Introduction

The term “spiro” in organic chemistry was firstly defined by Von Baeyer in the late 1890s. This term is used when two hydrocarbon rings are assembled on a shared carbon atom which is named the spiro carbon atom. Presently, spiro organic structures are considered in designing new pharmaceuticals. The special biological and conformational characteristics with the complexity and rigidity properties of the spiro compounds, make them good chiral candidates in drug discovery1,2,3,4.

Spirooxindole core is one of the most popular spiro compounds found in the structure of many alkaloids, bioactive synthetic compounds, and pharmaceuticals (Fig. 1)5. Spirooxindoles have shown various biological activities, including promising anticancer6, antimicrobial7, antiviral, antioxidant, anti-inflammatory, antileishmanial, and antiplasmodial agents8,9. Moreover, some spirooxindole-based compounds have been developed as inhibitors of microtubule assembly, such as spirotryprostatin A, alstonisine, and ptropodind. According to the importance of spirooxindoles in drug discovery, many researches have been directed to find new efficient synthetic routes furnishing molecules containing this core10,11.

Figure 1
figure 1

Selected spirooxindole natural products.

Full size image

Polyoxometalates (POMs), known as inorganic ligands, are discrete, anionic metal-oxide clusters of group V or VI transition metals in their highest oxidation state and exhibit a great diversity of sizes, nuclearities, and shapes12,13,14. POMs benefit from interesting structural skeletons including protons (Brönsted acids, can promote acid‐catalyzed reactions), oxygen atoms (with a high negative charge can be used in base‐catalyzed reactions), and metal ions with unoccupied orbitals (Lewis acids)15. The motivation for choosing POMs comes not only from their intriguing structural diversity, but also they contain several potential applications in many fields such as sorbent16,17 catalysis18,19,20, magnetic21, optical materials sensitive devices22, electro/photochromic systems23, sensors24 and medicine25. Lacunary POMs are defect derivatives of saturated ones, including one or more vacant sites such as mono-lacunary, di, or tri-lacunary structures26. The most common lacunary POMs are derivatives of the Keggin and Wells–Dawson ions, resulting frequently in sandwich-type clusters27,28. Totally, structural vacancies in the lacunary POMs lead to enhance surface reactivity, therefore, they can be substituted by metals with strong Lewis acidity, such as lanthanoids or transition metals like zirconium to generate Lewis acid catalysts29,30.

Lanthanoid-containing polyoxometalates (Ln-POMs), specially constructed from lacunary Keggin anions are structurally rigid clusters (Fig. 2) and showed higher stability, and have Lewis acid–base active sites compared with naked POMs. Also, a synergistic combination between the Ln and POM within one molecular structure can enhance their potential application in many fields such as luminescence, magnetism, and catalysis31,32. Furthermore, due to their easy synthetic procedure and their robustness nature in the solid and solution, they can be also used in acid/base-catalyzed reactions for laboratory research purposes and industrial applications. Although there are several examples of isatin-based compounds synthesized using POMs or POMs-based composites33,34,35,36, Lewis acid catalysts containing Ln-POMs have rarely been studied for them.

Figure 2
figure 2

Reproduced from ref40 with permission.

The structure of Ln-B2W22 nanocatalyst (Color code: W, dark blue; Ln, grey; O, red; B, purple).

Full size image

Herein, we have successfully synthesized a series of isostructural α-Keggin borotungstate dimers with Ln cations, [Ln(BW11O39)2]15− (Ln-B2W22, Ln = La, Ce, Nd, Sm, Gd, and Er). Then, the related nanocatalysts were prepared by the top-down approach using the ultrasonic technique. In continuation of our efforts towards advancing synthetic methods to achieve spirooxindoles, in this research, we want to introduce a highly efficient, environmentally benign, and simple one-pot method for the nano-Gd-B2W22-catalyzed synthesis of bioactive spirooxindole derivatives37,38,39.

Experimental

Chemicals and materials

The chemical compounds were purchased from Merck (Darmstadt, Germany, www.merckmillipore.com) and Sigma-Aldrich (St. Louis, MO, USA, www.sigmaaldrich.com) and used with no crystallization or purification.

Instrumentation

Electrothermal 9200 apparatus was employed to determine the melting point of products. Bruker Tensor 27 FT-IR spectrometer (400–4000 cm–1 region) was used to detect absorbance bands of organic products using a KBr disk containing the compounds. 1H NMR, 13C NMR spectra were recorded on a Bruker AQS 400-AVANCE spectrometer at 400 and 100 MHz, respectively, using TMS as an internal standard (DMSO solution). Also, the infrared spectra of catalysts were recorded in the range of 4000–400 cm–1 on a Thermo Nicolet/AVATAR 370 Elemental analysis (CHN) was performed using a Thermo Finnigan Flash EA 1112 microanalyzer. Metal content was measured by the Spectro Arcos ICP-OES spectrometer model 76004555 using in the range of 130–770 nm for ICP spectra. Powder X-ray diffraction (PXRD) data were collected on ASENWARE/AW-XDM300 X-ray powder diffractometer using Cu Kα (λ = 1.54184 Å) radiation at room temperature with the scan range 2θ = 3 to 40° and step size of 0.05° and step time of 1 s. The scanning electron microscope (SEM) analysis, EDS, and EDS mapping were recorded using LEO-1450 VP at an acceleration voltage of 10.00 kV and resolution of about 500 nm (Zeiss, Germany).

Preparation of catalysts

The mono-lacunary Keggin K9[BW11O39]·13H2O was synthesized according to a literature method and identified by FT-IR and elemental analysis41. Then, mono-lacunary Keggin can be stabilized by lanthanide centers in solution and in the solid-state to form sandwich-type polyoxometalates K15[Ln(BW11O39)2]·nH2O (Ln-B2W22, Ln = La, Ce, Nd, Sm, Gd, and Er)40,42.

General synthetic procedure for catalysts. A mixture of lanthanoid nitrate (0.085 mmol) and K9[BW11O39]·13H2O (0.155 mmol) in 20 mL of KCl (1 M) was stirred for 10 min in air and then the pH was adjusted to 5.0 by dropwise addition of 0.1 M KOH. The resulted mixture was stirred for a further 40 min at 50 °C. Pure crystals of the catalysts were obtained by slow evaporation of the solvent after several days.

Synthesis of nanocatalysts. The mixture solution of Ethanol (10 mL), water (15 mL), and Ln-B2W22 crystals (0.03 g) were subjected to ultrasonication (150 W). After 20 min, nanocatalysts were collected by the centrifuge and then washed with cold water (3 × 5 mL) under vacuum. FT-IR spectra (KBr pellet, cm−1) of nano-Ln-B2W22 were consistent with their spectra before doing the nano procedure (Fig. S2).

General procedure for the synthesis of spiro-2-amino-4H-pryans. A combination of 1,3-diketone, carbonyl compound (either isatin or acenaphtoquinone), α-cyano compound (either malononitrile or ethyl cyanoactetate), and Gd-B2W22 was stirred in water at ambient temperature until the complete formation of the product was traced by TLC (Fig. 3). Then, the crude product was filtered, washed with water and dissolved in hot ethanol for crystallization. Furthermore, all products were characterized and analyzed by melting points and FT-IR spectra, and the results were compared with those reported in the literature to prove the formation of target products.

Figure 3
figure 3

Synthesis of spiro-2-amino-4H-pryans.

Full size image

General procedure for the synthesis of uracil fused spirooxindoles. A combination of isatin, uracil derivative (either 1,3-dimethyl-6-aminouracil or 6-aminouracil), 1,3-diketone compounds, and Gd-B2W22 was stirred in refluxing water for 8–26 min (Fig. 4). Then, the mixture was filtered, washed well with water and dried at 80 °C. The product was recrystallized for further purification in hot ethanol. All products were characterized by melting point and the characterizations were compared with that of in literature.

Figure 4
figure 4

Synthesis of uracil-fused spirooxindoles.

Full size image

Synthesis of pyrroloacridine derivatives. A mixture of isatin, aniline, dimedone and nanocatalyst was refluxed in water for an appropriate time (Fig. 5). By the completion of the reaction, the mixture was cooled down and filtered. Then the crude product was washed well with hot water, and finally crystallized in hot EtOH. The characterization data of products were compared with that published in the literature.

Figure 5
figure 5

Synthesis of pyrroloacridine derivatives.

Full size image

Characterization data

Spectral data for catalyst:

  • La-B2W22. Colorless needle-like crystals in 52.6% yield (based on W). Anal. Calcd. for H52O104LaK15B2W22: K, 9.01; W, 65.2; La, 2.13; B, 0.33; H, 0.81%. Found: K, 9.37; W, 63.66; La, 2.38; B, 0.31; H, 0.77%. FT-IR (KBr pellet, cm−1): 3451, 1616, 1254, 997, 948, 887, 832, 777, 521.

  • Ce-B2W22. Orange needle-like crystals in 55% yield (based on W). Anal. Calcd. for H58O107CeK15B2W22: K, 8.94; W, 61.62; Ce, 2.13; B, 0.33; H, 0.89%. Found: K, 9.21; W, 60.75; Ce, 2.09; B, 0.31; H, 0.92%. FT-IR (KBr pellet, cm−1): 3446, 1616, 1252, 996, 947, 887, 831, 777, 522.

  • Nd-B2W22. Light purple needle-like crystals in 49% yield (based on W). Anal. Calcd. for H50O103NdK15B2W22: K, 9.03; W, 62.27; Nd, 2.22; B, 0.33; H, 0.78%. Found: K, 9.32; W, 63.41; Nd, 2.18; B, 0.31; H, 0.81%. FT-IR (KBr pellet, cm−1): 3441, 1617, 1243, 996, 984, 885, 832, 777, 520.

  • Sm-B2W22. Colorless needle-like crystals in 62% yield (based on W). Anal. Calcd. for H50O103SmK15B2W22: K, 9.02; W, 62.21; Sm, 2.31; B, 0.33; H, 0.78%. Found: K, 9.06; W, 63.41; Sm, 2.21; B, 0.31; H, 0.76%. FT-IR (KBr pellet, cm−1): 3438, 2917, 1611, 1253, 1000, 494, 884, 831, 778, 519.

  • Gd-B2W22. Colorless needle-like crystals in 65% yield (based on W). Anal. Calcd. for H60O108GdK15B2W22: K, 8.89; W, 61.30; Gd, 2.38; B, 0.33; H, 0.92%. Found: K, 9.01; W, 61.45; Gd, 2.31; B, 0.31; H, 98%. FT-IR (KBr pellet, cm−1): 3471, 1611, 1253, 1000, 948, 883, 832, 799, 517.

  • Er-B2W22. Colorless needle-like crystals in 53% yield (based on W). Anal. Calcd. for H52O104ErK15B2W22: K, 8.97; W, 61.88; Er, 2.56; B, 0.33; H, 0.80%. Found: K, 9.03; W, 61.51; Er, 2.51; B, 0.32; H, 0.91%. FT-IR (KBr pellet, cm−1): 3428, 1621, 1258, 997, 948, 886, 835, 780, 522.

Results and discussion

Characterization of catalysts

Firstly, six lanthanoid-containing polyoxometalate K15[Ln(BW11O39)2] (Ln-B2W22, Ln = La, Ce, Nd, Sm, Gd, and Er) crystals (microscopic size) of this study were obtained by reaction of the lanthanoid ion with the mono-lacunary Keggin [BW11O39]9− at pH 5 (Figs. 2 and S3). Next, the above crystals were solved and subjected to ultrasonication and then nanocatalysts were collected by the centrifuge (top-down approach). The distribution histograms reveal that the average particle size of catalysts is less than 100 nm upon 20 min of sonication (Fig. 6). Particle size distribution histogram of other nanocatalysts are given in the Supplementary Figs. S4–S8.

Figure 6
figure 6

Particle size distribution histogram of nano-Gd-B2W22.

Full size image

Also, the SEM showed that the dominant morphology for nanocatalysts is rod-like (Fig. 7). Furthermore, the presence of O, K, Gd, and W in the nanocatalysts is confirmed by the EDS spectrum (Fig. 8). SEM images of La-B2W22 and EDS spectra of other nanocatalysts are given in Supplementary Information (Figs. S9–S14).

Figure 7
figure 7

SEM images of nano-Gd-B2W22.

Full size image
Figure 8
figure 8

EDS spectrum of nano-Gd-B2W22.

Full size image

It is important to note that infrared spectroscopy is frequently employed technique for the characterization of POMs due to their characteristic metal–oxygen stretching vibrations that occur in the region between 400 and 1000 cm−1 which is known as the fingerprint region for the POMs. As shown in Figs. S1, S2, and Table 1, the overlaid IR spectra strongly suggest the same structural family for all crystalline and nano compounds. Also, the IR spectra of catalysts present a similar vibration pattern with the mono-lacunary Keggin [BW11O39]9−, confirming the presence of the [BW11O39]9− moiety in all compounds. Briefly, nano-Gd-B2W22 showed the absorption bands at 1610 and 3471 cm−1 which attributed to the water molecules. The band at around 1250 cm−1 is attributed for bending frequencies of O–B–O. Also, characteristic bands of the terminal oxygens νas(W–Ot) at 948 cm−1 showed a red shift in comparison with the naked [BW11O39]9− (995 cm−1) that indicated [BW11O39]9− anions coordinated to Ln3+ center (Fig. 9).

Table 1 Representation of important absorption bands (cm-1) for K15[Ln(BW11O39)2] (Ln = La, Ce, Nd, Sm, Gd, and Er) and naked [BW11O39]9− for comparison.
Full size table
Figure 9
figure 9

FTIR spectrum of nano-Gd-B2W22.

Full size image

Also, the powder XRD pattern of the catalysts appears at around 9–10° for a 2θ value (similar to other mono-lacunary Keggin anions)43 (Supplementary Fig. S15).

Catalytic activity

Ensuing this research, the catalytic activity of nano-Ln-B2W22 catalysts was tested in the model three-component reaction of isatin, malononitrile, and dimedone. To achieve eco-friendly optimized conditions, various factors were investigated. Initially, acidic catalysts including SSA, p-TSA, H3PW12O40, Lanthanum nitrate, K9[BW11O39]·13H2O and ZnO were chosen to compare the results obtained by catalysts (Table 2). The Gd-B2W22 nanocatalyst was selected for further tests. Next, the effect of solvent was studied by running the model reaction in polar and non-polar solvents. Finally, the amount of catalyst was optimized to achieve the highest amount of product. The reaction was also repeated with no catalyst furnishing trace amount of product. That’s while in the presence of 10 mol% of nanocatalyst, the target product was obtained in 96%. Therefore, ensuring by the effect of a catalyst in this reaction, the generalization was accomplished in water, in the presence of 10 mol% nano-Gd-B2W22 at reflux condition. It is important to note that the Lewis acidity (Z/r3; Z = charge and r = ionic radius) of lanthanoids decreases with an increase in the ionic radii44. However, among the Ln-B2W22 (Ln = La, Ce, Nd, Sm, Gd, and Er) catalysts examined, Gd-B2W22 showed better catalytic performance because by reducing the size from Gd to Er, the Er center was sterically hindered by two BW11 ligands and its Lewis acid site is not well accessible.

Table 2 Optimization of the reaction conditions.
Full size table

The one-pot reaction of isatin, α-cyano compound (either malononitrile or ethylcyanoacetate), and 1,3-diketone (either ethyl acetoacetate, dimedone, or barbituric acid) or 3-methyl-1H-pyrazol-5(4H)-one/ 4-hydroxycoumarin or α-naphtol/β-naphtol) gave the favorite products. Notwithstanding, the effect of substituent on isatin ring, the yield of products was found in good to high. By employing acenaphthenequinone instead of isatin, the expected spiro-4H-pyrans were formed in good to high yields. The products obtained from ethylcyanoacetate need a longer reaction time than those obtained from malononitrile that possibly is due to the lower reactivity of ethylcyanoacetate (Table 3). All products were known and identified by comparing their melting points with authentic literature. Some selected NMR spectra are presented in supplementary file (Figs. S16–S47).

Table 3 Synthesis of spiro-2-amino-4H-pryans37.
Full size table

In Scheme 1, we propose a sensible mechanism for the preparation of spirooxindole derivatives. First, the Gd-B2W22 catalyst, as a Lewis acid, activates the carbonyl group of the isatin molecule, and then malononitrile, due to alpha-activated hydrogens, will have a nucleophilic attack on activated carbon, which produces intermediate 1. This intermediate creates intermediate 2 by elimination of water, and finally the corresponding product was synthesized by adding dimedone to this intermediate.

Scheme 1
scheme 1

The reasonable mechanism for the synthesis of spirooxindole derivatives.

Full size image

To confirm the wide effectiveness of nano-Gd-B2W22 as a catalyst, this was used in the reaction of isatin derivatives, 6-amino-1,3-dimethyl uracil, and 1,3-diketone (either dimedone, 1,3dimethyl barbituric acid, or barbituric acid). These reactions were successfully catalyzed by nano-Gd-B2W22 in refluxing water under optimized conditions furnishing spiro-products in good efficiency. The results are summarized in Table 4.

Table 4 Synthesis of uracil-fused spirooxindoles10,38,39.
Full size table

Next, the catalytic effect of nano-Gd-B2W22 was studied in the production of pyrroloacridine compounds through the one-pot reaction of isatin, aromatic amines, and dimedone. The generalization of this reaction was considered using different aromatic amine-bearing electron-donating and electron-withdrawing substituents. The expected pyrroloacridine derivatives were formed in wonderful yield within short reaction times as summarized in Table 5 (Fig. 10).

Table 5 One-pot, three-component synthesis of pyrrolo[2,3,4-kl]acridin-1-one derivatives45,46.
Full size table
Figure 10
figure 10

Reusability of nano-Gd-B2W22.

Full size image

Catalyst recyclability

Heterogeneous Catalysts, play an important and effective role in industries and other applications in laboratory scale. Hence, recyclability of the catalyst to prevent waste generation is one of the most important factors in catalysis. Nevertheless, recoverability of nano-Gd-B2W22 was evaluated on the model reaction and it was recycled up to 6 runs by simple filtration with a gradual decrease in activity from 96 to 85% in the corresponding product (Fig. 10). In addition, to elucidate whether the recycling process can result in any change in the catalyst’s morphology and structure, the SEM image as well as FTIR spectra of the recycled nano-Gd-B2W22 catalyst were recorded (Fig. 11). These results support that the structure of the nano-Gd-B2W22 underwent several reactions was preserved, but some agglomeration is evident.

Figure 11
figure 11

(a) SEM image of recycled nano-Gd-B2W22; (b) FTIR overlay of the fresh and recycled nano-Gd-B2W22.

Full size image

Concluding remarks

In the present study, a series of isostructural lanthanoid-containing polyoxometalate nanocatalysts Ln-B2W22 (Ln = La, Ce, Nd, Sm, Gd, and Er) were synthesized and characterized using a suite of analytical techniques. Among these nanocatalysts, the gadolinium-containing POM (Gd-B2W22) showed remarkable catalytic performance for the synthesis of bioactive isatin derivatives including spiro-2-amino-4H-pryans, uracil fused spirooxindoles, and pyrroloacridine derivatives under the reflux condition in high yields and short reaction times (8–26 min). Also, further studies are underway in our laboratory to extend the application of these family nanocatalysts to other coupling reactions.