The synthesis of arylidene derivatives has attracted considerable attention from organic chemists for many years, because many of them have significant biological activity. More over, benzylidene malononitriles were reported to be effective anti-fouling agents, ungicides and insecticides. They are important intermediates for the synthesis of various organic compounds.1
Knoevenagel condensation of carbonyl compounds on molecules containing an active methylene group is an important method for the preparation of substituted alkenes. Several important key products, such as nitriles used in anionic polymerization and a,b-unsaturated ester intermediates employed in the synthesis of several therapeutic drugs, e.g., niphedipine and nitrendipine, etc have been synthesized via this condensation.1,2 Ylidenenitriles have increasing applications in industry, medicine, agriculture and biological science, and are precursors to heterocycles.3 The reaction is catalyzed by primary and secondary amines or their corresponding ammonium salts in harmful solvents or with Lewis acids, as TiCl4/ base4a, ZnCl44b, CdI24c and other catalysts.4e-h Most of these methods have not been entirely satisfactory owing to such drawbacks as low yields, long reaction times and effluent pollution.
In continuation of our program to develop reactions in eco-friendly conditions5 and our interest in the Knoevenagel reaction,6 in this communication we wish to disclose our results for this reaction using MCM-41 as catalyst.
RESULT AND DISCUSSION
Mesoporous molecular sieves, which was synthesized by Mobile group in 19927, have a space enough to accommodate the guest molecules. The MCM-41 exhibits a hexagonal array of cylindrical pores, cubic ordered pores and lamellar structure. Chemists have found this material as a brilliant host for the accommodation guest molecules and used it in catalytic reactions.8 Then, we decided to investigate knoevenagel condensation in the presence of MCM-41 as a solid acid catalyst.
To start our investigation, we performed the study of various solvents effect on the course of reaction of 4-chlorobenzaldehyde with malononitrile. This reaction was carried out in various solvents such as water, DMF, chloroform, Ethanol, CH2Cl2 and toluene and the best results in terms of yield and time obtained in CH2Cl2.
After optimizing the reaction condition, various aromatic aldehydes reacted very well with malononitrile and ethylcyanoacetate as active methylene compounds to give the corresponding arylidenes in good to excellent yields (Scheme 1). More interestingly, the reaction is highly streoselective affording alkenes in E-geometry. The results are summarized in Table 1. It is worthwhile to mention that Knoevenagel condensation with ketones in this condition does not occur. We used acetophenone as an example of ketones which did not react in these conditions even after prolonged reaction time (24h). The aromatic aldehydes readily condensed with malononitrile, while with ethylcyanoacetate, the reaction is slightly slow. This may be attributed to the fact that abstraction of a proton from the active methylene group of ethylcyanoacetate is more difficult due to its lower activity. As shown in Table 1, aromatic aldehydes containing both electron donating or withdrawing groups gave the desired products in good to excellent yields. Deactivated aldehydes such as nitrobenzaldehyde isomers and 4-chlorobenzaldehyde required shorter reaction time due to the electron withdrawing groups (Table 1, entries 2,3,8,9). Moreover, the steric hindrance seems to have significant effects on the reaction times and yields (Table 1, entries 6,13).
Table 1.(a) Isolated yields
The use of MCM-41 as a recyclable catalyst in this reaction allowed us to perform the condensation under mild conditions. In this case, yields were excellent. In order to show the merit of the present work in comparison with some reported procedures, we compared the result of the synthesis of olefinic compounds in the presence of HZSM-5,15 Proline,16 RE-NaY zeolite17 and MCM-41 with respect to the reaction times and yield of products (Table 2). The results show that MCM-41 promotes the reaction more effectively than HZSM-5. Reaction in the presence of other catalysts in Table 2 required longer reaction times.
In summery, we have demonstrated the MCM-41 catalyzed condensation between various aldehydes and active methylene compounds. This method offers some advantages in terms of simplicity of performance, easy work-up, use of inexpensive, available and easy to handle catalyst and high yields of products and relatively short reaction times.
Table 2.Comparison of the synthesis of trisubstituted alkenes using different catalysts.
All products are known compounds and were characterized by mp, IR, 1HNMR and GC/MS. Melting points were measured by using the capillary tube method with an electro thermal 9200 apparatus. 1HNMR spectra were recorded on a Bruker AQS AVANCE-300 MHz spectrometer using TMS as an internal standard (CDCl3 solution). IR spectra were recorded from KBr disk on the FT-IR Bruker Tensor 27. GC/MS spectra were recorded on an Agilent Technologies 6890 network GC system and an Agilent 5973 network Mass selective detector. Thin layer chromatography (TLC) on commercial aluminum-backed plates of silica gel, 60 F254 was used to monitor the progress of reactions. All products were characterized by spectra and physical data.
Preparation of MCM-41
MCM-41 was prepared according to the procedure described previously.18 A typical procedure was as follow: 1.8 g of fumed silica was added to a solution prepared from dissolving 0.6 g of NaOH in 25 ml of water. The resultant mixture was stirred for 2 h, and then 1.9 g of cetyltrimethyl ammonium bromide (CTABr) in 20 ml of water was added to this solution and stirred for one more hour. The resulting reaction mixture which has the molar composition of 1 SiO2, 7.5 Na2O, 5.2 CTABr, 2500 H2O was kept over night and poured into the teflon lined stainless steel autoclave to make crystallization under static condition at 100 ℃. The product was filtered, washed with distilled water, dried at 70 ℃ and calcined in air at 540 ℃ for 4 h.
General procedure for the Knoevenagel condensation
A mixture of carbonyl compound (1 mmol), ethylcyanoacetate or malononitrile (1mmol) and MCM-41 (0.1 g) was refluxed in CH2Cl2 for indicated time as required to complete the reaction (Table 1). Upon completion of the reaction, monitored by TLC, the reaction mixture was cooled to room temperature. The mixture was filtered off (removed the catalyst and catalyst was washed with methanol for reuse). Upon the evaporation of solvent, the crude product was recrystallized from ethanol to give the pure product.
Selected physical data
2-benzylidene malononitrile (3a). Mp: 85 ℃. 1HNMR (CDC13, 300 MHz) δH (ppm): 7.25-7.51 (m, 5 H), 7.95 (t, J=8.0 Hz, 1H). 13C NMR (CDCl3, Me4Si) δC (ppm): 68.61, 111.28, 111.48, 111.65, 112.41, 112.64, 127.18, 128.82, 131.92, 154.90.
2-(4-Nitrobenzylidene)malononitrile (3b). Mp: 102 ℃. 1HNMR (CDC13, 300 MHz) δH (ppm): 7.2 (2H, J=7.8, d), 7.9 (2H, J=7.8, d), 8.05 (t, J=7.9 Hz, 1H). 13C NMR (CDCl3, Me4Si) δC (ppm): 69.95, 111.55, 111.62, 122.19, 126.17, 130.35, 131.60, 131.98, 148.07, 155.92.
Ethyl-2-cyano-3-(4-hydroxyphenyl)acrylate (3j). Mp: 88 ℃. 1HNMR (CDC13, 300 MHz) δH (ppm): 1.71 (t, J=8.1, 3H), 4.01 (q, J=8.1, 2H), 5.15 (s, 1H), 7.2 (2H, J=7.8, d), 7.9 (2H, J=7.8, d), 8.06 (t, J=7.9 Hz, 1H). 13C NMR (CDCl3, Me4Si) δC (ppm): 14.15, 55.90, 87.38, 112.05, 115.42, 122.01, 123.21, 123.09, 126.07, 127.89, 153.54, 159.81.
Ethyl-2-cyano-3-(4-methoxyphenyl)acrylate (3m). Mp: 85 ℃. 1HNMR (CDC13, 300 MHz) δH (ppm): 1.71 (t, J=8.1, 3H), 3.71 (s, 3H), 4.01 (q, J=8.1, 2H), 7.2 (2H, J=7.8, d), 7.9 (2H, J=7.8, d), 8.15 (t, J=8.0 Hz, 1H). 13C NMR (CDCl3, Me4Si) δC (ppm): 14.25, 55.96, 61.12, 87.45, 112.55, 115.62, 122.19, 123.31, 123.69, 126.17, 127.8, 154.65, 159.91.
Reusability of MCM-41
Next, we investigated the reusability and recycling of MCM-41. At the end of the reaction, the catalyst could be recovered by a simple filtration. The recycled catalyst could be washed with methanol and subjected to a second run of the reaction process. To assure that catalysts were not dissolved in methanol, the catalysts were weighted after filteration and before using and reusing for the next reaction. The results show that these catalysts are not soluble in methanol. In Table 3, the comparison of efficiency of MCM-41 in synthesis of 3a after five times is reported. As it is shown in Table 3, the first reaction using recovered MCM-41 afforded similar yield to those obtained in the first run. In the second, third, fourth and fifth runs, the yield were gradually decreased.
Table 3.(a) Isolated yields