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Original article
12 (
8
); 5212-5222
doi:
10.1016/j.arabjc.2016.12.015

Synthesis and characterization of nanostructured Cu-ZnO: An efficient catalyst for the preparation of (E)-3-styrylchromones

, , , , ,
a P.G. and Research Centre, Radhabai Kale Mahila Mahavidyalaya, Ahmednagar 414 001, India
b P.G. and Research Centre, Yashavantrao Chavhan Institute of Science, Satara 415 001, India

⁎Corresponding author at: P.G. and Research Centre, Radhabai Kale Mahila Mahavidyalaya, Ahmednagar 414 001, India. Fax: 0241 243 0318. kgkanade@yahoo.co.in (Kaluram G. Kanade)

Received: , Accepted: ,
© The Author(s) 2017
Disclaimer:
This article was originally published by Elsevier and was migrated to Scientific Scholar after the change of Publisher.

Peer review under responsibility of King Saud University.

Abstract

We have explored nanocrystalline ZnO and Cu-ZnO catalyst for the preparation of 3-styrylchromones with trans selectivity derived from 3-formylchromones. Synthesis of ZnO and Cu-ZnO nano flakes (NFs) was carried by precipitation technique. The analytical techniques such as UV-Visible spectroscopy, X-ray diffraction (XRD), Brunauer-Emmett-Teller (BET), field emission scanning electron microscopy (FESEM) and energy-dispersive analysis X-ray spectroscopy (EDAX) were used to characterize the catalysts. The XRD pattern showed highly pure wurtzite ZnO and Cu-ZnO. The FESEM images showed nano flakes such as sunflower seed morphology in the range width of 9–34 nm and length 90–180 nm. Doping of copper in ZnO was employed to study the selectivity of Knoevenagel and Knoevenagel-Doebner reactions. Knoevenagel condensation was catalyzed efficiently by pure ZnO nano flakes, whereas the Cu-ZnO nano flakes facilitate the Knoevenagel-Doebner reaction. Present synthetic protocols are novel, very clean and high yielding for synthesis of 3-styrylchromones. Almost same yield was observed to the recycled catalyst up to four runs.

Keywords

Heterogeneous catalysis
ZnO nano flakes
Knoevenagel condensation
Knoevenagel-Doebner reaction
3-Styrylchromone
1

1 Introduction

Chromone derivatives are of great interest for chemists and biochemists, owing to their potential biological activities and natural occurrence (Faridoon et al., 2016). Styrylchromones are small group of natural flavonoids which have shown anticancer (Maicheen et al., 2013), antiviral (Vints and Rozen, 2014), antibacterial (Ghani et al., 2013) and antiallergic (Velema et al., 2013) activities. Natural 2-styrylchromone hormothamnione and 6-desmethoxyormothamnione were isolated from marine cryophyte Chrysophaeum taylori exhibited potent cytotoxic activity (Gerwick, 1989). The synthesis, reactivity and biological evaluation of styrylchromone derivatives become an important research, and among them the study of 3-styrylchromones is limited compared to 2-styrylchromone (Pawar et al., 2013; Sakagami et al., 2015).

A variety of strategies for the synthesis of 3-styrylchromones have been emerged; of these, the Wittig reaction of 3-formylchromones gives mixture of (E) and (Z) diastereomer (Sandulache et al., 2003). However, this synthetic strategy suffered from diastereomeric separation, use of toxic ylides and low atom economy. Heck reaction between 3-bromochromone and alkene gives structurally diverse 3-styrylchromone (Kim and Hong, 2011; Davies et al., 1987), but this process requires two key activation steps, formation of 3-bromochromone and Pd(0) catalyzed Heck coupling reaction. Knoevenagel condensation is one of the simplest and direct methods for the synthesis of 3-styrylchromone. Potassium tert-butoxide (Silva et al., 2008) and dry pyridine (Karale et al., 1999, 2003) were used for Knoevenagel condensation of 3-formylchromone. A modification in this route, a Knoevenagel-Doebner reaction between 3-formylchromones with 4-nitrophenylacetic acid which consists of condensation followed by decarboxylation reaction gives 3-styrylchromone (Karale et al., 2002; Shelke et al., 2009). However, these protocols are suffered by one of disadvantages such as deactivation of catalyst, extended reaction time and low yield. Hence, it would be imperative to develop an efficient method for synthesis of 3-styrylchromone.

The nanocatalysis in organic transformation is one of the distinctive catalytic phenomena which is well documented and explored by many researchers (Polshettiwar and Varma, 2010). Nanocrystalline metal oxides and metal doped metal oxides with controlled morphology showed enhanced catalytic activity over conventional catalysts (Algarni et al., 2016; Zhang et al., 2015; Hou and Li, 2016; Chattopadhyay et al., 2009; Gawande et al., 2012; Wachs, 2005; Kanade et al., 2007). Due to elevated catalytic ability, environmental stability and reasonable price, nanocrystalline ZnO is promising candidate as catalyst in various organic transformations (Hosseini-Sarvari, 2016; Indulkar et al., 2012; Kiamehr and Moghaddam, 2009). Recently, we explored the catalytic activity of nano sized ZnO in dihydropyrimidin-2(1H)-one/thione synthesis (Shinde et al., 2016).

A variety of methods have been utilized for the synthesis of morphological controlled nanocrystalline ZnO (Li et al., 2012, 2013a; Hong et al., 2006). Among these the simplest solution based on non-aqueous route is most appealing owing to a solvent controlling the crystal growth (Li et al., 2013b; Zeng et al., 2016; Kanade et al., 2006). Hence, herein we report the synthesis of morphological controlled nanocrystalline ZnO and Cu doped ZnO using amyl alcohol as reaction medium, as synthesized catalyst was used for Knoevenagel condensation of 2,4-dinitrotoluene and Knoevenagel-Doebner reaction of 4-nitrophenylacetic acid in synthesis of 3-styrylchromones (Schemes 2 and 4).

2

2 Experimental

2.1

2.1 Chemical and materials

Zinc acetate [Zn(CH3COO)2 2H2O], oxalic acid [H2C2O4 2H2O], copper chloride [CuCl2], amyl alcohol, acetone, dimethylformamide and deionized-water were used in the experiments. All reagents were procured from Alfa Aesar and were used without further purification.

2.2

2.2 Synthesis of ZnO and Cu doped ZnO nano flakes

The catalysts were prepared by following procedure as reported in our earlier communication with a few modifications (Kanade et al., 2007). In particular, zinc acetate (0.15 M) and oxalic acid (0.1 M) solutions were prepared in amyl alcohol. The precursor zinc oxalate and co-precipitate Cu-zinc oxalate were obtained by slow addition of oxalic acid solution into zinc acetate solution with constant stirring rate 250 rpm for 12 h at room temperature. The requisite quantity of copper acetate was added to dope copper (Cu = 5%) in ZnO. To remove impurity if any, the precipitate was washed with high purity water and acetone respectively. The decomposition of zinc oxalate and Cu-zinc oxalates was carried out at 450 °C for 5 h under aerobic conditions.

2.3

2.3 General procedure for synthesis of 3-styrylchromone catalyzed by ZnO and Cu-ZnO nano flakes

In a 50 mL of round bottom flask, a mixture of substituted 3-formylchromone (0.001 mol), 2,4-dinitrotoluene/4-nitrophenylacetic acid (0.001 mol), catalyst and DMF (10 mL) was taken. The reaction mixture was refluxed for specified times. The reaction progress was monitored by TLC. After completion, the reaction mixture was cooled to room temperature. An insoluble catalyst was recovered by filtration and recycled by oven drying. Furthermore, solvent was recovered on rotary evaporator under reduced pressure. The crude product was recrystallized in acetic acid. A structure of all synthesized products was confirmed by IR, 1H NMR, mass spectra and comparison of their physical constant.

2.4

2.4 Characterization techniques

The UV-vis spectra were obtained in high purity water as solvents on an ELICO SL 210 UV-vis spectrophotometer. The phase purity of catalysts was established by XRD pattern with Bruker axs-D8 Advance. Nanocatalyst was scanned over range of 2θ = 20–80° using Cu Kα radiation (λ = 1.5406 Å). Inductively coupled plasma optical emission spectrophotometer (ICP-OES) was used to establish the percentage of copper in Cu-ZnO. Brunauer-Emmett-Teller (BET) surface area of nanocatalysts was examined using Quantachrome v11.02 nitrogen instrument. The surface morphology and particle size were determined using a FESEM with FEI Nova NanoSEM 450. Fourier transform infrared (FTIR) spectra were recorded on a Shimadzu Affinity 1-S spectrophotometer. 1H NMR was recorded in DMSO-d6 and Chloroform-d solvent on a Bruker Advance-400 spectrometer with tetramethylsilane (TMS) as internal reference. The progress of reaction was monitored by TLC.

3

3 Result and discussion

3.1

3.1 Characterization of catalyst

3.1.1

3.1.1 Optical absorption

UV-Visible absorption spectra were examined by dispersing nanocatalysts in high purity water at room temperature Fig. 1. The UV absorption for pure ZnO and Cu-ZnO was observed at wavelength 372 nm and 391 nm respectively. The slight red shift in an absorption edge confirms the doping of Cu in ZnO (Mohan et al., 2012).

The UV-Visible absorption spectra of (a) ZnO and (b) Cu(5%)-ZnO nano flakes.
Figure 1
The UV-Visible absorption spectra of (a) ZnO and (b) Cu(5%)-ZnO nano flakes.

3.1.2

3.1.2 XRD analysis

The XRD patterns of ZnO and Cu(5%)-ZnO nano flakes are shown in Fig. 2. The crystal phase of catalyst was established from the position and intensity of XRD peaks using JCPDS card No.36-1451. It confirms hexagonal wurtzite arrangement for ZnO and Cu-ZnO. The average crystallite size is evaluated using FWHM of Scherrer’s formula (Holzwarth and Gibson, 2011). Peaks of Metallic Cu and its oxides were not observed in XRD. This indicates the incorporation of Cu ion into the Zn lattice site (Singhal et al., 2012; Kanade et al., 2007). The nanocrystalline ZnO and Cu-ZnO have an average crystallite sizes in range 15–27 and 20–37 nm respectively. Decreased peak intensities in Cu-ZnO indicated that pure ZnO is more crystalline.

XRD of (a) ZnO (b) Cu(5%)-ZnO nano flakes.
Figure 2
XRD of (a) ZnO (b) Cu(5%)-ZnO nano flakes.

3.1.3

3.1.3 Brunauer-Emmet-Teller (BET) method

The Nitrogen adsorption/desorption isotherm for ZnO and Cu doped ZnO nano flakes is shown in Fig. 3. They can be classified as type IV, which confirms the presence of mesoporous materials (Xu et al., 2007). However, at very high relative pressures the capillary condensation occurs and adsorption saturation is not considerably visible. The specific BET surface area ZnO and Cu(5%)-ZnO nano flakes were found to be 79.13 and 38.54 m2 g−1 respectively. Although the crystallite size of the ZnO and Cu-ZnO does not differ significantly, the BET surface area of the former is greater than that of the latter. A possible cause is agglomeration of the Cu-ZnO nano flakes. The FESEM images of Cu-ZnO support the same (Fig. 4a–d). The average pore diameters of pure ZnO and Cu(5%)-ZnO samples are 15.9 nm and 20.3 nm respectively (Fig. 3a and b, inset).

Nitrogen adsorption-desorption isotherms (a) ZnO and (b) Cu-ZnO nano flakes.
Figure 3
Nitrogen adsorption-desorption isotherms (a) ZnO and (b) Cu-ZnO nano flakes.
FESEM images of (a and b) ZnO and (c and d) Cu(5%)-ZnO nano flakes.
Figure 4
FESEM images of (a and b) ZnO and (c and d) Cu(5%)-ZnO nano flakes.

3.1.4

3.1.4 Morphology

The FESEM images of ZnO and Cu-ZnO displayed nano flake morphology (Fig. 4a–d). Pure ZnO appears evenly sized nano flakes such as sunflower seeds with a width and length in range of 9–14 nm and 100–180 nm respectively (Fig. 4a and b). On the other hand, Cu-ZnO showed agglomerated uneven sized nano flakes with a width and length in range of 9–35 nm and 114–180 nm respectively. A morphological distortion in Cu-ZnO is observed may be due to incorporation of Cu2+ in ZnO nano structure.

3.1.5

3.1.5 Compositional analysis by ICP-OES and EDAX

The elemental copper percentage in Cu-ZnO catalyst was examined with ICP-OES. Table 1 shows copper percentage in ZnO is in good agreement with the calculated values. The compositional analysis of the pure ZnO and Cu- ZnO nano flakes has been carried out using EDAX (Fig. S1).

Table 1 Elemental analysis of Cu (%) in Cu-ZnO.
Copper percentage
Calculated Observed
Cu-ZnO 5% 4.56

In order to optimize reaction condition, Knoevenagel condensation of 6-chloro-4-oxo-4H-chromene-3-carbaldehyde 1, 2,4-dinitrotoluene 2 and 30 mol% of catalyst was employed as standard model reaction for the synthesis of compound 3 (Scheme 1).

Standard model reaction.
Scheme 1
Standard model reaction.

The model reaction was optimized by various experimental parameters such as catalyst, solvent and temperature as outlined in Table 2.

Table 2 Optimization of reaction conditions for synthesis of (E)-3-(2,4-dinitrostyryl)-6-chloro-4H-chromen-4-one.a
Entry Catalyst Solvent Temp (°C) Time (h) Yield (%)b
1 ZnO NF Ethanol Reflux 10 Trace
2 ZnO NF Water Reflux 10 Trace
3 ZnO NF DMSO Reflux 5 58
4 ZnO NF Toluene, Reflux 6 29
5 ZnO NF DMF Reflux 2 77
6 Cu -ZnO NF DMF Reflux 2.5 65
7 ZnO bulk DMF Reflux 5 53
8 ZnO NF DMF RT 12 39
9 ZnO NF DMF 60 8 42
10 ZnO NF DMF 80 5 63
a Reaction condition: 6-chloro-4-oxo-4H-chromene-3-carbaldehyde (1 mmol) 1, 2,4-dinitrotoluene (1 mmol) 2, 30 mol% of catalyst.
b Isolated yield.

Initially, reaction was catalyzed by ZnO nano flakes in various solvents at reflux temperature. The reaction in ethanol and water afforded the expected 3-(2,4-dinitrostyryl)-6-chloro-4H-chromen-4-one in only traces with several side products and unreacted starting material (Table 2, entries 1–2). The reaction proceeded in DMSO and toluene with yield 58% and 29% respectively (Table 2, entries 3–4), while, in DMF good yield was achieved (Table 2, entry 5). It may be due to the enhancement of anionic process in polar aprotic solvent (Taha et al., 2008). The desired lower yield of the reported synthesis 3-styrylchromones, may be due to side reactions such as facile oxidation and dimerization of 2,4-dinitrotoluene (Farrissey et al., 1969).

To observe the catalytic effectiveness of bulk ZnO, ZnO and Cu doped ZnO nano flake, the reaction was carried out in DMF as a solvent at reflux temperature. The pure ZnO shows best catalytic activity over bulk ZnO and Cu(5%) ZnO (Table 2, entries 5–7).

To investigate appropriate concentration of catalyst nanocrystalline ZnO, standard model reaction was carried out using 5, 10, 15, 20, 30 and 50 mol% of the ZnO in DMF at 110 °C (Table 3, entries 1–5). The optimum amount of ZnO was 10 mol%, and use of catalyst more than 10 mol% ZnO nanoparticles has no significant effect yield of products.

Table 3 Influence of amount of ZnO catalyst in synthesis of (E)-3-(2,4-dinitrostyryl)-6-chloro-4H-chromen-4-one.a
Entry Catalyst (Mol%) Yield (%)b
1 5 69
2 10 78
3 15 76
4 20 74
5 30 73
6 50 70
a Reaction condition: 6-chloro-4-oxo-4H-chromene-3-carbaldehyde (1 mmol)1a, 2,4-dinitrotoluene (1 mmol) 2, catalyst.
b Isolated yield.

In particular, the developed protocol (Scheme 2) was extended to all substrates listed in Table 4.

Synthesis of (E)-3-(2,4-dinitrostyryl)-4H-chromen-4-ones 3a-h.
Scheme 2
Synthesis of (E)-3-(2,4-dinitrostyryl)-4H-chromen-4-ones 3a-h.

Similarly, the reaction parameters for condensation of 4-nitrophenylacetic acid (Scheme 3) were optimized through series of experiments as listed in Table 5. Interestingly, Cu-ZnO showed superior catalytic efficiency over ZnO nanoparticles (Table 5, entry 10).

Synthesis (E)-3-(4-nitrostyryl)-6-chloro-4H-chromen-4-one.
Scheme 3
Synthesis (E)-3-(4-nitrostyryl)-6-chloro-4H-chromen-4-one.
Table 4 Synthesis of (E)-3-(2,4-dinitrostyryl)-4H-chromen-4-ones.a
Compound Structure Yield (%)b Mp (°C) (lit.) References
3a 78 224 (225) Karale et al. (2002)
3b 69 231 (232) Karale et al. (2002)
3c 68 228 (228) Karale et al. (2002)
3d 80 282 (280) Karale et al. (2002)
3e 72 285 –
3f 70 252 (255) Karale et al. (2002)
3g 67 248 (248) Karale et al. (2002)
3h 80 253 –
a Reaction condition: 3-formylchromone(1 mmol)1a-h, 2,4-dinitrotoluene (1 mmol) 2, ZnO nano flakes.
b Isolated yield.
Table 5 Optimization of reaction conditions for synthesis of 3-(4-nitrostyryl)-6-chloro-4H-chromen-4-one.a
Entry Catalyst Solvent Temp (°C) Time (h) Yield (%)b
1 ZnO NF Ethanol Reflux 10 Trace
3 ZnO NF Water Reflux 10 Trace
4 ZnO NF DMSO Reflux 3 63
5 ZnO NF Toluene, Reflux 5 35
6 ZnO NF DMF Reflux 2.5 67
7 Cu-ZnO NF DMF Reflux 2 80
8 Cu-ZnO NF DMF RT 12 45
9 Cu-ZnO NF DMF 60 8 59
10 Cu-ZnO NF DMF 80 5 72
a Reaction condition: 6-chloro-4-oxo-4H-chromene-3-carbaldehyde (1 mmol)1a, 4-nitrophenylacetic acid (1 mmol) 4, catalyst.
b Isolated yield.

On optimized catalyst loading, 15 mol% of Cu(5%)-ZnO showed best result (Table 6).

Table 6 Influence of amount of Cu(5%)-ZnO catalyst in condensation of 4-nitrophenylacetic acid.a
Entry Catalyst (Mol%) Yield (%)b
1 5 67
2 10 75
3 15 84
4 20 82
5 30 80
a Reaction condition: 6-chloro-4-oxo-4H-chromene-3-carbaldehyde (1 mmol)1a, 4-nitrophenylacetic acid (1 mmol) 4, catalyst.
b Isolated yield.

We extended optimized reaction protocol for all other substrates listed in Table 7.

Table 7 Synthesis of 3-(4-nitrostyryl)-4H-chromen-4-ones.a
Compound Structure Yield (%)b Mp (°C) (lit.) References
5a 85 203 (202–204) Shelke et al. (2009)
5b 75 253 (250–252) Shelke et al. (2009)
5c 73 233 (232–234) Shelke et al. (2009)
5d 87 270 (269–271) Shelke et al. (2009)
5e 79 285 (283–285) Shelke et al. (2009)
5f 77 261 (262–264) Shelke et al. (2009)
5g 71 240 (240–242) Sandulache et al. (2003)
5h 88 262 (260–262) Shelke et al. (2009)
a Reaction condition: 3-formylchromone (1 mmol)1a-h, 4-nitrophenylacetic acid (1 mmol) 4, Cu-ZnO catalyst.
b Isolated yield.

3.2

3.2 Spectroscopic data of synthesized compounds

3.2.1

3.2.1 (E)-3-(2,4-dinitrostyryl)-6-chloro-4H-chromen-4-one(3a)

IR (cm−1):1647, 1593, 1556, 1517, 1463, 1436, 1342, 1328, 1307, 1265, 829, 738, 636. 1H NMR (400 MHz, DMSO-d6) δ 8.80 (s, 1H), 8.75 (d, J = 2.3 Hz, 1H), 8.52 (dd, J = 8.8, 2.2 Hz, 1H), 8.26 (d, J = 16.0 Hz, 1H), 8.18 (d, J = 8.8 Hz, 1H), 8.08 (d, J = 2.5 Hz, 1H), 7.89 (dd, J = 9.0, 2.6 Hz, 1H), 7.79 (d, J = 9.0 Hz, 1H), 7.36 (d, J = 16.0 Hz, 1H). LC-MS: 373 [M + 1]+ (Figs. S2–S4).

3.2.2

3.2.2 (E)-3-(2,4-dinitrostyryl)-6-chloro-7-methyl-4H-chromen-4-one(3b)

IR (cm−1): 1649, 1593, 1537, 1521, 1419, 1340, 1309, 1265, 1182, 1124, 904, 833, 786, 738, 717. 1H NMR (400 MHz, DMSO-d6) δ 8.80–8.70 (m, 1H), 8.50 (dd, J = 8.8, 2.2 Hz, 1H), 8.25 (d, J = 16.0 Hz, 1H), 8.16 (d, J = 8.9 Hz, 1H), 8.04 (s, 1H), 7.95 (s, 1H), 7.77 (s, 1H), 7.34 (d, J = 15.9 Hz, 1H), 2.48 (s, 3H). LC-MS: 387 [M + 1]+ (Figs. S5–S7).

3.2.3

3.2.3 (E)-3-(2,4-dinitrostyryl)-6,8-dimethyl-4H-chromen-4-one(3c)

IR (cm−1): 1647, 1593, 1517, 1348, 1469, 1321, 1267, 966, 831, 788, 740, 721. 1H NMR (400 MHz, DMSO-d6) δ 8.84–8.66 (m, 2H), 8.51 (dd, J = 8.8, 2.5 Hz, 1H), 8.30 (d, J = 15.9 Hz, 1H), 8.20 (d, J = 8.8 Hz, 1H), 7.78 (d, J = 2.0 Hz, 1H), 7.53 (s, 1H), 7.37 (d, J = 15.9 Hz, 1H), 2.45 (s, 3H), 2.41 (s, 3H). LC-MS: 367 [M + 1]+ (Figs. S8–S10).

3.2.4

3.2.4 (E)-3-(2,4-dinitrostyryl)-6,8-dichloro-4H-chromen-4-one(3d)

IR (cm−1): 1651, 1595, 1556, 1531, 1519, 1448, 1348, 1327, 1269, 1174, 972, 869, 740, 688. 1H NMR (400 MHz, DMSO-d6) δ 8.89 (s, 1H), 8.77 (d, J = 2.4 Hz, 1H), 8.54 (dd, J = 8.8, 2.4 Hz, 1H), 8.33–8.16 (m, 3H), 8.07 (d, J = 2.5 Hz, 1H), 7.36 (d, J = 15.9 Hz, 1H). LC-MS: 406 [M + 1]+ (Figs. S11–S13).

3.2.5

3.2.5 (E)-3-(2,4-dinitrostyryl)-7-methyl-4H-chromen-4-one (3f)

IR (cm−1):1645, 1593, 1529, 1516, 1479, 1346, 1332, 1315, 1271, 1315, 1271, 966, 825, 740, 719. 1H NMR (400 MHz, Chloroform-d) δ 8.83 (s, 1H), 8.44 (d, J = 8.8 Hz, 1H), 8.21 (d, J = 10.0 Hz, 2H), 8.10 (d, J = 14.5 Hz, 2H), 7.98 (d, J = 8.7 Hz, 1H), 7.53 (d, J = 8.5 Hz, 1H), 7.41 (d, J = 8.6 Hz, 1H), 2.49 (s, 3H). LC-MS: 353 [M + 1]+ (Figs. S14–S16).

3.2.6

3.2.6 (E)-3-(4-nitrostyryl)-6-chloro-4H-chromen-4-one (5a)

IR (cm−1):3066, 1651, 1591, 1558, 1510, 1504, 1463, 1440, 1336, 1323, 1273 1161, 1109, 873, 833, 786, 746. 1H NMR (400 MHz, DMSO-d6) δ 8.79 (s, 1H), 8.29–8.20 (m, 2H), 8.08 (d, J = 2.6 Hz, 1H), 7.98–7.86 (m, 2H), 7.80 (dd, J = 8.9, 6.9 Hz, 3H), 7.36–7.25 (m, 1H). LC-MS: 328 [M + 1]+ (Figs. S17–S19).

3.2.7

3.2.7 (E)-3-(4-nitrostyryl)-6, 8-dichloro-4H-chromen-4-one (5d)

IR (cm−1): 1656, 1595, 1556, 1506, 1446, 1336, 1327, 1305, 1274, 1172, 1107,972, 744. 1H NMR (400 MHz, DMSO-d6) δ 8.87 (s, 1H), 8.31–8.19 (m, 3H), 8.04 (d, J = 2.5 Hz, 1H), 7.91 (d, J = 16.4 Hz, 1H), 7.8–7.77 (m, 2H), 7.29 (d, J = 16.4 Hz, 1H). LC-MS: 362 [M + 1]+ (Figs. S20–S22).

3.2.8

3.2.8 (E)-3-(4-nitrostyryl)-6-methyl-4H-chromen-4-one (5g)

IR (cm−1):1651, 1616, 1589, 1510, 1622, 1450, 1417, 1332, 1319, 1269, 1174, 1107, 995,866, 746. 1H NMR (400 MHz, Chloroform-d) δ 8.26–8.18 (m, 2H), 8.14–8.06 (m, 1H), 7.85 (dd, J = 16.3, 4 Hz, 1H), 7.64 (dd, J = 8.8, 2.2 Hz, 2H), 7.51 (dd, J = 8.5, 1.9 Hz, 1H), 7.39 (d, J = 8.5 Hz, 1H), 7.28 (s, 1H), 7.07 (dd, J = 16.3, 4 Hz, 1H), 2.49 (s, 3H). LC-MS: 308 [M + 1]+ (Figs. S23–S25).

In order to check the reusability of a catalyst, the reaction (Schemes 2 and 4) was repeated several times. The catalyst was recycled and reused up to four runs without any significant loss in catalytic activity (Fig. 5).

Synthesis of 3-(4-nitrostyryl)-4H-chromen-4-ones 5a-h.
Scheme 4
Synthesis of 3-(4-nitrostyryl)-4H-chromen-4-ones 5a-h.
Reusability of ZnO and Cu-ZnO nano flakes in synthesis of (E)-3-styrylchromone.
Figure 5
Reusability of ZnO and Cu-ZnO nano flakes in synthesis of (E)-3-styrylchromone.

Although precise role of nanocrystalline ZnO and Cu-ZnO for condensation of 2,4-dinitrotoluene (Scheme 2) and 4-nitrophenylacetic acid (Scheme 4) respectively, is not clearly understood, we have attempted to understand fact from characterization of catalysts, pure ZnO shows well dispersed uniform nano flakes with higher degree of crystallinity, higher surface area and less structural defects over Cu-ZnO. The catalytic activity is dependent on morphology, surface area and crystallinity of nanoparticles (Chattopadhyay et al., 2009). Hence, the catalytic activity of pure ZnO nano flakes for condensation reaction was found to be higher.

Patai et al.(1954) have investigated, in Knoevenagel-Doebner reaction major part of condensation occurs prior to the decarboxylation and no simple relationship exists between condensed product and the rate of the decarboxylation. We assumed pure ZnO nano flakes catalyses the condensation reaction significantly, but no significant effect on rate of decarboxylation. The high π -complexing ability of Cu2+ present in Cu-ZnO inductively stabilizes the negative charge which increases rate of decarboxylation (Cairncross et al., 1970). Hence, Cu doped ZnO has better catalytic efficiency toward Knoevenagel-Doebner reaction of 4-nitrophenylacetic acid.

4

4 Conclusion

We report for the first time implementation of ZnO and Cu doped ZnO nano flakes as catalyst for preparation of (E)-3-styrylchromones. Very Significantly, pure ZnO catalyzes Knoevenagel condensation of 2,4-dinitrotoluene efficiently and Cu(5%)-ZnO catalyzes Knoevenagel-Doebner condensation of 4-nitrophenylacetic acid better. The Knoevenagel-Doebner reaction by doping (Cu) the catalyst is novel and is of great potential for further applications. These protocols are advantageous as they are economical and efficiently achieve excellent yield and catalyst can be reused up to four runs without loss of catalytic activity.

Acknowledgment

One of the authors SPK gratefully acknowledges UGC New Delhi for award of senior research fellowship. Author is thankful to parent institute Rayat Shikshan Sanstha, Satara.

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Appendix A

Supplementary material

Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.arabjc.2016.12.015.

Appendix A

Supplementary material

Supplementary data 1

Supplementary data 1


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