Table of Contents
Organic Chemistry International
Volume 2014, Article ID 851924, 8 pages
http://dx.doi.org/10.1155/2014/851924
Research Article

Microwave-Assisted Three-Component “Catalyst and Solvent-Free” Green Protocol: A Highly Efficient and Clean One-Pot Synthesis of Tetrahydrobenzo[b]pyrans

Department of Chemistry, Visva-Bharati (A Central University), Santiniketan 731235, India

Received 26 July 2014; Revised 2 September 2014; Accepted 2 September 2014; Published 17 September 2014

Academic Editor: Ashraf Aly Shehata

Copyright © 2014 Sougata Santra et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Abstract

A green and highly efficient method has been developed for the one-pot synthesis of tetrahydrobenzo[b]pyrans via a three-component condensation of aldehydes, 1,3-cyclic diketones, and malononitrile under MW irradiation without using any catalyst and solvent. This transformation presumably occurs by a sequential Knoevenagel condensation, Michael addition, and intramolecular cyclization. Operational simplicity, solvent and catalyst-free conditions, the compatibility with various functional groups, nonchromatographic purification technique, and high yields are the notable advantages of this procedure.

1. Introduction

Development of environmentally benign and clean synthetic procedures has become the goal of organic synthesis in recent times [15]. The multicomponent reactions (MCRs) are one of the most powerful and efficient tools in organic synthesis for the invention of biologically important scaffolds in the viewpoint of green chemistry [69]. One-pot multicomponent reactions (MCRs) have attracted considerable attention from the viewpoint of ideal synthesis by virtue of their efficiency, facile implementation, and generally high yield of the products [1013]. Indeed, the concept of environmental factor (E-factor) and atom economy have gradually become included into conventional organic synthesis in both industry and academia. Solvents are the main reason for an insufficient E-factor, especially in synthesis of fine chemicals and pharmaceutical industries [14, 15]. As a result, it has become imperative both in academia and industry to design catalyst- and solvent-free MCRs, as these processes are rendered green with reduction of waste, time, manpower, and cost [1621].

Recently MW-assisted chemistry has become a useful technique for a variety of applications in organic synthesis and transformations [2225]. Microwave (MW)-promoted MCRs have been attracting research interest from chemists because these reactions exhibit some particular or unexpected reactivities and also because of their significant usefulness in green chemistry [5, 26]. In continuation of our research, we have reported few MW-promoted multicomponent coupling reactions in various chemical transformations for the synthesis of useful heterocyclic compounds [2732]. Very recently, we have reported an efficient synthesis of pyrano[3,2-c]coumarin derivatives via copper(II) triflate catalyzed tandem reaction of 4-hydroxycoumarin with α, β-unsaturated carbonyl compounds under solvent-free conditions [33]. 4H-Benzo[b]pyrans are ubiquitous to a variety of biologically active molecules and have been shown to a wide range of pharmacological activities and biological properties, for example, spasmolytic, diuretic, anticoagulant, anticancer, and anti-anaphylactic activities [3436]. In the last decade numerous methods have been developed for the synthesis of 4H-benzo[b]pyrans [3767] by using a broad variety of toxic nitrogen-containing bases [3747], electrolytic multicomponent transformation [48], hazardous and volatile organic solvents [49, 50], different types of metal catalysts particularly, MgO [50, 51], nano-MgO in [bmIm]BF4 [54], SiO2NPs [55], ZnO NPs [56, 57], biocatalyst [58], and mesoporous material [60]. Few solvent-free and microwave assisted methodologies have also been reported for the preparation of this moiety with limited substrates scope [6163]. Very recently meglumine [64], urea [65], ZnFe2O4 [66], and Fe3O4NPs [67] have been used for the synthesis of these compounds. Regardless of their efficiency and reliability, most of these methodologies are not satisfactory in view of green chemistry by means of using large amount of volatile solvents, toxic and uneasily available catalyst, longer reaction times, and lower yields. To avoid these limitations, there is a need for a simple, efficient, and cost-effective “green protocol” for the synthesis of 4H-benzo[b]pyran derivatives under environmentally friendly conditions.

2. Materials and Methods

Reactions carried out under scientific microwave reactor (Biotage, Initiator EXP EU 355301). Melting points were determined on a glass disk with an electric hot plate and are uncorrected. 1H NMR (400 MHz) and 13C NMR (100 MHz) spectra were run in DMSO- and CDCl3 solutions. IR spectra were taken as KBr plates in a Shimazdu 8400S FTIR. Commercially available substrates were freshly distilled before the reaction. Solvents, reagents, and chemicals were purchased from Aldrich, Fluka, Merck, SRL, Spectrochem and Process Chemicals.

2.1. General Procedure for Tetrahydrobenzo[b]pyran (4)

A equimolar mixture of aldehyde (1 mmol), malononitrile (1 mmol) and 1,3-cyclic diketone (1 mmol) was taken in a microwave vessel. The reaction mixture was irradiated under scientific microwave (Biotage, Initiator EXP EU 355301) at 80°C for a certain period of time to complete the reaction. The reaction mixture was then washed with ethanol (10 mL) to afford the crude product as solid, which was recrystallized from EtOH to get the analytically pure product.

2.1.1. 2-Amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4aa)

White solid (88%), m.p.: 230-231°C ([53] 230°C); 1H NMR (400 MHz, DMSO-): (m, 2H, H-Ar), 7.12–7.06 (m, 3H, H-Ar), 6.93 (brs, 2H, NH2), 4.10 (s, 1H, H-4), 2.45-2.44 (m, 2H, CH2), 2.18 (d,  Hz, 1H, H-6′), 2.02 (d,  Hz, 1H, H-6), 0.96 (s, 3H, CH3), 0.88 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.6, 158.5, 144.8, 128.4, 127.2, 126.6, 119.8, 112.8, 58.3, 50.0, 35.6, 31.8, 28.4, 26.8 ppm; IR (KBr): 3435, 3318, 2913, 2198, 1672 cm−1.

2.1.2. 2-Amino-7,7-dimethyl-5-oxo-4-p-tolyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ba)

White solid (88%), m.p.: 216-217°C ([53] 218°C); 1H NMR (400 MHz, CDCl3): (m, 4H, H-Ar), 4.56 (brs, 2H, NH2), 4.35 (s, 1H, H-4), 2.43 (s, 2H, CH2), 2.25 (s, 3H, CH3), 2.21 (d,  Hz, 2H, CH2), 1.10 (s, 3H, CH3), 1.03 (s, 3H, CH3) ppm; 13C NMR (100 MHz, CDCl3): , 161.3, 157.3, 140.2, 136.6, 129.2, 127.3, 118.7, 114.1, 50.6, 40.6, 35.1, 32.1, 28.8, 27.7, 21.0 ppm; IR (KBr): 3413, 3324, 2956, 2191, 1664 cm−1.

2.1.3. 2-Amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ca)

White solid (84%), m.p.: 208-209°C ([53] 206°C); 1H NMR (400 MHz, DMSO-): (d,  Hz, 2H, H-Ar), 7.19 (d,  Hz, 2H, H-Ar), 4.59 (br, 2H, NH2), 4.40 (s, 1H, H-4), 2.46 (s, 2H, CH2), 2.23 (d,  Hz, 2H, CH2), 1.13 (s, 3H, CH3), 1.04 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.6, 158.5, 143.7, 131.1, 129.1, 128.2, 119.5, 112.3, 57.7, 49.9, 35.1, 31.7, 28.3, 26.8 ppm; IR (KBr): 3390, 3321, 3253, 3211, 2962, 2190, 1739, 1681, 1654, 1604, 1213, 1039, 844 cm−1.

2.1.4. 2-Amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4da)

Yellow solid (82%), m.p.: 178–180°C ([47] 180–182°C); 1H NMR (400 MHz, DMSO-): (d,  Hz, 2H, H-Ar), 7.37 (d,  Hz, 2H, H-Ar), 7.12 (brs, 2H, NH2), 4.29 (s, 1H, H-4), 2.47–2.44 (m, 2H, CH2), 2.20 (d,  Hz, 1H, H-6′), 2.04 (d,  Hz, 1H, H-6), 0.97 (s, 3H, CH3), 0.89 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 158.6, 152.3, 146.3, 128.7, 125.6, 123.7, 119.4, 111.7, 57.0, 49.9, 35.7, 31.9, 28.3, 27.0 ppm; IR (KBr): 3436, 3324, 2196, 1668 cm−1.

2.1.5. 2-Amino-4-(4-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ea)

Yellow solid (83%), m.p.: 201–203°C ([53] 203°C); 1H NMR (400 MHz, DMSO-): (d,  Hz, 2H, H-Ar), 6.95 (br, 2H, NH2), 6.84 (d,  Hz, 2H, H-Ar), 4.12 (s, 1H, H-4), 3.71 (s, 3H, OCH3), 2.50-2.49 (m, 2H, CH2), 2.22-2.21 (m, 2H, CH2), 1.03 (s, 3H, CH3), 0.94 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.1, 158.4, 157.9, 136.8, 128.2, 119.8, 113.6, 113.0, 58.5, 55.0, 50.1, 34.7, 31.7, 28.4, 26.7 ppm; IR (KBr): 3376, 3316, 2955, 2194, 1683, 1140, 1035, 842 cm−1.

2.1.6. (E)-2-Amino-7,7-dimethyl-5-oxo-4-styryl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4fa)

White solid (74%), m.p.: 187-188°C ([47] 183–185°C); 1H NMR (400 MHz, DMSO-): (d,  Hz, 2H, H-Ar), 7.21 (t,  Hz, 2H, H-Ar), 7.16–7.14 (m, 1H, H-Ar), 7.01 (brs, 2H, NH2), 6.30 (d,  Hz, 1H, CH), 6.03–5.98 (m, 1H, CH), 3.75 (d,  Hz, 1H, H-4), 2.36-2.35 (m, 2H, CH2), 2.17 (d,  Hz, 2H, CH2), 0.96 (s, 3H, CH3), 0.93 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.8, 159.6, 136.8, 131.5, 129.6, 129.0, 127.8, 126.6, 120.3, 112.2, 55.5, 50.5, 33.2, 32.2, 28.6, 27.3 ppm; IR (KBr): 3436, 3321, 2933, 2193, 1673 cm−1.

2.1.7. 2-Amino-4-(3-hydroxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ga)

White solid (76%), m.p.: 228-229°C ([53] 225°C); 1H NMR (400 MHz, DMSO-): (s, 1H, OH), 7.01–6.92 (m, 3H, H-Ar), 6.51–6.47 (m, 3H, H-Ar, NH2), 3.99 (s, 1H, H-4), 2.44-2.43 (m, 2H, CH2), 2.19 (d,  Hz, 1H, H-6′), 2.03 (d,  Hz, 1H, H-6), 0.96 (s, 3H, CH3), 0.89 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.8, 158.9, 157.7, 146.5, 129.6, 120.2, 118.1, 114.4, 113.9, 113.2, 58.7, 50.3, 35.8, 32.2, 28.8, 27.1 ppm; IR (KBr): 3431, 3335, 2915, 1668 cm−1.

2.1.8. 2-Amino-4-(4-hydroxy-3-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ha)

White solid (82%), m.p.: 242–244°C ([47] 240–242°C); 1H NMR (400 MHz, DMSO-): (s, 1H, OH), 6.92 (s, 2H, NH2), 6.68–6.64 (m, 2H, Ar-H), 6.53–6.50 (m, 1H, Ar-H), 4.07 (s, 1H, H-4), 3.71 (s, 3H, OCH3), 2.51–2.48 (m, 2H, CH2), 2.25 (d,  Hz, 1H, H-6′), 2.10 (d,  Hz, 1H, H-6), 1.03 (s, 3H, CH3), 0.97 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.2, 158.4, 147.3, 145.2, 135.8, 119.9, 119.4, 115.3, 113.0, 111.4, 58.8, 55.6, 50.0, 35.0, 31.8, 28.5, 26.6 ppm; IR (KBr): 3416, 3341, 2923, 2192, 1662 cm−1.

2.1.9. 2-Amino-4-(benzo[d][1,3]dioxol-5-yl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ia)

White solid (88%), m.p.: 224-225°C; 1H NMR (400 MHz, DMSO-): (brs, 2H, NH2), 6.81 (d,  Hz, 1H, H-Ar), 6.62–6.59 (m, 2H, H-Ar), 5.97 (s, 2H, CH2), 4.10 (s, 1H, H-4), 2.50 (s, 2H, CH2), 2.24 (d,  Hz, 1H, H-6′), 2.12 (d,  Hz, 1H, H-6), 1.03 (s, 3H, CH3), 0.96 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 162.4, 158.4, 147.2, 145.9, 138.9, 120.3, 112.7, 108.0, 107.5, 100.9, 58.5, 50.0, 35.2, 31.8, 28.3, 26.9 ppm; IR (KBr): 3435, 3318, 2923, 2198, 1670 cm−1; Anal. Calcd. for C19H18N2O4: C, 67.44; H, 5.36; N, 8.28%; Found: C, 67.36; H, 5.31; N, 8.22%.

2.1.10. 2-Amino-4-(4-bromophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ja)

White solid (85%), m.p.: 210–212°C ([53] 215°C); 1H NMR (400 MHz, DMSO-): (d, Hz, 2H, H-Ar), 7.10 (d,  Hz, 2H, H-Ar), 7.06 (brs, 2H, NH2), 4.18 (s, 1H, H-4), 2.51-2.50 (m, 2H, CH2), 2.25 (d,  Hz, 1H, H-6′), 2.10 (d,  Hz, 1H, H-6), 1.03 (s, 3H, CH3), 0.94 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 163.0, 158.8, 144.5, 131.6, 129.9, 120.0 (2C), 112.6, 58.0, 50.3, 35.6, 32.2, 28.7, 27.2 ppm.

2.1.11. 2-Amino-4-(furan-2-yl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ka)

White solid (72%), m.p.: 224–226°C ([47] 220–222°C); 1H NMR (400 MHz, DMSO-): (m, 1H, CH), 7.01 (brs, 2H, NH2), 6.26-6.25 (m, 1H, CH), 5.99-5.98 (m, 1H, CH), 4.25 (s, 1H, H-4), 2.48–2.39 (m, 2H, CH2), 2.22 (d,  Hz, 1H, H-6′), 2.10 (d,  Hz, 1H, H-6), 0.97 (s, 3H, CH3), 0.91 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 163.4, 159.4, 155.8, 141.8, 119.6, 110.5, 110.4, 105.1, 55.4, 49.9, 31.9, 29.0, 28.5, 26.6 ppm; IR (KBr): 3441, 3314, 2923, 2182, 1665 cm−1.

2.1.12. Typical Procedure for the Synthesis of 2-Amino-4-cyclohexyl-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4la)

A mixture of cyclohexanecarbaldehyde (1 mmol), malononitrile (1 mmol), and dimedone (1 mmol) was taken in a microwave vessel. The reaction mixture was irradiated under scientific microwave (Biotage, Initiator EXP EU 355301) at 80°C for 7 min to complete the reaction. The reaction mixture was then washed with ethanol (10 mL) to afford the crude product as solid, which was recrystallized from EtOH to get the analytically pure product as white solid (88%). M.p.: 203-204°C; 1H NMR (400 MHz, CDCl3): (br, 2H, NH2), 3.31 (s, 1H, H-4), 2.38-2.37 (m, 2H, CH2), 2.29-2.28 (m, 2H, CH2), 1.75–1.63 (m, 4H, CH2), 1.49–1.41 (m, 2H, CH2), 1.34–1.31 (m, 1H, CH2), 1.11–1.09 (m, 9H, CH2 and CH3), 0.95–0.92 (m, 1H, CH2) ppm; 13C NMR (100 MHz, CDCl3): , 163.2, 159.9, 120.3, 114.1, 58.5, 50.8, 43.7, 40.6, 34.7, 32.0, 30.4, 29.2, 27.7, 27.3, 26.5, 26.2, 26.1 ppm. IR (KBr): 3409, 3326, 2923, 2192, 1662, 1373, 1255, 1213, 1033, 945 cm−1; Anal. Calcd. for C18H24N2O2: C, 71.97; H, 8.05; N, 9.33%; Found: C, 71.91; H, 8.02; N, 9.27%.

2.1.13. 2-Amino-5-oxo-4-phenyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4ab)

White solid (85%), m.p.: 234–236°C ([48] 238–240°C); 1H NMR (400 MHz, DMSO-): (m, 2H, H-Ar), 7.20–7.14 (m, 3H, H-Ar), 6.99 (s, 2H, NH2), 4.18 (s, 1H, H-4), 2.65–2.59 (m, 2H, CH2), 2.35–2.22 (m, 2H, CH2), 1.99–1.90 (m, 2H, CH2) ppm; 13C NMR (100 MHz, DMSO-): , 164.9, 158.8, 145.1, 128.7, 127.5, 126.9, 120.2, 114.1, 58.6, 36.7, 35.8, 26.8, 20.2 ppm; IR (KBr): 3446, 3324, 2903, 2184, 1665 cm−1.

2.1.14. 2-Amino-5-oxo-4-p-tolyl-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4bb)

White solid (86%), m.p.: 227-228°C ([48] 223–225°C); 1H NMR (400 MHz, DMSO-): (m, 4H, H-Ar), 6.96 (brs, 2H, NH2), 4.14 (s, 1H, H-4), 2.62–2.59 (m, 2H, CH2), 2.27–2.24 (m, 5H, CH2 and CH3), 1.94–1.84 (m, 2H, CH2) ppm; 13C NMR (100 MHz, DMSO-): , 164.7, 158.8, 142.2, 136.0, 129.2, 127.4, 120.2, 114.3, 58.7, 36.7, 35.4, 26.8, 20.9, 20.2 ppm; IR (KBr): 3437, 3345, 2924, 1678 cm−1.

2.1.15. 2-Amino-4-(4-chlorophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile (4cb)

White solid (88%), m.p.: 230–232°C ([48] 226–228°C); 1H NMR (400 MHz, DMSO-): (m, 2H, H-Ar), 7.19–7.16 (m, 2H, H-Ar), 7.05 (brs, 2H, NH2), 4.20 (s, 1H, H-4), 2.62–2.58 (m, 2H, CH2), 2.31–2.24 (m, 2H, CH2), 1.97–1.87 (m, 2H, CH2) ppm; 13C NMR (100 MHz, DMSO-): , 165.0, 158.8, 144.1, 131.5, 129.5, 128.6, 120.0, 113.7, 58.1, 36.6, 35.4, 26.8, 20.1 ppm; IR (KBr): 3451, 3314, 2913, 2197, 1682 cm−1.

2.1.16. 2-Amino-5-oxo-4-p-tolyl-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (4bc)

White solid (86%), m.p.: 253–255°C ([47] 257–259°C); 1H NMR (400 MHz, DMSO-): (m, 2H, H-Ar), 7.60–7.56 (m, 2H, H-Ar), 7.44–7.31 (m, 4H, H-Ar), 7.02 (s, 2H, NH2), 4.40 (s, 1H, CH), 2.23 (s, 3H, CH3) ppm; 13C NMR (100 MHz, DMSO-): , 158.0, 153.3, 152.3, 140.4, 131.9, 129.1, 128.7, 126.6, 123.8, 118.1, 116.0, 113.0, 104.2, 58.2, 36.6, 35.7, 20.7 ppm; IR (KBr): 3451, 3310, 2935, 1685 cm−1.

2.1.17. 2-Amino-4-(4-chloro-phenyl)-5-oxo-4H,5H-pyrano[3,2-c]chromene-3-carbonitrile (4cc)

White solid (85%), m.p.: 262–264°C ([47] 265–267°C); 1H NMR (400 MHz, DMSO-): (m, 2H, H-Ar), 7.58–7.53 (m, 2H, NH2), 7.32–7.28 (m, 4H, H-Ar), 7.13 (d,  Hz, 2H, H-Ar), 4.48 (s, 1H, CH) ppm; 13C NMR (100 MHz, DMSO-): , 158.0, 152.4, 142.4, 140.3, 131.6, 129.7, 128.7, 127.9, 124.1, 123.5, 118.8, 115.8, 103.6, 57.6, 36.5, 35.8 ppm; IR (KBr): 3456, 3306, 2934, 2191, 1686 cm−1.

3. Results and Discussion

As a part of our ongoing research to provide greener methodologies under solvent and catalyst-free conditions [27, 29, 6871] we have found that the three-component condensation of aldehyde, 1,3-cyclic diketone, and malononitrile under MW irradiation without using any catalyst and solvent produced 4H-benzo[b]pyran derivatives in high yields within short reaction times (Scheme 1). Indeed, to the best of our knowledge, this is the first report of synthesis of 4H-benzo[b]pyran under catalyst and solvent-free conditions.

851924.sch.001
Scheme 1: Synthesis of 4H-benzo[b]pyran derivatives under MW irradiation.

Initially, we commenced our study taking benzaldehyde, malononitrile, and dimedone as the model substrates at 80°C (conventional heating) under catalyst and solvent-free conditions for 3 hours; however, a mixture of products was obtained. By increasing the temperature and time, the progress of the reaction was not satisfactory. We, then, turned our attention towards MW irradiation instead of conventional heating. Gratifyingly, the desired product was obtained in 88% in a microwave reactor (Biotage, Initiator EXP EU 355301) after 7 min at 80°C. By increasing the time and temperature the yield decreased. This may be due to the decomposition of the product at higher temperature. Finally, reaction conditions were optimized using benzaldehyde (1 mmol), malononitrile (1 mmol), and dimedone (1 mmol) at 80°C under microwave irradiation for 7 min. A wide range of structurally varied aldehydes and 1,3-cyclic diketones were subjected under optimized reaction conditions to provide the corresponding 4H-benzo[b]pyran derivatives as summarized in Scheme 3.

It can be seen that electron-rich and electron-deficient aldehydes reacted efficiently to afford the desired products with good yields. The chloro- and bromo-substituted benzaldehydes gave the corresponding 4ca, 4ja, and 4cb in 84%, 85%, and 88% yields, respectively. Aldehyde containing electron donating –OMe groups on the aromatic ring were well-tolerated (4ea and 4ha). 3-Hydroxybenzaldehyde afforded the corresponding product (4ga) with good yield. In addition, aldehyde containing two electron donating functional groups (–OH and –OMe) reacted very well (4ha). Heteroaryl aldehydes such as furfural also participated in the multicomponent reaction to produce the desired product in moderate yield without affecting the heterocyclic moiety (4ka). We are delighted to find that the α,β-unsaturated aldehyde, such as cinnamaldehyde, was tolerated under our present reaction conditions (4fa). Acid-sensitive substrate, such as piperonal, produced the desired condensation product 4ia in excellent yield. Notable advantage of this method is its efficiency for the synthesis of 4H-benzo[b]pyrans derivative from aliphatic aldehyde with high yields (4la). In addition, 4-hydroxycoumarin also afforded the corresponding products (4bc and 4cc). In general the reactions are clean and reaction procedure is very simple. To provide the analytically pure 4H-benzo[b]pyran derivatives, only ethanol was employed for recrystallization. Moreover, we have developed greener reaction conditions bearing lower E-factor [14, 15, 72] of 0.21 and 0.25 in the cases of synthesizing 4aa and 4ba, respectively.

The plausible mechanism for the reaction is exposed in Scheme 2. Based on the literature [61], we assume that Knoevenagel condensation, Michael addition, and intramolecular cyclization are involved subsequently in the synthesis of 4H-benzo[b]pyran derivatives. In the first step, the aldehyde undergoes a Knoevenagel condensation reaction with malononitrile to afford cyano olefin [A] [61, 73], which endures a Michael addition reaction with the tautomeric enolic form of dimedone [B] to give the intermediate [C]. The intermediate C on intramolecular cyclization produces the final product 4.

851924.sch.002
Scheme 2: Plausible reaction mechanism.
851924.sch.003
Scheme 3: Synthesis of various 4H-benzo[b]pyran derivatives. Reaction conditions: 1 mmol of 1, 1 mmol of 2, and 1 mmol of 3 under microwave irradiation at 80°C for 7 min. All are isolated yields.  mmol of aldehyde, 1 mmol of 2, and 1 mmol of 4-hydroxycoumarin under microwave irradiation at 80°C for 7 min.

4. Conclusions

In summary, we have developed an environmentally benign one-pot strategy for the synthesis of 4H-benzo[b]pyran derivatives in high yields under microwave irradiation using a mixture of aldehydes, malononitrile, and 1,3-cyclic diketones. Operational simplicity, solvent and catalyst-free conditions, compatibility with various functional groups, and nonchromatographic purification technique are notable advantages of this procedure. Lower E-factor values make this protocol better and a more practical alternative to the existing methodologies. The combination of solvent and catalyst-free conditions under microwave irradiation makes this procedure truly environmentally benign.

Conflict of Interests

The authors declare that there is no conflict of interests regarding the publication of this paper.

Acknowledgments

Alakananda Hajra is pleased to acknowledge the financial support from CSIR, Government of India (Grant no. 02(0168)/13/EMR-II). The authors are thankful to DST-FIST and UGC-SAP. Sougata Santra thanks UGC, New Delhi, India, for his fellowship.

References

  1. D. J. Ramón and M. Yus, “Asymmetric multicomponent reactions (AMCRs): the new frontier,” Angewandte Chemie—International Edition, vol. 44, no. 11, pp. 1602–1634, 2005. View at Publisher · View at Google Scholar · View at Scopus
  2. M. O. Simon and C. J. Li, “Green chemistry oriented organic synthesis in water,” Chemical Society Reviews, vol. 41, no. 4, pp. 1415–1427, 2012. View at Publisher · View at Google Scholar · View at Scopus
  3. M. B. Gawande, A. Velhinho, I. D. Nogueira, C. A. A. Ghumman, O. M. N. D. Teodoro, and P. S. Branco, “A facile synthesis of cysteine-ferrite magnetic nanoparticles for application in multicomponent reactions—a sustainable protocol,” RSC Advances, vol. 2, no. 15, pp. 6144–6149, 2012. View at Publisher · View at Google Scholar · View at Scopus
  4. Y. Gu, “Multicomponent reactions in unconventional solvents: State of the art,” Green Chemistry, vol. 14, no. 8, pp. 2091–2128, 2012. View at Publisher · View at Google Scholar · View at Scopus
  5. M. B. Gawande, V. D. B. Bonifácio, R. Luque, P. S. Branco, and R. S. Varma, “Benign by design: catalyst-free in-water, on-water green chemical methodologies in organic synthesis,” Chemical Society Reviews, vol. 42, no. 12, pp. 5522–5551, 2013. View at Publisher · View at Google Scholar · View at Scopus
  6. P. Anastas and J. C. Warner, Green Chemistry: Theory and Practice, Oxford University Press, New York, NY, USA, 1998.
  7. M. Poliakoff and P. Anastas, “A principled stance,” Nature, vol. 413, no. 6853, p. 257, 2001. View at Publisher · View at Google Scholar · View at Scopus
  8. J. M. DeSimone, “Practical approaches to green solvents,” Science, vol. 297, no. 5582, pp. 799–803, 2002. View at Publisher · View at Google Scholar · View at Scopus
  9. R. A. Gross and B. Kalra, “Biodegradable polymers for the environment,” Science, vol. 297, no. 5582, pp. 803–807, 2002. View at Publisher · View at Google Scholar · View at Scopus
  10. J. Zhu and H. Bienayme, Multicomponent Reactions, Wiley-VCH, Weinheim, Germany, 2005.
  11. A. Dömling, “Recent developments in isocyanide based multicomponent reactions in applied chemistry,” Chemical Reviews, vol. 106, no. 1, pp. 17–89, 2006. View at Publisher · View at Google Scholar · View at Scopus
  12. L. Weber, K. Illgen, and M. Almstetter, “Discovery of new multi component reactions with combinatorial methods,” Synlett, no. 3, pp. 366–374, 1999. View at Google Scholar · View at Scopus
  13. R. W. Armstrong, A. P. Combs, P. A. Tempest, S. D. Brown, and T. A. Keating, “Multiple-component condensation strategies for combinatorial library synthesis,” Accounts of Chemical Research, vol. 29, no. 3, pp. 123–131, 1996. View at Google Scholar
  14. R. A. Sheldon, “E factors, green chemistry and catalysis: an odyssey,” Chemical Communications, no. 29, pp. 3352–3365, 2008. View at Publisher · View at Google Scholar · View at Scopus
  15. R. A. Sheldon, “Fundamentals of green chemistry: efficiency in reaction design,” Chemical Society Reviews, vol. 41, no. 4, pp. 1437–1451, 2012. View at Publisher · View at Google Scholar · View at Scopus
  16. M. Ješelnik, R. S. Varma, S. Polanc, and M. Kocevar, “Catalyst-free reactions under solvent-free conditions: Microwave-assisted synthesis of heterocyclic hydrazones below the melting points of neat reactants,” Chemical Communications, no. 18, pp. 1716–1717, 2001. View at Google Scholar · View at Scopus
  17. M. S. Singh and S. Chowdhury, “Recent developments in solvent-free multicomponent reactions: a perfect synergy for eco-compatible organic synthesis,” RSC Advances, vol. 2, no. 11, pp. 4547–4592, 2012. View at Publisher · View at Google Scholar · View at Scopus
  18. K. Tanaka and F. Toda, “Solvent-free organic synthesis,” Chemical Reviews, vol. 100, no. 3, pp. 1025–1074, 2000. View at Publisher · View at Google Scholar · View at Scopus
  19. G. W. V. Cave, C. L. Raston, and J. L. Scott, “Recent advances in solventless organic reactions: towards benign synthesis with remarkable versatility,” Chemical Communications, no. 21, pp. 2159–2169, 2001. View at Google Scholar · View at Scopus
  20. A. K. Chakraborti, S. Rudrawar, K. B. Jadhav, G. Kaur, and S. V. Chankeshwara, ““on water” organic synthesis: a highly efficient and clean synthesis of 2-aryl/heteroaryl/styryl benzothiazoles and 2-alkyl/aryl alkyl benzothiazolines,” Green Chemistry, vol. 9, no. 12, pp. 1335–1340, 2007. View at Publisher · View at Google Scholar · View at Scopus
  21. S. V. Chankeshwara and A. K. Chakraborti, “Catalyst-free chemoselective N-tert-butyloxycarbonylation of amines in water,” Organic Letters, vol. 8, no. 15, pp. 3259–3262, 2006. View at Publisher · View at Google Scholar · View at Scopus
  22. B. A. Roberts and C. R. Strauss, “Toward rapid, “green”, predictable microwave-assisted synthesis,” Accounts of Chemical Research, vol. 38, no. 8, pp. 653–661, 2005. View at Publisher · View at Google Scholar · View at Scopus
  23. C. O. Kappe, “Controlled microwave heating in modern organic synthesis,” Angewandte Chemie, vol. 43, no. 46, pp. 6250–6284, 2004. View at Publisher · View at Google Scholar · View at Scopus
  24. D. Dallinger and C. O. Kappe, “Microwave-assisted synthesis in water as solvent,” Chemical Reviews, vol. 107, no. 6, pp. 2563–2591, 2007. View at Publisher · View at Google Scholar · View at Scopus
  25. J. S. Yadav, B. V. S. Reddy, K. S. Shankar, T. Swamy, and K. Premalatha, “Microwave-accelerated solvent and catalyst free synthesis of 3-indolylhydroquinones,” Bulletin of the Korean Chemical Society, vol. 29, no. 7, pp. 1418–1420, 2008. View at Publisher · View at Google Scholar · View at Scopus
  26. V. Polshettiwar and R. S. Varma, “Microwave-assisted organic synthesis and transformations using benign reaction media,” Accounts of Chemical Research, vol. 41, no. 5, pp. 629–639, 2008. View at Publisher · View at Google Scholar · View at Scopus
  27. M. Rahman, A. K. Bagdi, S. Mishra, and A. Hajra, “Functionalization of an sp3 C—H bond via a redox-neutral domino reaction: diastereoselective synthesis of hexahydropyrrolo[2,1-b]oxazoles,” Chemical Communications, vol. 50, no. 22, pp. 2951–2953, 2014. View at Publisher · View at Google Scholar · View at Scopus
  28. D. Kundu, A. Majee, and A. Hajra, “Task-specific ionic liquid catalyzed efficient microwave-assisted synthesis of 12-alkyl or aryl-8,9,10,12-tetrahydrobenzo[a]xanthen-11-ones under solvent-free conditions,” Green Chemistry Letters and Reviews, vol. 4, no. 3, pp. 205–209, 2011. View at Publisher · View at Google Scholar · View at Scopus
  29. S. Das, S. Santra, A. Roy, S. Urinda, A. Majee, and A. Hajra, “One-pot multicomponent synthesis of polyhydroquinolines under catalyst and solvent-free conditions,” Green Chemistry Letters and Reviews, vol. 5, no. 1, pp. 97–100, 2012. View at Publisher · View at Google Scholar · View at Scopus
  30. A. Hajra, D. Kundu, and A. Majee, “An efficient one-pot synthesis of naphthoxazinones by a three-component coupling of naphthol, aldehydes, and urea catalyzed by zinc triflate,” Journal of Heterocyclic Chemistry, vol. 46, no. 5, pp. 1019–1022, 2009. View at Publisher · View at Google Scholar · View at Scopus
  31. M. Rahman, A. Majee, and A. Hajra, “Microwave-assisted brønsted acidic ionic liquid-promoted one-pot synthesis of heterobicyclic dihydropyrimidinones by a three-component coupling of cyclopentanone, aldehydes, and urea,” Journal of Heterocyclic Chemistry, vol. 47, no. 5, pp. 1230–1233, 2010. View at Publisher · View at Google Scholar · View at Scopus
  32. M. Rahman, A. K. Bagdi, D. Kundu, A. Majee, and A. Hajra, “Zwitterionic-type molten salt-catalyzed multicomponent reactions: One-pot synthesis of substituted imidazoles under solvent-free conditions,” Journal of Heterocyclic Chemistry, vol. 49, no. 5, pp. 1224–1228, 2012. View at Publisher · View at Google Scholar · View at Scopus
  33. A. K. Bagdi, A. Majee, and A. Hajra, “Regioselective synthesis of pyrano[3,2-c]coumarins via Cu(II)-catalyzed tandem reaction,” Tetrahedron Letters, vol. 54, no. 29, pp. 3892–3895, 2013. View at Publisher · View at Google Scholar · View at Scopus
  34. L. L. Andreani and E. Lapi, “Aspects and orientations in modern pharmacognosy,” Bollettino Chimico Farmaceutico, vol. 99, pp. 583–587, 1960. View at Google Scholar
  35. Y. L. Zhang, B. Z. Chen, K. Q. Zheng, M. L. Xu, and X. H. Lei, “Chemotherapeutic studies on schistosomiasis—XXV. Derivatives of substituted coumarin-3-carboxylic esters and amides,” Chemical Abstracts, vol. 96, Article ID 135383e, 1993. View at Google Scholar
  36. L. Bonsignore, G. Loy, D. Secci, and A. Calignano, “Synthesis and pharmacological activity of 2-oxo-(2H) 1-benzopyran-3-carboxamide derivatives,” European Journal of Medicinal Chemistry, vol. 28, no. 6, pp. 517–520, 1993. View at Publisher · View at Google Scholar · View at Scopus
  37. T.-S. Jin, J.-C. Xiao, S.-J. Wang, T.-S. Li, and X.-R. Song, “An efficient and convenient approach to the synthesis of benzopyrans by a three-component coupling of one-pot reaction,” Synlett, no. 13, pp. 2001–2005, 2003. View at Google Scholar · View at Scopus
  38. S. Balalaie, M. Sheikh-Ahmadi, and M. Bararjanian, “Tetra-methyl ammonium hydroxide: An efficient and versatile catalyst for the one-pot synthesis of tetrahydrobenzo[b]pyran derivatives in aqueous media,” Catalysis Communications, vol. 8, no. 11, pp. 1724–1728, 2007. View at Publisher · View at Google Scholar · View at Scopus
  39. S. Balalaie, M. Bararjanian, M. Sheikh-Ahmadi, S. Hekmat, and P. Salehi, “Diammonium hydrogen phosphate: an efficient and versatile catalyst for the one-pot synthesis of tetrahydrobenzo[b]pyran derivatives in aqueous media,” Synthetic Communications, vol. 37, no. 7, pp. 1097–1108, 2007. View at Publisher · View at Google Scholar · View at Scopus
  40. S.-B. Guo, S.-X. Wang, and J.-T. Li, “D,L-proline-catalyzed one-pot synthesis of pyrans and pyrano[2,3-c]pyrazole derivatives by a grinding method under solvent-free conditions,” Synthetic Communications, vol. 37, no. 13, pp. 2111–2120, 2007. View at Publisher · View at Google Scholar · View at Scopus
  41. B. C. Ranu, S. Banerjee, and S. Roy, “A task specific basic ionic liquid, [bmIm]OH-promoted efficient, green and one-pot synthesis of tetrahydrobenzo[b]pyran derivatives,” Indian Journal of Chemistry B, vol. 47, no. 7, pp. 1108–1112, 2008. View at Google Scholar · View at Scopus
  42. X.-Z. Lian, Y. Huang, Y. Q. Li, and W. J. Zheng, “A green synthesis of tetrahydrobenzo[b]pyran derivatives through three-component condensation using N-methylimidazole as organocatalyst,” Monatshefte fur Chemie, vol. 139, no. 2, pp. 129–131, 2008. View at Publisher · View at Google Scholar · View at Scopus
  43. L.-Q. Yu, F. Liu, and Q.-D. You, “One-pot synthesis of tetrahydrobenzo[b]pyran derivatives catalyzed by amines in aqueous media,” Organic Preparations and Procedures International, vol. 41, no. 1, pp. 77–82, 2009. View at Publisher · View at Google Scholar · View at Scopus
  44. J. Zheng and Y. Li, “Basic ionic liquid-catalyzed multicomponent synthesis of tetrahydrobenzo[b]pyrans and pyrano[c]chromenes,” Mendeleev Communications, vol. 21, no. 5, pp. 280–281, 2011. View at Publisher · View at Google Scholar · View at Scopus
  45. A. Hasaninejad, M. Shekouhy, N. Golzar, A. Zare, and M. M. Doroodmand, “Silica bonded n-propyl-4-aza-1-azoniabicyclo[2.2.2]octane chloride (SB-DABCO): a highly efficient, reusable and new heterogeneous catalyst for the synthesis of 4H-benzo[b]pyran derivatives,” Applied Catalysis A: General, vol. 402, no. 1-2, pp. 11–22, 2011. View at Publisher · View at Google Scholar · View at Scopus
  46. M. Khoobi, L. Ma’mani, F. Rezazadeh et al., “One-pot synthesis of 4H-benzo[b]pyrans and dihydropyrano[c]chromenes using inorganic-organic hybrid magnetic nanocatalyst in water,” Journal of Molecular Catalysis A: Chemical, vol. 359, pp. 74–80, 2012. View at Publisher · View at Google Scholar · View at Scopus
  47. M. G. Dekamin, M. Eslami, and A. Maleki, “Potassium phthalimide-N-oxyl: a novel, efficient, and simple organocatalyst for the one-pot three-component synthesis of various 2-amino-4H-chromene derivatives in water,” Tetrahedron, vol. 69, no. 3, pp. 1074–1085, 2013. View at Publisher · View at Google Scholar · View at Scopus
  48. M. N. Elinson, A. S. Dorofeev, S. K. Feducovich et al., “The implication of electrocatalysis in MCR strategy: electrocatalytic multicomponent transformation of cyclic 1,3-diketones, aldehydes and malononitrile into substituted 5,6,7,8-tetrahydro-4H-chromenes,” European Journal of Organic Chemistry, no. 19, pp. 4335–4339, 2006. View at Publisher · View at Google Scholar · View at Scopus
  49. X.-S. Wang, D.-Q. Shi, S.-J. Tu, and C.-S. Yao, “A convenient synthesis of 5-oxo-5,6,7,8-tetrahydro-4H-benzo-[b]-pyran derivatives catalyzed by KF-alumina,” Synthetic Communications, vol. 33, no. 1, pp. 119–126, 2003. View at Publisher · View at Google Scholar · View at Scopus
  50. M. M. Heravi, Y. S. Beheshtiha, Z. Pirnia, S. Sadjadi, and M. Adibi, “One-pot, three-component synthesis of 4H-pyrans using cu(II) oxymetasilicate,” Synthetic Communications, vol. 39, no. 20, pp. 3663–3667, 2009. View at Publisher · View at Google Scholar · View at Scopus
  51. M. Seifi and H. Sheibani, “High surface area MgO as a highly effective heterogeneous base catalyst for three-component synthesis of tetrahydrobenzopyran and 3,4-dihydropyrano[c] chromene derivatives in aqueous media,” Catalysis Letters, vol. 126, no. 3-4, pp. 275–279, 2008. View at Publisher · View at Google Scholar · View at Scopus
  52. L.-M. Wang, J.-H. Shao, H. Tian, Y.-H. Wang, and B. Liu, “Rare earth perfluorooctanoate [RE(PFO)3] catalyzed one-pot synthesis of benzopyran derivatives,” Journal of Fluorine Chemistry, vol. 127, no. 1, pp. 97–100, 2006. View at Publisher · View at Google Scholar · View at Scopus
  53. R. Hekmatshoar, S. Majedi, and K. Bakhtiari, “Sodium selenate catalyzed simple and efficient synthesis of tetrahydro benzo[b]pyran derivatives,” Catalysis Communications, vol. 9, no. 2, pp. 307–310, 2008. View at Publisher · View at Google Scholar · View at Scopus
  54. D. Kumar, V. B. Reddy, B. G. Mishra, R. K. Rana, M. N. Nadagouda, and R. S. Varma, “Nanosized magnesium oxide as catalyst for the rapid and green synthesis of substituted 2-amino-2-chromenes,” Tetrahedron, vol. 63, no. 15, pp. 3093–3097, 2007. View at Publisher · View at Google Scholar · View at Scopus
  55. S. Banerjee, A. Horn, H. Khatri, and G. Sereda, “A green one-pot multicomponent synthesis of 4H-pyrans and polysubstituted aniline derivatives of biological, pharmacological, and optical applications using silica nanoparticles as reusable catalyst,” Tetrahedron Letters, vol. 52, no. 16, pp. 1878–1881, 2011. View at Publisher · View at Google Scholar · View at Scopus
  56. P. Bhattacharyya, K. Pradhan, S. Paul, and A. R. Das, “Nano crystalline ZnO catalyzed one pot multicomponent reaction for an easy access of fully decorated 4H-pyran scaffolds and its rearrangement to 2-pyridone nucleus in aqueous media,” Tetrahedron Letters, vol. 53, no. 35, pp. 4687–4691, 2012. View at Publisher · View at Google Scholar · View at Scopus
  57. S. Banerjee and A. Saha, “Free-ZnO nanoparticles: a mild, efficient and reusable catalyst for the one-pot multicomponent synthesis of tetrahydrobenzo[b]pyran and dihydropyrimidone derivatives,” New Journal of Chemistry, vol. 37, no. 12, pp. 4170–4175, 2013. View at Publisher · View at Google Scholar · View at Scopus
  58. K. Pradhan, P. Bhattacharyya, S. Paul, and A. R. Das, “Synthesis of 3,4-dihydropyridin-2-one derivatives in convergent mode applying bio catalyst vitamin B1 and polymer supported catalyst PEG-SO3H from two different sets of building blocks,” Tetrahedron Letters, vol. 53, no. 44, pp. 5840–5844, 2012. View at Publisher · View at Google Scholar · View at Scopus
  59. I. A. Azath, P. Puthiaraj, and K. Pitchumani, “One-pot multicomponent solvent-free synthesis of 2-amino-4H-benzo[b]pyrans catalyzed by per-6-amino-β-cyclodextrin,” ACS Sustainable Chemistry and Engineering, vol. 1, no. 1, pp. 174–179, 2013. View at Publisher · View at Google Scholar · View at Scopus
  60. J. Mondal, A. Modak, M. Nandi, H. Uyama, and A. Bhaumik, “Triazine functionalized ordered mesoporous organosilica as a novel organocatalyst for the facile one-pot synthesis of 2-amino-4H-chromenes under solvent-free conditions,” RSC Advances, vol. 2, no. 30, pp. 11306–11317, 2012. View at Publisher · View at Google Scholar · View at Scopus
  61. I. Devi and P. J. Bhuyan, “Sodium bromide catalysed one-pot synthesis of tetrahydrobenzo[b]pyrans via a three-component cyclocondensation under microwave irradiation and solvent free conditions,” Tetrahedron Letters, vol. 45, no. 47, pp. 8625–8627, 2004. View at Publisher · View at Google Scholar · View at Scopus
  62. N. M. A. El-Rahman, A. A. El-Kateb, and M. F. Mady, “Simplified approach to the uncatalyzed knoevenagel condensation and michael addition reactions in water using microwave irradiation,” Synthetic Communications, vol. 37, no. 22, pp. 3961–3970, 2007. View at Publisher · View at Google Scholar · View at Scopus
  63. G. Kaupp, M. R. Naimi-Jamal, and J. Schmeyers, “Solvent-free Knoevenagel condensations and Michael additions in the solid state and in the melt with quantitative yield,” Tetrahedron, vol. 59, no. 21, pp. 3753–3760, 2003. View at Publisher · View at Google Scholar · View at Scopus
  64. R.-Y. Guo, Z.-M. An, L.-P. Mo et al., “Meglumine: a novel and efficient catalyst for one-pot, three-component combinatorial synthesis of functionalized 2-amino-4 H-pyrans,” ACS Combinatorial Science, vol. 15, no. 11, pp. 557–563, 2013. View at Publisher · View at Google Scholar · View at Scopus
  65. G. Brahmachari and B. Banerjee, “Facile and one-pot access to diverse and densely functionalized 2-amino-3-cyano-4H-pyrans and pyran-annulated heterocyclic scaffolds via an eco-friendly multicomponent reaction at room temperature using urea as a novel organo-catalyst,” ACS Sustainable Chemistry & Engineering, vol. 2, no. 3, pp. 411–422, 2014. View at Publisher · View at Google Scholar · View at Scopus
  66. P. Das, A. Dutta, A. Bhaumik, and C. Mukhopadhyay, “Heterogeneous ditopic ZnFe2O4 catalyzed synthesis of 4H-pyrans: further conversion to 1,4-DHPs and report of functional group interconversion from amide to ester,” Green Chemistry, vol. 16, no. 3, pp. 1426–1435, 2014. View at Publisher · View at Google Scholar · View at Scopus
  67. A. Fallah-Shojaei, K. Tabatabaeian, F. Shirini, and S. Z. Hejazi, “Multi-walled carbon nanotube supported Fe3O4NPs: an efficient and reusable catalyst for the one-pot synthesis of 4H-pyran derivatives,” RSC Advances, vol. 4, no. 19, pp. 9509–9516, 2014. View at Publisher · View at Google Scholar · View at Scopus
  68. M. Rahman, D. Kundu, A. Hajra, and A. Majee, “Formylation without catalyst and solvent at 80 °C,” Tetrahedron Letters, vol. 51, no. 21, pp. 2896–2899, 2010. View at Publisher · View at Google Scholar · View at Scopus
  69. B. C. Ranu, A. Hajra, and S. S. Dey, “A practical and green approach towards synthesis of dihydropyrimidinones without any solvent or catalyst,” Organic Process Research & Development, vol. 6, no. 6, pp. 817–818, 2002. View at Publisher · View at Google Scholar · View at Scopus
  70. B. C. Ranu, S. S. Dey, and A. Hajra, “Solvent-free, catalyst-free Michael-type addition of amines toelectron-deficient alkenes,” Arkivoc, vol. 2002, no. 7, pp. 76–81, 2002. View at Google Scholar · View at Scopus
  71. B. C. Ranu and A. Hajra, “A simple and green procedure for the synthesis of α-aminophosphonate by a one-pot three-component condensation of carbonyl compound, amine and diethyl phosphite without solvent and catalyst,” Green Chemistry, vol. 4, no. 6, pp. 551–554, 2002. View at Publisher · View at Google Scholar · View at Scopus
  72. A. Kamal, V. Srinivasulu, B. N. Seshadri, N. Markandeya, A. Alarifi, and N. Shankaraiah, “Water mediated Heck and Ullmann couplings by supported palladium nanoparticles: importance of surface polarity of the carbon spheres,” Green Chemistry, vol. 14, no. 9, pp. 2513–2522, 2012. View at Publisher · View at Google Scholar · View at Scopus
  73. M. Saha, S. Roy, S. K. Chaudhuri, and S. Bhar, “Microwave-assisted ammonium formate-mediated knoevenagel reaction under solvent-free conditions: a green method for C—C bond formation,” Green Chemistry Letters and Reviews, vol. 1, no. 2, pp. 113–121, 2008. View at Publisher · View at Google Scholar · View at Scopus