A Systematic Study on Knoevenagel Reaction and Nazarov Cyclization of Less Reactive Carbonyl Compounds Using Rare Earth Triflates and Its Applications

Abstract

A systematic study of Knoevenagel reaction and Nazarov cyclization was made on variety of less reactive carbonyl compounds such as ${\beta}$-ketoesters, 1,3-diketones and cyclic active methylene compounds using $Yb(OTf)_3$ as the catalyst. Recycling study confirms reusability of the catalyst without much loss of activity.

EXPERIMENTAL

All yields reported were based on isolated compounds. TLC separations were carried out on silica gel plates with UV indicator from Aldrich; visualization was by UV fluorescence or by staining with iodine vapor. IR spectra were recorded on a FT-IR Bruker Vector 22 Infrared spectrophotometer using KBr disks. NMR spectra were recorded on FT-NMR Bruker 400/200 MHz spectrometer as CDCl3 solutions with TMS as internal reference.

General Experimental Procedure for the Synthesis of Substituted Alkenes

To a mixture of an aromatic aldehyde (5.0 mmol) and active methylene compound (5.5 mmol), Yb(OTf)3 (0.5 mmol) was added and the resulting reaction mixture was heated at 80–85 ℃ in an oil bath for required time. The progress of the reaction was monitored by thin layer chromatography (TLC, silica gel, hexane: ethyl acetate, 8:2). After completion of the reaction, the reaction mixture was diluted by adding ethyl acetate (10 mL) and washed with water (2×5 mL) and brine solution (5 mL). The organic layer was dried (Na2SO4) and concentrated under reduced pressure. The residue was purified by column chromatography (silica gel, hexane-EtOAc, 8.5:1.5) to afford the substituted olefin in very good yield. All the new products obtained were fully characterized by spectroscopic methods such as IR, 1H NMR, 13C NMR and mass spectroscopy. If the compound is already known in the literature, IR, and 1H NMR values are given, compared with the spectral data already known, and a suitable reference is also mentioned.

2-(4-Methoxybenzylidene)malononitrile (3a)

The reaction was carried out according to the general experimental procedure using 4-methoxybenzaldehyde (250 mg, 1.836 mmol), malononitrile (145 mg, 2.197 mmol) and Yb(OTf)3 (114 mg, 0.184 mmol). Conditions: 80-85 ℃, 2 h. The title compound 3a was obtained (330 mg, 98%) as a solid, mp 114-116 ℃. The spectral data for the compound 3a was in agreement with the values already reported in the literature.

1H NMR (400 MHz, DMSO) d 3.88 (s, 3H, -OCH3), 7.19 (m, 2H, ArH), 7.97 (m, 2H, ArH) and 8.40 (s, 1H, ArCH=). IR (KBr) 2224, 1605, 1571, 1513, 1370, 1319, 1279, 1185, 1022, 834 cm-1.

2-(4-Methoxy-benzylidene)-malonic acid diethyl ester (3i)

The reaction was carried out according to the general procedure using 4-methoxy benzaldehyde (100 mg, 0.735 mmol), diethyl malonate (129 mg, 0.808 mmol) and Yb(OTf)3 (52 mg, 0.074 mmol) conditions: 80-85 ℃, 6 h. The title compound 3i was obtained (171 mg, 84%) as an oily mass. The spectral data of the compound 3i was in agreement with the values reported in the literature.

1H NMR (400 MHz, CDCl3) δ 1.20 (t, J=7.2 Hz, 3H, -CH2CH3), 1.25 (t, J=7.2 Hz, 3H, -CH2CH3), 3.44 (s, 3H, -OCH3), 4.26 (m, J=7.2, 16.4 Hz, 4H, -CH2CH3), 7.49 (m, 4H, ArH), 7.72 (s, 1H, ArCH=). IR (KBr) 2982, 1723, 1628, 1448, 1375, 1255, 1196 cm-1.

2-Cyano-3-(3,4,5-trimethoxy-phenyl)-propionic acid ethyl ester (4)

The reaction was carried out 2-cyano-3-(3,4,5-trimethoxyphenyl)-acrylic acid ethyl ester (250 mg, 0.860 mmol), Hantzsch ester (240 mg, 0.950 mmol), and silica (2.5 g) in presence of toluene (5 mL) at reflex condition for 5 h. The title compound 4 was obtained (275 mg, 94%). The spectral data of the compound 4 was in agreement with the values reported in the literature.

1H NMR (400 MHz, CDCl3) δ 1.22 (t, J=7.2 Hz, 3H, -CH2CH3), 3.12 (m, J=5.6, 13.6 Hz, 2H, Ar-CH2), 3.64 (t, J=5.6, 1H, ArCH2-CH-), 3.76 (s, 3H, -OCH3), 3.78 (s, 6H, 2-OCH3), 4.19 (q, J=7.2, 14. Hz, 2H, -CH2CH3), 6.42 (s, 2H, ArH). IR (KBr) 2942, 2253, 1743, 1592, 1509, 1463, 1245, 1127, 1005, 851 cm-1.

2-(2-Chloro-benzylidene)-3-oxo-butyric acid ethyl ester (7d)

The reaction was carried out according to the general procedure using 2-chloro benzaldehyde (100 mg, 0.711 mmol), ethylacetoacetate (102 mg, 0.783 mmol) and Yb(OTf)3 (50 mg, 0.071 mmol) conditions: 80-85 ℃, 15 h. The crude product was purified by column chromatography (silica gel, hexane: ethyl acetate. 9:1). First eluted was unreacted starting material 1d (40 mg, 40%). Second eluted was the title compound 7d was obtained (54 mg, 57%, yield calculated based on the recovered starting material) as an oilic mass. The spectral data of the compound 7d was in agreement with the values reported in the literature.

1H NMR (400 MHz, CDCl3) δ 1.17 (t, J=7.2 Hz, 3H, -CH2CH3), 2.42 (s, 3H, -COCH3), 4.24 (q, J=7.6, 14.0 Hz, 2H, -CH2CH3), 7.35 (m, 4H, ArH). 7.85(s, 1H, ArCH=). IR (KBr) 1724, 1669, 1618, 1468, 1439, 1376, 1239, 1200 cm-1.

3-(4-Nitrobenzylidene)pentane-2,4-dione (7f)

The reaction was carried out according to the general experimental procedure using 4-nitrobenzaldehyde (250 mg, 1.650 mmol), acetyl acetone (182 mg, 1.820 mmol) and Yb(OTf)3 (103 mg, 0.165 mmol). Conditions: 80-85℃, 18 h. The title compound 7f was obtained (131 mg, 34%) as a clear oilic mass. The spectral data for the compound 7f was in agreement with the values already reported in the literature.

1H NMR (400 MHz, CDCl3) δ 2.23 (s, 3H, -COCH3), 2.39 (s, 3H, -COCH3), 3.76 (s, 3H, -OCH3), 7.43 (s, 1H, ArCH=), 7.48 (d, 2H, ArH), 8.17 (d, 2H, ArH). IR (KBr) 1709, 1653, 1595, 1518, 1345, 1237, 1172, 858 cm-1.

5-(4-Hydroxy-3-methoxybenzylidene)pyrimidine-2,4,6 (1H,3H,5H)-trione (13c)

The reaction was carried out according to the general experimental procedure using vaniline (250 mg, 1.645 mmol), barbituric acid (210 mg, 1.645 mmol) and Yb(OTf)3 (102 mg, 0.165 mmol). Conditions: 80-85 ℃, 5 h. The title compound 13c was obtained (415 mg, 96%) as a solid, mp 309-311 ℃. The spectral data for the compound 13c was in agreement with the values already reported in the literature.

1H NMR (400 MHz, DMSO) δ 3.82 (s, 3H, -OCH3), 6.89 (d, 1H, ArH), 7.79 (m, 1H, ArH), 8.21 (s, 1H, ArCH=), 8.47 (d, 1H, ArH), 10.56 (s, 1H, -OH), 11.13 (s, 1H, -NH), 11.26 (s, 1H, -NH). IR (KBr) 3456, 3195, 3130, 3052, 2847, 1729, 1692, 1657, 1542, 1498, 1398, 1299, 1273, 1255, 1178, 1136, 1024, 851, 791, 516 cm-1.

2,2-Dimethyl-5-(4-methoxybenzylidene)-1,3-dioxane-4,6-dione (14b)

The reaction was carried out according to the general experimental procedure using 4-methoxybenzaldehyde (250 mg, 1.836 mmol), meldrumsacid (317 mg, 2.201 mmol) and Yb(OTf)3 (115 mg, 0.185 mmol). Conditions: 80-85 ℃, 4.5 h. The title compound 14b was obtained (320 mg, 67%) as a solid, mp 127-129 ℃. The spectral data for the compound 14b was in agreement with the values already reported in the literature.

1H NMR (400 MHz, DMSO) δ 1.72 (s, 6H, -CH3), 3.87 (s, 3H, -OCH3) 7.09 (d, 2H, ArH), 8.22 (d, 2H, ArH), 8.30 (s, 1H, ArCH=).

IR (KBr) 3101, 2997, 2840, 1747, 1714, 1574, 1514, 1429, 1391, 1203, 1171, 1019, 933, 837, 798 cm-1.