The sulfoxidation of sulfides have received special attention in organic synthesis especially in medical chemistry because compounds containing S=O bonds (sulfoxides) are privileged structural scaffolds for building pharmacologically and biologically active molecules. Magnetic separation is an efficient strategy for the rapid separation of catalysts from reaction medium and an alternative to time-, solvent-, and energy-consuming separation techniques. In recent times, many protocols based on using magnetically recoverable nano-catalysts have been reported for the oxidation of sulfides to the sulfoxides. This review is focused on metal complexes, acid, and bromine reagents supported on magnetic nanoparticles and their applications as magnetically recoverable nano-catalysts in the sulfoxidation reactions.
Catalysis research campaign is a hot research topic in modern organic synthesis. In this category, magnetic separation has received profound attention because catalysts immobilized on magnetic nanoparticles can be readily separated from reaction medium using an external magnet, without the need for filtration, centrifugation, or other tedious work-up processes , , , , . Furthermore, recent literature studies clearly have shown that magnetic nanoparticles possess several admirable advantages such as high surface area to bulk ratios, low toxicity, high activity and thermal stability, and the capability of surface modifications and easy dispersion , , , , , , . In fact, magnetic separable catalysts are a well-favored and fascinating strategy to bridge the split between heterogeneous and homogenous catalysis , . In describing the magnetic nanoparticles, it can be said that the magnetic separation is, in fact, an admirable and valuable victory in the research of chemistry catalysis. On the other hand, catalysis research under green solvents or solvent-free conditions is always a popular theme from the viewpoint of green synthesis . Considering these issues, in recent times, most of organic chemists focused on organic-synthesis catalysis based on using magnetically recoverable catalysts under benign mediums and mild conditions.
Organosulfur chemistry is always a fascinating research field in organic synthesis because sulfur-containing compounds are prevalent in a broad spectrum of active biological, pharmaceutical, and natural molecules , , . Sulfoxides are an important class of sulfur-containing compounds that received special attention in organic synthesis. The first report for the synthesis of sulfoxides was presented in 1856 by Marcker . Since then, studies on the sulfoxide applications and activities in various fields, especially medical chemistry, have noticeably increased, and a large number of methods have been reported for their preparation. Organic sulfoxides have been widely applied as a ligand and an oxotransfer reagent in asymmetric synthesis of organic compounds , , . They play also a vital role in the synthesis of natural products, valuable physiologically and pharmacologically active molecules . Furthermore, sulfoxide derivatives are also prevalent structural motifs in many drugs and biologically active molecules , . A nice category of valuable pharmaceutical and biological molecules containing S=O bonds, modafinil (1), adrafinil (2), CRL-40,941 or fladrafinil (3), fipronil (4), oxydemeton-methyl (5), omeprazole (6), pantoprazole (7), lansoprazole (8), and rabeprazole (9), is listed in Figure 1 , , , , , , . The oxidation of sulfides is the most straightforward strategy for the preparation of the sulfoxides . Sulfoxidation catalysis is a well-known and valuable reaction in organic synthesis . Up to now, a wide variety of catalysts have been reported for the oxidation of sulfides to the sulfoxides. However, here, a significant blind spot is glaring, namely, the recovery and reusability of the catalyst because catalysts are often expensive or toxic. The separation of a catalyst from the sulfoxide products or reaction mixture is a difficult, tedious, and time-consuming task and needs a series of costly and specific techniques. Therefore, the search for new recoverable catalysts is a real challenge in sulfoxidation catalysis. In recent times, a wide number of protocols based on using magnetically recoverable nano-catalysts have been reported for the oxidation of sulfides to sulfoxides. Here, in this review, we provided a scientific effort to list the magnetically recoverable nano-catalysts reported in the literature and their activity in sulfoxidation reactions.
2 Magnetically recoverable metallic catalysts
Catalysis research based on using transition metal complexes is a well-known research field in organic synthesis because transition metal complexes activate various sites in substrates where reactions can readily take place , . Over the last decade, the catalytic activity of transition metal complexes immobilized on solid supports (such as silica, alumina, zeolites, and mesoporous materials) in various chemical reactions were extensively studied by organic chemists , , . Although these protocols are valuable, there are several drawbacks such as tedious work-up routes, unsatisfactory yields, and difficult separation or recovery. Furthermore, in these catalytic strategies, fewer sites present on the surface are accessible for catalysis; less reactive and selective is the catalytic system, and sometimes, the support, itself, can act as the catalyst of side reactions , . Therefore, the development of new support materials and heterogenization strategies for organic synthesis is an important challenge in modern catalysis science. More recently, transition metal complexes immobilized on magnetic nanoparticles have emerged as efficient magnetically recoverable catalysts to overcome these drawbacks. In recent times, a wide number of the sulfoxidation reactions have been developed based on using transition metals complexes immobilized on magnetic nanoparticles as the catalyst.
3 Copper catalysts
Copper complexes are well-known and promising catalysts for oxidation reactions because copper is a less toxic, readily available, and inexpensive metal compared with other transition metals , . During the recent years, a number of magnetically recoverable copper catalysts have been investigated for the oxidation of sulfides to sulfoxides. Ghorbani-Choghamarani and his team reported the sulfoxidation of aliphatic and aromatic sulfides based on using Cu(II)-Schiff base complex-functionalized magnetic Fe3O4 nanoparticles (MNPs 1, Figure 2) as magnetically recoverable catalysts. The structure of MNPs 1 was characterized by FT-IR spectroscopy, TGA, and SEM . The SEM analysis showed that the MNPs 1 is prepared in nanometer-sized particles (70–80 nm). Sulfoxidation reaction failed in the absence of the catalyst (MNPs 1). Solvent-free sulfoxidation reactions were catalyzed by MNPs 1 (0.02 mmol) in the presence of H2O2 at ambient temperature (Scheme 1). By this catalytic system, a variety of sulfides can be successfully converted to the corresponding sulfoxides in excellent yields (in less than 180 min). Furthermore, the copper nano-catalyst was found to be highly stable with noticeable catalytic activities, even after 10 runs. In 2015, Ghorbani-Choghamarani and his co-workers developed a magnetically recoverable copper catalyst (MNPs 2, Figure 2) for the sulfoxidation reactions. The structure of the as-prepared catalyst was characterized by TG/DTG, FT-IR, TEM, VSM, ICP, AAS, XRD, EDS, and SEM spectroscopic techniques . Diameters of approximately 10–20 nm for the final MNPs 2 were observed in SEM and TEM images. Successful preparation of the MNPs 2 was well confirmed by EDS analysis. The MNPs 2 was then tested for the oxidation of sulfides to sulfoxides in the presence of H2O2 under thermal ethanol (Scheme 2). Under the described conditions, a variety of asymmetrical and unsymmetrical sulfides were tested, and the desired products were afforded in moderate to high yields. Interestingly, product separation was readily performed using an external magnet, and the recovered catalyst was reused for 12 runs without any notable loss in catalytic activity. In another study, the catalytic activity of chitosan-Schiff base complex of Cu(II) supported on supramagnetic Fe3O4 nanoparticles (MNPs 3, Figure 2) for the oxidation of sulfides was also evaluated by Naghipour and Fakhri. The as-synthesized MNPs 3was characterized by XRD, FT-IR, TGA, SEM, and EDX spectroscopic techniques . The sulfoxidation reaction failed when the process was performed in the absence of MNPs 3. To identify the best reaction medium, a series of solvents such as H2O, EtOH, CH3CN, CH2Cl2, and ethyl acetate was tested. However, the maximums yield of the desired sulfoxide was observed under solvent-free conditions. Under the optimized conditions, room temperature oxidation of a nice library of sulfides with 30% H2O2 leads to afford the corresponding sulfoxides in excellent yields (Scheme 3). This catalytic system was not efficient for the oxidation of diphenylsulfide because the desired product was obtained in unsatisfactory yield even after 48 h. Recycling studies have shown that the recovered catalyst can be reused for four cycles without significant loss of activity. Comparison FT-IR spectra of used MNPs 3 with the fresh MNPs 3 show that the structure of MNPs 3 remained intact after the successive four runs of recoveries.
The same group also reported the preparation of magnetically recoverable Fe3O4@chitoasn-based Cu complex (MNPs 4, Figure 2) for the catalytic oxidation of sulfides. The structure of the as-prepared catalyst was characterized by a series of spectroscopic techniques such as XRD, FT-IR, TGA, SEM, and EDX . TGA analysis also showed a high thermal stability of the magnetic nano-catalyst. The EDS analysis confirmed the successful immobilization of copper complex on Fe3O4@chitosan-bond-2-hydroxy-1-naphthaldehyde. The catalytic performance of the MNPs 4 for the oxidation of sulfides to sulfoxides has been evaluated in the presence of H2O2 as the oxidant. The obtained results showed that the target sulfoxides were obtained in excellent yields in less than 90 min. By this catalytic strategy, diphenylsulfoxide was also afforded satisfactory yields (Scheme 4). The catalyst was recycled by simple magnetic separation and reused for four runs without significant loss in efficiency.
More recently, Ghorbani-Choghamarani et al. reported the fabrication of Cu-S-(propyl)-2-aminobenzothioate immobilized on magnetic Fe3O4 nanoparticles (MNPs 5) for the catalytic sulfoxidation of sulfides. The as-fabricated catalyst has been characterized by TEM, SEM, EDS, AAS, ICP-OES, XRD, FT-IR, VSM, and TGA spectroscopic techniques . The SEM and TEM images of MNPs 5 show that the catalyst consisted of nanometer-sized particles (20 nm). The catalytic activity of MNPs 5 was then examined in the oxidation of sulfides to sulfoxides. Solvent-free sulfoxidation reactions in the presence of H2O2were catalyzed by MNPs 5 at room temperature. As shown in Scheme 5, the desired sulfoxides were obtained in 91–97 yields (in less than 190 min). The catalyst was quickly recovered from solution using handheld magnets. The authors also indicated that this catalyst was recovered and reused for at least five runs without any significant loss of activity.
A plausible reaction mechanism for the sulfoxidation reactions catalyzed by organometallic catalysts containing Cu(II) metal ions, based on published previous reports in literature , , is depicted in Scheme 6. The reaction of H2O2 with the Cu catalyst leads to intermediate A, which is converted to active oxidant B. In the next step, nucleophilic attack of the sulfide on this intermediate gives cation C, which produces the corresponding sulfoxide .
4 Vanadium catalysts
Vanadium catalysts are found to be efficient catalysts for the selective oxidation sulfides to the sulfoxides with oxidizing agents such as H2O2 and UHP. Most of the catalysts investigated for the sulfoxidation reactions are based on the chemistry of vanadium oxides. During recent years, several magnetically recoverable catalysts have been investigated for the sulfoxidation of sulfides. In 2012, Bagherzadeh and co-workers reported the preparation of a magnetically recyclable vanadium (V) catalyst through the covalent anchoring of VO(salen)Cl on silica-coated magnetic Fe3O4 nanoparticles (MNPs 6). The as-prepared catalyst was characterized by FT-IR, XRD, DLS spectroscopic techniques . The DLS analysis demonstrates that the Fe3O4@SiO2@VO(salen) catalyst has particles, which are exactly in the nanosize range. The catalytic potential of the MNPs 6 was evaluated in the sulfoxidation of sulfides in CH2Cl2/MeOH at room temperature using UHP as the oxidant. As shown in Scheme 7, the conversion of diphenyl sulfide, as a result of the electronic and steric effect of the aryl groups, was lower than the others. The catalyst can be recycled six times without a negligible decrease in activity.
Rostami and Atashkar reported that the oxidation of sulfides to the sulfoxides was successfully catalyzed by the chiral oxo-vanadium (+)-pseudoephedrine complex immobilized on magnetic Fe3O4 nanoparticles (MNPs 7). The as-fabricated catalyst has been characterized by UV-Vis spectrophotometer, SEM, XRD, TGA, FT-IR, EDX, and AGFM. The SEM image of the VO(Pseudoephedrine)@MNPs (MNPs 7) confirmed that the catalyst was made up of uniform nanometer-sized particles less than 33 nm . The catalytic performance of the MNPs 7 strongly depends on the amount of catalyst and H2O2. Also, a series of solvents were tested on the sulfoxidation reactions, but the best results were obtained in the absence of a solvent. Under the standardized conditions (Scheme 8), a nice library of alkyl aryl and diaryl sulfides was smoothly converted to the corresponding sulfoxides in good to excellent yields. The catalyst (MNPs 7) was easily recovered by an external magnetic decantation and reused for 20 reaction times without any significant loss of activity and enantioselectivity.
Ghorbani-Choghamarani et al. reported the fabrication of oxovanadium(IV)-glycine imine supported on magnetic Fe3O4 nanoparticles (MNPs 8). The structure of the as-synthesized MNPs 8 was well characterized by a series of spectrum techniques such as FT-IR, SEM, TEM, XRD, TGA, VSM, and ICP-OES. The SEM image of MNPs 8 confirmed the formation of nanoparticles with spherical shape and slight agglomeration. TEM analysis of MNPs 8 showed the formation of particles with spherical morphology and size varying in the range of 10–32 nm . MNPs 8 was used in the sulfoxidation reaction of alkylaryl and dialkyl sulfides by H2O2under solvent-free conditions (25–220 min); the corresponding products are furnished in excellent yields (90–99%) (Scheme 9). The MNPs 8 could be used for five successive times without any significant decrease in activity.
Very recently, Ghorbani-Choghamarani and Norouzi reported amine-functionalized magnetic Fe3O4 nanoparticles (MNPs 9) for the immobilization of oxovanadium(IV) complex. As-prepared MNPs 9 was characterized by FT-IR, TGA, XRD, SEM, and EDX spectroscopic techniques. The SEM micrograph of the MNPs 9 exhibits a relatively homogeneous grain distribution, which was made up of particles with the size in the nanosized range (8–20 nm) . Unfortunately, the exact structure of MNPs 9 was not reported in this paper. The catalytic behavior of the MNPs 9 has been evaluated in the sulfoxidation of sulfides. Solvent-free sulfoxidation reaction in the presence of MNPs 9 as the catalyst and H2O2 as the oxidant at ambient temperature was considered as the best conditions for further investigation. A fascinating category of aliphatic and aromatic sulfides was performed well under the described conditions, and the corresponding products were obtained in good to excellent yields in satisfactory times (Scheme 10). Recycling of the catalytic system was also investigated in the oxidation of methyl phenyl sulfide; only a slight loss of activity was observed after 12 times.
The proposed mechanism for this process involves the nucleophilic attack of sulfide on MNPs@VO (salen) complex to form intermediate A. The reaction of intermediate A with hydrogen peroxide affords the sulfoxide and regenerates the vanadium salen complex (Scheme 11) .
5 Manganese catalysts
Oxidation catalysis based on using manganese complexes is a fascinating research field in organic synthesis. A number of manganese complexes are well-known to catalyze the oxidation of sulfides to sulfoxides . In 2014, Ghorbanloo and his colleagues reported the oxidation of sulfides to sulfoxides based on using Mn(II) complex (complex: porphyrin) supported on (3-chloropropyl)-trimethoxysilane-functionalized silica-coated magnetic nanoparticles (MNPs 10, Figure 3) as a magnetically separable catalyst. The structure of the MNPs 10 was characterized by FT-IR, VSM, and SEM spectroscopic techniques . The SEM image of MNPs 10 shows that the catalyst is formed of nanometer-sized particles (63 nm). The oxidation reactions were carried out in the presence of H2O2 under thermal acetonitrile (Scheme 12). The highest yield was observed in the oxidation of methyl phenyl sulfide. The MNPs 10 was readily recovered by simple magnetic decantation and can be reused for four cycles without considerable loss in catalytic activity.
The utilization of imidazole-functionalized silica-coated magnetic Fe3O4 nanoparticles as support for the immobilization of manganese porphyrin (MNPs 11, Figure 3) was also reported by Bagherzadeh and Mortazavi-Manesh. A series of spectroscopic techniques such as FT-IR, XRD, and UV-Vis was used to analyze the MNPs 11. The SEM image of MNPs 11 confirmed that the particles were well distributed with dimensions about 33 nm . The catalytic activity of the MNPs 11 was checked in sulfoxidation reactions (Scheme 13). Good to high conversions of the sulfoxide products was observed at room temperature for 3 h (UHP as the oxidant and CH2Cl2/MeOH as reaction medium). The MNPs 11 was successively reused for about six runs without significant loss of efficiency.
Very recently, Bagherzadeh et al. prepared a Mn MNPs catalyst via the covalent anchoring of Mn(II)-substituted phosphotungstate on ammonium-modified silica-coated magnetic Fe3O4 nanoparticles. The resulting nanocomposite was characterized by a series of spectroscopic techniques such as FT-IR, XRD, SEM, and EDX. The EDX analysis confirmed the successful synthesis of Fe3O4@SiO2-MnPOWnanocomposite (MNPs 12). The SEM analysis indicates that the particles are approximately spherical with an average diameter of about 50 nm . The MNPs 12 was then tested as a heterogeneous catalyst for room temperature oxidation of sulfides in the presence of UHP as the oxidant under CH2Cl2/MeOH. The collected results are listed in Scheme 14. Under the described conditions, methyl-phenyl sulfide and diethyl sulfide were effectively oxidized to the desired sulfoxides. Leaching and recycling experiments showed that the MNPs 12 can be reused for at least four cycles without any notable loss of activity. Unfortunately, any proposed mechanism for the sulfoxidation reactions using MNPs-Mn (II) catalysts was not reported.
6 Other transition metal catalysts
During recent years, the catalytic activity of a number of other metal complexes immobilized on magnetic nanoparticles has been investigated for the oxidation of sulfides to the sulfoxides. In 2015, Bagherzadeh and his team reported the fabrication of two magnetically separable molybdenum catalysts via the immobilization of the molybdenum complex, [MoO2Cl2(DMSO)2], on amino propyl and Schiff base-modified magnetic Fe3O4@SiO2 nanoparticles by covalent linkage (MNPs 13 and 14, Figure 4). The characterization of the as-fabricated catalysts was accomplished by FT-IR, TGA, SEM, ICP/OES, VSM, TEM, EDX, and XPS spectroscopic techniques. The SEM and TEM images of MNPs 13 and 14 show that the catalyst is formed of nanometer-sized particles. The presence of molybdenum in the nanoparticle structure was confirmed by EDX analysis. The loading amount of molybdenum was about 0.7 mmol g−1, which is determined by ICP/OES analysis . The catalytic behavior of MNPs 13 and 14 was then explored in the oxidation of sulfides to sulfoxides. UHP and CH2Cl2/MeOH were used as the oxidant and solvent, respectively. As shown in Scheme 15, the best results in terms of conversion and selectivity were observed when MNPs 14 was used as the catalyst. The catalyst (MNPs 14) could be reused for four times without obvious decrease in the catalytic activity.
In another publication, Bezaatpour and his coworkers reported the immobilization of a molybdenum N4-type Schiff base complex on magnetic Fe3O4 nanoparticles (MNPs 15) as a novel heterogeneous catalyst for the oxidation of sulfides. Stepwise preparation of the Mo-salenSi@Si-CoFe2O4 catalyst is shown in Scheme 5. The structure of MNPs 15 was characterized fully by FT-IE, SEM, TEM, DRS, EDX, XRD, XRD, and VSM spectroscopic techniques. The SEM and TEM images of MNPs 15 show that the catalyst is formed of nanometer-sized particles (17–20 nm) . The catalytic activity of MNPs 15 was then tested in the oxidation of various sulfides using H2O2 as the oxidant. The oxidation reactions are performed in the absence of a solvent at 55°C. As shown in Scheme 16, a wide range of sulfides could be efficiently transformed to the corresponding sulfoxides with high chemoselectivity (in less than 5 min). The catalyst could be recycled six times by magnetic separation without any decrease in the catalytic activity.
In 2014, Hashemi’s research team reported the preparation of iron(II) acetylacetonate immobilized on amine-modified magnetic Fe3O4@SiO2 nanoparticles (MNPs 16) as an efficient and recyclable heterogeneous catalyst for the selective oxidation of sulfides to the corresponding sulfoxides using H2O2 as a green oxidant at room temperature. The as-prepared catalyst was completely characterized by spectroscopic techniques of TEM, SEM, XRD, EDS, FTIR, TGA, ICP-AES, VSM, and elemental analysis (CHN). The presence of Fe complex in the nanocomposite structure was confirmed by EDX analysis. The Fe content grafted on the Fe3O4@SiO2-amine was measured by plasma atomic emission analysis (ICP-AES), about 0.11 mmol g−1 . The MNPs 16 enabled to catalyze the sulfoxidation of symmetrical and unsymmetrical sulfides with high yield and excellent selectivity in ethanol (Scheme 17). The MNPs 16 can be readily recovered and reused for eight reaction runs without detectable loss of efficiency. Based on suggested mechanism (Scheme 18), the presence of Fe(IV)=O complex (high-valent iron oxo complex) is essential in oxidation reactions. First, the reaction of Fe(II) complex (A) with H2O2 as oxidizing agent, which leads to the formation of intermediate B. Then, the O–O bond homolysis/heterolysis of intermediate B results in the formation of intermediate C, which is involved in the oxygen atom transfer reaction for the oxidation of sulfides to sulfoxides .
In another report, Zohreh and co-workers successfully synthesized a heterogeneous tungstate-based catalyst via the immobilization of high amounts of WO4−2 onto the cross-linked poly(ammonium ethyl acrylamide)-coated magnetic Fe3O4 nanoparticles (MNPs 17). FT-IR, TEM, TGA, VSM, XRD, EDX, and CHN spectroscopic techniques were used for the characterization of the MNPs 17. RDP analysis of MNPs 17 also confirmed the crystalline plans of magnetic Fe3O4 nanoparticles. The data obtained by ICP-OES showed that the loading amount of WO42− ions is 0.89 mmol g−1 . The as-synthesized catalyst exhibited high activity in the oxidation of sulfides to sulfoxides. A variety of sulfides were subjected to sulfoxidation reactions in CH3CN/H2O at room temperature (H2O2 used as oxidant). The sulfoxide products were obtained in good to excellent yields (Scheme 19). The MNPs 17 could be readily recovered by magnetic separation six times without loss of catalytic activity.
Later, Ghorbani-Choghamarani and co-workers reported that M-Salen complexes (M=Ni, Co, Cr, Zn or Cd) supported on magnetic Fe3O4 nanoparticles (MNPs 18–22) are efficient recoverable catalysts for the oxidation of sulfides to the sulfoxides. The as-prepared catalysts were characterized by FT-IR, TGA, XRD, SEM, and EDX spectroscopic techniques. The EDX analysis confirmed the successful immobilization of M-Salen complexes on magnetic Fe3O4 nanoparticles. The size of the catalysts was evaluated using scanning electron microscopy; most of the particles formed were nanometer-sized with an average diameter about 15 nm . The application of these catalysts (MNPs 18–22) was tested in the solvent-free sulfoxidation of sulfides by H2O2. The collected results are listed in Scheme 20. The obtained results revealed clearly that all the nano-catalysts are very efficient in these sulfoxidation processes because, generally, all the target sulfoxide products were obtained in both excellent yields (up to 99%) and suitable times (in less than 120 min). The MNPs catalysts were magnetically recovered and reused for 10 cycles without noticeable loss of catalytic activity. Also, the chemo-selective oxidative coupling of thiols to the disulfides in excellent yields could be successfully performed by this catalytic system.
Very recently, Hajjami and Kolivand immobilized copper and zirconium oxide complexes on imine-bonded magnetic Fe3O4 nanoparticles (MNPs 23–24) and their activity as magnetically separable catalysts evaluated in the oxidation of sulfides to sulfoxides. The as-prepared catalysts were well confirmed by a series of spectroscopic techniques such as XRD, FT-IR spectroscopy, TGA, TEM, SEM, EDX, and VSM . The TEM images disclose the spherical shapes of the MNP catalysts with an average size of 12 nm, which exhibits a close agreement with the values calculated from the XRD analysis. Optimized results were obtained with 0.03 g of catalysts (MNPs 23–24) and use of H2O2 as an oxidant under thermal ethanol. All results are listed in Scheme 21. The collected results in Scheme 21 demonstrated well that both MNPs 23 and 24 are efficient catalysts for the oxidation of sulfides to sulfoxides. Furthermore, the catalysts can be reused many times without any loss in activity.
In a nice publication, tungstate-based poly(ionic liquid) entrapped magnetic nanoparticles (MNPs 25) were reported, by Pourjavadi and his research team, as a new and robust magnetically recoverable catalyst for oxidation reactions. The resulting catalyst was characterized by a series of spectroscopic techniques such as FTIR, TGA, SEM, TEM, XRD, XRF, CHN, VSM, and AAS. The TEM analysis confirmed that the size of MNP particles was about 10 nm, and they were dispersed into the polymeric matrix. It was found that the loading amount of WO4 ion in MNP@PILW was calculated by atomic absorption spectroscopy (AAS), about 0.61 mmol g−1 . The catalytic activity of MNPs 25 was evaluated in the oxidation of sulfides to sulfoxides. In the absence of a catalyst (MNPs 25), the sulfoxidation reaction failed. To identify the optimal conditions, the effect of catalyst loading and solvent nature was well studied. Under the standardized conditions, as shown in Scheme 22, a series of symmetrical and unsymmetrical sulfides was smoothly oxidized under solvent-free conditions, and the desired sulfoxides were obtained in high to excellent yields. Under this catalytic system (MNPs 25/H2O2), a fascinating category of substrates including alcohols and olefins were also selectively oxidized with excellent yields. The MNPs 25 can be easily recovered and reused for at least 10 runs without notable loss of activity.
Nickel ferrite (NiFe2O4), with an inverse spinel structure, is an important soft magnetic material with remarkable thermal stability, large magnetic anisotropy, and moderate saturation magnetization . In 2014, Desai and coworkers described a fascinating and efficient protocol for the oxidation of sulfides to sulfoxides using magnetically recoverable and reusable NiFe2O4 nanoparticles (MNPs 26). The prepared MNPs 26 was characterized by SEM, TEM, XRD, AAS, and hysteresis loop. The TEM analysis exhibits cubic morphology of the nanoparticles of size ranging between 14 and 20 nm . The synthesized catalyst in the presence of H2O2 showed high catalytic activity and chemoselectivity in the oxidation of symmetrical and unsymmetrical sulfides to sulfoxides. The sulfoxidation reactions were performed in acetonitrile at ambient temperature (Scheme 23). The maximum yield was observed in the sulfoxidation of diphenyl sulfide. It is noteworthy that this oxidizing catalytic system is compatible with the other functional groups in the molecule especially highly sensitive imine bonds that are well retained under this mild condition The MNPs 26 was magnetically separated and reused for five runs without noticeable loss of catalytic activity. Also, the chemo-selective oxidative coupling of thiols to the respective disulfides could be efficiently carried out by this catalytic system in high yields.
7 Magnetically recoverable acidic catalysts
From past to present, the catalysis of chemical and organic reactions by acids has been always a hot research target in organic-synthesis catalysis. As the acids are often liquid or expensive, their separation from the reaction media is the most important concern of organic chemists . Therefore, the designing and fabricating of strong solid acids and their utility as a catalyst in organic reactions is a nice response to the future of organic synthesis, in particular, from the green chemistry point of view , . In fact, the immobilization of the acidic functional groups on the magnetic nanoparticles and their catalytic utility in chemical and organic reactions can be considered an ideal and fascinating solution to overcome this blind spot because the catalyst can be readily separated from the reaction media by an external magnet. Inspired by this strategy, the oxidation of sulfides to sulfoxides have recently been investigated by several research teams. In 2013, Rostami and his research team evaluated the catalytic behavior of N-propylsulfamic acid supported onto magnetic Fe3O4 nanoparticles (MNPs 27) in the oxidation of sulfides to sulfoxides. The as-prepared acidic catalyst was characterized by SEM, XRD, and FT-IR spectroscopic techniques. Sulfoxidation catalysis under solvent-free conditions was the key milestone to attain the optimal conditions. Under the described conditions, a nice category of aliphatic and aromatic sulfides are smoothly and selectively oxidized to the corresponding sulfoxides in high yields (Scheme 24). The MNPs 27 could be recovered via magnetic attraction and could be recycled at least 10 times without appreciable decrease in activity. The SEM images of both the fresh and reused catalyst also revealed that no detectable changes of the catalyst occurred during the reaction and the recycling stages .
The ability of silica sulfuric acid-coated Fe3O4 magnetic nanoparticles (MNPs 28) to catalyze the sulfoxidation of sulfides was investigated by Rostami and co-workers. The structure of MNPs 28 was characterized by FT-IR spectroscopy, TGA, XRD, and SEM spectroscopic techniques. The SEM image of MNPs 28 shows that the catalyst was formed of nanometer-sized particles . The well grafting of sulfuric acid on Fe3O4 is verified by TGA analysis. Among a number of tested solvents, the highest yield and shortest times were observed under water as the solvent. Sulfoxidation reactions could be then conducted under aqueous medium at ambient temperature; good to excellent yields of the target products were obtained in less than 20 min (Scheme 25). Recycling of the catalytic system was evaluated in the oxidation of methyl phenyl sulfide, and only a slight loss of activity was observed after 15 runs.
In another study, Ghorbani-Choghamarani and his research team immobilized dopamine sulfamic acid on magnetic Fe3O4 nanoparticles (MNPs 29) to fabricate a magnetically recoverable catalyst for the sulfoxidation of sulfides. The structure of MNPs 29 was characterized by FT-IR, XRD, TGA, SEM, TEM, and VSM spectroscopic techniques. TEM analysis confirmed that most of the particles are quasispherical with homogeneous average diameter of about 15–25 nm . The MNPs 29 provided satisfactory yields for a nice library of substrates containing alkylaryl and dialkyl sulfides. Sulfoxidation reactions were carried out by H2O2 in ethanol at ambient temperature. As shown in Scheme 26, excellent yields were observed in the sulfoxidation of asymmetrical sulfides (95–98%) in less than 55 min. The MNP 29 can be magnetically recovered after the reaction and can be reused for four runs with only a minimal loss of activity (first run 97% yield, fourth run 87% yield).
In 2016, Ghorbani-Choghamarani and Azadi demonstrated that the MNPs 30 consisting of sulfamic acid heterogenized on amine-functionalized magnetic Fe3O4 nanoparticles can be successfully used as a magnetically recoverable catalyst for the sulfoxidation of sulfides. In the absence of a catalyst, only 31% yield of the desired sulfoxide was observed after 65 min. By using MNPs 30, the oxidation of a variety of symmetrical and unsymmetrical sulfides to sulfoxides has been investigated in the presence of H2O2 in thermal ethanol (Scheme 27). High to excellent yields (82–97%) were observed in less than 85 min. The nano-structure of MNPs 30 was characterized by FT-IT, TGA, XRD, SEM, and TEM spectroscopic techniques. The TEM analysis confirmed that most of the particles were monodispersed and uniform with a spherical shape (45 nm) . The authors reported that the MNPs 30could be reused up to 10 times, and no significant loss of activity was observed.
Very recently, Shiri and his research team reported the immobilization of sulfamic acid on magnetic Fe3O4 nanoparticles via diethylenetriamine ligand (MNPs 31). The structure of the resulting catalyst was well characterized by a series of spectroscopic techniques such as FT-IR, SEM, EDX, VSM, TGA, and XRD. The SEM analysis revealed the formation of uniformly sized magnetic nanoparticles with spherical morphology and an average size range of 17–21 nm. The catalytic activity of MNPs 31 was then tested in sulfoxidation reactions; it was observed that 15 mg of MNPs 31 and 0.5 ml of hydrogen peroxide (30%) in CH3CN at room temperature are the standardized conditions for further investigation . As shown in Scheme 28, a diverse range of symmetrical and unsymmetrical sulfides was successfully oxidized to the corresponding sulfoxides in high yields (86–98%). It is noteworthy that the activity of the MNPs 31 remains unaltered even after six runs without any significant loss in yield. Furthermore, MNPs 31 showed the high catalytic activity in the oxidative coupling of thiols Knoevenagel condensation of aromatic aldehydes with active methylene compounds.
A plausible mechanistic path for the sulfoxidation reactions is depicted in Scheme 29. The elucidation for this process is the in situ formation of peroxyacid (A) by the reaction of MNPs-NSO3H or OSO3H with hydrogen peroxide, followed by the oxygen transfer to the organic substrate .
8 Magnetically recoverable bromine catalysts
Catalysis research under bromine is a well-known topic in organic synthesis . However, the hazardous and toxic nature of bromine and its negative and deleterious effects on human health has caused organic chemists to avoid working on bromine , . Furthermore, the separation of bromine source catalysts from the desired products or reaction media is a difficult, tedious, and time-consuming task and needs a series of costly and specific techniques , . The heterogenization of bromine sources on magnetic nanoparticles can be considered as an efficient and fascinating catalytic system to overcome this drawback because the catalyst can be readily separated from the reaction mixture by an external magnet. In this section, we focused on sulfoxidation reactions catalyzed by magnetically recoverable bromine sources.
In 2014, Rostami and co-workers reported a new strategy to immobilize tribromide ion on surface-functionalized magnetic Fe3O4 nanoparticles (MNPs 32, Figure 5) leading to a magnetically recoverable catalyst, which exhibits high catalytic efficiency in a series of sulfoxidation reactions. The as-prepared MNPs 32 was subject to characterization by a series of spectroscopic techniques such as FT-IR, XRD, TGA, EDX, SEM, and VSM. The SEM analysis of MNPs 32 was revealed that the catalyst was made up of uniform nanometer-sized particles less than 26 nm . The presence of bromine in nanocomposite structure was confirmed by EDS analysis. Eighty percent yield of the sulfoxide product was observed when the process was performed under catalyst-free conditions for 24 h. MNPs 32/H2O2 was an efficient catalytic system for the solvent-free oxidation of aliphatic and aromatic sulfides to the sulfoxide at room temperature (Scheme 30). It is noteworthy that the target sulfoxide products were furnished in excellent yields in less than 70 min. The MNPs 32 could be recovered via magnetic attraction and could be reused for at least 15 runs without significant decrease in activity.
One year later, Kolvari and his research team described the fabrication of imidazole tribromide supported on magnetic (Y-Fe2O3) nanoparticles (MNPs 33, Figure 5). A series of spectroscopic techniques such as XRD, TGA, FT-IR, EDX, SEM, TEM, and VSM was used to confirm the structure of MNPs 33. The average diameter determined from SEM and TEM images is 10±5 nm for MNPs 33. The strong magnetization of MNPs 33 (54 emu g−1) was also revealed by vibrating sample magnetometer analysis (VSM) . MNPs 33 was then tested in the solvent-free sulfoxidation of sulfides using hydrogen peroxide as the oxidant at room temperature. A nice category of aliphatic and aromatic sulfides were examined, and it was found that the substrate kind had a key role in these transformations (Scheme 31). MNPs 33 can be readily recovered by magnetic decantation and used for sulfoxidation for up to five runs without any detectable loss of activity.
In 2016, Shiri and Tahmasbi found that tribriomide ion heterogenized on diethylenetriamine-functionalized magnetic Fe3O4 nanoparticles (MNPs 34, Figure 5) is a highly efficient and selective heterogeneous catalyst for sulfoxidation reactions. MNPs 34 was characterized by FT-IR spectroscopy, TGA, VSM, XRD, TEM, and SEM spectroscopic techniques. The VSM analysis showed that MNPs 34 has a saturated magnetization value of 47.86 emu g−1. The XRD analysis of MNPs 34 showed that the surface modification of the Fe3O4 nanoparticles do not lead to their phase change . The as-synthesized MNPs 34 was then tested for the solvent-free oxidation of sulfides to sulfoxides in the presence of hydrogen peroxide in ethanol at room temperature. By the described catalytic system, a broad spectrum of symmetrical and unsymmetrical sulfides subjected to the sulfoxidation reactions and with high to excellent yields (91–98%) of the desired sulfoxide products were observed in less than 50 min (Scheme 32). Recycling studies on the oxidation of methyl phenyl sulfide have shown that the catalyst can be readily recovered and reused six times without significant loss of activity.
In a fascinating publication, Shiri et al. used Fe3O4 MNPs functionalized with (3-aminopropyl)trimethoxysilane as support for the immobilization of a bromine source (MNPs 35, Figure 5). The as-synthesized MNPs 35 was comprehensively characterized by FT-IR, TGA, VSM, XRD, EDX, and SEM spectroscopic techniques. The SEM analysis revealed that the catalyst was made up uniformly, and the average size of MNPs 35 was about 10 nm. The EDX analysis confirmed the successful immobilization of the bromine source on the surface of magnetic Fe3O4nanoparticles . The catalytic activity of MNPs 35 was investigated for the sulfoxidation of sulfides. The oxidation of methyl phenyl sulfide was chosen as a model substrate, and the reaction conditions were optimized (catalyst concentration, solvent effect, etc.). Among all the tested solvents, ethanol proved to be the most efficient in this reaction. Sulfoxidation reactions were carried out by hydrogen peroxide and MNPs 35 in EtOH at ambient temperature. Under the described conditions, a nice library of aliphatic and aromatic sulfides was oxidized to the corresponding sulfoxides in excellent yields (87–97%). As seen in Scheme 33, chemoselectivity of this catalytic system tested in the oxidation of several sulfides containing oxidation-prone functional groups and the obtained results revealed well that these functional groups remained intact during the oxidation process. The recovery of MNPs 35 was simply performed using an external permanent magnet, and no noticeable loss of catalytic efficiency consequently was observed during the five consecutive reactions. The SEM image of the recovered catalyst shows that there are no significant differences in the morphologies of the freshly prepared catalyst and the recovered catalyst after an operation for five consecutive reaction times . Furthermore, MNPs 35 exhibited a high activity in the selective oxidative coupling of thiols to the disulfides.
A plausible mechanism for the oxidation of sulfides to sulfoxides in the presence of MNPs-Br3 is outlined in Scheme 34.
9 Other report
Finally, Malakooti and Atashin reported the preparation of magnetic Fe3O4 nanoparticles embedded in an SBA-15 silica wall (MNPs 36) as new magnetically recoverable catalyst for oxidation reactions. The structure of MNPs 36 was analyzed by a series of spectroscopic techniques such as FT-IR, XRD, UV/Vis, TEM, VSM, and ICP-AES. The TEM analysis of MNPs 36 confirmed an ordering cylindrical pore structure, which is in accordance with XRD data . Sulfoxidation of a series of sulfides was successfully achieved with a high conversion using H2O2 as the oxidant in the presence of a catalytic amount of MNPs 36 at 80°C in water within 2–8 h (Scheme 35). Separation from the reaction mixture was easily achieved by applying an external permanent magnet, and the separated catalyst could be recycled for at least six runs without any appreciable loss in activity. MNPs 36 exhibited also a high catalytic activity in the oxidation of alcohols to aldehydes.
10 Comparison of the catalytic activity of MNPs nano-catalysts
Table 1 enlists a comparative data of various parameters in the sulfoxidation of sulfides catalyzed by MNPs nano-catalysts. As shown in Table 1, in most cases, H2O2 was used as the oxidant. Most of the sulfoxidation reactions were performed in EtOH or under solvent-free conditions. In this category (MNPs nano-catalysts), magnetic Fe3O4 nanoparticles were found to be the most successful. Except for a few cases, in most cases, MNPs were able to catalyze the sulfoxidation of sulfides in satisfactory yields. From the economic point of view, MNPs 7, 26, and 29 are the most reusable catalysts. The highest yields were observed in the presence of MNPs 1, 8, 17, 18, and 31.
|Entry||MNPs (cat.)||Conditions||Recovery||Time||Yield (%)|
|1||MNPs 1||H2O2, solvent free, 40°C||10||10–180 min||90–99|
|2||MNPs 2||H2O2, EtOH, 60°C||12||20–360 min||58–99|
|3||MNPs 3||H2O2, solvent free, r.t.||4||10 min–48 h||53–97|
|4||MNPs 4||H2O2, solvent free, 35oC||4||5 min–24 h||75–98|
|5||MNPs 5||H2O2, solvent free, r.t||5||5–190 min||91–97|
|6||MNPs 6||UHP, CH2Cl2/MeOH, r.t.||6||4–24 h||60–99|
|7||MNPs 7||H2O2, solvent free, r.t.||20||5–300 min||80–97|
|8||MNPs 8||H2O2, solvent free, r.t.||5||25–220 min||90–99|
|9||MNPs 9||H2O2, solvent free, r.t.||12||7–600 min||70–98|
|10||MNPs 10||H2O2, CH3CN, 60°C||4||120 min||23–75|
|11||MNPs 11||UHP, CH2Cl2/MeOH, r.t.||6||3 h||75–90|
|12||MNPs 12||UHP, CH2Cl2/MeOH, r.t.||4||2 h||11–95|
|13||MNPs 13||UHP, CH2Cl2/MeOH, r.t.||4||30 min||45–85|
|14||MNPs 14||UHP, CH2Cl2/MeOH, r.t.||4||30 min||50–99|
|15||MNPs 15||H2O2, solvent free, 55°C||6||5 min||40–100|
|16||MNPs 16||H2O2, EtOH, r.t.||8||150–480 min||76–96|
|17||MNPs 17||H2O2, CH3CN/H2O, r.t.||6||1–4 h||68–96|
|18||MNPs 18||H2O2, solvent free, 35°C||10||5–60 min||90–99|
|19||MNPs 19||H2O2, solvent free, 35°C||10||5–120 min||91–99|
|20||MNPs 20||H2O2, solvent free, 35°C||10||5–30 min||92–97|
|21||MNPs 21||H2O2, solvent free, 35°C||10||5–60 min||88–96|
|22||MNPs 22||H2O2, solvent free, 35°C||10||5–30 min||90–96|
|23||MNPs 23||H2O2, EtOH, 35°C||7||10 min–20 h||62–99|
|24||MNPs 24||H2O2, EtOH, 45°C.||7||5 min–24 h||53–99|
|25||MNPs 25||H2O2, solvent free, r.t.||10||60–150 min||82–99|
|26||MNPs 26||H2O2, CH3CN, r.t.||5||90–210 min||84–92|
|27||MNPs 27||H2O2, solvent free, r.t.||10||5–120 min||85–95|
|28||MNPs 28||H2O2, water, r.t.||15||2–20 min||83–97|
|29||MNPs 29||H2O2, EtOH, r.t.||4||5–60 min||70–98|
|30||MNPs 30||H2O2, EtOH, 60°C||10||5–85 min||82–98|
|31||MNPs 31||H2O2, CH3CN, r.t.||6||15–310 min||86–98|
|32||MNPs 32||H2O2, solvent free, r.t.||15||5–70 min||82–97|
|33||MNPs 33||H2O2, solvent free, r.t.||5||15–45 min||80–97|
|34||MNPs 34||H2O2, solvent free, r.t.||6||5–50 min||90–98|
|35||MNPs 35||H2O2, EtOH, r.t.||5||10–280 min||87–97|
|36||MNPs 36||H2O2, H2O, 80°C||7||2–8 h||70–100|
11 Summary and outlook
The concept of magnetic nanoparticle supporting of catalysts has rapidly developed in recent times. These MNPs nano-catalysts possess a series of admirable advantages such as simple preparation, simplicity of operation, high catalytic activity, easy separation and reusability, and being nontoxic. Magnetic nanoparticles can be readily separated from reaction medium using an external magnet, without the need for filtration, centrifugation, or other tedious work-up processes. The most important aspect in magnetic catalysis is the design and fabrication of a catalyst for a specific organic reaction, considering the mechanistic pathway and the feasibility to perform reactions on a laboratory scale with potential for industrial applications . The recyclability aspects are the salient features due to their popularity and sustainable applications . A broad library of high-pressure and high-temperature organic reactions can be carried out on nano-magnetite supported catalysts because they are highly stable . Because of the sturdy interaction between the support (magnetic nanoparticles) and metals, leaching of metals can be avoided or minimized . The catalytic activity of a large number of MNPs in the oxidation of sulfides to sulfoxides, which are prevalent structural scaffolds in many drugs and biologically active molecules, was investigated in this paper. In most cases, the sulfoxide products were afforded in reasonable yields. The sulfoxidation reactions are often performed under mild conditions without using solvents or by applying ethanol, methanol, and water. Therefore, they are an efficient tool for green chemistry and probably for industrial and pharmaceutical application. Also, the MNP nano-catalysts can be simply separated by magnetic decantation and reused many times. In this category (MNPs nano-catalysts), magnetic Fe3O4 nanoparticles were widely used as a support for the immobilization of catalysts. Accordingly, MNPscan be regarded as an efficient separation technology with great possibilities for applications in the field of catalysis. The combination of two metals (bimetallic) on the magnetic supports (which are difficult to prepare and expensive) would be a new generation of highly stable and selective magnetically recoverable catalysts in the near future, which can be evaluated for a variety of important and valuable chemical reactions from economic and medicinal points of view.
About the authors
Mosstafa Kazemi was born in Ilam, Iran. He 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 Assist Prof. Lotfi Shiri the Ilam University, where he is working on the synthesis and applications of magnetic nanoparticle-supported nano-catalysts in organic synthesis.
Massoud Ghobadi was born in Ilam, Iran. He received MS degree in Inorganic Chemistry from Ilam University in 2012. Currently, he is working toward his PhD under the supervision of Assoc. Prof. Mohsen Nikoorazm at the Department of Chemistry of Ilam University. His current interests are focused on the development of new strategies for the fabrication of heterogeneous nano-catalysts and their application in chemical reactions.
This work was supported by the research facilities of Ilam University, Ilam, Iran.
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