Aqueous starfruit juice catalyzed a simple and efficient Knoevenagel condensation of aromatic aldehydes with malononitrile has been developed under visible light. Products were obtained in yields up to 98% after short reaction times and they were isolated by simple filtration in pure crystallization states. The method is green and economically viable. A plausible mechanism for photochemical Knoevenagel condensation reaction catalyzed by starfruit juice was also predicted.
Knoevenagel condensation, first demonstrated by Emil Knoevenagel in 1894 [
In the past two decades, classical organic chemistry had been rewritten around new approaches that search for products and processes in the chemical industry that are environmentally acceptable [
An attractive area in organic synthesis involves photochemical reactions particularly using visible light in environment-friendly solvent like water or aqueous ethanol and is generally considered as a clean and green procedure. This type of photo-activation of substrate very often minimizing the formation of by-products and for this reason, photochemical reactions occupy an interesting position and excellent reviews/paper have been pub- lished [
A number of organic reactions using natural catalysts such as clay [
Starfruit (Averrhoa carambola) (
The main ingredients [
We have described a potentially efficient, absolutely clean, and high yielding eco-friendly methodology, for the photochemical Knoevenagel condensation of various aromatic aldehydes with malononitrile catalyzed by aque- ous starfruit juice. The present protocol devoid of any toxic catalysts, solvents or solid supports and may be considered as an excellent improvement over the existing methods.
All reactions were run in dried glassware. Reagents were purchased (Spectrochem or SRL or LOBA) and used without further purification. Melting points were determined on a Kofler block and uncorrected. Reactions were irradiated in a 200 W tungsten lamp (Philips India Ltd). 1H and 13C NMR and spectra were obtained in CDCl3 or DMSO-d6 on a Bruker AV-300 (300 MHz) spectrometers using TMS as an internal standard. Analytical samples were dried in vacuo at room temperature. The carbon, hydrogen and nitrogen percentages in synthesized prod- ucts were analyzed by Perkin-Elmer 2400 series II C, H, N analyzers. Thin layer chromatography was carried out on silica gel.
The mature green starfruit were purchased from the local market. The starfruit were cut into pieces with the help of knife. The hard green material (20 g) was boiled with water (50 ml), cooled and it was centrifuged using mi- cro centrifuge (REMI RM-12C). The clear portion of the aqueous extract (pH = 3.5) of the starfruit was used as catalyst for the reactions.
Different aromatic aldehydes (1a-s) (10 mmol) or (1t) (5 mmol), malononitrile (10 mmol), and aqueous starfruit juice (5 ml, pH = 3.5) were taken in a round bottomed flask and irradiated with a 200 W tungsten lamp (Philips India Ltd). The reaction time varied from 2 - 7 min monitored by TLC. Upon completion of the reaction, the re- action mixture was cooled and the crystalline products (3a-t) so obtained was filtered, washed with water and dried in vacuo. The Knoevenagel condensation products were isolated in excellent yields in essentially pure form.
2-(3-Hydroxyphenylmethylene)malononitrile (3e): Yellow crystal, Yield: 95%, mp. 165˚C; 1H NMR (300 MHz, DMSO-d6): δ 7.08 (d, 7.5 Hz, 1H), 7.35 - 7.44 (m, 3H), 8.44 (s, 1H, H-C=C), 10.12 (s, 1H, OH); Anal. Calcd. for C10H6N2O, C, 70.58; H, 3.55; N, 16.46%, found C, 70.22; H, 3.87; N, 16.21%.
2-(4-Benzoyloxyphenylmethylene)malononitrile (3n): Colorless crystal, Yield: 94%, mp. 152˚C - 154˚C; 1H NMR (300 MHz, CDCl3): δ 7.43 (d, 8.7 Hz, 2H), 7.54 (t, 7.5 Hz, 2H), 7.66 - 7.78 (m, 1H), 7.78 (s, 1H, H-C=C), 8.01 (d, 8.7 Hz, 2H), 8.20 (d, 7.8 Hz, 2H); 13C NMR (75 MHz, CDCl3): δ 82.54 (=C<), 112.49 (CN), 113.61 (CN), 123.12, 128.41, 128.53, 128.75, 130.29 (-CH=), 132.37, 134.20, 155.56, 158.56, 164.24 (ester carbonyl); DEPT - 90 (75 MHz, CDCl3): 123.11, 128.74, 130.28, 132.35, 134.18, 158.52; DEPT - 135 (75 MHz, CDCl3): 123.11, 128.74, 130.28, 132.35, 134.18, 158.51; Anal. Calcd. for C17H10N2O2, C, 74.45; H, 3.67; N, 10.21%, found C, 74.11; H, 3.81; N, 10.43%.
2-(4-Benzoyloxy-3-methoxyphenylmethylene)malono-nitrile (3o): Colorless crystal, Yield: 92%, mp. 140˚C - 141˚C; 1H NMR (300 MHz, CDCl3): δ 3.89 (s, 3H, OMe), 7.34 (d, 8.4 Hz, 1H), 7.43 (dd, 8.7 and 1.8 Hz, 1H), 7.53 (t, 7.5 Hz, 2H), 7.64 - 7.69 (m, 1H), 7.74 (d, 1.8 Hz, 1H), 7.76 (s, 1H, H-C=C), 8.20 (d, 8.7 Hz, 2H);
Anal. Calcd. for C18H12N2O3, C, 71.05; H, 3.97; N, 9.21%, found C, 70.85; H, 4.02; N, 9.52%.
2-(3,4-Methylenedioxyphenylmethylene)malononitrile (3p): Yellow crystal, Yield: 98%, mp. 200˚C - 202˚C; 1H NMR (300 MHz, CDCl3): δ 6.12 (s, 2H, -O-CH2-O-), 6.93 (d, 8.1 Hz, 1H), 7.32 (dd, 8.1 and 1.5 Hz, 1H), 7.59 (s, 1H, H-C=C), 7.60 (s, 1H); Anal. Calcd. for C11H6N2O2, C, 66.67; H, 3.05; N, 14.14%, found C, 67.01; H, 3.21; N, 14.32%.
2-[{p-3, 3’-Bis(2-methylindolyl)methyl}phenyl-methylene]malononitrile (3s): Pale-yellow crystal, Yield: 78%, mp. 320˚C - 322˚C; 1H NMR (300 MHz, CDCl3): δ 2.09 (s, 6H, Me), 6.04 (s, 1H, Ar-CH), 6.84 - 6.93 (m, 4H), 7.06 (t, 6.9 Hz, 2H), 7.28 (d, 9.0 Hz, 2H), 7.44 (d, 8.1 Hz, 2H), 7.72 (s, 1H, H-C=C), 7.80 (d, 8.7 Hz, 2H), 7.80 (br. s, 2H, NH); Anal. Calcd. for C29H22N4, C, 81.67; H, 5.20; N, 13.14%, found C, 81.33; H, 5.40; N, 13.25%.
p-Bis-2-(phenylmethylene)malononitrile (3t): White crystal, Yield: 95%, mp. 300˚C; 1H NMR (300 MHz, DMSO-d6): δ 8.09 (s, 4H), 8.63 (s, 2H, H-C=C); 13C NMR (75 MHz, DMSO-d6): δ 84.71 (=C<), 112.14 (CN), 113.80 (CN), 130.83 (-CH=), 135.32 (aromatic quarternary), 159.80 (aromatic -CH=); DEPT - 90 (75 MHz, DMSO-d6): 130.83, 159.81; DEPT - 135 (75 MHz, DMSO-d6): 130.84, 159.81; Anal. Calcd. for C14H6N4, C, 73.04; H, 2.63; N, 24.34%, found C, 72.95, H, 2.76; N, 24.45%.
Financial assistance from the UGC Minor Research Project No. PSW-130/11-12 (ERO), New Delhi, India is gratefully acknowledged.