Nanoscience and nanomaterial have become an area of tremendous importance in the field of catalysis because of the extremely smaller particle size and higher surface area available for the reaction [1-4]. During the recent years, several kinds of nanomaterials, their synthetic approaches and advanced characterization tools are being developed all over the globe. Within the known materials, magnetic iron oxide (Fe3O4 and Fe2O3) nanoparticles have been acknowledged as eco materials and have been successfully used as a magnetically recyclable catalyst for various organic transformations including degradation of organic compounds [5-7]. Surface modification/ functionalization of these magnetic nanoparticles plays an important role in the field of catalysis as it prevents the oxidation and agglomeration of the nanoparticles to make them compatible for organic reactions [8-9]. Although a vast number of surface functionalized magnetically recoverable catalysts based on iron oxide supports have been known [10-12], but lanthanide cerium cations/complexes immobilized nanoparticles have never been realized in the literature.
Magnetically responsive maghemite (Fe2O3, γ-Fe2O3), also known as Fe(II) deficient magnetite nanoparticles with similar spinel ferrite structure, has gained considerable attention mainly due to their efficient and economical synthesis, scalability and nontoxic nature [13-14]. These nanoparticles can readily be synthesized by the oxidation of magnetite nanoparticles, using a co-precipitation method followed by heat treatment at different temperatures ranging from 200 to 300 oC [15-16]. Maghemite nanoparticles because of their low cost, biocompatibility and higher chemical stability have been widely explored in pharmaceutical and nano-medicines related applications [17]. However, a few reports are known related to their uses as support matrix in the area of heterogeneous catalysis for organic transformations [18-19]. Importantly, Rathi et al. [20-21] reported maghemite decorated with ultra-small palladium nanoparticles (γ-Fe2O3-Pd) for Heck-Mizoroki olefination, Suzuki reaction and allylic oxidation of alkenes. In a subsequent report, this similar group described the hydrogenation of nitroarenes, azides and alkenes using maghemite-Pd nanocomposites in a continuous flow reactor system. However, the Rathi’s nanoparticles involve nanofabricated catalytic Pd-surface engineered maghemite nanoparticles, which were synthesized by following a simple two-step co-precipitation method. Further, the Rathi’s catalytic double phase iron oxide nanoparticles have not been analyzed via surface charge (zeta potential values), which is very important to study and check the aggregation behavior of the nanoparticles before and after any involved catalytic reaction. In contrast, herein we report a quite innovative Lewis acid-based chemical activity of novel highly positively charged (z potential: +45.7 mV), non-aggregated ultra-small (6.61±2.04 nm-sized) super-paramagnetic Ce3/4+-cation/complex-doped maghemite (γ-Fe2O3) nanoparticles, which are synthesized by an innovative high-power ultrasonication methodology [22-23]. Global Design Of Experiment (DoE, MINITAB®16 DoE software) optimization provided quite innovative surface engineering of maghemite nanoparticles doped with Lewis acid Ce3/4+ cations/complexes based on the well-known coordinative/ligand exchange chemistry. Indeed, both inductive coupled plasma-atomic emission spectroscopy (ICP-AES) and transmission electron microscopy-energy dispersive X-ray spectroscopy (TEM-EDAX) analyses demonstrated the presence of various exchangeable Ce3/4+ cations/ligands (NO3- anions, H2O & OH species) onto the maghemite nanoparticles surface that might be effectively exchanged by any organic species containing Lewis base N/S/O-relating groups [22-23] (Israel et al. 2014; Lellouche et al. 2014). These Ce3/4+cations/complexes-doped maghemite (γ-Fe2O3) nanoparticles have never been realized as reusable catalyst in organic synthesis.
The aza-Michael addition of amines with α,β-unsaturated carbonyl compounds, nitriles to provide β-amino derivatives is one of the important reactions for the formation of carbon-nitrogen bond in organic chemistry [24-25]. These β-amino derivatives have found wide spread applications as synthetic intermediates in the preparation of numerous pharmaceuticals, natural products and medicinally important precursors [26-27]. The diversified applications of these β-amino compounds have led to the development of several unique and novel synthetic approaches for aza-Michael addition in recent decades. Conventionally, these addition reactions are carried out in the presence of strong base or/and acid. However these methods are associated with the drawbacks such as harsh reaction conditions and formation of by-products [28]. In order to overcome these limitations, subsequently a number of milder approaches utilizing Lewis acid catalysts such as PtCl4·5H2O, Cu(OTf)2, InCl3, Yb(OTf)3, LiClO4, Bi(NO3)3·5H2O, FeCl3.6H2O, CeCl3.7H2O, ZnO, MgO, silica-supported perchloric acid and sulfated zirconia have been reported in the literature [29-36]. In addition, some metal-free catalysts for example, cyclodextrin in water [37], polyethylene glycol [38] and boric acid in water [39] have been developed. Furthermore, heterogeneous catalysts such as graphene oxide, Amberlyst-15, modified mesoporous SBA-15, copper nanoparticles, silica gel, KF/Al2O3 have been explored for this important transformation [40-42]. However, most of the catalytic methods are associated with the drawbacks, such as requirement of excessive amount of acidic reagents that causes serious environmental hazard, use of volatile and toxic organic solvents, inefficient recovery of the catalyst by conventional filtration and prolonged reaction times. Moreover, most of the methods were found to be ineffective for the aromatic amines owing to their less reactivity as compared to the aliphatic ones. Ceric ammonium nitrate (CAN), a well-known one electron transfer reagent has been used as a promoter in various C-C and C-N bond forming reactions [43-44]. The main advantages of using CAN include its lower toxicity, inexpensiveness, easy handling, and solubility in most of the organic solvents. Duan et al. [45] reported the use of CAN (ceric ammonium nitrate) as an efficient promoter for aza-Michael addition of aromatic and aliphatic amines with α,β-unsaturated carbonyl compounds using ultrasound irradiation under solventless condition. However, ineffective mixing of the reagents during the reaction along with the poor recovery of CAN after the reaction, leaves a scope for the further development of a novel catalytic methodology for this transformation.
Accordingly, we herein report the use of highly hydrophilic, non-aggregated and strongly positively charged ceric ammonium nitrate-stabilized maghemite (CAN-γ-Fe2O3) nanoparticles (Ce3/4+ cations/complexes doping) in water as an efficient and reusable nanoscaled catalyst for the addition of various amines with α,β-unsaturated carbonyl and nitrile compounds to give corresponding β-amino compounds under ultrasonic irradiation (Scheme 1).
All the substrates and solvents were commercially available and used as received without further purification. Hydrophilic CAN-stabilized maghemite (γ-Fe2O3) Nanoparticles were synthesized by following the procedure reported in the previous literature [22].
Typical experimental procedure for the aza-Michael addition
A reaction mixture containing amine (2 mmol) and Michael acceptor compound, i.e. methyl acrylate (2.5 mmol) into a round bottom flask was added aqueous solution of CAN-γ-Fe2O3 nanoparticles (0.5 ml; [Ce] 0.0075 mmol). The resulting mixture was sonicated at room temperature for 2 h. The progress of the reaction was monitored by thin-layered chromatography (TLC). After 2 hrs of sonication, the mixture was diluted with dichloromethane followed by isolation of the product simply by liquid-liquid extraction. The obtained crude product was purified by column chromatography using EtOAc/hexane (6:4) as eluent. Conversion of the products was analyzed by GC-FID model-Varian CP3800 (Column specification: Stabilwax® w/Integra-Guard®, length-30 m, 0.25 ID) at the flow rate 0.5 mL min-1, injector temp 250 °C, FID detector temperature 275 °C. The structural identity of the products was established by comparing their spectral data, i.e. both 1H and 13C nuclear magnetic resonance (1H & 13C NMR) with those of authentic compounds.
3-(Butylamino)propanenitrile [46]: 1H NMR (CDCl3, 500 MHz) δ 2.92 (2H, t), 2.58 (2H, t), 2.48 (2H, t), 2.34 (1H, s, -NH), 1.72 -1.48 (2H, m), 1.21-1.12 (2H, m), 0.88 (3H, t); 13C NMR: δ 112, 61, 53, 47, 36, 29, 20, 15; ESIMS (m/z) = 127 (M+1).