nature was well studied. Under the standardized conditions, as shown in Scheme 25, a broad spectrum of aromatic aldehydes containing both electron-withdrawing and electron-donating groups reacted smoothly with ethylcyanoacetat under thermal water/ethanol, and the desired products were obtained in good to excellent yields (in <30 min). The CF-MNPs 20 can be magnetically recovered after the reaction and can be reused for four runs with only a minimal loss of activity (first run with 95% yield and fourth run with 88% yield).
CF-MNPs 20 catalyzed Knoevenagel condensation of aldehydes and ethylcyanoacetat.
2.4.3 Ritter reaction
Amides are key structural units in a variety of natural compounds and pharmacologically active molecules and display a broad range of biological activity [139], [140], [141]. The Ritter reaction is a commonly used strategy for the preparation of amides [139]. In 2014, Zhang and his colleagues evaluated the catalytic behavior of magnetic CoFe2O4 nanoparticle immobilized diamine-N-sulfamic acid (CoFe2O4@SiO2–DASA) in the synthesis of amides via the Ritter reaction under solvent-free conditions. Stepwise preparation of the CoFe2O4@SiO2-DASA is depicted in Scheme 26. CoFe2O4@SiO2-DA (CF-MNPs 21) was generated through the reaction of (N-(2-aminoethyl)-3-aminopropyl)tris-(2-ethoxy)silane with CoFe2O4@SiO2 in refluxing toluene. Amine groups tethered on the surface of CoFe2O4@SiO2 were treated with chlorosulfonic acid in CH2Cl2 to give the final catalyst (CoFe2O4@SiO2-DASA). The structure of CF-MNPs 21 was confirmed by a series of spectroscopic techniques such as FT-IR, XRD, SEM, TEM, and EDX spectroscopic techniques. EDX analysis confirmed the successful immobilization of sulfamic acid on CoFe2O4@SiO2 nanoparticles. The SEM and TEM images of CF-MNPs 21 show that the catalyst is formed of nanometer-sized particles (25–30 nm) [142]. Ritter reaction failed in the absence of a catalyst. The catalyst concentration was a key factor to achieve the optimal conditions. As shown in Scheme 27, a broad spectrum of structurally diverse alcohols (1 mmol) and nitriles (1 mmol) was subjected to the Ritter reaction, and the target amides were furnished in high yields. The CF-MNPs 21 could be easily recovered by applying an external magnet and recycled for six runs without significant loss of its catalytic performance.
General route for the preparation of magnetic CoFe2O4 nanoparticle immobilized diamine-N-sulfamic acid (CF-MNPs 21).
CF-MNPs 21 catalyzed the synthesis of amides via the Ritter reaction.
2.4.4 N-formylation
Formylation of amines is an important strategy of protecting a broad spectrum of amines and amino acids; the desired products, namely, formamides, are key intermediates in the synthesis of biologically and pharmaceutically active molecules [143], [144], [145]. In 2015, Kooti and Nasiri reported the preparation of the magnetic CoFe2O4@SiO2-PTA catalyst (CF-MNPs 22) by the wet impregnation method, dissolving the phosphotungstic acid (PTA) on silica-coated cobalt ferrite nanoparticles (PTA) in aqueous medium (Scheme 28). The resultant CF-MNPs 22 was characterized by a series of spectroscopic techniques, including FT-IR, XRD, SEM, EDX, ICP-AES, and VSM. SEM and TEM analyses revealed uniformly sized MNPs with spherical morphology [146]. The catalytic activity of CF-MNPs 22 was tested in the N-formylation of amines under solvent-free conditions at room temperature. As shown in Scheme 28, a wide range of aliphatic and aromatic amines was subjected to the N-formylation, and moderate to high yields of the desired formamides were obtained in <60 min. Easy recovery of the CF-MNPs 22 using an external magnet and high yields with up to five times make the protocol sustainable and economic.
Preparation and catalytic activity of CoFe2O4@SiO2-PTA (CF-MNPs 22) in the N-formylation of amines.
2.4.5 Synthesis of 5-hydroxymethylfurfural
Finally, surface modification of cobalt ferrite nanoparticles with dicarboxylic acids (CoFe2O4@Carboxilic acid) as a magnetically recoverable acidic catalyst (CF-MNPs 23) for the synthesis of 5-hydroxymethylfurfural (HMF) have been published by Ranganath and his research team. The structure of CoFe2O4@Carboxilic acid was characterized by FT-IR, XRD, XPS, SEM, and TEM. TEM analysis of the CF-MNPs 23 shows that the average size of the catalyst was 11 nm in diameter and 18 nm after recyclability [147]. As shown in Scheme 29, high conversion and good selectivity in the process was observed after 1 h. No significant loss of catalytic efficiency was consequently observed during the five consecutive reactions.
Preparation and catalytic activity of CoFe2O4@carboxilic acid (CF-MNPs 23) in the synthesis of 5-hydroxymethylfurfural (HMF).
In order to show the efficiency of CF-MNPs catalysts, the obtained results for the alcoholysis of epoxides, Knoevenagel condensation, Ritter reaction, N-formylation, and synthesis of HMF were compared with the previously reported procedures in the literature (Table 9). As shown in Table 9, the reaction time and product yield is better than other protocols reported in literature. Also, the CF-MNPs catalyst is comparable in terms of stability, non-toxicity, and easy separation.
Comparison of the activity of CF-MNPs catalysts with other reported catalysts in the alcoholysis of epoxides (entries 1–3), Knoevenagel condensation (entries 4–6), Ritter reaction (entries 7–9), N-formylation (entries 10–12), and synthesis of 5-hydroxymethylfurfural (entries 13–14).
| Entry | Catalyst | Examples | Recovery | Time | Yield (%) |
|---|---|---|---|---|---|
| 1 | CF-MNPs 19 | 14 | 8 | 15–30 min | 78–92 [134] |
| 2 | K5[CoW12O40]·3H2O | 35 | No | 15–30 min | 42–100 [148] |
| 3 | ZrO(NO3)2 nH2O | 11 | 3 | 20 min | 78–97 [149] |
| 4 | CF-MNPs 20 | 14 | 4 | 2–30 min | 68–96 [138] |
| 5 | NP-SnO2 | 16 | 7 | 1–3 h | 80–96 [150] |
| 6 | I2 | 16 | No | 2–90 min | 70–98 [151] |
| 7 | CF-MNPs 21 | 27 | 6 | 3–4 h | 79–96 [142] |
| 8 | TfOH | 35 | No | 2–4 h | 84–98 [152] |
| 9 | In(OTf)3 | 14 | No | 24 h | 48–97 [153] |
| 10 | CF-MNPs 22 | 14 | 5 | 30–60 min | 50–97 [146] |
| 11 | Iridium | 13 | No | 5–10 h | 41–95 [154] |
| 12 | MnO2 | 20 | No | 12 h | 27–93 [155] |
| 13 | CF-MNPs 23 | 1 | 5 | 1 h | 96 [147] |
| 14 | [AEMIM]BF4 | 1 | No | 8 h | 68 [156] |
3 Summary and outlook
The modern age of organic synthesis is changing towards the path of creative synthetic strategies which principally focus on environmental aspects of chemical or organic synthetic reactions such as the use of green materials and catalysts. Nowadays, magnetic separation has emerged as a robust, highly efficient, easy, and rapid separation technique for products and catalysts. Cobalt spinel ferrites (CoFe2O4) are attractive candidates due to their strong anisotropy, high coercivity, moderate saturation magnetization, and good mechanical and excellent chemical stabilities at higher temperature. The most important aspect in CoFe2O4 catalysis is the design and fabrication of a catalyst for specific organic reaction, considering the mechanistic pathway and the feasibility to perform reactions on a laboratory scale with potential for industrial applications [19]. A broad library of high-pressure and high-temperature organic reactions can be carried out on nanomagnetite supported catalysts, because they are highly stable [19]. Due to sturdy interaction between the support (MNPs) and metals, leaching of metal can be avoided or minimized [19]. This review has submitted a comprehensive overview on the applications of recoverable cobalt ferrites nanocatalysts in a nice category of catalytic processes: oxidation, coupling, condensation, reduction, and multicomponent reactions. Amino-functionalized cobalt ferrite nanoparticles are the best support for the immobilization of metals and organocatalysts. A variety was first modified with amine ligands and was then reacted with metal complexes or acids to give the target nanocatalysts. These cobalt ferrite nanocatalysts possess a series of admirable advantages such as simple preparation, simplicity of operation, high catalytic activity, easy separation, and reusability. Accordingly, future attempts for more efficient protocols will still focus on the stability, sustainability, and environmental impacts due to the growing needs of the industry. These efforts enable a wide variety of industrial applications for cobalt ferrites nanocatalysts in the future.
About the authors
Mosstafa Kazemi was born in Ilam, Iran. He has received his MS degree in organic chemistry from Ilam University in 2013. He is presently preparing his PhD in the Nanosciences and Catalysis research group of Assistant Prof. Lotfi Shiriat at the Ilam University, where he is working on the synthesis and applications of magnetic nanoparticles-supported nanocatalysts in organic synthesis.
Massoud Ghobadi was born in Ilam, Iran. He has received his MS degree in inorganic chemistry from Ilam University in 2012. Currently, he is working towards his PhD under supervision of Assoc. Prof. Mohsen Nikoorazm at the Department of Chemistry of the Ilam University. His current interests are focused on the development of new strategies for the fabrication of heterogeneous nanocatalysts and their application in chemical reactions.
Ali Mirzaie was born in Ilam, Iran. He has received his MS degree in organic chemistry from Payame Noor University of Zanjan in 2013. His current interests are focused on the development of new strategies for the fabrication of heterogeneous nanocatalysts and their application in chemical reactions.
Acknowledgments
This work was supported by the research facilities of Islamic Azad University of Ilam, Iran.
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