Experimental and DFT studies on the green synthesis of 2-amino-4H-chromenes using a recyclable GOQDs-NS-doped catalyst

Abstract

This research presents an innovative approach for synthesizing 2-amino-4H-chromene derivatives, utilizing 30 mg of NS-doped graphene oxide quantum dots (GOQDs) as a catalyst in a one-pot, three-component reaction conducted in ethanol. The NS-doped GOQDs were synthesized using a cost-effective bottom-up method through the condensation of citric acid (CA) with thiourea and the reaction was carried out at 185 $_{}^{\circ }$C, resulting in the elimination of water. The catalytic performance of the synthesized NS-doped GOQDs resulted in high product yields, achieving up to 98% for the 2-amino-4H-chromene derivatives from aromatic aldehydes, malononitrile, resorcinol, $\beta$-naphthol, and dimedone. The reaction showcased rapid completion time (typically < 2 h), low-cost reagents, and easy work-up procedures. In addition, the study integrates experimental and theoretical analyses, including density functional theory (DFT) calculations, to investigate the electronic properties of the synthesized compounds. Calculated HOMO and LUMO energies indicate efficient charge transfer within the molecular structure. The FT-IR spectra of compound 4c were recorded in the range of 4000–500 cm$_{}^{-1}$, and vibrational frequencies were computed at the B3LYP/6-311+G(d,p) level, correlating well with experimental data. Detailed analyses, including Mep surfaces, Mulliken population analysis, and Natural Bond Orbital (NBO) analysis, provide further insights into the electronic distribution and reactivity of the compounds. Furthermore, comparative $_{}^1$H and $_{}^{13}$C NMR analyses of compound 4c reveal strong agreement between computational and experimental findings. This research not only validates the synthetic method but also emphasizes the dual experimental and computational approach in understanding the structural and electronic characteristics of the 4c compound.

Introduction

One-pot multi-component reactions are favored in chemistry for their efficiency and practicality compared to traditional multi-step syntheses¹. Important reactions like the Ugi, Mannich, Hantsch, and Passerini reactions have contributed to the popularity of multi-component reactions (MCRs)^2–5. This approach is widely used in organic chemistry to synthesize important heterocyclic compounds economically in a single step. Researchers focus on these reactions to design products efficiently, especially in the preparation of compounds possessing biological and pharmacological activity^6–8. Oxygen-containing heterocyclic molecules like 4H-chromene moieties are essential in pharmaceuticals; the various pharmacological activities of this group of molecules, such as anticancer⁹, anti-tumor¹⁰, antioxidant¹¹, antibacterial¹², and antiviral¹³ properties of 2-amino-4H-chromene compounds have sparked significant interest among chemists and biologists, with Fig. 1 showcasing various biologically active examples. The extraction of these compounds from plants, which serve as natural sources of these valuable molecules, is of significant importance^14,15. Therefore, Because of the wide range of applications for 2-amino-4H-chromens, numerous synthetic methods can be found in published literature, a straightforward approach to synthesizing substituted 2-amino-4H-chromenes is by conducting a three-component condensation reaction with reactants including malononitrile, aromatic aldehyde, , and an activated phenol in the presence of an essential catalyst, many catalysts have been used for this preparation, such as ${\text{ZnAl}}_2{\text{O}}_4$-${\text{Bi}}_2{\text{O}}_3$ nano powder¹⁶, ${\text{NiFe}}_2{\text{O}}_4$@propyl-Pip¹⁷, TEOA¹⁸, hydrotalcite (HT)¹⁹ and nanocrystalline MgO²⁰. Many existing methods face challenges, for instance, extended reaction times, the use of volatile or hazardous organic solvents, complex work-up procedures, additional energy requirements, and the need for large amounts of catalysts, resulting in difficulties in product separation and the inability to recycle the catalyst. Therefore, there is still a high demand for straightforward, adaptable, and eco-friendly methods to synthesize 2-amino-4H-chromenes under uncomplicated conditions.

Fig. 1.

Some biologically active examples of chromenes.

Nowadays, catalysts are essential in chemical reactions. Scientists are seeking catalysts that reduce pollution and toxicity while also being cost-effective^21,22. It has been shown that GO can be used as a heterogeneous metal-free catalyst in organic reactions²³. NS-doped-GOQDS is a carbocatalyst that is efficient, inexpensive, non-toxic, stable, and reusable. NS-doped-GOQDS, a carbocatalyst known for its efficiency, affordability, non-toxicity, stability, and reusability, emerges as a fully biocompatible green catalyst for organic synthesis. The development of metal-free catalysts, such as organocatalysts (OC) and carbocatalysts (CC), has significantly impacted catalysis by offering environmentally friendly alternatives. In a continuation of our previous work on synthesizing the NS-doped-GOQDS catalyst and its application in reactions, we introduce a novel method for producing dihydro-4H-chromenes. Our earlier research involved the synthesis of pyrazole and pyridine derivatives synthesized through four- and five-component reactions, respectively, utilizing GOQDs-NS-Doped as a catalyst²⁴. Herein, we present a convenient approach for the efficient synthesis of innovative 2-amino-4H-chromenes using GOQDs-NS-Doped as a catalyst (Fig. 2). Additionally, a computational study was conducted, and the B3LYP/6-311+G(d,p) method was employed to calculate the properties of the 4c derivative in its ground state.

Fig. 2.

Catalytic synthesis of 2-amino-4H-chromenes using GOQDs-NS-Doped.

Experimental results and discussion

The green and nanocatalyst based on graphene oxide was characterized using various techniques. Fourier transform infrared (FTIR) spectra were recorded with a Spectrum 400 PerkinElmer FT-IR spectrophotometer. Material characterization was performed using X-ray powder diffraction (XRD, Bruker D8 advance, Cu Ka radiation), scanning electron microscopy (SEM, FE-SEM/STEM SU 9000), UV–Vis spectroscopy (Shimadzu UV-160A), thermogravimetric analysis (TGA, Rheometric Scientific STA 1500), and Brunauer–Emmett-Teller (BET) analysis (PHSCHINA PHS 1020 Porosity Analyzer). Reagent-grade chemicals were sourced from Merck for the synthesis of the compounds. The starting materials included benzaldehyde (${\text{C}}_6{\text{H}}_5\text{CHO}$, 99% purity), malononitrile (${\text{NCCH}}_2\text{CN}$, 99% purity), resorcinol (${\text{C}}_6{\text{H}}_6{\text{O}}_2$, 99% purity), and $\beta$-naphthol (${\text{C}}_{10}{\text{H}}_8\text{O}$, 99% purity). Characterization was carried out using FTIR spectroscopy (Spectrum 400 PerkinElmer FT-IR spectrophotometer) and NMR spectroscopy (Bruker Inova instrument, DMSO-d₆). NMR spectra were acquired at varying frequencies: $_{}^1$H (300 and 500 MHz) and $_{}^{13}$C (75, 100, and 500 MHz).

Preparation of (NS-doped-GOQDS)

The synthesis of the NS-doped-GOQDS catalyst, as depicted in Fig. 3, was conducted following the procedure detailed in our earlier publication²⁴. Nitrogen-sulfur-doped graphene oxide quantum dots NS-doped-GOQDS were prepared using a one-pot synthesis method. Citric acid served as the carbon source, and thiourea acted as the nitrogen and sulfur source. The synthesis involved a two-step process. First, graphene oxide (GO) was prepared by pyrolyzing citric acid under controlled conditions, resulting in a black solid. This solid was then purified by washing, crumbling, and dissolving it in a NaOH solution. After neutralization, it was filtered, and dried to obtain pure graphene oxide (GO) powder. To synthesize the NS-doped-GOQDS, 30 mg of citric acid was heated until melted, at which point thiourea was added to the reaction mixture. The mixture was then heated and stirred for 30 minutes, yielding a black viscous product. This product was further purified through cooling, washing, crumbling, and dissolving in a NaOH solution, followed by neutralization, filtration, and drying. This final step yielded the NS-doped-GOQDS powder.

Fig. 3.

Preparation of GOQDs-NS-doped carbocatalysts.

General approach to 2-amino-4H-chromenes synthesis

A mixture containing 1 mmol of aldehydes, 1.1 mmol of various nucleophiles (resorcinol, $\beta$-naphthol and dimedone), and 1 mmol of malononitrile, along with 30 mg of the catalyst, was placed in a flask and stirred at 40 $_{}^{\circ }$C in EtOH for the specified time according to Table 1. The progress of the reaction was monitored using Thin-layer chromatography (TLC) with a solvent system of ethyl acetate/n-hexane (1:3). We used a water bath called a Stuart SW15D. It’s like a little heater with a stirrer that helps keep the temperature even. We put a thermometer right in the flask so we could watch the temperature all the time. We adjusted the water bath temperature as needed to keep it at 40 $_{}^{\circ }$C. After reaction completion, as monitored by TLC, the heterogeneous GOQDs-NS-Doped catalyst was removed by filtration using filter paper. The reaction mixture was then cooled, leading to the formation of product crystals. These crystals were collected by filtration, recrystallized from ethanol, and then filtered again to isolate the purified product.

Table 1.

Optimization of catalyst for 2-amino-4H-chromenes reaction.

Entry	Catalyst	Solvent	Catalyst amount (mg)	Temperature ($_{}^{\circ }$C)	Time (min)	Yield (%)	Ref
1	${\text{Fe}}_3{\text{O}}_4$@${\text{SiO}}_2$@$({\mathrm{CH}}_2)_3^{}$ NHCO- adenine sulfonic acid	Solvent-free	40	110	120-170	90	28
2	Tungstic acid-SBA-15	${\text{H}}_2\text{O}$	30	100	12 h	86	29
3	${\text{Fe}({\text{HSO}}_4)}_3$	MeCN	0.1 mmol	reflux	4 h	83	30
4	[Choline][Ac]	[Choline][Ac]	-	rt	120	70	31
5	GO/$\alpha$-Fe2O3/CuL	Solvent free	60	70	60	98	32
6	Pd@GO	EtOH	10	80	15	92	33
7	Urea	EtOH/${\text{H}}_2\text{O}$	10 mol%	rt	720	73	34
8	${\text{Fe}}_3{\text{O}}_4-\mathrm{GO}-\mathrm{SO}3\mathrm{H}$	EtOH	30	40	120	70	This work
9	${\text{Fe}}_3{\text{O}}_4-\mathrm{GO}-\mathrm{Cu}$	EtOH	30	40	30	82	This work
10	${\text{Fe}}_3{\text{O}}_4-\mathrm{GO}-\mathrm{Pd}$	EtOH	30	40	30	78	This work
11	${\text{Fe}}_3{\text{O}}_4-\text{GO}$	EtOH	30	40	45	70	This work
12	NS-doped-GOQDS	EtOH	30	40	9	98	This work

SEM measurement

Figure 4 presents scanning electron microscopy (SEM) images of graphene oxide (GO), graphene quantum dots (GOQDs), and GOQDs-NS-doped structures. Figure 4a,b showcase the SEM images of GO, revealing its characteristic morphology. The SEM images of GOQDs-NS-doped structures (Fig. 4c–f) demonstrate distinct features. The material consists of separated sheets with defined edges (Fig. 4c), indicating a layered structure. Notably, the images reveal crumpled areas (Fig. 4c) and layered structures with lamellae (Fig. 4d–f). The thickness of these lamellae ranges from 95 nm to 348 nm. The increased layer thickness in GOQDs-NS-doped structures is attributed to interlayer interactions facilitated by the presence of oxygen, nitrogen, and sulfur functional groups on the graphene oxide layers. These heteroatoms are primarily concentrated at the edges of the NS-doped GOQDs, resulting in thicker edges within the sheets.

Fig. 4.

Comparison of GO and GOQDs-NS-doped structures using SEM. Panels (a) and (b) show the characteristic amorphous structure of GO. Panels (c) through (f) illustrate the formation of a layered structure in the GOQDs-NS-doped.²⁵.

XRD analysis

The presence of thiourea within the GOQDs-NS-doped structure is suggested by XRD analysis. Figure 5a shows the SAXS pattern of GOQDs, exhibiting a broad peak at 2$\theta$ = 22$_{}^{\circ }$. The XRD pattern of thiourea is shown in Fig. 5b²⁶, revealing characteristic peaks. Figure 5c demonstrates the XRD pattern of amorphous NS-doped GOQDs, exhibiting a peak at 2$\theta$ = 26$_{}^{\circ }$. Comparison of these patterns indicates the presence of thiourea within the NS-doped GOQDs structure.

Fig. 5.

XRD patterns of (a) amorphous GOQDs, (b) thiourea, and (c) NS-doped GOQDs.²⁵.

FT-IR analysis

The FTIR spectrum of Graphene Oxide Quantum Dots (GOQDs) exhibits a broad band around 3433 $c{m}^{-1}$, indicative of hydrogen bonding associated with O-H stretching in water. Additionally, the C=O stretching is evidenced by peaks at 1709 $c{m}^{-1}$ and 1763 $c{m}^{-1}$, which confirm the existence of carboxyl and carbonyl functional groups. Furthermore, C-O stretching bands characteristic of phenolic, alcohol, or ether groups occur at 1385 $c{m}^{-1}$ and 1021 $c{m}^{-1}$, while a faint absorption at 1271 $c{m}^{-1}$ points to the presence of epoxy groups (as depicted in Fig. 6a).

Fig. 6.

Spectra showing (a) (GOQDs) and (b) (NS-doped-GOQDs).²⁵.

In the FTIR spectrum of nitrogen-sulfur-doped GOQDs, weak peaks appear at 3738 $c{m}^{-1}$ and 3897 $c{m}^{-1}$, indicating the presence of free OH and NH groups on the surface. A broad peak in the range of 3200–3500 $c{m}^{-1}$ corresponds to O–H and NH stretching, suggesting that hydroxyl and amine groups are present in the NS-doped GOQDs. The peaks at 2927 $c{m}^{-1}$ and 3072 $c{m}^{-1}$ are attributed to sp$_{}^3$and sp$_{}^2$ C–H groups, respectively. Additionally, bands at 1709 $c{m}^{-1}$, 1635 $c{m}^{-1}$, and 1403 $c{m}^{-1}$ relate to C=O, C=N, and C–N functional groups. A resonance peak at 875 $c{m}^{-1}$ can be assigned to the bending vibration of OH groups from adsorbed water molecules on the graphene oxide. The peak at 1356 $c{m}^{-1}$ corresponds to the C–OH group, while the peaks at 1116 $c{m}^{-1}$ and 1173 $c{m}^{-1}$ indicate C–O–C and C–N–C stretching vibrations. Lastly, the peak at 747 $c{m}^{-1}$ reflects H–C–H bending (shown in Fig. 6b).

Thermogravimetric studies

Figure 7a,b) illustrate the thermogravimetric analysis (TGA) curves for (GOQDs) and (NS-doped GOQDs). For GOQDs, two significant mass loss events occur at approximately 225 and 435 $_{}^{\circ }$C, while for NS-doped GOQDs, these correspond to around 290 $_{}^{\circ }$C and 410 $_{}^{\circ }$C. The initial mass loss is associated with the oxidation of amorphous carbon, while the subsequent loss relates to the combustion of the carbon bonds in both GOQDs and NS-doped GOQDs. The structure of GOQDs, which contains a higher carbon content and fewer polar groups, leads to reduced interlayer spacing. Consequently, it melts at a lower temperature of 225 $_{}^{\circ }$C and exhibits layer slipping at the higher temperature of 435 $_{}^{\circ }$C due to the strong interactions between the graphene oxide layers. The TGA curve of GOQDs demonstrates a notable release of large exothermic heat resulting from carbon combustion. In contrast, NS-doped GOQDs exhibit an increased interlayer distance due to the functional groups’ presence. This results in a melting temperature of 290 $_{}^{\circ }$C and a lower combustion temperature of 410 $_{}^{\circ }$C, as the oxidation of carbon occurs more rapidly in the presence of heteroatoms.

Fig. 7.

TGA spectra for (a) GOQDs and (b) NS-doped GOQDs. 3.5 EDX analysis.²⁵.

The elemental composition analyzed via EDX is presented in Fig. 8a,b. The analysis reveals the presence of oxygen, sulfur, and nitrogen on the surface of NS-doped GOQDs, with sulfur being particularly prominent in the EDX findings. It is important to note that while this technique can demonstrate the presence of these elements, it does not provide precise quantification of their amounts.

Fig. 8.

EDX spectra showing (a) GOQDs and (b) NS-doped GOQDs.²⁵.

Ultraviolet (UV) measurement

The UV–Vis spectrum presented in Fig. 9 illustrates the characteristics of NS-doped GOQDs, and GOQDs, respectively shown as curves b and a. The GOQDs exhibit distinct absorption peaks around 240 nm and 300 nm, while the NS-doped GOQDs display a notable increase in absorbance at approximately 225 nm and 330 nm. This enhancement indicates the presence of unpaired electrons from nitrogen and sulfur, which are available for $n-\pi$ interactions.

Fig. 9.

UV-Vis absorption spectra of (a) GOQDs and (b) NS-doped-GOQDs.²⁵.

BET analysis

The analysis performed using BET indicated that NS-doped GOQDs has a specific surface area of 16.482 $m^2/g$ . In comparison, the specific surface area of raw graphite powder with particle sizes under 20 $\mathrm{\mu }$m was only 7.65 m$_{}^2$/g²⁷. This demonstrates an increase of over twofold. Furthermore, the pore size distribution revealed that the enhancement in specific surface area primarily resulted from pores with diameters averaging 14.13 nm.

Reaction condition optimization for 2-amino-4H-chromene synthesis

In the assessment of the catalytic activity of GOQDs-NS-Doped, a one-pot synthesis of a multi-component reaction was chosen, involving malonitrile, aldehyde, active nucleophiles (such as resorcinol, $\beta$-naphthol, and dimedone), to yield 2-amino 4H-chromenes. 4-Nitro benzaldehyde (1c) and resorcinol (3a) were specifically selected as model compounds for optimizing the reaction. Comparing the yield and time of the reaction catalyzed by GOQDs-NS-Doped with other catalysts) revealed significant improvements (Table 1). GOQDs-NS-Doped, serving as a heterogeneous basic catalyst, notably enhanced the yield under mild conditions. In contrast, catalysts like ${\text{Fe}({\text{HSO}}_4)}_3$ required reflux conditions for 4 hours to offer the product (Table 1, entry 3). Other catalysts, such as urea, [Choline][Ac], Tungstic acid-SBA-15, and GO/$\alpha$-${\text{Fe}}_2{\text{O}}_3/\text{CuL}$, suffered from prolonged reaction times(Table 1, entries 1, 2, and 4). Additionally, Pd@GO (Table 1, entry 6) required higher temperatures of 80 $_{}^{\circ }$C, while GOQDs-NS-Doped exhibited superiority in terms of reaction conditions, time, and yield. ${\text{Fe}}_3{\text{O}}_4-\text{GO}$, ${\text{Fe}}_3{\text{O}}_4$-GO-Pd, ${\text{Fe}}_3{\text{O}}_4$-GO-Cu, and ${\text{Fe}}_3{\text{O}}_4$-GO-${\text{SO}}_3\text{H}$ were explored as Lewis acidic and Brønsted acidic catalysts, with varying results. Notably, ${\text{Fe}}_3{\text{O}}_4$-GO-Cu and ${\text{Fe}}_3{\text{O}}_4$-GO-Pd improved yield and reduced reaction time (Table 1, entries 9 and 10). By incorporating nitrogen and sulfur into graphene oxide, the yield of the reaction was boosted to 98% in just 9 min while also lowering the required temperature to 40 $_{}^{\circ }$C and demonstrating high reusability. This highlights the advantageous features of GOQDs-NS-Doped detailed in (Table 1, entry 12), making it a promising catalyst for further research and application.

In our investigation (Table 2, entries 1-6), we explored the impact of varying temperatures on the reaction. Starting at room temperature, we achieved a 60% yield after 45 minutes (Table 2, entry 1). Augmenting the temperature to 35 $_{}^{\circ }$C significantly improved the production yield to 88% (Table 2, entry 2). The optimal outcome was observed at 40 $_{}^{\circ }$C, where the yield soared to an impressive 98% (Table 2, entry 4). Interestingly, maintaining the temperature at 45 $_{}^{\circ }$C did not influence the yield (Table 2, entry 5), but a further increase to 50 $_{}^{\circ }$C saw a slight decrease to 95% yield (Table 2, entry 6). Next, we investigated the effect of catalyst loading from 0 mg to 35 mg (Table 2, entries 7–13). The absence of a catalyst resulted in no product formation even after 60 min (Table 2, entry 7). With catalyst loading increasing from 10 mg to 20 mg, the production yield improved from 40 to 80% (Table 2, entries 8-10). Moreover, the reaction time was reduced from 40 minutes to 15 min with the addition of the catalyst (Table 2, entries 8–11). Notably, at 30 mg of catalyst, the yield peaked at 98% (Table 2, entry 12), beyond which increasing the loading to 35 mg did not impact the yield (Table 2, entry 13). The influence of solvents on the reaction was also assessed (Table 2, entries 14-21). The use of ethanol as a green solvent led to a remarkable increase in yield. However, the addition of water to ethanol reduced the yield from 98 to 74% (Table 2, entry 15). Water used alone in the reaction yielded a lower percentage (Table 2, entry 16). Methanol yielded a 91% product. Aprotic solvents such as tetrahydrofuran, toluene, and dimethylformamide did not enhance the yield as effectively as alcohols (Table 2, entries 18, 19, and 21). Interestingly, conducting the reaction under solvent-free conditions did not significantly impact the yield (Table 2, entry 20).

Table 2.

Optimization of 2-amino-4H-chromenes reaction for 4c derivative$_{}^a$.

Entry	Catalyst	Solvent	$\times$ (mg)	Temperature ($_{}^{\circ }$C)	Time (min)	Yield (%)$_{}^b$
Effect of temperature
1	GOQDs-NS-Doped	EtOH	30	rt	45	60
2	GOQDs-NS-Doped	EtOH	30	30	30	78
3	GOQDs-NS-Doped	EtOH	30	35	15	88
4	GOQDs-NS-Doped	EtOH	30	40	30	98
5	GOQDs-NS-Doped	EtOH	30	45	30	98
6	GOQDs-NS-Doped	EtOH	30	50	30	95
Effect of catalyst loading
7	GOQDs-NS-Doped	EtOH	0	40	60	NR
8	GOQDs-NS-Doped	EtOH	10	40	40	40
9	GOQDs-NS-Doped	EtOH	15	40	30	60
10	GOQDs-NS-Doped	EtOH	20	40	30	80
11	GOQDs-NS-Doped	EtOH	25	40	15	92
12	GOQDs-NS-Doped	EtOH	30	40	9	98
13	GOQDs-NS-Doped	EtOH	35	40	9	98
Effect of solvent
14	GOQDs-NS-Doped	EtOH	30	40	9	98
15	GOQDs-NS-Doped	${\text{H}}_2\text{O}$/EtOH (1:1)	30	40	15	74
16	GOQDs-NS-Doped	${\text{H}}_2\text{O}$	30	40	80	35
17	GOQDs-NS-Doped	MeOH	30	40	9	91
18	GOQDs-NS-Doped	THF	30	40	60	30
19	GOQDs-NS-Doped	Toluene	30	40	110	23
20	GOQDs-NS-Doped	Solvent-free	30	40	20	55
21	GOQDs-NS-Doped	DMF	30	40	15	70

$_{}^{\text{a}}$Reaction conditions: 1 mmol of 4-Nitrobenzaldehyde, 1 mmol of malononitrile, 1.1 mmol of resorcinol, and varying weight amounts of NS-Doped-GOQDS catalyst.

$_{}^{\text{b}}$Yields refer to isolated products.

Preparation of substituted 2-amino-4H-chromens in the presence of NS-Doped-GOQDS

Following optimization, the reaction’s scope and generality were investigated. Aromatic aldehydes bearing electron-donating and electron-withdrawing substituents at ortho-, meta-, or para-positions on the aromatic ring were successfully converted into 2-amino-4H-chromen derivatives with excellent yields as shown in Table 3. Notably, a wide range of functional groups, such as 4-nitro, remained stable under the reaction conditions, as evidenced by the successful transformations in the entries 3, 8, and 15 of Table 3. Moreover, aromatic aldehydes with methyl and methoxy groups as electron-donating substituents exhibited excellent performance, affording the desired products in yields of up to 90% as shown in Table 3 (refer to entries 7, 16, and 18). Additionally, halide groups like 4-Cl and 4-Br were efficiently converted into the desired products with high yields. Aromatic aldehydes with a combination of electron-withdrawing and electron-donating substituents showed promising results in the reaction. Notably, substituents with stronger electron-withdrawing characteristics demonstrated increased efficiency and reaction rates, showcasing enhanced electron withdrawal capabilities.

Table 3.

Synthesis and physical data of 2-amino-4H-chromenes 4a-r catalyzed by NS-Doped-GOQDS$_{}^a$.

Entry	R	Nucleophiles	Product	Time (min)	Yield (%)$_{}^b$	Mp ($_{}^{\circ }$C)
						Found	Reported
1	${\text{C}}_6{\text{H}}_5$ 1a	Resorcinol 3a	4a	15	95	235–236	234–23635
2	3-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1b	Resorcinol 3a	4b	12	94	169–170	170–17236
3	4-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1c	Resorcinol 3a	4c	9	98	211–213	212–21037
4	4-${\text{ClC}}_6{\text{H}}_4$ 1d	Resorcinol 3a	4d	12	96	160–162	160–16135
5	2,4-${\text{Cl}}_2{\text{C}}_6{\text{H}}_4$ 1e	Resorcinol 3a	4e	15	92	254–256	256–25838
6	4-${\text{BrC}}_6{\text{H}}_4$ 1f	Resorcinol 3a	4f	15	91	224–226	223–22539
7	4-${\text{BrC}}_6{\text{H}}_4$ 1g	Resorcinol 3a	4g	20	90	183–186	184–18640
8	4-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1h	$\beta$-Naphthol 3b	4h	12	97	184–186	185–18641
9	3-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1i	$\beta$-Naphthol 3b	4i	13	90	234–236	232–23442
10	2-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1j	$\beta$-Naphthol 3b	4j	12	92	278–280	258–26243
11	4-${\text{ClC}}_6{\text{H}}_4$ 1k	$\beta$-Naphthol 3b	4k	10	96	209–211	210–21218
12	2,6-${\text{Cl}}_2{\text{C}}_6{\text{H}}_4$ 1l	$\beta$-Naphthol 3b	4l	25	85	206	205–20744
13	${\text{C}}_6{\text{H}}_4$ 1m	Dimedone 3c	4m	10	95	222–225	224–22645
14	4-${\text{ClC}}_6{\text{H}}_4$ 1n	Dimedone 3c	4n	12	96	210–212	210–21246
15	4-${\text{NO}}_2{\text{C}}_6{\text{H}}_4$ 1o	Dimedone 3c	4o	8	97	181–183	180–18247
16	2-${\text{OCH}}_3{\text{C}}_6{\text{H}}_4$ 1p	Dimedone 3c	4p	14	94	203–205	203–20748
17	4-${\text{BrC}}_6{\text{H}}_4$ 1q	Dimedone 3c	4q	13	92	200–204	201–20349
18	2-${\text{CH}}_3{\text{C}}_6{\text{H}}_4$ 1r	Dimedone 3c	4r	12	90	210–213	211–21350

$_{}^{\text{a}}$Reaction condition: Benzaldehyde 1 mmol (151.0 mg), malononitrile 1 mmol (0.66 mg), resorcinol 1.1 mmol (0.121 mg), and 30 mg of NS-Doped-GOQDS catalyst at 40$_{}^{\circ }$C.

$_{}^{\text{b}}$Yields refer to isolated products

Proposed mechanism of GOQDs-NS-Doped catalyzed 2-amino-4H-chromenes synthesis

The mechanism of preparation of 2-amino-4H-chromenes catalyzed by GOQDs-NS-Doped was demonstrated in Fig. 10. It begins with malononitrile (2), an active methylene compound, undergoing tautomerization in the presence of the catalyst. This allows for a Knoevenagel condensation with an aldehyde derivatives 1 in ethanol, producing arylidene malononitrile M in high yield. The condensation process involves the elimination of water. Subsequently,  compound 3 undergoes ortho C-alkylation with M, forming compound N. Compound N then undergoes tautomerization to yield O. The hydroxyl group of O participates in a nucleophilic addition reaction with the cyano (CN) group, leading to an intramolecular cyclization, resulting in the formation of 2-imino-chromene P. Finally, intramolecular proton transfer within P generates the substituted 2-amino-4H-chromene 4. The GOQDs-NS-Doped catalyst plays a crucial role in facilitating the tautomerization of malononitrile, enabling the Knoevenagel condensation. The reaction proceeds through a series of well-defined steps involving nucleophilic addition, cyclization, and proton transfer.

Fig. 10.

The proposed mechanism for the synthesis of 2-amino 4H-chromenes catalyzed by NS-Doped-GOQDS.

Recycling of the catalyst

The recovery and reusability of each catalyst play a key role in reducing the overall cost of the product. A carefully optimized model reaction was conducted to investigate the catalyst’s reusability. Upon completion of the reaction, the mixture was cooled to room temperature, and the product was separated through filtration using filter paper. The product was washed with ethanol and water multiple times, dried under vacuum, and subsequently reintroduced for further use. Figure 11. demonstrates that the catalyst maintained its activity even after being reused four times in the reaction, indicating minimal loss of effectiveness.

Fig. 11.

Re-usability of GOQDs-NS-Doped catalyst.

Selected spectroscopic data

2-Amino-7-hydroxy-4-phenyl-4H-chromene-3-carbonitrile C₁₆H₁₂N₂O₂ (4a):

yield 98 (%), m.p 235–236 $_{}^{\circ }$C, $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 4.57 (s, 1H), 6.36 (s, 1H), 6.44 (s, 1H), 6.80 (s, 3H), 7.13 (s, 3H), 7.26 (s, 2H), 9.64 (s, 1H); $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 37.60, 67.0, 102.8, 112.9, 114.3, 121.1, 127.2, 127.9, 129.1, 130.4, 146.9, 149.5, 160.6, 166.8. ; IR (KBr $c{m}^{-1}$): 3698, 3589, 3496, 2192, 1651, 1500, 1150, 850.

2-Amino-7-hydroxy-4-(3-nitrophenyl)-4H-chromene-3-carbonitrile C₁₆H₁₁N₃O₄ (4b):

yield 94 (%) m.p 169–170 $_{}^{\circ }$C, $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 4.94 (s, 1H), 6.48 (d, J = 1.8 Hz, 1H), 6.52 (dd, J = 1.8 Hz, J = 8.4 Hz, 1H), 6.85 (d, J = 8.4 Hz, 1H), 7.05 (s, 2H), 7.62 (m, 2H), 8.05 (s, 1H), 8.10 (d, J = 8.1 Hz, 1H), 9.79 (s, 1H); $_{}^{13}$C NMR (DMSO-d₆, 75 MHz), $\delta$ C (ppm) 39.85, 66.88, 102.84, 113.05, 113.11, 120.88, 122.25, 122.32, 130.11, 130.46, 134.75, 148.43, 149.03, 149.43, 163.70, 166.12.; IR (KBr, $c{m}^{-1}$): 3740, 3505, 3414, 2195, 1656, 1400, 1294, 1150, 847.

2-Amino-7-hydroxy-4-(4-nitrophenyl)-4H-chromene-3-carbonitrile C₁₆H₁₁N₃O₄ (4c):

yield 95 (%), m.p 210–213 $_{}^{\circ }$C , $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 4.88 (s, 1H), 6.47 (s, 1H), 6.51 (d, J = 8.1 Hz, 1H), 6.81 (d, J = 8.1 Hz, 1H), 7.03 (s, 2H), 7.48 (d, J = 8.1 Hz, 2H), 8.19 (d, J = 8.1 Hz, 2H), 9.08 (s, 1H); $_{}^{13}$C NMR (DMSO-d₆, 75 MHz), $\delta$ C (ppm): 38.50, 74.20, 102.90, 112.83, 113.15, 120.74, 124.46, 129.15, 130.40, 146.85, 149.47, 1546.23, 161.12, 165.40.; IR (KBr, $c{m}^{-1}$): 3793, 3688, 3596, 2186, 1651, 1512, 1418, 1344, 1162, 1109, 850, 797.

2-Amino-4-(4-chlorophenyl)-7-hydroxy-4H-chromene-3-carbonitrile C₁₆H₁₁Cl₂N₂O₂ (4d):

yield 93 (%), m.p 160–162 $_{}^{\circ }$C, $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm) 4.67 (s, 1H), 6.33 (d, J = 1.8 Hz, 1H), 6.48 (dd, J = 1.8 Hz, J = 8.4 Hz, 1H), 6.77 (d, J = 8.4 Hz, 1H), 6.93 (s, 2H), 7.25 (d, J = 8.1 Hz, 2H), 7.38 (d, J = 8.1 Hz, 2H), 9.76 (s, 1H), $_{}^{13}$C NMR (DMSO-d₆, 75 MHz), $\delta$ C (ppm) 38.5 68.37, 102.76, 112.92, 113.63, 120.95, 129.01, 129.70, 130.33, 131.71, 149.34, 162.82, 166.70.; IR (KBr, $c{m}^{-1}$): 3701, 3527, 3414, 2189, 1651, 1503, 1462, 1153, 1106, 859, 800.

2-amino-4-(2,4-dichlorophenyl)-7-hydroxy-4H-chromene-3- carbonitrile C₁₆H₁₁Cl₂N₂O₂ (4e):

yield 92 (%), m.p 254–256 $_{}^{\circ }$C.; $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 5.15 (s, 1H), 6.43 (d, J = 1.8 Hz, 1H), 6.49 (dd, J = 2.1 Hz, J = 8.4 Hz, 1H), 6.72 (d, J = 8.4 Hz, 1H), 6.98 (s, 2H), 7.22 (d, J = 8.4 Hz, 1H), 7.39 (dd, J = 1.8 Hz, J = 8.4 Hz, 1H), 7.59 (d, J = 1.8 Hz, 1H), 9.78 (s, 1H).; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 37.39, 68.93, 102.71, 112.32, 113.01, 120.66, 128.54, 129.58, 129.73, 132.63, 132.79, 133.21, 142.32, 149.55, 164.82, 168.91.; IR (KBr, $c{m}^{-1}$): 3710, 3547, 3469, 2189, 1506, 1506, 1465, 1156, 1103, 809, 856.

2-Amino-4-(4-bromophenyl)-7-hydroxy-4H-chromene-3-carbonitrile C₁₆H₁₁BrN₂O₂ (4f):

yield 91 (%) ; m.p 224–226 $_{}^{\circ }$C; $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm) 4.62 (s, 1H), 6.37 (s, 1H), 6.46 (d, J = 8.5 Hz, 1H), 6.76 (d, J = 8.5 Hz, 1H), 6.88(s, 2H), 7.10 (d, J = 8.5 Hz, 2H), 7.48 (d, J = 8.5 Hz, 2H), 9.70 (s, 1H).; $_{}^{13}$C NMR (DMSO-d₆, 100 MHz):$\delta$ C (ppm) 40.1, 66.70, 102.8, 113.1, 113.7, 120.3, 130.4, 132.0, 146.3, 149.4, 162.7, 166.8.; IR (KBr, $c{m}^{-1}$): 3693, 3508, 3417, 2189, 1648, 1503, 1165, 797.

2-Amino-7-hydroxy-4-(p-tolyl)-4H-chromene-3-carbonitrile C₁₇H₁₄N₂O₂ (4g):

yield 90 (%) m.p 183–186 $_{}^{\circ }$C , $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 2.27 (s, 3H), 4.58 (s, 1H), 6.42 (d, J = 1.8 Hz, 1H), 6.48 (dd, J = 1.8 Hz, J = 8.4 Hz, 1H), 6.78 (m, 2H), 7.45 (d, J = 8.1 Hz, 2H), 7.87 (d, J = 8.1 Hz, 2H). 9.68 (s, 1H),$_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 21.01, 69.90, 102.66, 112.89, 113.98, 121.13, 127.77, 129.23, 129.56, 136.13, 143.93, 149.24, 163.42, 166.60.; IR (KBr, $c{m}^{-1}$): 3699, 3577, 3408, 2192, 1648, 1509, 1465, 1159, 1115, 859, 812.

3-Amino-1-(4-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile C₂₀H₁₃N₃O₃ (4h):

yield 97 (%), m.p: 185–186 $_{}^{\circ }\text{C}$ $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm) 8.13–8.11 (d, J=8.45 Hz, 2H,), 7.97–7.95 (d, J=9 Hz, 2H), 7.91(d, J=7.45 Hz, 2H), 7.74–7.72 (m, 2H), 7.66 (d, J=8.25 Hz, 1H), 7.47–7.34 (m, 6H), 7.12 (s, 2H), 5.54 (s, 1H), $_{}^{13}$C NMR (DMSO-d₆, 500 MHz): $\delta$ C (ppm) 37.71, 66.61, 116.86, 114.43, 122.55, 120.19, 125.94, 123.32, 128.55, 127.36, 145.93, 130.8, 129.22, 134.56, 131.43, 136.67, 145.18, 166.91.; IR (KBr, $c{m}^{-1}$): 3548, 3450, 3329, 2189, 1656, 1582, 1517, 1416, 1339, 1228, 821.

3-Amino-1-(3-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile, C₂₀H₁₃N₃O₃ (4i):

yield 90 (%), m.p:234–236 $_{}^{\circ }\text{C}$ IR (KBr): $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm) 8.07 (s, 1H), 8.03–8.01 (d, J=7.8 Hz, 1H), 7.98–7.96 (d, J=8.9 Hz, 1H), 7.92–7.91 (d, J=7.6Hz, 1H), 7.87–7.85 (d, J=8.1 Hz, 1H), 7.67–7.66 (d, J=7.4 Hz, 1H), 7.58–7.55 (m, 1H), 7.46–7.37 (m, 3H), 7.15 (s, 2H), 5.62 (s, 1H). $_{}^{13}$C NMR (DMSO-d₆, 500 MHz): $\delta$ C (ppm) 37.33, 66.99, 114.51, 116.80, 147.99, 120.11, 121.38, 121.87, 123.46, 125.11, 127.30, 128.59, 129.92, 130.41, 130.80, 133.65, 146.91, 147.89, 167.61.; IR (KBr, $c{m}^{-1}$): 3542, 3444, 3393, 2867, 2183, 1659,1589, 1527, 1403,1342, 1219, 813.

3-Amino-1-(2-nitrophenyl)-1H-benzo[f]chromene-2-carbonitrile C₂₀H₁₃N₃O₃ (4j):

yield 92 (%) m.p:278–280 $_{}^{\circ }\text{C}$ $_{}^1$H NMR ( DMSO-d₆, 500 MHz): $\delta$ H (ppm) 8.01–7.93 (m, 2H), 7.75–7.73 (d, J= 8.1 Hz, 1H), 7.51–7.35 (m, 5H), 7.14 (s, 2H), 7.01 (d, J=7.75, 1H), 6.11 (s, 1H).; $_{}^{13}$C NMR (DMSO-d₆, 500 MHz): $\delta$ C (ppm): 33.30, 69.17, 113.53, 116.80, 119.64, 122.67, 124.71, 125.17, 127.66, 128.20, 128.72, 130.84, 130.06, 134.22, 139.13, 147.47, 147.55, 166.77.; IR (KBr, $c{m}^{-1}$): 3505, 3402, 3187, 2186, 1656, 1598, 1514, 1403, 1342, 1228, 815.

3-Amino-1-(4-chlorophenyl)-1H-benzo[f] chromene-2-carbonitrile, C₂₀H₁₃ClN₂O (4k):

yield 96 (%), m.p: 209–211 $_{}^{\circ }\text{C}$; $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm) 7.96–7.91 (m, 2H), 7.82 (d, J= 7.7 Hz, 1H), 7.48–7.41 (m, 2H), 7.36–7.31 (m, 3H), 7.23–7.21 (m, 2H), 7.03 (s, 2H), 5.37 (s, 1H) $_{}^{13}$C NMR (DMSO-d₆, 500 MHz): $\delta$ C (ppm) 37.33, 66.9, 114.5, 116.8, 120.1, 121.8, 123.4, 125.1, 127.3, 128.5, 129.9, 130.4, 130.8, 133.6, 146, 147.9, 166.0.; IR (KBr, cm⁻¹): 3533, 3358, 3127, 2934, 2213, 1641, 1585, 1480, 1413, 1363, 1086, 818, 615.

3-Amino-1-(2,6-dichlorophenyl)-1H-benzo[f] chromene-2-carbonitrile C₂₀H₁₂Cl₂N₂O (4l):

yield 85 (%), m.p: 217–219 $_{}^{\circ }\text{C}$ $_{}^1$H NMR (DMSO-d₆, 500 MHz): $\delta$ H (ppm): 7.95-7.91 (m, 2H), 7.63 (d, J=6.85 Hz, 1H), 7.53 (d, J=8 Hz, 1H), 7.45–7.38 (m, 2H), 7.29–7.22 (m, 3H), 7.08 (s, 2H), 6.15 (s, 1H).; $_{}^{13}$C NMR (DMSO-d₆, 500 MHz): $\delta$ C (ppm) 37.30, 68.18, 114.60, 117.8, 121.20, 121.78, 123.34, 12660., 127.13, 128.45, 129.89, 131.30, 130.70, 133.23, 146.12, 147.80, 167.02.; IR (KBr, $c{m}^{-1}$): 3548, 3418, 3392, 2919, 2186, 1659, 1588, 1514, 1400, 1231, 769.

2-Amino-7,7-dimethyl-5-oxo-4-phenyl-5,6,6,7,8-tetrahydro-4H-chromene-3-carbonitrile C₁₈H₁₈N₂O₂ (4m):

yield 95 (%), m.p: 221–223$_{}^{\mathit{circ}}$C $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 7.31–7.21 (m, 3H), 7.21–7.07 (m, 2H), 6.98 (s, 2H), 4.15 (s, 1H), 2.49 (s, 2H), 2.23 (d, J = 16.3 Hz, 1H), 2.07 (d, J = 15.6 Hz, 1H), 1.01 (s, 3H), 0.93 (s, 3H).; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 26.79, 28.39, 31.80, 35.57, 49.96, 68.29, 112.73, 119.72, 126.56, 127.14, 128.32, 144.74, 158.48, 162.49, 195.64.; IR (KBr, $c{m}^{-1}$): 3580, 3520, 3332, 2870, 2175, 1410, 1240, 780.

2-Amino-4-(4-chlorophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile C₁₈H₁₇ClN₂O₂ (4n):

yield 96 (%), m.p: 210–213$_{}^{\mathit{circ}}$c $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 7.33 (d, J = 8.0 1Hz, 2H), 7.17 (d, J = 8.1 Hz, 2H), 7.02 (s, 2H), 4.20 (s, 1H), 2.49 (s, 2H), 2.23 (d, J = 16.1 Hz, 1H), 2.09 C 13 (d, J = 16.0 Hz, 1H), 1.01 (s, 3H), 0.93 (s, 3H).; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 195.56, 162.54, 158.48,143.68, 131.09, 129.07, 128.23, 119.48, 112.33, 57.80, 49.92, 35.10, 31.72, 28.27, 26.83.; IR (KBr, $c{m}^{-1}$): 3580, 3450, 3323, 2880, 2123, 1520, 1300, 770.

2-Amino-7,7-dimethyl-4-(4-nitrophenyl)-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile C₁₈H₁₇N₃O₄ (4o):

yield 97 (%), m.p: 180–182$_{}^{\mathit{circ}}$C $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 8.15 (d, J = 8.2 Hz, 2H), 7.43 (d, J = 8.2 Hz, 2H), 7.12 (s, 2H), 4.35 (s, 1H), 2.52 (s, 2H), 2.24 (d, J = 16.2 Hz, 1H), 2.09 (d, J = 15.9 Hz, 1H), 1.02 (s, 3H), 0.94 (s, 3H).; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 195.65, 163.07, 158.55, 152.22, 146.22, 128.56, 123.60, 119.25, 111.69, 57.00, 49.82, 35.63, 31.75, 28.19, 26.89.; IR (KBr, $c{m}^{-1}$): 3487, 3456, 3341, 2878, 2166, 1490, 1356, 787.

2-Amino-4-(2-methoxyphenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile C₁₉H₂₀N₂O₃ (4p):

yield 94 (%), m.p: 203–207$_{}^{\mathit{circ}}$C $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 7.14 (m, 1H), 7.03–6.89 (m, 3H), 6.79 (s, 2H), 4.48 (s, 1H), 3.73 (s, 3H), 2.48 (d, J = 13.5 Hz, 1H), 2.23 (d, J = 16.4 Hz, 1H), 2.05 (d, J = 16.5 Hz, 1H), 1.03 (s, 3H), 0.95.; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 195.49, 13 (s, 3H). 163.02, 158.95, 156.79, 132.12, 128.51, 127.75, 120.28, 119.77, 111.86, 111.43, 57.38, 55.56, 50.01, 31.70, 30.30, 28.56, 26.51.; IR (KBr, $c{m}^{-1}$): 3470, 3453, 3351, 2872, 2200, 1807, 1532, 764.

2-Amino-4-(4-bromophenyl)-7,7-dimethyl-5-oxo-5,6,7,8-tetrahydro-4H-chromene-3-carbonitrile C₁₈H₁₇BrN₂O₂ (4q):

yield 92 (%), m.p: 201–203$_{}^{\mathit{circ}}$C $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 7.47 (d, J = 8.3 Hz, 1H), 7.10 (d, J = 8.4 Hz, 2H), 7.03 (s, 2H), 4.17 (s, 1H), 2.49 (s, 2H), 2.23 (d, J = 16.1 Hz, 1H), 2.09 (d, J = 16.0 Hz, 1H), 1.02 (s, 3H), 0.93 (s, 3H). $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 195.60, 162.57, 158.45, 144.12, 131.15, 129.47, 119.56, 119.48, 112.23, 57.68, 49.91, 35.16, 31.74, 28.26, 26.83. IR (KBr, $c{m}^{-1}$): 34770, 3460, 3363, 2830, 2185, 1610, 1476, 783.

2-Amino-7,7-dimethyl-5-oxo-4-(o-tolyl)-5,6,7,8-tetrahydro-4-H-Chromene-3-carbonitrile C₁₉H₂₀N₂O₂ (4r):

yield 90 (%), m.p: 211–213$_{}^{\mathit{circ}}$C $_{}^1$H NMR (DMSO-d₆, 300 MHz): $\delta$ H (ppm) 7.16–6.97 (m, 3H), 6.94 (d, J = 7.1 Hz, 1H), 6.90 (s, 2H), 4.47 (s, 1H), 2.46 (s, 3H), 2.23 (d, J = 16.1 Hz, 1H), 2.06 (d, J = 16.0 Hz, 1H), 1.03 (s, 2H), 0.95 (s, 3H).; $_{}^{13}$C NMR (DMSO-d₆, 75 MHz): $\delta$ C (ppm) 195.70, 162.45, 158.26, 143.53, 134.74, 129.86, 127.25, 126.33, 126.15, 119.69, 113.45, 58.28, 49.92, 31.81, 30.92, 28.39, 26.76, 19.03.; IR (KBr, $c{m}^{-1}$ ): 3493, 3420, 2896, 2220, 1598, 1798, 1446, 774.

Computational studies on the derivative 4c

The focus of the study shifted towards utilizing DFT calculations to investigate the physicochemical properties of compound 4c due to promising experimental results. Since the compound 4c has the highest yield among all of our derivatives, DFT studies are presented only for this one. DFT methods are well-known for their reliability, cost-effectiveness, and widespread use in various chemical applications. These methods are beneficial for determining electrophilicity index and spectral data. Among the different DFT methods available, the B3LYP functional^51–53 was chosen for its direct approach and improved computational accuracy without a sign of increasing computing time. This method has been proven to reveal important about systems in previous studies. The Gaussian 09 package was used to perform all the geometry optimizations and frequency calculations for the desired product 4c at the B3LYP functional and the 6-311+G(d,p) basis set. We performed computational studies to determine the physicochemical properties of 4c, such as its structure, NMR and FT-IR spectra, NBO charge distribution, thermodynamics, and electrophilicity index.

Electronic properties of 4c

The optimized structure and molecular properties of compound 4c have been investigated and characterized in Fig. 12 and Table 4. The dipole moment of 4c is reported to be 21.45 Debye, indicating its significant dipolar characteristics. The obtained electronic data includes the energies of the HOMO and LUMO, ${\mathrm{E}}_{\mathrm{HOMO}}$ and ${\mathrm{E}}_{\mathrm{LUMO}}$. The HOMO represents the electron-donating capability of the molecule, while the LUMO signifies its ability to accept electrons. These orbitals play crucial roles in the chemical reactivity of the compound.

Fig. 12.

Optimized geometry of 4c: Frontal view/ side view.

Table 4.

The electronic data of 4c.

Parameter	Value
Energy (Hartree)	− 1081.06
$\mu$D (Deby)	21.45
$E_{\mathit{HUMO}}$ (ev)	− 0.1661
${\mathrm{E}}_{\mathrm{LUMO}}$ (ev)	− 0.1170
Eg (ev)	0.0491
Point group	C1
Ionization potential	0.1170
Electron affinity	0.1170
Chemical potential ($\mu$) (ev)	0.1416
Chemical Hardness ($\eta$) (ev)	0.0246
Electrophilicity ($\omega$) (ev)	0.4059
Polarizability ($\alpha$)	621.33

For 4c, as shown in Fig. 13, Point group C1 indicates a high level of asymmetry in the molecule, and the ${\mathrm{E}}_{\mathrm{HOMO}}$and ${\mathrm{E}}_{\mathrm{LUMO}}$ values are − 0.1661 eV and − 0.1170 eV, respectively, with a calculated band gap (Eg) between them of 0.0491 eV. As the HOMO energy level increases and approaches the energy level of the LUMO, the molecular ionization potential decreases. Additionally, as the energy level of the LUMO orbital decreases, the electron affinity of the molecule increases. Moreover, the chemical hardness is another calculated parameter that increases with the increase in the energy difference of the HOMO and LUMO orbitals. Additionally, the Global Electrophilicity Index (GEI) is mentioned as a metric that reflects the molecule’s electron-accepting capacity. This index is fundamental in understanding various aspects of the molecule, such as its structure, reactivity, and toxicity. In 1999, Parr and colleagues defined the GEI (u)⁵⁴ as a parameter used to predict and analyze properties like aromaticity, Lewis acidity⁵⁵, and dynamics of molecules. This index serves as a valuable tool in studying the reactivity and behavior of chemical compounds. The Global Electrophilicity Index (GEI), chemical potential, and Chemical hardness are calculated by equations (1), (2), and (3), respectively. According to the rules and laws of quantum mechanics, the ionization potential and electron affinity of molecular orbitals have inverse relationships with the energy levels of the frontier molecular orbitals Table 4.

$$\begin{array}{cc}& \omega =\frac{{\mu }^2}{2\eta }\hfill \end{array}$$ 1

$$\begin{array}{cc}& \mu =\frac{{\mathrm{E}}_{\mathrm{LUMO}}+{\mathrm{E}}_{\mathrm{HOMO}}}{2}\hfill \end{array}$$ 2

$$\begin{array}{cc}& \eta =\frac{{\mathrm{E}}_{\mathrm{LUMO}}-{\mathrm{E}}_{\mathrm{HOMO}}}{2}\hfill \end{array}$$ 3

Fig. 13.

Frontier orbitals (HOMO and LUMO) for 4c calculated at B3LYP/6-311+G(d,p) level of theory.

Thermodynamic parameters of 4c derivative

Frequency calculations were performed to determine the standard molar energy (E), enthalpy (H), entropy (S), Gibbs free energy (G), and molar heat capacity (Cv) for the 4c derivative. The results are presented in Table 5. According to these results, the negative values of relative energy, Gibbs free energy, and enthalpy for the chromene derivative indicate that 4c is stable in the gas phase.

Table 5.

Quantum molecular descriptors for the molecule 4c.

Parameter	Value
Relative energy (E) ((kcal/mol)	− 678643.425
Standard enthalpies (H) (kcal/mol)	− 678642.957
Entropies (S) (cal/molK)	0.285799
Gibbs free energy (G) (kcal/mol)	− 678454.529
molar heat capacity (Cv) (cal/molK)	0.147157

Vibrational analysis of 4c

The structure of the synthesized chromone, 2-amino-7-hydroxy-4-(4-nitrophenyl)-4H-chromene-3-carbonitrile (4c), was experimentally confirmed by FTIR spectroscopy. To further support the structural assignment, theoretical vibrational frequencies were calculated using the B3LYP/6-311+G(d,p) level of theory. A potential energy distribution (PED) analysis⁵⁶ was then performed using the VEDA 4 program to gain a deeper understanding of the vibrational modes. The PED for each normal mode is follow as below equation:

$$\begin{array}{c}\hfill \text{PED}=\frac{{\sum }_i{F}_{\mathit{ii}}·L_{\mathit{ik}}^2}{{\lambda }_k}\end{array}$$ 4

where $F_{\mathit{ii}}$ represents the force constant, $L_{\mathit{ik}}$ is the normalized amplitude of the corresponding element (i, k), and $\lambda k$ is the eigenvalue associated with the vibrational frequency of element k. Only PED contributions exceeding 40% for each observed frequency were included in this study. The observed frequencies, along with the assignments of various vibrational modes, calculated frequencies, and PED values, are presented in Table 6. The important peaks of this compound related to $N{H}_2$, C–N, and OH bonds are as follows: The peak at 2186 $c{m}^{-1}$ indicates the stretching frequency of the C–N bond, while the peaks at 3596 and 3688 $c{m}^{-1}$ represent the symmetric and asymmetric stretching vibrations of the N–H group in $N{H}_2$ Additionally, the peak at 1630 $c{m}^{-1}$ corresponds to the H–N–H bending vibration. The absorption at 3793 $c{m}^{-1}$ corresponds to the stretching bond of the phenolic OH bond. In aromatic compounds, C–H stretching frequencies appear in the range of 3100-3000 $c{m}^{-1}$. In the compound 4c, the C–H stretching vibrations are predicted at 3193- 3226 $c{m}^{-1}$ for DFT- B3LYP/6-311+G(d,p) level of theory. Additionally, the paired stretching absorptions of the C=C bonds in the aromatic ring appear at around 1587 and 1487 $c{m}^{-1}$ , and an absorption at 1651 $c{m}^{-1}$ indicates the presence of a vinyl C=C double bond, as clearly seen in Fig. 14. In the computational calculation, the stretching vibrations of the N–H bonds are predicted at 3587 $c{m}^{-1}$ and 3709 $c{m}^{-1}$, the C–N bond at 2285 $c{m}^{-1}$, the OH bond at 3802 $c{m}^{-1}$, and the C=C bonds for aromatic and vinyl at 1547 $c{m}^{-1}$ and 1650 $c{m}^{-1}$, respectively. The observed and calculated frequencies are in good agreement, and the PED analysis indicates that these modes are pure.

Table 6.

Assignments, experimental, DFT-B3LYP-6311+G(d,p) and PED (%) calculated ir frequencies of compound 4c.

Assignments	Frequencies ($c{m}^{-1}$) experimental	Theoretical	(%PED)
O–H sym.stretching	3793	3802	100%
C–N sym.stretching	2186	2285	89%
N–H sym.stretching	3596	3587	48%
N–H asym.stretching	3688	3709	52%
Aromatic C–H asym.stretching	3197	3201	92%
Aromatic C–H sym.stretching	3180	3192	99%
H–N–H in-plane-bending(scissoring)	1630	1630	45%
H–O–C in-plane-bending(rocking)	1109	1187	41%
H–C–C–C torsion	850	850	61%
O–C–O–N Out-of-plane bending(wagging)	680	688	63%
O–N–C in-plane-bending(rocking)	530	537	49%

The largest P.E.D. contributions are reported.

Fig. 14.

Both the experimental (FT-IR) spectrum (shown in the top portion) and the calculated spectrum (shown in the bottom portion) for the compound labeled 4c.

The experimental and computational study of 4c consists of $_{}^1$H NMR and $_{}^{13}$C NMR spectra

The successful determination of a molecule’s NMR spectra based on its ground state geometry using the DFT/GIAO method highlights the reliability and precision of quantum chemical calculations in predicting NMR properties. By accurately modeling the molecular structure and its interactions, DFT calculations can offer valuable insights into the chemical shifts observed in NMR spectra. This demonstrates the utility of computational methods in elucidating the relationship between molecular properties and experimental observations in spectroscopy⁵⁷. The discrepancy between experimental and computed chemical shift values (Table 7) can be influenced by factors such as intermolecular hydrogen bonding and solvent effects. Specifically, the computational recording of OH (H33) and N–H (H29, H30) proton signals at a lower chemical shift compared to the experimental spectra may be attributed to hydrogen bonding interactions involving these functional groups with other molecules within the same compound or with solvent molecules (such as DMSO). This interaction can enhance proton mobility, leading to an increase in the observed chemical shift. In the computational state, H7 is recorded at 6.60 ppm, while in the experimental state, it is observed in the range of 6.47 ppm, appearing as a single signal in both the experimental and computational states. For H10, in the experimental state, it is displayed as a doublet at 6.51 and 6.54 ppm, whereas in the computational state, it appears as a singlet at a chemical shift of 6.90 ppm. On the aromatic ring, H17 and H19 exhibit a doublet with peaks at 7.58 and 7.48 ppm in the experimental state, respectively. In the computational state, these peaks are seen at 7.48 and 7.57 ppm, respectively. H9 appears as a doublet at 6.81 and 6.84 ppm in the experimental state, while in the computational chemical shift, it is observed at 7.82 ppm. For H21 and H22 in the experimental state, a doublet is observed at chemical shifts of 8.19 and 8.21 ppm, respectively. In contrast, in the theoretical state, they are predicted at 8.01 and 8.18 ppm. Overall, these findings suggest a strong experimental and computational data agreement. These key findings from the $_{}^{13}$C-NMR the spectra match the experimental data . The CN carbon is observed at approximately 123.6 ppm in the experimental state and at 124.4 ppm in the computational state. The carbon connected to OH (Ar, C–OH) appears around 161.1 ppm in the experimental state and at 165.4 ppm in the computational state. Additionally, the carbon linked to ${\text{NH}}_2$ is detected at 165.4 ppm in the experimental state and at 168.3 ppm in the computational state.

Table 7.

The $_{}^1$H-NMR and $_{}^{13}$C-NMR chemical shifts of compound 4c were determined experimentally and calculated utilizing the B3LYP/6-311+G(d,p) theoretical approach for purposes of comparison.

Atom$_{}^a$	Theoretical chemical shift/ppm	Experimental chemical shift/ppm	|${\delta }_{\mathit{exp}}-{\delta }_{B3LYP}|$
H33	5.06	9.08	4.02
H29	5.51	7.03	1.52
H30	5.41	7.03	1.62
H7	6.60	6.47	0.13
H10	6.90	6.51	0.39
H10	6.90	6.54	0.36
H17	7.57	7.48	0.12
H19	7.48	7.58	0.1
H9	7.82	6.81	1.01
H9	7.82	6.84	0.98
H21	8.01	8.19	0.18
H22	8.18	8.21	0.03
C11 (C-CN)	79.4	74.2	3.2
C26 (CN)	123.6	124.4	0.8
C1 (C-OH)	165.4	161.1	4.3
C12 (C-${\text{NH}}_2$)	168.3	165.4	2.9

$_{}^{\text{a}}$Figure 12 for the atom numbering.

Mulliken charge analysis, (MEP) surface and NBO charge distribution

The Mulliken atomic charges reveal the electronic charge distribution in the molecule, influencing its dipole moment, polarizability, and reactivity. C13, located in the central ring, shows a high positive charge due to its position in an electron-poor region, electron-withdrawing effects from nearby groups, and neighboring pi-bonds potentially delocalizing electrons away from it. As seen in Fig. 15, C13 is para to an ${\text{NO}}_2$ group, which is strongly electron-withdrawing and meta-directing for electrophilic aromatic substitution. The ${\text{NO}}_2$ group pulls electrons from the ring through inductive and resonance effects, creating electron deficiency particularly at ortho and para positions. This results in C13’s partial positive charge. The molecule’s structure, with multiple electron-withdrawing groups, enhances this effect, making C13 notably electron-deficient according to the Mulliken analysis.

Fig. 15.

The Mulliken atomic charges of 4c.

The MEP surface represents the molecular electrostatic potential as a powerful visualization tool commonly utilized to map a molecule’s reactivity towards nucleophilic and electrophilic attacks during chemical reactions. By representing the distribution of electrostatic potential around a molecule, the MEP surface provides valuable insights into the regions of positive and negative charge density, which in turn indicate sites favorable for nucleophilic or electrophilic interactions^58,59.

As depicted in Fig. 16, various parts of molecule 4c are color-coded in green, yellow, blue, and light red, indicating a lack of significant electrophilic and nucleophilic characteristics in these areas. The dark blue regions of the molecule correspond to hydrogen atoms attached to nitrogen and oxygen atoms, which are highly electronegative and exhibit the strongest electrophilic properties in the molecule. Conversely, bright red areas around the O24 and O25 atoms display notable nucleophilic behavior.

Fig. 16.

The molecular surfaces of 4c molecule were analyzed for computed Molecular electrostatic potentials, defined according to the electronic density’s 0.0004 electrons/b3 contour. Color representation is as follows: blue indicates regions more positive than 0.010 atomic units (a.u.); green represents values between 0.010 and 0 a.u.; yellow indicates values between 0 and − 0.010 a.u.; and red indicates regions more negative than − 0.010 a.u.

To validate the findings from the Molecular Electrostatic Potential (MEP) calculations, Natural Bond Orbital (NBO) studies were conducted. Atomic charges in molecules play a crucial role in understanding phenomena such as charge transfer and electronegativity equalization during chemical reactions⁶⁰. NBO atomic charges influence the electronic structure, molecular polarizability and dipole moment. In the NBO calculations displayed in Fig. 17, atoms H29, H30, H33, and H7, identified as blue and electrophilic in the MEP map, exhibit predominantly high positive electric charges. Moreover, oxygen atoms in the molecule, identified as nucleophilic regions based on their electrostatic potential, display negative charges in the NBO analysis. Notably, all nitrogen atoms in the molecule, except N23 in the nitro group, exhibit negative electric charges. The positive charge on the nitrogen atom in ${\text{NO}}_2$ (N23) is attributed to the higher electronegativity of the surrounding oxygen atoms compared to nitrogen, resulting in the nitrogen atom carrying a less negative charge in ${\text{NO}}_2$ relative to its oxygen counterparts.

Fig. 17.

NBO charge distribution of compound 4c.

The polarity of a bond is directly influenced by the electronegativity of the atoms involved in the bond formation. For instance, in the sigma bond (C12–N28), the calculated polarity values of $\sigma$ = 0.6373 (sp$_{}^{2.09}$) + 0.7706 (sp$_{}^{1.57}$) reveal that the electron density concentration around atom N28 is higher than that around atom C12 due to the nitrogen’s higher electronegativity relative to carbon. Based bond polarity, another significant result of these calculations is the determination of bond occupancy, representing the number of bonding electrons between the nuclei of the atoms in the bond. It is anticipated that bond occupancy in pi bonds may decrease notably in resonance structures compared to sigma bonds (as observed in entry 1 and 2 of the Table 8). In chemical reactions, as bonds between molecules become more effective and powerful, they tend to be more stable and exhibit more negative energy. Based on the data provided in entry 4 of the table, in molecule C4, the sigma bond (C1–O32) displays the lowest energy level of − 23.16342 atomic units (a.u.), indicating high stability. This bond also exhibits a notable occupancy factor of 0.99731, ranking third after the N23–O24 and N23–O25 bonds in terms of occupancy. Among carbon-carbon bonds, the sigma bond C15–C18 (entry 28) exhibits the highest occupancy factor of 0.98806, while the sigma bond C14–C16 has the lowest bond energy of − 21.31549 in entry 25.

Table 8.

NBO Calculations for determining polarizability coefficients and hybridization of atoms in the molecule 4c.

Entry	Occupancy (a.u.)	Bond (A-B)$_{}^a$	Energy (a.u.)	NBO	S (%) (A)	S (%) (B)	P (%) (A)	P (%) (B)
1	0.98751	$\sigma$(C1–C2)	− 0.74149	0.7064(sp$_{}^{1.66}$)+0.7078(sp$_{}^{1.86}$)	37.51	35.00	62.45	64.95
2	0.82876	$\pi$(C1–C2)	− 0.28633	0.6961(sp$_{}^{1.00}$)+0.7179(sp$_{}^{1.00}$)	–	–	100	100
3	0.98722	$\sigma$(C1–C6)	-1.48836	0.7128(sp$_{}^{1.69}$)+0.7014(sp$_{}^{1.96}$)	37.11	33.74	62.76	66.21
4	0.99731	$\sigma$(C1–O32)	23.18934	0.5769(sp$_{}^{2.97}$)+0.8168(sp$_{}^{1.85}$)	25.11	35.02	74.67	64.90
5	0.98560	$\sigma$(C2–C3)	− 1.42291	0.7013(sp$_{}^{1.86}$)+0.7128(sp$_{}^{1.59}$)	34.99	38.56	64.96	61.41
6	0.98694	$\sigma$(C2–H7)	− 0.53786	0.7866(sp$_{}^{2.33}$)+0.6174(sp)	30.01	99.95	69.96	0.05
7	0.98568	$\sigma$(C3–C4)	− 0.88036	0.7059(sp$_{}^{1.57}$)+0.7083(sp$_{}^{2.18}$)	38.89	31.44	61.07	68.51
8	0.81620	$\pi$(C3–C4)	− 5.25953	0.7165(sp$_{}^{1.00}$)+0.6976(sp$_{}^{99.99}$)	–	0.01	99.97	99.94
9	0.99419	$\sigma$(C3–O31)	− 1.10011	0.5524(sp$_{}^{3.48}$)+0.8336(sp$_{}^{1.99}$)	22.28	33.43	77.47	66.52
10	0.98208	$\sigma$(C4–C5)	− 0.70536	0.7217(sp$_{}^{1.89}$)+0.6922(sp$_{}^{1.89}$)	34.56	34.55	65.41	65.41
11	0.98237	$\sigma$(C4–C8)	− 0.77381	0.7084(sp$_{}^{1.95}$)+0.7058(sp$_{}^{2.03}$)	33.91	32.99	66.06	66.97
12	0.98684	$\sigma$(C5–C6)	− 1.28362	0.7059(sp$_{}^{1.75}$)+0.7083(sp$_{}^{1.72}$)	36.39	36.76	63.57	63.20
13	0.86360	$\pi$(C5–C6)	− 0.36178	0.7113(sp$_{}^{1.00}$)+0.7029(sp$_{}^{1.00}$)	–	–	99.95	99.94
14	0.98799	$\sigma$(C5–H9)	− 0.53452	0.7847(sp$_{}^{2.45}$)+0.6199(s)	28.98	99.95	70.97	0.05
15	0.98854	$\sigma$(C6–H10)	− 0.53368	0.7844(sp$_{}^{2.39}$)+0.6203(s)	29.58	99.95	70.38	0.05
16	0.97878	$\sigma$(C8–C11)	− 0.70328	0.6969(sp$_{}^{2.18}$)+0.7172(sp$_{}^{1.74}$)	31.44	36.29	68.52	63.69
17	0.82418	$\pi$(C8C11)	− 0.28941	0.7123(sp$_{}^{99.99}$)+0.7019(sp$_{}^{1.00}$)	0.03	0.02	99.95	99.99
18	0.98561	$\sigma$(C8–C13)	− 0.74520	0.7109(sp$_{}^{1.82}$)+0.7032(sp$_{}^{2.03}$)	35.50	32.96	64.47	67.01
19	0.98324	$\sigma$(C11–C12)	− 0.76434	0.7112(sp$_{}^{2.08}$)+0.7030(sp$_{}^{1.38}$)	32.42	42.02	67.53	57.95
20	0.98766	$\sigma$(C11–C26)	− 0.75798	0.7172(sp$_{}^{2.20}$)+0.6968(sp$_{}^{0.88}$)	31.24	53.14	68.71	46.83
21	0.99430	$\sigma$(C12–N28)	− 0.94235	0.6373(sp$_{}^{2.09}$)+0.7706(sp$_{}^{1.57}$)	32.32	38.94	67.60	61.00
22	0.99433	$\sigma$(C12–O31)	− 1.01954	0.5662(sp$_{}^{2.94}$)+0.8243(sp$_{}^{2.03}$)	25.33	33.03	74.44	66.90
23	0.98537	$\sigma$(C13–C14)	− 0.69849	0.7168(sp$_{}^{1.97}$)+0.6972(sp$_{}^{1.85}$)	33.61	35.10	66.36	64.85
24	0.98576	$\sigma$(C13–C15)	− 0.69912	0.7152(sp$_{}^{1.99}$)+0.6989(sp$_{}^{1.86}$)	33.40	34.99	66.56	64.97
25	0.98736	$\sigma$(C14–C16)	− 0.97206	0.7040(sp$_{}^{1.78}$)+0.7102(sp$_{}^{1.71}$)	35.96	36.92	63.99	63.04
26	0.88554	$\pi$(C14–C16)	− 21.31549	0.7247(sp$_{}^{99.99}$)+0.6891(sp$_{}^{99.99}$)	0.05	0.01	99.90	99.91
27	0.98896	$\sigma$(C14–H17)	− 1.41904	0.7834(sp$_{}^{2.48}$)+0.6215(s)	28.76	99.96	71.19	0.04
28	0.98806	$\sigma$(C15–C18)	− 0.72875	0.7061(sp$_{}^{1.76}$)+0.7081(sp$_{}^{1.72}$)	36.17	36.77	63.79	63.19
29	0.88265	$\pi$(C15–C18)	− 0.34385	0.7254(sp$_{}^{99.99}$)+0.6883(sp$_{}^{99.99}$)	0.06	0.02	99.88	99.91
30	0.98849	$\sigma$(C15–H19)	− 3.43203	0.7833(sp$_{}^{2.47}$)+0.6217(s)	28.76	99.96	71.19	0.04
31	0.98687	$\sigma$(C16–C20)	− 0.74667	0.6980(sp$_{}^{1.96}$)+0.7161(sp$_{}^{1.67}$)	33.75	37.43	66.21	62.53
32	0.98782	$\sigma$(C16–H21)	− 0.53373	0.7880(sp$_{}^{2.40}$)+0.6156(s)	29.38	99.95	70.58	0.05
33	0.98684	$\sigma$(C18–C20)	− 0.86970	0.6974(sp$_{}^{1.96}$)+0.7162(sp$_{}^{1.67}$)	33.79	37.44	66.16	62.53
34	0.98780	$\sigma$(C18–H22)	− 4.12443	0.7880(sp$_{}^{2.40}$)+0.6157(s)	29.42	99.95	70.54	0.05
35	0.99482	$\sigma$(C20–N23)	− 0.85720	0.6093(sp$_{}^{2.99}$)+0.7929(sp$_{}^{1.63}$)	25.02	38.08	74.86	61.88
36	0.99782	$\sigma$(N23–O24)	− 1.05763	0.7018(sp$_{}^{2.24}$)+0.7124(sp$_{}^{3.20}$)	30.85	23.75	69.02	76.11
37	0.83854	$\pi$(N23–O24)	− 0.44006	0.6641(sp$_{}^{1.00}$)+0.7477(sp$_{}^{1.00}$)	–	–	99.94	99.98
38	0.99783	$\sigma$(N23–O25)	− 1.06307	0.7018(sp$_{}^{2.24}$)+0.7124(sp$_{}^{3.21}$)	30.84	23.75	69.02	76.11
39	0.99665	$\sigma$(C26–N27)	− 1.07240	0.6527(sp$_{}^{1.13}$)+0.7578(sp$_{}^{1.12}$)	46.96	47.06	53.02	52.57
40	0.99087	$\pi$(C26–N27)	− 0.36768	0.6770(sp$_{}^{99.99}$)+0.7359(sp$_{}^{99.99}$)	0.18	0.04	99.76	99.65
41	0.98751	$\pi$(C26–N27)	− 0.36532	0.6516(sp$_{}^{99.99}$)+0.7587(sp$_{}^{1.00}$)	0.03	0.02	99.89	99.68
42	0.99231	$\sigma$(N28–H29)	− 0.70789	0.8459(sp$_{}^{2.26}$)+0.5333(s)	30.71	99.93	69.25	0.07
43	0.99346	$\sigma$(N28–H30)	− 0.71818	0.8463(sp$_{}^{2.31}$)+0.5327(s)	30.22	99.94	69.74	0.06
44	0.99355	$\sigma$(O32–H33)	− 0.46545	0.8656(sp$_{}^{3.66}$)+0.5007(s)	21.43	99.87	78.48	0.13

Furthermore, the NBO calculations provide insights into the contributions of s and p orbitals from atoms participating in bond formation. For example, in the sigma bond (C15–H19), atom C15 contributes 28.76 and 71.19 in s and p orbitals, respectively, while H19 contributes 99.96 in s orbitals with negligible contribution (0.04) in p orbitals, as indicated in entry 30. Notably, there is no p orbital contribution from atom H19 in its bond with C15.

Conclusion

In conclusion, we have devised a novel and effective synthetic approach for producing 2-amino-4H-chromene and its derivatives by reacting aldehydes, malononitrile, and various nucleophiles, such as resorcinol, $\beta$-naphthol and dimedone, with a GOQDs-NS-Doped Catalyst. The GOQDs-NS-Doped, a nano-material known for its high surface area and reusability, exhibited exceptional catalytic efficiency, resulting in high yields of the 2-amino-4H-chromene compounds in a short time frame using ethanol as the solvent. the catalyst could be utilized again for up to four cycles without a significant loss in its catalytic activity. This approach presents clear advantages over traditional methods for synthesizing these compounds. In addition, the theoretical calculations supported the experimental results. The synthesized 4c derivative underwent detailed analysis using the DFT method. Extensive structural studies, including electronic characterizations also the HOMO and LUMO energies were determined and NMR and IR data analysis for the synthesized compound 4c were conducted, all at the B3LYP/6-311+G(d,p) level of theory.”

Author contributions

P.B. designed and conducted the experimental work, synthesized 2-amino-4H-chromene derivatives from resorcinol. P.B. wrote the initial draft of the manuscript and contributed to all subsequent revisions. Sh.R. provided overall project supervision and access to lab facilities. A.P.M. oversaw the manuscript writing and editing process, ensuring clarity and conciseness. A.B. provided the computational data and analysis, reviewed and edited the computational section of the manuscript. A.G. synthesized 2-amino-4H-chromene derivatives from $\beta$-naphthol, and provided the spectral data. Z.A. synthesized 2-amino-4H-chromene derivatives from dimedone, and provided the spectral data. All authors reviewed and approved the final manuscript.

Data availability

The datasets used and/or analysed during the current study available from the corresponding author on reasonable request.

Declarations

Competing interests

The authors declare no competing interests.