Ultrasound assisted eco-friendly synthesis of 3-cinnamoyl coumarins using N,N'-(1,2-phenylene)bis(2-aminobenzamide) dichloro cobalt immobilized on mesoporous Al-SBA-15 as a new and recyclable catalyst

ABSTRACT A novel mesoporous Al-SBA-15 modified by N,N'-(1,2-phenylene)bis(2-aminobenzamide) dichloro cobalt has been prepared and applied as a reusable catalyst in the 3-cinnamoyl coumarins synthesis via three-component reaction between benzaldehydes, salicylaldehydes and ethyl acetoacetate by the assist of ultrasonic irradiation. By using of the nanocatalyst and also ultrasound irradiation, the easiness and velocity of the abovementioned reaction were enhanced and an environment friendly condition was provided to synthesis of various 3-cinnamoyl coumarin compounds. The properties and structure of nanocatalyst have been specified by methods including powder X-ray diffraction (XRD), energy dispersive spectroscopy (EDS), Fourier transform infrared spectroscopy (FT-IR), transmission electron microscopy (TEM), nitrogen adsorption–desorption analysis and scanning electronic microscopy (SEM). Superiority of this novel and viable method is due to mild reaction condition, short reaction times, high yields of 3-cinnamoyl coumarins, environmentally benign, recoverability of the CoCl2N,N'-(1,2-phenylene)bis(2-aminobenzamide)/Al-SBA-15 catalyst and reusability with important preservation in its catalytic activity. GRAPHICAL ABSTRACT


Introduction
Ordered structures of mesoporous silica containing SBAn, MSU-X and M41S as significant category of compounds have been raised for different applications inclusive separation and catalysis (1,2). SBA-15 has attracted attention of many researchers because of its great specific surface area, prominent mechanical and thermal stability, high pore dimensions, particular surface acidity and monotonous size of pores (3)(4)(5)(6)(7)(8).
The elements like zirconium, titanium and aluminum can be inserted into hexagonal silica structure to create substances with extensive applications in ion exchange and catalysis processes. Between these metal-incorporated mesoporous materials, aluminum substituted-silica have revealed prominent potential as acidic catalyst in numerous reactions (9)(10)(11). Various studies have incorporated organic-inorganic multifunctional groups into heterogeneous catalysts, which can applied to catalyze chemical processes (12,13). A lot of active sites in the functional organic-inorganic hybrid catalysts seriously enhanced the rate of organic reactions (14). These materials represent imperative applications in biomedicine, catalysis, biotechnology and drug delivery (15)(16)(17). Mesoporous silica functionalized by organic-inorganic hybrid structures has been used to catalyze some organic reactions by creating environmentally mild condition, excellent surface area, effective recovery from reactants and reusability (18,19).
In the recent years, sonochemistry has attracted many attention as an effective method which was preponderantly employed in the numerous fields including catalyzed reactions (e.g. heterogeneous catalysis, enzymatic reactions, using ion exchange resin, phase transfer catalysis and biphasic reactions) (20)(21)(22)(23)(24). The application of ultrasonic technique in organic synthesis has plentiful significant advantages such as milder reaction condition, lower reaction length, higher efficiencies of products, nontoxic and environmental friendly solvent, minimization of side products, saving money and energy in comparison with the conventional heating methods (25)(26)(27)(28)(29). In fact, ultrasound irradiation accelerates numerous reactions by means of the making, development and implosive bubbles collapse in liquid phase (30). Cavitational collapse generates high pressures, extreme heating and too small lifetimes. Cavitation as a condensing agent cooperates to dispersion the ultrasound energy (31,32).
Thus, to avoid these problems, the development of an alternative method with convenient and environment friendly procedure by a catalyst with high catalytic performance for 3-cinnamoyl coumarins synthesis is still favored. The target of ongoing investigation is minimalization of aforementioned limitations for the 3-cinnamoyl coumarins preparation by a three-component, one-pot procedure using functionalized mesoporous Al-SBA-15 and also with the assist of ultrasound irradiation. Therefore, the novel hybrid CoCl 2 N,N'-(1,2-phenylene) bis(2aminobenzamide)/Al-SBA-15 (CoCl 2 NN'PhBIA/Al-SBA-15) was prepared, identified and used in the sonochemically synthesis of 3-cinnamoyl coumarin compounds by threecomponent reaction of salicylaldehyde, ethyl acetoacetate and benzaldehydes (Scheme 1). This method makes many superiorities such as providing sustainable and green condition, using simple one-pot way to prepare 3-cinnamoyl coumarins, benign reaction condition, recoverable and reusable hybrid nano-catalyst.         Distinguished surface area of mesoporous Al-SBA-15 and Al-SBA-15 modified by CoCl 2 NN'PhBIA were determined via nitrogen adsorption-desorption analysis as indicated in Figure 6(a). The curves of N 2 adsorption-desorption designate a typical isotherm with a H 1 hysteresis node (commencing from P/P 0 = 0.6) of type IV. These results are in well consistency with mesostructure compounds. The nanocatalyst CoCl 2 NN'PhBIA/Al-SBA-15 surface area was measured at 582 m 2 /g which was fewer than Al-SBA-15 specific surface area (815 m 2 /g) owing to insertion of NH 2 , phenyl and cobalt into the Al-SBA-15 hexagonal channels. The pore dimension distribution diagrams for modified and bare Al-SBA-15 (depicted in Figure 6(b)) represent the bounded pore size dispensation for them placed on 3.17 and 5.52 nm, respectively. Decrement in CoCl 2 NN'PhBIA/Al-SBA-15 pore dimension is an approval for modification of Al-SBA-15 surface by the ligand and Co atom.

Preparation of 3-cinnamoyl coumarins
In the course of our efforts to introduce a green and sustainable route for the synthesis of 3-cinnamoyl coumarin compounds, we here in report three-component reaction of ethyl acetoacetate, salicylaldehyde and different substituted benzaldehydes at presence the CoCl 2 NN'PhBIA/Al-SBA-15 nano-catalyst under ultrasonic irradiation.
In order to optimization the reaction condition, the preparation of 4e was designated as a model reaction. The influence of various catalysts, solvents, amount of catalyst and different reaction condition on the model reaction were investigated ( Table 1). The entries 1-6 of this  [6][7][8][9][10][11][12] demonstrated that the model reaction effectively proceeded using CoCl 2 NN'PhBIA/Al-SBA-15 as catalyst. Also the maximum yield for 4e was afforded by 3 mg of it (entry 14 of Table 1). According to the results of this table, the ultrasonic irradiation provided higher yields of 4e at shorter reaction times in comparison with the heating process. Ultrasound condition caused to increasing the number and dimension of active cavitation bubbles resulted in upper maximum collapse temperature, thus the preparation of 3-cinnamoyl coumarin derivatives was accelerated under ultrasonic irradiation. Also, sonication technique is more environmental friendly owing to its fundamental green chemistry conception.
To find the best power of ultrasonic irradiation for the three-component reaction of salicylaldehydes, benzaldehydes and ethyl acetoacetate this reaction was performed under various powers of ultrasonic irradiation ( Table 2). The resultants prove the reaction is impressively improved by 3 mg of  groups on the salicylaldehyde and aromatic aldehydes (Table 3).

Proposed mechanism
Scheme 4 demonstrates a conceivable pathway for the three-component synthesis of 3-cinnamoyl coumarins using the organic-inorganic hybrid catalyst. It is presumed that cobalt from the hybrid catalyst as a Lewis acid activates carbonyl groups of the substrates by coordination to the oxygen (from carbonyl groups). At first, the O from the carbonyl group of salicylaldehyde created a complex with Co from CoCl 2-NN'PhBIA/AL-SBA-15 which then reacts with active methylene group of ethyl acetoacetate. 3-acetylcoumarin is produced by transesterification reaction in this intermediate and following dehydration. Afterward, oxygen from carbonyl groups of 3-acetyl coumarin coordinates to Co from CoCl 2 NN'PhBIA/AL-SBA-15 and enol is generated. Next, the aldol condensation takes place between aromatic aldehyde and the enol form. Nitrogen atoms in NN'PhBIA ligand forms hydrogen bonding with the hydrogens of enol hydroxyl and accelerates this reaction. At the end of process, dehydration reaction yields 3-cinnamoyl coumarins. The CoCl 2 NN'PhBIA/Al-SBA-15 improves the reaction condition due to its high surface area. Moreover, the coordination of carbonyls to cobalt atoms accelerates the conjugation and the nucleophile addition to the reactants. Also, the N atoms of NN'PhBIA ligand can activate the aldol condensation and dehydration through hydrogen bonding. These routes is significantly proceed by the cavitation effect of ultrasonic irradiation. Under ultrasonic irradiation, the electronic currents in the conjugated systems of reactants and intermediates are improved and reaction is quickly proceeded to formation of product. Generally, faster electronic resonances in reactants lead to faster nucleophile attacks resulted in increasing the rate of various steps in the proposed mechanism specially transesterification and aldol condensation. In addition, better electronic resonances provide more effective adsorption of reactants on the catalyst surface. Moreover, the electronic resonance in NN'PhBIA of catalyst is increased by ultrasound irradiation and ability of hydrogen bonding formation in N atoms of NN'PhBIA is raised. Also, the bonding energy is created by shock wave and small hot bubbles generated through the cavitation effect of ultrasonic irradiations in liquid phase. These occurrences enhance the adsorption of reactants on the CoCl 2 NN'PhBIA/Al-SBA-15 surface through enhancing mass transportation of substrates from the liquid phase to the catalyst surface.      the other three-component reaction of ethyl acetoacetate, salicylaldehyde and benzaldehydes for eight times. Negligible loss in catalytic activity was afforded and the 3-cinnamoyl coumarins were formed by great yields (Figure 7).
The structure of mesoporous CoCl 2 NN'PhBIA/Al-SBA-15 nanocatalyst was investigated using FT-IR spectra before apply in the reaction and after recovery of eight cycles ( Figure 8). As seen in this figure, the corresponding peaks are equal for CoCl 2 NN'PhBIA/Al-SBA-15 organic-inorganic hybrid before and after the corresponding reaction. These results approve the excellent stability of the mesoporous nanocatalyst in reaction.

Conclusion
In the ongoing investigation a convenient, advanced and eco-friendly procedure was presented to prepare 3-cinnamoyl coumarin compounds through tree component condensation of ethyl acetoacetate, salicylaldehyde and different benzaldehydes using mesoporous CoCl 2 NN'PhBIA/Al-SBA-15 organic-inorganic hybrid as a recoverable and very effectual catalyst under sonication condition. Green technique, the catalyst recyclability and facile reusability for eight times, moderate condition, providing supreme efficiencies of 3-cinnamoyl coumarin compounds at low times are several significant superiorities of the introduced method.

Chemicals and devices
The chemicals were bought from Sigma-Aldrich and Merck corporations and were consumed in experiments without extra purification methods.
The ultrasound energy was employed in reactions using a multi-wave ultrasound producer (Sonicator 3200; Bandelin, MS 73, made in Germany), involving a transformer and Ti vibrator (horn) (diameter = 12.5 mm), working at 20 kHz and eldest output power is equal to 200 W. The ultrasound producer advice spontaneously regulates the power balance. Products melting points were determined by using an Electro thermal 9200 device. A Magna 550 device was employed for gaining the FT-IR peaks of products with KBr plates. Mass spectrometer (Finnigan-MAT-8430) was used to obtain electron ionization mass spectral (EIMS) by 70 eV in m/z. The NMR spectrums were achieved by assist of a spectrometer (Bruker Avance-400 MHz) at present tetramethylsilane (TMS) internal standard. Nanomaterials morphologies were imagined by SEM (MIRA 3 TESCAN). Structure and particle size of the organic-inorganic CoCl 2 NN'PhBIA/Al-SBA-15 hybrid were imagined with a Philips CM10 transition electron microscope (30 kV). The nanocatalysts XRD pattern were recorded on a PHILIPS PW 1510 (made in Netherland) X-ray diffractometer by Cu-Kα light (λ = 0.154056 nm, 2θ = 0.8°-10°). By using a Sigma ZEISS (Oxford Instruments Field Emission) EDS of the nanocatalyst were visualized. The nitrogen adsorption/desorption analyses of bare mesoporous Al-SBA-15 and CoCl 2 NN'PhBIA/Al-SBA-15 hybrid catalyst were carried out by an automatical gas adsorption device (BEL SORP mini II) at 120°C.

Synthesis of mesoporous Al-SBA-15
Solution A was made by dissolving of pluronic acid (P123 = 2 g) in hydrochloric acid (75 ml, pH = 1.5) and mixing on magnetic stirrer for 6 h at 40°C. Solution B was prepared via mixing tetraethylorthosilicate (TEOS) (3.2 ml), HCl solution (5 ml, pH = 1.5) and aluminum triisopropoxide (0.22 g). The solution B was intensely mixed in a restricted flask during 1.5 h up to being transparent. Subsequently solution B was poured in the solution A and strongly mixed at 40°C in 20 h. The attaining transparent solution was heated to 100°C for 20 h in an autoclave (Teflon-covered stainless steel). The resulting white powder was separated by filtration, cleaned 3 times with water and 2 times by EtOH and then was dried at 100°C during 12 h. The pluronic acid was eliminated through calcination (temperature = 550°C, heating ramp = 1°C /min) for 10 h.   A suspension involving 12.9 mg CoCl 2 , 0.2 g of NN'PhBIA/ Al-SBA-15 and 30 ml of absolute ethanol was mixed by stirrer for 24 h at 25°C. The resultant solid was centrifuged, frequently cleaned by absolute EtOH and subsequently dried using infrared radiation at 25°C to yield CoCl 2 NN'PhBIA/Al-SBA-15 organic-inorganic hybrid.

Preparation of 3-cinnamoyl coumarins
A blend of corresponding salicylaldehyde (1 mmol), aromatic aldehyde (1 mmol), ethyl acetoacetate (1.2 mmol) and CoCl 2 NN'PhBIA/Al-SBA-15 (3 mg) nanocatalyst in ethanol (5 ml) was ultrasonicated at 55 W power for adequate times. After the reaction was terminated (examined by TLC), the blend (with temperature = 65°C) was cooled to 25°C. Then it was poured in acetone and the precipitated catalyst was isolated by centrifuging, washed several times with hot EtOH to separate the residue product and dried for reuse. Also, the residual 3-cinnamoyl coumarin was purified through recrystallization in ethanol.