A Green Recyclable Poly(4-vinylpyridine)-Supported Copper Iodide Nanoparticles Catalyst for the Multicomponent Synthesis of 3,4-dihydropyrimidin-2(1H)-ones/thiones

EXPRIMENTAL

General Procedure

A mixture of aldehyde (1 mmol), β-ketoester (1 mmol), urea or thiourea (1 mmol) and P4VPy-CuI (0.1 g) was heated in an oil bath (80 °C) for the appropriate time (Table 1). After completion of the reaction as followed by TLC, 5 mL of ethyl acetate was added to the mixture and filtered. The catalyst washed with ethyl acetate, dried and stored for another consecutive reaction run. Evaporation of the solvent from the filtrate and recrystallization of the solid residue from hot ethanol afforded the pure products in high yields.

Scheme 1.Synthesis of dihydropyrimidone derivatives catalyzed by P4VPy-CuI.

Table 1.aIsolated yield. bProducts were characterized by comparison of their spectroscopic data (NMR and IR) and melting points with those reported in the literature.16−27

Determination of the Copper Content in P4VPy-CuI

The P4VPy-CuI (100 mg) was extracted with concentrated HCl (5×2 mL) in a screw-capped vessel, followed by treatment with concentrated nitric acid (2 mL) to digest the metal complex. The mixture was then transferred into a volumetric flask (100 mL), diluted 1:50 for the second time and was analyzed by the ICP analysis. The copper concentration was determined from the atomic emissions (324.754 nm) by reference to a linear (R=0.99) calibration curve of (1−4 ppm) of CuI prepared in a manner identical to the sample preparation. The loading of supported catalyst was calculated to be 1.32 mmol CuI·g−1 of prepared catalyst. The same procedure was used to measure the leaching accounts of supported catalyst after 8 consecutive runs.

 

RESULT AND DISCUSSIONS

The copper iodide nanoparticles immobilized on poly (4-vinylpyridine) (P4Py-CuI) was readily prepared in a one-step procedure. Poly(4-vinylpyridine) was refluxed with a solution of CuI under an N2 atmosphere in EtOH for the synthesis of polymer-supported CuI nanoparticles. This method was developed for the effective synthesis of copper nanoparticles incorporated heterogeneously as catalyst in the some organic reactions.32 Scanning electron microscopy (SEM), X-ray diffraction (XRD) analysis, atomic absorption and IR experimental techniques were used to characterize the catalyst. At first, for the optimization of the reaction conditions, the condensation of benzaldehyde, ethyl acetoacetate and urea was investigated under solvent-free conditions. The best result was achieved by carrying out the reaction of benzaldehyde, ethyl acetoacetate and urea (with 1: 1: 1 : mol ratio) in the presence of 0.1 g of P4VPy-CuI at 80 °C for 12 min under solventfree conditions (Table 1, entry 1). Using these optimized conditions, the reaction of various aromatic aldehydes was explored (Table 1). Aromatic aldehydes containing both electron-donating and electron-withdrawing groups were also reacted under the same reaction conditions to produce the corresponding DHPMs in good to high yields. Methyl acetoacetate and thiourea were also used with similar success to provide the corresponding products. Aliphatic aldehydes remain intact under the same reaction conditions (Table 1 entries 19, 20). Therefore, the method can be useful for the chemoselective Biginelli condensation of aromatic aldehydes in the presence of aliphatic ones (Table 1, entry 21). The experimental procedure with this catalyst is very simple and the catalyst can be removed easily by filtration. The solid products were easily recrystallized from hot ethanol and were obtained in good to high yields during short reaction times. Very low amount of the catalyst is needed. Moreover, our procedure is environmentally friendly as it does not use any toxic auxiliary or solvent. A distinct characterization of the present method, illustrated in this work is the formation of corresponding products without by-product. It is also noteworthy that catalyst do not suffer from extensive mechanical degradation after running. For a true heterogeneous catalyst, supported catalyst shouldn’t leach to the reaction mixture. Moreover the recyclability of the supported catalyst is also important and shouldn’t suffer from mechanical degradation. To investigate these properties, the reaction of benzaldehyde, ethyl acetoacetate and urea was selected again as the model (Table 2). After completion of the reaction, the recovered catalyst was washed with acetone and after dryness was reused in the next similar run. This procedure was repeated for 10 consecutive runs and the the desired product was obtained in high yields after 1-10 runs, respectively (Tables 2). Next we checked the leaching of CuI into the reaction mixture from the poly(4-vinyl pyridine)-support using ICP-AES. The difference between the copper content of the fresh catalyst and the used catalyst (10th run) is only 3% which indicates the low leaching amount of copper iodide catalyst into the reaction mixture.

Table 2.aIsolated yield.

Finally, in order to evaluate the efficiency and superiority of our introduced catalyst, we began to run the reaction between benzaldehyde, ethyl acetoacetate and urea in the presence of pure CuI at the same conditions. The obtained results showed that pure CuI could not be reused, need much more reaction time and gave the product in the low yield. In conclusion, we have described a simple and green procedure for the synthesis of DHPMs catalyzed by P4VPy–CuI under solvent-free conditions. The introduced catalyst can promote the yields and reaction times over 10 runs without appreciable loss in its activity and efficiency. Moreover, high yields of products, short reaction times, ease of work-up and clean procedure, will make this procedure a useful addition to the available methods.