A facilely designed, highly efficient green synthetic strategy of a peony flower-like SO₄²⁻–SnO₂-fly ash nano-catalyst for the three component synthesis of a serendipitous product with dimedone in water

Abstract

For the first time we have successfully found a serendipitous product 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione and its derivatives by utilizing a SO₄²⁻–SnO₂-fly ash nano-catalyst in water. We have designed a SO₄²⁻–SnO₂-fly ash nano-catalyst which is easily separable, has good catalytic activity, good reusability and notable industrial applications. The catalytic role of Sn–O involves its high affinity with the carbonyl group of dimedone. The major component of fly ash (SiO₂) may enhance the catalytic activity of oxidation processes. With these properties, the conversion of product could be rapid and high yielding. The facilely designed, SO₄²⁻–SnO₂-fly ash nano-catalyst was characterized by Fourier transform infrared spectroscopy (FT-IR), confocal Raman spectroscopy, powder X-ray diffraction (PXRD), field emission electron microscopy (FE-SEM), energy dispersive X-ray spectroscopy (EDS and elemental color mapping), high resolution transmission electron microscopy (HR-TEM) and UV-Visible diffuse reflectance spectroscopy (UV-Vis DRS) techniques. The nano-cube and peony flower like morphologies were found in the FE-SEM and HR-TEM images. The flower like SO₄²⁻–SnO₂-fly ash catalyst’s highly stable nature is favorable for organic reactions. The crystalline nature, surface morphology, chemical composition and morphology of the reused SO₄²⁻–SnO₂-fly ash nano-catalyst were proved by PXRD, FE-SEM, EDAX and HR-TEM analyses respectively. The facilely designed SO₄²⁻–SnO₂-fly ash nano-catalyst is versatile from both environmental and economical points of view. The synthesized serendipitous product derivatives and byproducts were characterized by FT-IR, nuclear magnetic resonance (NMR) and high resolution-mass spectrometry (HR-MS).

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Introduction

Modern organic chemists enthuse over an environmental and economical abundant source of water, which is practically reliable for organic synthesis. It leads to organic synthesis in a significantly more green manner.^1,2 In recent decades, researchers have paid much more interest to green synthetic approaches with green nano-catalysts for organic reactions.^3–5 With this in mind, we have taken the environmental pollutant and thermal power plant waste residue of fly ash to assist us as a catalyst for organic reactions. Our group earlier reported a SO₄²⁻-fly ash catalyst for crossed-Aldol condensation.⁶ This catalyst has not shown good catalytic activity in the present investigation. With the demand for a suitable catalyst, we have concentrated on SnO₂ based catalysts. Since they have low toxicity, high selectivity, good catalytic activity, and are thermally stable, less moisture nature, easily separable and reusable catalysts, many can be found in recent articles.^7–11 The SnO₂ catalyst has been utilized for numerous organic syntheses such as 2,4-diphenyl-4,6,7,8-tetrahydrochromen-5-one,¹² 4-hydroxycoumarin o-alkylation,¹³ 3,4-dihydropyrimidin-2(1H)-ones,¹⁴ ketoesters,¹⁵ esterification,^16,17 esterification of free fatty acids,¹⁸ 1H-imidazole¹⁹ and Mukaiyama aldol condensation.²⁰

Carbazole based derivatives are appropriate for medicinal application in breast cancer metastases, brain tumours,²¹ and HIV.²² The carbazole moiety possesses another vast application in organic electronics such as organic solar cells, organic light-emitting diodes (OLEDs),^23,24 organic thin film transistors (OTFTs),²⁵ and organic semiconductors.²⁶ In this context our present investigation is to study the three component reaction with dimedone. In organic synthesis, dimedone is the key intermediate for the synthesis of various products. In general, the three component reaction of dimedone, aniline and aldehydes has given all the possible products, which have all been reported previously, such as 1,8-dioxodecahydroacridines,^27–32 acridine³³ and N-acridines.³⁴ Our present investigation deals with the reaction between 3-amino-9-ethylcarbazole, dimedone and aldehyde in the presence of a SO₄²⁻–SnO₂-fly ash catalyst. The products of this reaction, denoted as 6, 7 and 8 were not obtained. The unexpected products 4 and 5 were produced, as shown in Scheme 1. Based on an earlier literature report, the electron withdrawing nature of a m-substituent amine (3-amino-9-ethylcarbazole) and the steric hindrance of electron density may defer the reaction.³⁵ Due to this possibility products 6, 7 and 8 were not produced. The reaction was performed in the presence of aniline (9) under optimized conditions and the corresponding product, 2,2-dimethyl-9-phenyl-2,3,9,10-tetrahydroacridin-4(1H)-one (10) was obtained. This led the authors to believe that the reaction involved a domino sequence of Knoevenagel condensation and Michael addition reactions (Scheme 1).

Scheme 1 Screening of the SO₄²⁻–SnO₂-fly ash nano-catalyst for the serendipitous product.

To the best of our knowledge, for the first time, we have found the serendipitous product of 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione and its derivatives in water. The heterogeneous catalyst SO₄²⁻–SnO₂-fly ash nano-catalyst has numerous advantages such as being easily separable, economically reliable, eco-friendly and having notable industrial applications.

Results and discussion

Fourier transform infrared spectroscopy (FT-IR)

The FT-IR spectra of the facilely designed SnO₂-fly ash, and SO₄²⁻–SnO₂-fly ash catalyst are depicted in Fig. 1a and b. The absorption band which appears at 968 cm⁻¹ is assigned to the Si–O–Si band. The intense peak appearing at 443 cm⁻¹ indicates the Sn–O stretching vibration. The broad vibration peak observed at 643 cm⁻¹ refers to the Sn–O–Sn symmetric and asymmetric frequency band and is in close agreement with earlier literature.³⁶ In addition to that the broad peak also appearing at 1124 cm⁻¹ is assigned to the asymmetric stretching vibration of SO₄²⁻ which is in good agreement with earlier reports.^37,38

Fig. 1 FT-IR spectra of (a) SnO₂-fly ash, and (b) SO₄²⁻–SnO₂-fly ash.

Confocal Raman spectroscopy

Fig. 2a and b depict the confocal Raman spectra of SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash respectively. Fig. 2a illustrates that the SiO₂ stretching vibration observed at 859 cm⁻¹ is close to the earlier reported value.³⁹ Fig. 2b reveals that the SiO₂ stretching band, which is at 1122 cm⁻¹, was shifted to the higher region. The broad high intensity peak observed at 794 cm⁻¹ is assigned to the SO₄²⁻ bending vibration which is in good agreement with the earlier reported Raman shift.⁴⁰ From the Fig. 2a and b spectra results, SnO₂ Raman shifts were found at 614, 816, 1270 and 2211 cm⁻¹ which represent the Sn–O–Sn stretching mode coinciding with earlier literature.^41,42

Fig. 2 Confocal Raman spectra of (a) SnO₂-fly ash, and (b) SO₄²⁻–SnO₂-fly ash.

Powder X-ray diffraction (PXRD)

The crystalline phases of the SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash catalyst were analysed using the powder XRD patterns illustrated in Fig. 3a and b. The diffraction patterns of the catalysts were indexed by mullite, quartz and calcite^43,44 (Fig. 3a and b). The diffraction peak which appears at 37.10 2θ (°) reveals the presence of SnO₂. The corresponding d spacing value of this 2θ (°) value is 1.11 Å and the plane is [3 3 1] which is in good agreement with earlier literature of SnO₂ (JCPDS card no. 50-1429 space group Pa3 and a₀ = 4.870 Å).⁴⁵ A new diffraction peak was observed at 64.98 2θ (°) which may represent the SO₄²⁻.⁴⁶ In addition to that diffraction peaks were also observed at 14.10 and 29.28 2θ (°) (indicated by a cube). The corresponding planes are [1 1 1] and [2 2 2] with d spacing values of 2.81 and 1.40 Å respectively. As a result these values confirm the presence of the crystalline phase of SnO₂ which were good agreement with earlier literature values of SnO₂ (JCPDS card no. 50-1429).

Fig. 3 PXRD-patterns of (a) SnO₂-fly ash and (b) SO₄²⁻–SnO₂-fly ash.

Field emission scanning electron microscopy (FE-SEM)

The nano-structure and surface morphology of the SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash catalyst were analyzed by FE-SEM and are depicted in Fig. 4a–d.

Fig. 4 FE-SEM images of (a) SnO₂-fly ash and (b–d) SO₄²⁻–SnO₂-fly ash, scale bars (a) 2 μm, (b) 1 μm (c and d) 200 nm.

The Fig. 4a image reveals that the SnO₂-fly ash, forms cubic structures which coexist on a globular shape with a size of 2 μm. Fig. 4b–d show the surface morphology of the facilely designed SO₄²⁻–SnO₂-fly ash catalyst. Fig. 4b shows the fine shape nano-structure of cubes and numerous of peony like (peony flower inset) nano-flowers (indicated with a yellow sphere) which are shown to be about 1 μm in size. The close up view of the nano-cube and nano-flower individual surface morphologies are shown in Fig. 4c and d. Fig. 4c shows that the flower like regular arrangement was attained and the flower petals are uniformly dispersed, within the constructed space. The Fig. 4d image shows that the fine shape and smooth face of the nano-cube can be clearly observed. In this observation the concise, designed catalyst is regularly arranged within the particles.

Field emission scanning electron microscopy elemental color mapping

The chemical composition of SO₄²⁻–SnO₂-fly ash was examined by FE-SEM elemental color mapping. The results provide evidence of the uniform distribution of chemical composition, as illustrated in Fig. 5. The individual elemental color mappings are (S, Sn, Si, Al, O, Ca, Fe and Mg) are displayed in Fig. 5b–i. The fly ash comprises a variety of elements (Al, O, Si, Ca, Fe and Mg) and after preparation, Sn and S elements are present in the facilely modified catalyst. It is noteworthy no other element appeared, this result further confirmed that the SO₄²⁻–SnO₂-fly ash catalyst is in its pure form.

Fig. 5 FE-SEM elemental color mapping of (a) SO₄²⁻–SnO₂-fly ash mix, (b) S, (c) Sn, (d) Si, (e) Al, (f) O, (g) Ca, (h) Fe, and (i) Mg, scale bars (a–i) 20 μm.

Energy dispersive X-ray spectroscopy (EDS)

The chemical compositions of SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash were examined by EDS analysis. The facilely designed SnO₂-fly ash components of Si, Mg, Ca, Al, Fe, O and Sn are shown in Fig. 6a. Fig. 6b shows that chemical composition of the facilely designed SO₄²⁻–SnO₂-fly ash peony like nano-flower and nano-cube catalyst is similar to Fig. 6a, however in addition to that the element S also appeared. The catalysts’ elemental composition, weight and atomic weight percentage are given in ESI Table S1.†

Fig. 6 EDS spectra of (a) SnO₂-fly ash and (b) SO₄²⁻–SnO₂-fly ash.

High resolution transmission electron microscopy (HR-TEM)

The nano-cube and peony like nano-flower morphology of SO₄²⁻–SnO₂-fly ash was examined by high resolution transmission electron microscopy (HR-TEM) shown in Fig. 7a–f. Fig. 7a reveals that a flawless nano-cube was observed, showing smooth edges of the cube’s length, width and height. The nano-cube parameters are displayed, the nano-cube length is 520 nm, width is 89 nm and height is 459 nm. The peony flower like nano-flower morphology was observed to be 0.2 μm in size. The Fast Fourier Transform (FFT) (inset) shows flower like uniform spots as shown in Fig. 7b. In order to further assess the nano-flower, the nano-flower petals of about 50 nm were observed (nano-petals FFT inset) as shown in Fig. 7c. The nano-flower morphology has modest properties such as strongly bound atoms, a highly stable nature, a high exposure of active facets, a high degree of dispersion and a highly crystalline nature.⁴⁷ The HR-TEM lattice fringes were measured to be 0.21 nm and 0.27 nm (FFT inserted) as depicted in Fig. 7d. Fig. 7e depicts the growth direction of the cube orientation planes [1 1 1], [2 2 2] and [3 3 1] which are displayed in the SAED pattern (FFT inset). Fig. 7f is the magnified SAED pattern which shows a cube like regular diffraction pattern. These results suggest that the facilely designed SO₄²⁻–SnO₂-fly ash catalyst has a good crystalline phase.

Fig. 7 HR-TEM images of SO₄²⁻–SnO₂-fly ash (a) nano-cube, (b) nano-flower, (c) nano-petal, (scale bars (a) 100 nm, (b) 0.2 μm, (c) 50 nm), (d) HR-TEM 5 nm, (e and f) SAED patterns 2 1/nm.

Optimization of the reaction conditions

To find the optimum reaction conditions for the unusual product 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (4a) and by product (E)-N-benzylidene-9-ethyl-9H-carbazol-3-amine (5a) in water, the reaction mixture of dimedone (1 mmol 152 mg), benzaldehyde (1 mmol 0.1 mL) and 3-amino-9-ethylcarbazole (1 mmol 210 mg) in the absence of a catalyst was prepared in water and kept at 95 °C for 8 h. The result in the absence of a catalyst was that no product was formed, see Table 1 entry 1. Then we attempted the reaction in the presence of a fly ash catalyst at 95 °C for 5 h. The result was that a trace amount of 4a and a 12% yield of 5a was obtained, as shown in Table 1 entry 2. The next examination of the reaction was carried out in the presence of the SO₄²⁻-fly ash catalyst at 85 °C for 3.5 h. Under these reaction conditions, only a trace amount of 4a and 17% of 5a were isolated, as shown in Table 1 entry 3.

Table 1 Optimization of the reaction conditions^a

Entry	Catalyst^b	Time (h)	Temperature (°C)	Yield^c (%)
				4a	5a
a Reaction conditions: dimedone, benzaldehyde and 3-amino-9-ethylcarbazole (1 mmol scale).b Preformed catalyst.c Isolated yield.
1	No catalyst	8	95	No reaction
2	Fly ash	5	95	Trace	12
3	SO4^2−-fly ash	3.5	85	Trace	17
4	H2SO4	8	85	Trace	10
5	CuO	10	85	—	26
6	Bi2O3	7	95	7	33
7	ZrO	12	95	—	18
8	MgO	12	95	—	10
9	SnO2	2.5	95	10	40
10	CuO-fly ash	12	95	—	38
11	Bi2O3-fly ash	3	85	13	47
12	ZrO-fly ash	8	85	—	28
13	SnO2-fly ash	1	80	18	63
14	SO4^2−–Bi2O3-fly ash	1	80	20	26
15	SO4^2−–SnO2-fly ash 1 wt%	20 (min)	80	56	16
16	SO4^2−–SnO2-fly ash 3 wt%	10 (min)	80	68	14
17	SO4^2−–SnO2-fly ash 5 wt%	10 (min)	80	73	12

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During this observation we found the catalytic efficiency of sulfuric acid in this reaction, however it did not promote the reaction. Next we screened the catalytic efficiency of five metal oxides (CuO, ZrO, MgO, Bi₂O₃ and SnO₂) in the present reaction. Using CuO, ZrO and MgO catalysts, only 5a was obtained in the yields shown in Table 1 entries 5, 7 and 8. However these catalysts (CuO, ZrO, MgO) did not support the production of both products 4a and 5a. The good selectivity of the Sn and Bi metals relates to their high oxidation activity, which is addressed in earlier literature.⁴⁸ In connection with this, the presence of Bi₂O₃ and SnO₂ catalysts affords a trace amount of 4a. Next we examined metal oxide loaded fly ash catalysts (CuO-fly ash and ZrO-fly ash). These catalysts only produced 5a in the yield depicted in Table 1 entry 10. During this continuous screening only in Table 1 entries 3, 4, 9, 11 and 13 was the unusual product of compound 4a obtained. As shown by the results and as is well documented, in the presence of fly ash, the super acidic conditions with the SnO₂ catalyst supports the production of compound 4a. With this in mind we have focused on a sulfated metal oxide fly ash catalyst for the present reaction. The reaction was performed with a SO₄²⁻–Bi₂O₃-fly ash catalyst which supports the 20% yield of compound 4a that was obtained and illustrated Table 1 entry 14. Our prime aim was to improve the yield of the unusual product, with this intention we studied different weight percentages of the SO₄²⁻–SnO₂-fly ash (SnO₂ 1, 3, 5 wt%) catalyst in the reaction. The yield of 4a was successfully improved to 56% by using 1 wt% of SO₄²⁻–SnO₂-fly ash. In observing this, the use of the SO₄²⁻–SnO₂-fly ash catalyst (3, 5 wt%) in the reaction was evaluated as shown in Table 1 entries 16 and 17 (Fig. 8).

Fig. 8 Optimization of the catalyst (a) fly ash, (b) SO₄²⁻-fly ash, (c) metal oxides (CuO, Bi₂O₃, ZrO, MgO, and SnO₂), (d) metal oxide loaded fly ash (CuO, Bi₂O₃, ZrO, MgO, and SnO₂), (e and f) SO₄²⁻–SnO₂-fly ash (3, 5 wt%).

Inspired by the optimal reaction conditions, we designed further entries, of electron withdrawing and donating substitute groups of aromatic benzaldehyde as shown in Table 2 (entries 1–7). The electron withdrawing groups of the benzaldehydes smoothly caused the reaction to proceed, with good yields. The bulky group of 2-chloro-6-methoxy-3-quinolinecarboxaldehyde gave a good yield within 10 min (Table 2 entry 2). Specifically the electron withdrawing groups of 4-trifluoromethylbenzaldehyde, 4-methylthiobenzaldehyde and 2-thiophenecarboxaldehye afforded excellent yields of the unusual product within short reaction times as shown in Table 2 entries 3, 5 and 7. When the substrate contained an electron donating group the lowest amount of time was required to convert it to product.

Table 2 Synthesis of 2-((2-((9-ethyl-9H-carbaol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione derivatives (4a–4g)

Entry	R-CHO^a	Product^b		Time (min)	Yield^c (%)
					4a–4g	5a–5g
a Reactions were carried out with dimedone, substituted benzaldehyde, 3-amino-9-ethylcarbazole (1 mmol scale), SO4^2–SnO2-fly ash catalyst (50 mg) and water as solvent at 80 °C for 5–10 min.b Isolated product.c Isolated yield.
1		4a	5a	10	73	12
2		4b	5b	10	79	10
3		4c	5c	5	85	7
4		4d	5d	5	70	14
5		4e	5e	7	87	9
6		4f	5f	8	68	22
7		4g	5g	5	85	10

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Reusability of the SO₄²⁻–SnO₂-fly ash green nano-catalyst

To evaluate the reusability efficacy of the heterogeneous SO₄²⁻–SnO₂-fly ash nano-catalyst, in the formation of the unusual 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(4-(trifluoromethyl)phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione under optimized conditions, the reused solid SO₄²⁻–SnO₂-fly ash nano-catalyst was recovered from the reaction mixture by employing a filtration technique. The catalyst was purified with DCM (20 mL) and dried in a hot air oven at 130 °C for 1 h. The same processes were continued up to the sixth consecutive run. The reusability potential of the SO₄²⁻–SnO₂-fly ash catalyst was examined up to the sixth reaction cycle. The conversion yield was evaluated in the terms of turn-over number (TON₁, TON₂) and turn-over frequency (TOF₁, TOF₂) which are shown in Table 3. The recycling experiment of the SO₄²⁻–SnO₂-fly ash nano-catalyst is shown in Fig. 9. The concise account of the results are that after six cycles the catalyst did not show any significant decay in its catalytic activity compared to the fresh catalyst. However the time required before the conversion of the product increased slightly.

Table 3 Recycling potential of the SO₄²⁻–SnO₂-fly ash catalyst for the production of 4c and 5c

Entry	No. of runs	Time (min)	Yield (%)	TON1	TON2	TOF1	TOF2
1	Fresh	5	85 : 7	5.1	0.42	0.017	0.001
2	Run 1	5	85 : 7	5.1	0.42	0.017	0.001
3	Run 2	5	85 : 7	5.1	0.42	0.017	0.001
4	Run 3	7	84 : 7	5.0	0.42	0.011	0.001
5	Run 4	8	84 : 7	5.0	0.42	0.010	0.0008
6	Run 5	8	83 : 7	4.9	0.42	0.010	0.0008
7	Run 6	8	83 : 6	4.9	0.36	0.010	0.0007

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Fig. 9 Recycling experiment of the SO₄²⁻–SnO₂-fly ash nano-catalyst.

Characterization techniques of the reused SO₄²⁻–SnO₂-fly ash catalyst

The crystalline nature and surface morphology of the SO₄²⁻–SnO₂-fly ash nano-catalyst after the sixth reaction run was examined by Powder XRD, FE-SEM, EDS and HR-TEM techniques. Fig. 10a depicts the powder XRD diffraction pattern of the SO₄²⁻–SnO₂-fly ash catalyst after six cycles which appears similar to the fresh catalyst diffraction pattern. The morphology of the catalyst after the sixth consecutive run was examined by the FE-SEM technique. In Fig. 10b and c, the FE-SEM reveals the peony like nano-flower and nano-cube morphologies respectively. The nano-flower and nano-cube have regular arrangements of 200 nm particles which is similar to the fresh catalyst’s morphology. From these morphology studies we can observe, that the reused SO₄²⁻–SnO₂-fly ash catalyst undergoes no morphology changes even after sixth successive run. The EDS spectrum shown in Fig. 10d and the chemical composition weight percentages are given in ESI Table S1.† The HR-TEM images show that after the sixth reaction run the catalyst possesses no noticeable changes in the nano-flower morphology (FFT images inset), shown in Fig. 10e. The smooth face of the nano-cubic morphology is displayed in Fig. 10f. As shown by the results, after the sixth reaction run the morphology of the catalyst was not affected. The SAED pattern diffraction spots show a regular arrangement illustrated in Fig. 10h.

Fig. 10 The SO₄²⁻–SnO₂-fly ash catalyst after the sixth run (a) powder XRD (b and c) FE-SEM images (d) EDS (e–g) HR-TEM images and (h) SAED patterns 2 1/nm.

Probing of the SO₄²⁻–SnO₂-fly ash for the serendipitous product

There is significant scope of the major component (SiO₂) of the fly ash (type C) to enhance, the catalytic activity for oxidation processes.⁴⁹ The catalytic role of SnO₂ is to activate the electrophilic nature of the carbonyl groups (dimedone and aldehyde). The main important character of Sn metal was to eagerly coordinate with the carbonyl group and readily form an adduct. In this respect Sn metal has a good selectivity for oxidation processes.⁵⁰ In addition, the catalytic nature of SO₄²⁻ could improve the oxidation reaction.⁵¹ With both of these components, the production of the serendipitous product could be rapid with a high yield.

Plausible mechanism of formation of the serendipitous product

The plausible mechanism pathway describes the formation of the serendipitous product of 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione. The electronic configurations of Sn and Si with empty d orbitals are promising for the formation of bonds with electronegative dimedone.⁵² The catalytic probe of Sn/SiO₂ would assist in improving the reactive species and product selectivity.^53,54 With this favorable observation in mind, we have designed a SO₄²⁻–SnO₂-fly ash nano-catalyst supporting this reaction under optimal conditions, resulting in the rapid formation of the product in a high yield. The plausible mechanism sequence is outlined in Scheme 2. It includes the well documented, reaction pathway of dimedone with benzaldehyde to afford adduct IV through Knoevenagel condensation. In addition to that the SO₄²⁻–SnO₂-fly ash eagerly coordinates with the carbonyl groups of dimedone and benzaldehyde which enhances the reaction activity.

Scheme 2 Plausible reaction mechanism of the serendipitous product.

The next step is nucleophilic Michael addition followed by enol formation, subsequently the catalyst was regenerated. Then catalyst is again involved in the reaction and coordinates with dimedone and then it finishes with the product V. In the next step VI (3-amino-9-ethylcarbazole) reacts with compound V, which undergoes the 1,4-Michael addition affording compound VII. The subsequent reaction between unreacted benzaldehyde and VI might lead to (E)-N-benzylidene-9-ethyl-9H-carbazol-3-amine. The final step is enol formation, followed by deprotonation to afford compound IX and catalyst regeneration.

Conclusions

In summary, for first time we have successfully found the serendipitous product of 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione and its derivatives (4a–4g) in water. With the environmental challenge of reprocessing solid waste residue in mind, fly ash was successfully utilised in the SO₄²⁻–SnO₂-fly ash as a heterogeneous green catalyst of the three component reaction. In addition, we have demonstrated that the nano-catalyst is attractive in view of its low cost, simple preparation method, ease of handling, reusability, easy separation from the reaction mixture and notable industrial applications. After six consecutive runs the SO₄²⁻–SnO₂-fly ash green nano-catalyst provided a good yield for the unusual 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(4-(trifluoromethyl)phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione product. The crystalline nature and surface morphology of the SO₄²⁻–SnO₂-fly ash nano-catalyst after six successive runs was examined by PXRD, FE-SEM, EDS and HR-TEM. From these studies we observed no appreciable changes in the catalytic nature.

Experimental

Materials and methods

All the chemicals were procured from Sigma Aldrich, Merck and SRL India. Fly ash material (type C) was collected from Thermal Power Plant-II, Neyveli Lignite Corporation (NLC), Neyveli, Tamil Nadu, India.

The Fourier transform infrared spectra (KBr 4000–400 cm⁻¹) were recorded on an Avatar-300 Fourier transform spectrophotometer. The confocal Raman spectra of the samples were collected using a Wi Tec Alpha 300. The crystalline phase of SnO₂-fly ash and the SO₄²⁻–SnO₂-fly ash catalyst, were characterized by powder X-ray diffraction (PXRD) using a D8 Advance Bruker diffractometer operating at 230 V, 50 Hz, 6.5 kV Å with a Cu Kα source (λ = 1.5418 Å). The nano-structure and surface morphology of SO₄²⁻–SnO₂-fly ash were characterized by field-emission scanning electron microscopy (FE-SEM) (55, Carl Zeiss). The elemental composition was analyzed using an energy dispersive X-ray spectrometer (EDS) working at 200 kV. The samples were prepared by a dispersion method on a glass slide with a gold coating. The nano-cube and peony like nano-flower SO₄²⁻–SnO₂-fly ash catalyst morphologies were recorded, using high resolution transmission electron microscopy (HR-TEM) (Tecnai G2 operating at 120 kV). The samples were dispersed on a carbon-coated TEM grid (200 mesh). The UV-visible diffuse reflectance spectra (UV-Vis DRS) of SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash were recorded using a Shimadzu UV-3600 UV-visible-NIR spectrometer. UV-Vis DRS samples were prepared by a pellet assembly method, using BaSO₄ as a reference. The melting points of the synthesized compounds were determined in open glass capillaries with a Mettler FP51 melting point apparatus and were uncorrected. The NMR spectra of unknown compounds were recorded with a Bruker AVIII 5000 spectrometer, operating at 400 MHz for ¹H NMR spectra and 125 MHz for ¹³C NMR spectra in CDCl₃ solvent using tetramethylsilane as the internal standard. HR-MS (ESI) analyses were recorded with a Bruker Maxis instrument (Maxis 10138).

The facilely designed SO₄²⁻–SnO₂-fly ash nano-catalyst

In a typical procedure for the synthesis of the SO₄²⁻–SnO₂-fly ash catalyst, SnCl₄·5H₂O (331 mg 5 wt%) was measured into a clean beaker, and dissolved in 10 mL of conductivity water. This solution was added drop-wise to the sulfated fly ash mixture and it was vigorously stirred for 10 min. This was followed by the addition of an aqueous ammonia solution (5 mL) which was added into the mixture under constant stirring. The mixture was maintained at 80 °C for 1 h. After 1 h with continuous stirring the mixture was transferred into a Teflon-lined stainless-steel autoclave, sealed and heated at 116 °C for 3 h. During the process, the pressure was maintained at 30 psi. Then the mixture was dried in an oven at 130 °C for 1 h and calcined at 500 °C for 3 h in a muffle furnace to obtain the final catalyst. The catalyst preparation method is shown in Fig. 11 as a schematic representation.

Fig. 11 Preparation method of the SO₄²⁻–SnO₂-fly ash nano-catalyst.

General procedure for the synthesis of the serendipitous products

A clean 25 mL round-bottom flask was charged with the dimedone (1 mmol), substituted benzaldehyde (1 mmol), 3-amino-9-ethylcarbazole (1 mmol) and SO₄²⁻–SnO₂-fly ash catalyst (50 mg) in water (15 mL) and the mixture was refluxed at 80 °C for 5–10 minutes. The completion of the reaction was monitored by TLC (ethyl acetate and hexane as an eluent 20%). After completion, the reaction mixture was cooled to ambient temperature. Then dichloromethane (20 mL) was added to the reaction mixture to separate the organic and aqueous layers. The insoluble, solid SO₄²⁻–SnO₂-fly ash nano-catalyst was washed with water and dichloromethane (20 mL) then dried in a hot air oven at 130 °C for 1 h. The dried SO₄²⁻–SnO₂-fly ash was reused for further reaction runs up to the sixth run. The organic layer was filtered, dried on anhydrous Na₂SO₄ and the organic solvent was removed using a rotary evaporator. The crude product was purified by Column chromatography, through silica gel (200 mesh) with (20%) 80 : 20 hexane–ethyl acetate as the eluent to the desired pure unusual products 4a–4g and 5a–5g. The purified compounds were confirmed, by physical constants and spectral techniques (FT-IR, NMR and HR-MS).

Characterization data for 2-((2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione derivatives (4a–4g) and (E)-N-benzylidene-9-ethyl-9H-carbazol-3-amine derivatives (5a–5g) (Table 2 entry 1–7)

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 1, 4a). Isolated as a yellow solid yield (414 mg 73%); m.p. 82–83 °C; FT-IR 3046, 2931, 1594 cm⁻¹; ¹H NMR (400 MHz, CDCl₃), δ_H; 8.02 (s, 1H, NH), 7.51 (d, J = 8.0 Hz, 1H, ArH), 7.44–7.42 (m, 3H, ArH), 7.36 (d, J = 8.0 Hz, 2H, ArH), 7.30 (s, 3H, ArH), 7.23 (d, J = 8.5 Hz, 1H, ArH), 7.18 (t, J = 7.0, 1H, ArH), 7.17–7.13 (m, 1H, ArH), 6.94–6.91 (dd, J₁ = 4.0 Hz, J₂ = 4.0 Hz, 1H, ArH), 5.59 (s, 1H, CH), 4.33 (q, J = 7.2 Hz, 2H, CH₂), 2.51 (d, J = 4.0 Hz, 1H, CH₂), 2.43 (d, J = 4.0 Hz, 3H, CH₂), 2.32 (t, J = 8.0 Hz, 2H, CH₂), 2.36 (s, 1H, CH₂), 2.18 (s, 1H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃), 1.27 (d, J = 12.0 Hz, 5H, CH₃), 1.17 (d, J = 8.0 Hz, 2H, CH₃), 1.12 (s, 3H, CH₃), 0.99 (s, 1H, CH₃), 0.92 (s, 1H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 190.54, 189.43, 158.09, 140.43, 138.91, 138.14, 134.57, 128.26, 128.10, 126.93, 126.83, 126.35, 125.88, 125.46, 125.39, 123.69, 122.49, 122.43, 120.43, 118.03, 115.62, 109.02, 108.38, 106.41, 50.33, 49.78, 47.10, 46.49, 37.53, 32.80, 31.44, 29.75, 29.68, 28.63, 28.20, 27.43, 13.86; HRMS (ESI/[M + H]⁺) calcd for C₃₇H₄₀N₂O₃; 561.3109, found 561.3117.

2-((2-Chloro-3,4-dihydroquinolin-3-yl)(2-((9-ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 2, 4b). Isolated as a brown solid yield (554 mg 79%); m.p. 112–113 °C; FT-IR 3046, 2931, 1621, 536 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 9.07 (s, 1H, NH), 8.87 (s, 1H, ArH), 8.10 (t, J = 4.0 Hz, 1H, ArH), 7.88 (d, J = 8 Hz, 1H, ArH), 7.54 (d, J = 4.0 Hz, 1H, ArH), 7.47 (t, J = 8.0 Hz, 2H, ArH), 7.39–7.35 (m, 3H, ArH), 7.24 (s, 2H, ArH), 7.08 (s, 1H, ArH), 5.26 (s, 1H, CH), 4.31 (q, J = 7.0 Hz, 2H, CH₂), 3.87 (s, 3H, OCH₃), 2.57 (d, J = 8.0 Hz, 2H, CH₂), 2.44 (d, J = 8.0 Hz, 2H, CH₂), 2.33 (d, J = 12.0 Hz, 2H, CH₂), 2.19 (d, J = 8.0, 2H, CH₂), 1.41 (t, J = 4.0 Hz, 3H, CH₃), 1.22 (s, 3H, CH₃), 1.17 (s, 3H, CH₃), 1.11 (d, J = 8.0 Hz, 4H, CH₃), 1.04 (s, 1H, CH₃), 0.93 (s, 1H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 200.11, 196.42, 158.30, 152.47, 147.72, 144.29, 142.83, 140.53, 139.31, 136.52, 135.67, 129.56, 129.22, 128.33, 127.97, 126.46, 126.12, 124.41, 123.51, 123.34, 122.99, 122.38, 120.65, 120.07, 119.14, 118.22, 113.44, 108.92, 108.81, 105.77, 55.63, 53.54, 50.14, 44.92, 37.70, 32.21, 31.83, 28.42, 28.33, 27.10, 13.88; DEPT 152.47, 136.51, 135.66, 129.55, 129.21, 126.46, 120.64, 120.07, 119.26, 119.14, 118.22, 113.43, 109.03, 108.91, 108.80, 105.77, 105.28, 55.62, 53.55, 50.14, 49.45, 41.07, 37.69, 29.29, 28.41, 28.34, 27.52, 13.88.

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(4-(trifluoromethyl)phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 3, 4c). Isolated as a brown solid yield (540 mg 85%); m.p. 98–99 °C; FT-IR 3046, 2936, 1594, 1326 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.71 (s, 1H, NH), 8.15–8.06 (m, 3H, ArH), 7.87 (s, 1H, ArH), 7.75 (d, J = 8.0 Hz, 1H, ArH), 7.59–7.49 (m, 3H, ArH), 7.44 (d, J = 12.0 Hz, 2H, ArH), 7.39 (t, J = 8.0 Hz, 1H, ArH), 7.29–7.22 (m, 2H, ArH), 5.83 (s, 1H, CH), 4.41 (q, J = 7.1 Hz, 2H, CH₂), 2.52 (d, J = 8.0 Hz, 2H, CH₂), 2.47 (s, 2H, CH₂), 2.42 (t, J = 4.0 Hz, 3H, CH₂), 2.36 (s, 1H, CH₂), 2.29 (d, J = 12.0 Hz, 1H, CH₂), 1.46 (t, J = 4.0 Hz, 3H, CH₃), 1.25 (d, J = 8.0 Hz, 5H, CH₃), 1.20 (d, J = 12.0 Hz, 3H, CH₃), 1.12 (s, 2H, CH₃), 0.99 (s, 2H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 200.68, 195.69, 177.99, 166.76, 155.70, 142.96, 140.61, 139.24, 128.67, 127.18, 126.09, 125.67, 123.56, 123.08, 122.38, 120.62, 120.03, 119.10, 117.82, 115.16, 114.09, 112.95, 110.89, 108.89, 50.24, 47.05, 46.48, 44.70, 41.07, 37.75, 33.76, 32.30, 31.45, 29.59, 29.33, 27.41, 13.87; DEPT 155.70, 128.67, 127.23, 127.18, 126.45, 126.09, 125.67, 125.63, 125.23, 125.13, 123.83, 120.68, 120.62, 120.03, 119.11, 119.11, 119.26, 117.82, 112.95, 108.89, 50.13, 47.53, 46.49, 44.41, 41.30, 37.66, 33.50, 32.21, 31.43, 29.61, 29.35, 27.73, 13.77; HRMS (ESI/[M + H]⁺) calcd for C₃₈H₃₉F₃N₂O₃; 629.2992, found 629.2991.

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(3-methoxyphenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 4, 4d). Isolated as a brown solid yield (415 mg 70%); m.p. 81–82 °C; FT-IR 3052, 2931, 2838, 1589 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.65 (s, 1H, NH), 8.11 (d, J = 4.0 Hz, 1H, ArH), 8.04 (d, J = 1.6 Hz, 1H), 7.60 (s, 1H, ArH), 7.51 (t, J = 4.0 Hz, 2H, ArH), 7.44–7.40 (m, 3H, ArH), 7.24 (d, J = 6.2 Hz, 1H, ArH), 7.06–7.03 (dd J₁ = 4.0 Hz, J₂ = 4.0 Hz, 1H, ArH), 6.83 (t, J = 8.0 Hz, 1H, ArH), 6.72 (d, J = 8.0 Hz, 1H, ArH), 5.30 (s, 1H, CH), 4.42 (q, J = 7.1 Hz, 2H, CH₂), 3.93 (s, 3H, OCH₃), 2.61 (d, J = 4.0 Hz, 1H, CH₂), 2.48 (d, J = 4.0 Hz, 1H, CH₂), 2.43 (d, J = 4.0 Hz, 3H, CH₂), 2.37 (s, 2H, CH₂), 2.34 (s, 1H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃), 1.24 (s, 3H, CH₃), 1.18 (s, 3H, CH₃), 1.15 (s, 3H, CH₃), 0.97 (s, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 200.62, 190.45, 177.87, 166.18, 160.05, 157.94, 143.73, 140.56, 138.87, 138.21, 129.72, 128.91, 128.90, 123.51, 123.11, 122.42, 122.12, 120.60, 120.00, 119.17, 118.92, 112.61, 111.66, 108.79, 55.47, 50.28, 49.77, 47.09, 46.44, 41.02, 37.73, 33.61, 31.39, 29.69, 29.45, 27.35, 13.88; DEPT 154.11, 133.85, 129.65, 126.18, 125.47, 124.84, 122.99, 120.70, 120.63, 120.59, 120.42, 120.08, 119.20, 118.04, 113.17, 108.97, 108.89, 106.35, 53.26, 49.73, 46.79, 44.43, 39.14, 37.37, 33.84, 30.01, 29.41, 28.65, 28.25, 26.19, 13.91; HRMS (ESI/[M + H]⁺) calcd for C₃₈H₄₂N₂O₄; 591.3224, found 591.3223.

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(4-(methylthio)phenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 5, 4e). Isolated as a brown solid yield (524 mg 87%); m.p. 85–86 °C; FT-IR 3041, 2915, 2865, 1594 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.60 (s, 1H, NH), 8.10 (d, J = 7.7 Hz, 1H, ArH), 8.01 (d, J = 1.8 Hz, 1H, ArH), 7.86 (d, J = 8.3, 1H, ArH), 7.49–7.47 (m. 1H, ArH), 7.45 (d, J = 1.8, 1H, ArH), 7.41 (s, 1H, ArH), 7.39 (s, 1H, ArH) 7.33 (s, 1H, ArH), 7.31 (s, 1H, ArH), 7.21 (d, J = 1.0 Hz, 1H, ArH), 7.15 (d, J = 8.3 Hz, 1H, ArH), 7.0 (d, J = 8.0, 1H, ArH), 5.48 (s, 1H, ArH), 4.37 (q, J = 7.2 Hz, 2H, CH₂), 2.54 (s, 3H, SCH₃), 2.51 (d, J = 2.9, 1H, CH₂), 2.46–2.42 (m, 3H, CH₂), 2.35 (t, J = 2.7 Hz, 2H, CH₂), 2.32 (s, 1H, CH₂), 2.28 (s, 1H, CH₂), 1.44 (t, J = 7.2 Hz, 3H, CH₂), 1.36 (t, J = 7.2 Hz, 1H, CH₃), 1.21 (s, 4H, CH₃) 1.13 (s, 2H, CH₃), 1.09 (s, 3H, CH₃), 0.95 (s, 1H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 190.65, 189.43, 157.25, 143.82, 142.46, 140.57, 138.81, 135.54, 133.47, 128.93, 127.45, 126.81, 125.93, 123.53, 123.13, 120.63, 120.04, 118.94, 115.56, 112.61, 108.84, 108.75, 53.37, 47.10, 46.54, 40.72, 37.71, 32.45, 31.43, 29.69, 29.44, 27.44, 15.20, 13.93; DEPT 157.31, 128.87, 127.36, 126.84, 125.85, 120.58, 120.00, 118.88, 112.48, 108.76, 108.67, 47.07, 46.47, 37.73, 32.40, 29.66, 27.39, 15.24, 13.88; HRMS (ESI/[M + H]⁺) calcd for C₃₈H₄₂N₂O₃S; 607.2995, found 607.2992.

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(2-nitrophenyl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 6, 4f). Isolated as a brown solid yield (415 mg 68%); m.p. 95–96 °C; FT-IR 3254, 2920, 1517, 1363 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 9.16 (s, 1H, NH), 8.44 (d, J = 6.7 Hz, 1H, NH), 8.17–8.14 (m, 2H, ArH), 7.96 (d, J = 7.4 Hz, 1H, ArH), 7.77 (t, J = 7.7 Hz, 1H, ArH), 7.61–7.57 (m, 2H, ArH), 7.52 (t, J = 1.0 Hz, 2H, ArH), 7.47–7.44 (m, 2H, ArH), 7.30 (t, J = 4.4 Hz, 1H, ArH), 6.30 (s, 1H, CH), 4.42 (q, J = 7.1 Hz, 2H, CH₂), 2.60 (s, 1H, CH₂), 2.49 (s, 3H, CH₂), 2.33 (s, 1H, CH₂), 2.19 (s, 2H, CH₂), 2.05 (d, J = 4.0 Hz, 1H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃), 1.25 (s, 3H, CH₃), 1.17 (s, 3H, CH₃), 1.00 (s, 3H, CH₃), 0.88 (s, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 200.25, 195.90, 188.17, 152.59, 149.25, 142.67, 140.60, 139.43, 138.63, 134.09, 133.51, 131.60, 130.64, 129.56, 126.51, 126.10, 124.57, 123.06, 120.71, 120.12, 119.16, 118.08, 113.62, 108.91, 50.17, 48.94, 44.76, 40.80, 37.77, 33.06, 31.83, 30.55, 29.96, 28.48, 28.30, 26.32, 13.88; DEPT 188.16, 152.59, 133.50, 131.25, 130.63, 129.56, 126.51, 126.35, 124.56, 124.13, 120.71, 120.12, 119.16, 118.09, 113.61, 108.90, 50.17, 48.95, 44.75, 40.80, 37.77, 30.55, 29.96, 28.48, 28.30, 26.31, 13.88; HRMS (ESI/[M + H]⁺) calcd for C₃₇H₃₉N₃O₅; 606.2969, found 606.2968.

2-((2-((9-Ethyl-9H-carbazol-3-yl)amino)-4,4-dimethyl-6-oxocyclohex-1-en-1-yl)(thiophen-2-yl)methyl)-5,5-dimethylcyclohexane-1,3-dione (Table 2, entry 7, 4g). Isolated as a brown solid, yield (487 mg 85%); m.p. 113–114 °C; FT-IR 3046, 2926, 1578, 745 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.02 (s, 1H, NH), 7.51 (d, J = 8.0 Hz, 1H, ArH), 7.44–7.42 (m, 3H, ArH), 7.36 (d, J = 8.0 Hz, 2H, ArH), 7.30 (s, 3H, ArH), 6.94–6.91 (dd J₁ = 4.0 Hz, J₂ = 4.0 Hz, 1H, ArH), 5.59 (s, 1H, CH), 4.33 (q, J = 7.2 Hz, 2H, CH₂), 2.51 (d, J = 4.0 Hz, 1H, CH₂), 2.43 (d, J = 4.0 Hz, 3H, CH₂), 2.32 (t, J = 8.0 Hz, 3H, CH₂), 2.18 (s, 1H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃), 1.27 (d, J = 12.0 Hz, 5H, CH₃), 1.17 (d, J = 8.0 Hz, 2H, CH₃), 1.12 (s, 3H, CH₃), 0.99 (s, 1H, CH₃), 0.92 (s, 1H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 200.02, 195.60, 178.49, 160.09, 150.72, 143.57, 143.24, 140.59, 138.88, 131.36, 129.49, 127.76, 125.96, 123.51, 123.12, 122.44, 120.70, 120.62, 119.96, 119.24, 118.98, 118.98, 112.81, 108.83, 108.76, 50.13, 49.74, 44.85, 40.91, 37.72, 31.87, 31.22, 29.95, 29.27, 27.09, 13.90. DEPT 150.76, 131.32, 129.49, 127.72, 125.91, 120.59, 119.95, 118.93, 112.74, 108.77, 108.70, 50.32, 49.67, 44.78, 40.86, 37.95, 31.16, 29.93, 29.27, 28.58, 27.38, 27.01, 13.89; HRMS (ESI/[M + H]⁺) calcd for C₃₅H₃₈N₂O₃S; 567.2682, found 567.2682.

(E)-N-Benzylidene-9-ethyl-9H-carbazol-3-amine (Table 2, entry 1, 5a). Isolated as a brown solid yield (36 mg 12%); m.p.82–83 °C; FT-IR 1610 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.71 (s, 1H, C=N), 8.23 (d, J = 8.0 Hz, 1H, ArH), 8.15 (d, J = 4.0 Hz, 1H, ArH), 8.07–8.05 (m, 2H, ArH), 7.57–7.53 (m, 1H, ArH), 7.42 (t, J = 8.0 Hz, 4H, ArH), 7.35 (m, 2H, ArH), 4.35 (q, J = 7.1 2H, CH₂), 1.45 (t, J = 8 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 158.02, 143.82, 140.59, 136.81, 130.93, 128.83, 128.65, 125.98, 123.54, 123.15, 120.66, 120.05, 118.98, 112.69, 108.87, 108.78, 37.70, 13.92.

(E)-N-((2-Chloro-6-methoxyquinolin-3-yl)methylene)-9-ethyl-9H-carbazol-3-amine (Table 2, entry 2, 5b). Isolated as a brown solid yield (42 mg 10%); m.p. 182–183 °C; FT-IR 1616, 597 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 9.11 (s, 1H, C=N), 8.92 (d, J = 8.0 Hz, 1H, ArH), 8.15–8.11 (m, 2H, ArH), 7.92 (d, J = 8.0 Hz, 1H, ArH), 7.57 (dd J₁ = 1.6 Hz, J₂ = 1.6, 1 Hz, ArH), 7.48 (t, J = 8.0 Hz, 1H, ArH), 7.41 (t, J = 8.0 Hz, 1H, ArH), 4.37 (q, J = 7.1 Hz, 2H, CH₂), 3.92 (s, 3H, OCH₃) 1.44 (t, J = 8.0 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 158.37, 152.62, 147.81, 144.39, 142.97, 140.58, 139.34, 135.69, 129.67, 128.38, 128.09, 126.50, 126.11, 124.41, 123.57, 123.05, 120.66, 120.05, 119.15, 113.44, 108.90, 108.78, 105.81, 55.63, 37.72, 13.87; HRMS (ESI/[M + H]⁺) calcd for C₂₅H₂₀ClN₃O; 414.1374, found 414.1373.

(E)-9-Ethyl-N-(4-(trifluoromethyl)benzylidene)-9H-carbazol-3-amine (Table 2, entry 3, 5c). Isolated as a brown solid yield (23 mg 7%); m.p. 108–109 °C; FT-IR 1621, 1326 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.71 (s, 1H, C=N), 8.15–8.06 (m, 4H, ArH), 7.75 (d, J = 8.0 Hz, 2H, ArH), 7.54–7.49 (m, 2H, ArH), 7.43 (d, J = 8.0 Hz 2H, ArH), 7.27 (s, 1H, ArH), 4.41 (q, J = 7.3 Hz, 2H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 155.73, 142.98, 140.60, 139.23, 128.65, 126.07, 125.71, 123.07, 120.61, 120.01, 119.09, 112.90, 108.86, 108.78, 37.76, 13.87; HRMS (ESI/[M + H]⁺) calcd for C₂₂H₁₇F₃N₂; 367.1423, found 367.1422.

(E)-9-Ethyl-N-(3-methoxybenzylidene)-9H-carbazol-3-amine (Table 2, entry 4, 5d). Isolated as a brown solid yield (48 mg 14%); m.p. 78–79 °C; FT-IR 1589, 2915 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.60 (s, 1H, C=N), 8.12 (d, J = 8.0 Hz, 1H, ArH), 8.01 (s, 1H, ArH), 7.92 (d, J = 8.0 Hz, 2H, ArH), 7.48–7.40 (m, 4H, ArH), 7.23 (d, J = 8.0 Hz, 1H, ArH), 7.00 (d, J = 12.0 Hz, 2H, ArH), 4.41 (q, J = 7.2 Hz, 2H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C 161.89, 157.63, 144.22, 140.52, 138.59, 130.22, 129.80, 125.81, 123.49, 123.11, 120.57, 119.99, 118.80, 114.18, 112.31, 108.72, 108.63, 55.43, 37.70, 13.88.

(E)-9-Ethyl-N-(4-(methylthio)benzylidene)-9H-carbazol-3-amine (Table 2, entry 5, 5e). Isolated as a brown solid yield (33 mg 9%); m.p. 138–139 °C; FT-IR 1589, 2871 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.61 (s, 1H, C=N), 8.12 (d, J = 8.0 Hz, 1H, ArH), 8.02 (s, 1H, ArH), 7.88 (d, J = 8.0 Hz, 2H, ArH), 7.48 (t, J = 8.0 Hz, 2H, ArH), 7.42–7.40 (m, 2H, ArH), 7.34 (d, J = 8.0 Hz, 2H, ArH), 7.25 (d, J = 8.0 Hz, 1H, ArH), 4.38 (q, J = 4.6 Hz, 2H, CH₂), 2.55 (s, 3H, SCH₃), 1.45 (t, J = 8.0 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C: 157.28, 143.87, 142.40, 138.78, 133.50, 128.88, 125.86, 123.52, 123.11, 120.58, 119.99, 118.88, 112.50, 108.76, 108.67, 37.72, 15.23, 13.87; HRMS (ESI/[M + H]⁺) calcd for C₂₂H₂₀N₂S; 345.1426, found 345.1425.

(E)-9-Ethyl-N-(2-nitrobenzylidene)-9H-carbazol-3-amine (Table 2, entry 6, 5f). Isolated as a brown solid yield (77 mg, 22%); m.p. 140–141 °C; FT-IR 1610, 1331 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 9.15 (s, 1H, C=N), 8.43 (d, J = 8.0 Hz, 1H, ArH), 8.15 (t, J = 8.0 Hz, 2H, ArH), 8.08 (d, J = 8.0 Hz, 1H, ArH), 7.75–7.55 (m, 1H, ArH), 7.50 (d, J = 8.0 Hz, 2H, ArH), 7.49–7.42 (m, 1H, ArH), 7.28 (d, J = 8.0 Hz, 2H, ArH), 7.25 (d, J = 8.0 Hz, 1H, ArH), 4.39 (q, J = 4.0 Hz, 2H, CH₂), 1.48 (t, J = 8.0 Hz, 3H, CH₃) ¹³C NMR (125 MHz, CDCl₃) δ_C 152.57, 149.28, 142.70, 140.61, 139.44, 133.48, 130.61, 129.57, 126.10, 124.56, 123.56, 123.07, 120.71, 120.11, 119.16, 113.62, 108.89, 108.77, 37.77, 13.87.

(E)-9-Ethyl-N-(thiophen-2-ylmethylene)-9H-carbazol-3-amine (Table 2, entry 8, 5g). Isolated as a brown solid yield (33 mg, 10%); m.p. 210–211 °C; FT-IR 1605, 717 cm⁻¹; ¹H NMR (400 MHz, CDCl₃) δ_H; 8.78 (s, 1H, C=N), 8.12 (d, J = 8.0 Hz, 1H, ArH), 8.00 (d, J = 1.4 Hz, 1H, ArH), 7.50–7.47 (m, 3H, ArH), 7.43–7.39 (m, 1H, ArH), 7.22 (d, J = 8.0 Hz, 2H, ArH), 7.15 (t, J = 4.0 Hz, 1H, ArH), 4.37 (q, J = 7.2 Hz, 2H, CH₂), 1.45 (t, J = 8.0 Hz, 3H, CH₃); ¹³C NMR (125 MHz, CDCl₃) δ_C; 150.76, 143.52, 143.23, 140.54, 138.84, 131.30, 129.49, 127.71, 125.91, 123.47, 123.09, 120.60, 119.95, 118.92, 112.73, 108.75, 108.69, 37.72, 13.89; HRMS (ESI/[M + H]⁺) calcd for C₁₉H₁₆N₂S; 305.1113, found 305.1111.

Acknowledgements

The authors gratefully acknowledge Prof. M. Periasamy, School of Chemistry, University of Hyderabad for the benevolent donation of the laboratory facility and are also grateful to the UGC Networking Resource Centre, School of Chemistry, University of Hyderabad for providing the characterization facility. One of the authors K.T. is grateful to the School of Physics, University of Hyderabad for providing the FE-SEM characterization and also grateful to the Centre for Nano Technology, University of Hyderabad for providing the HR-TEM measurement.

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Footnote

† Electronic supplementary information (ESI) available: UV-Vis DRS spectra of SnO₂-fly ash and SO₄²⁻–SnO₂-fly ash. EDS spectra of the catalyst, atomic and weight percentage in Table S1. The NMR spectra of the synthesized compounds 4a–4g and 5a–5g, and the HRMS spectra of selected compounds (4a, 4c, 4d, 4e, 4f, 4g and 5b, 5c, 5e, 5f and 5g). See DOI: 10.1039/c5ra04006j

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