Nano-ferrite supported glutathione as a reusable nano-organocatalyst for the synthesis of phthalazine-trione and dione derivatives under solvent-free conditions

Abstract

Nano-organocatalyzed one-pot four-component reactions for the synthesis of phthalazine-trione/dione derivatives have been devised for the first time from easily accessible starting materials under solvent-free conditions. This methodology showed very good substrate scope and high degree of tolerance for a variety of aldehydes (including aliphatic and heteroaromatic aldehydes) and active methylene compounds. Moreover, the catalyst can be easily separated from the reaction mixture because of its highly paramagnetic nature, by using an external magnet, and can be reused in five more consecutive runs without much decrease in catalytic activities. Other significant advantages of this method are shorter reaction time, good yield, simple work-up procedure, easy catalyst handling etc.

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Introduction

Organocatalysis, the term which refers to a form of catalysis where the rate of a reaction is increased by the addition of an organic catalyst, is an attractive and productive enterprise in modern-day chemistry. Organocatalysis, including fabrication, characterization, modification and application, is an emerging field which is extensively applied for many name reactions like Michael reaction,¹ Mannich reaction,² enantioselective C–N, C–C, C–O bond formation etc.³ However, catalyst separation and reusability are the major drawbacks. So, there is a lack of application under sustainable circumstances. Nanoparticles (NPs) are striking candidates as solid supports for immobilization of homogeneous organocatalysts like glutathione,⁴ L-proline,⁵ dopamine,⁶ sulphonic acids etc.^7,8 Compared to other solid supports like silica, alumina, metal oxides, polymers etc., NPs are non-arguably the most widely investigated because their separation and reusability are easy. Green chemistry has attracted substantial attention for overcoming several troubles relating to environmental pollution. Application of heterogeneous nano-organocatalysts is considered as a sustainable approach because: (i) they are well dispersed in reaction media thereby providing high surface area for ready access of substrate molecules and (ii) they can be easily separated from the reaction mixture by filtration, centrifugation or by using an external magnetic field, and can then be reused in several more consecutive runs without much decrease in catalytic activity. Metal NPs attached to enzymes or biomolecules are a newer class of catalysts with excellent activities in diversity orientated synthesis (DOS) of small molecules having skeletons found in many important natural products.^9,10

Heterocyclic compounds possessing an aza group are widely distributed in nature and of a large number of N-containing compounds, those which possess hydrazine moiety at the fusion site are of immense importance because of their pharmacological, clinical and agrochemical applications.^11,12 Many of these phthalazine derivatives possess anti-fungal,¹³ anti-cancer,¹⁴ anti-tumor¹⁵ and many other pharmaceutical activities.¹⁶ Because of these biological and medicinal activities, many researchers developed powerful and promising methods for synthesizing phthalazene molecules by using various types of catalysts like inorganic–organic hybrid materials Al-SBA-15-TPI/H₆P₂W₁₈O₆₂,¹⁷ TBBDA,¹⁸ Cs₂CO₃@magnetic nanoparticles,¹⁹ SO₃H-FMSM,²⁰ SBA@BiPy²⁺2Cl⁻,²¹ [TMG][Ac],²² RH@[SiPrDABCO@BuSO₃H]HSO₄,²³ CoFe₂O₄–CS–SO₃H (ref. 24) etc. in sequential multi-component reactions (MCRs). These reported methods possess many advantages, but at the same time have many disadvantages too. Previously our group also reported Ni(0) NP-catalyzed synthesis of phthalazine derivatives²⁵ but major difficulties were agglomeration of Ni NPs and inability to form phthalazine molecules with aliphatic and heteroaromatic aldehydes. To address these problems, we applied a surface-stabilized magnetic nano-organocatalyst. In this way we prevented the chance of agglomeration and increased the catalytic activity of nanocomposite. From a literature study we have chosen glutathione as a stabilizer as well as organocatalyst, because it binds to NPs by using –SH functionality while –NH₂ and –COOH groups are free for catalysis.¹⁰ Furthermore, for easy separation we have selected magnetic NPs (Fe₃O₄) which avoids the involvement of cumbersome procedures like centrifugation and filtration. Moreover, application of nano-FGT (nano-ferrite-supported glutathione) also made the methodology more efficient as compared to the one reported, since the reaction went smoothly with a diverse range of active methylene compounds and aldehydes including heteroaromatic and aliphatic aldehydes.

So, the chief purpose behind this presented protocol is to highlight the synergetic effects of combined nano-organocatalyst operated in an enhanced synthetic procedure, MCR and solvent-free reaction conditions for development of a new eco-compatible methodology. Therefore, we have chosen nano-FGT as a proficient, harmless, mild, magnetically recyclable, highly stable, powerful solid nano-organocatalyst and have applied it for the synthesis of phthalazine derivatives in four-component MCR under solvent-free conditions.

Results and discussion

Magnetic nano-FGT, consisting of Fe₃O₄ spheres as the core and glutathione as the outer shell, was prepared by a simple, low-cost and convenient method.⁴ Magnetite NPs were synthesized by the co-precipitation of Fe²⁺ and Fe³⁺ in ammonia solution.²⁶ To improve the chemical stability of magnetite NPs, their surface modification was successfully performed by the suitable deposition of glutathione onto the NPs' surface by ultrasonic irradiation at room temperature (Scheme 1).

Scheme 1 Synthesis of nano-FGT.

Formation of nano-FGT was confirmed by FT-IR, TEM, SEM and EDX analyses. Its thermal stability was analyzed by TGA. Functional group identification was done by the FT-IR technique. The FT-IR spectrum of nano-FGT (Fig. SI 1†) showed broad peaks at 3349 and 3166 cm⁻¹ which indicated the presence of –OH and –NH₂ groups. The peaks at 1643 and 1634 cm⁻¹ correspond to the carbonyl stretching of acid and amide groups of glutathione respectively. The peak at 2926 cm⁻¹ is due to C–H stretching. The absorption band at 599 cm⁻¹ corresponds to Fe–O stretching of Fe₃O₄ NPs and absence of S–H stretching band at around 2525 cm⁻¹ clearly indicated that glutathione is successfully anchored onto the surface of Fe₃O₄ NPs via thiol group.²⁷ Morphology and particle size of nano-FGT were determined from TEM images. TEM images showed the presence of a dark spherical NP core of size 10–20 nm uniformly coated with a layer of glutathione [Fig. 1a and b]. SAED (selected area electron diffraction) of freshly prepared nano-FGT showed spotty diffraction, thereby proving its crystalline character [Fig. 1c]. EDX analysis was carried out to determine the elemental composition of nano-FGT. This confirmed the presence of Fe, O, S, N and C (Fig. 2). SEM images were obtained with the help of a JSM-6360 (JEOL) scanning electron microscope. The SEM image also confirmed the spherical morphology of nano-FGT (Fig. 3).

Fig. 1 TEM images of nano-FGT (a) at 0.2 μm and (b) at 5 nm. (c) SAED pattern of nano-FGT.

Fig. 2 EDX of nano-FGT.

Fig. 3 SEM image of nano-FGT at 200 nm.

The prepared nano-FGT was also characterized by the powder XRD technique. The XRD diffraction pattern (Fig. 4) shows characteristic 2θ peaks at around 30.33°, 35.61°, 43.19°, 57.28°, 62.83°, 74.30° which are in good agreement with those reported in the literature.⁴

Fig. 4 Powder XRD pattern of nano-FGT.

In order to gain information regarding thermal stability of nano-FGT, we performed TGA (thermogravimetric analysis). There is a weight loss at around 53 °C in the thermogram which corresponds to the degradation of solvent molecules trapped in the catalyst. There is another weight loss at 185 °C which is due to the degradation of glutathione molecules of nano-FGT. These observations prove that the catalyst is stable below 180 °C and therefore can be easily applied in reactions under that temperature (Fig. 5).

Fig. 5 Nano-FGT TGA thermogram.

ICP-OES analysis was performed in order to determine the actual amount of Fe in the nano-ferrite heterogeneous catalyst. It was found to be 63.23%.

After successful surface modification and characterization of nano-FGT, we explored its applicability for the synthesis of phthalazine dione and trione derivatives by four-component condensation reaction of phthalic anhydride (1), hydrazinium hydroxide (2), active methylene compounds (3, 4 or 5) and aldehydes (6a–q) (Scheme 2). Initially a mixture of phthalic anhydride (1) (1 mmol), hydrazinium hydroxide (2) (1.2 mmol), dimedone (3) (1 mmol) and 4-chlorobenzaldehyde (6a) (1 mmol) was taken as a pilot reaction. Following which the reaction was stirred at room temperature without any catalyst. It was found that without catalyst the reaction failed to form the desired product even after 24 h of constant stirring. Only starting materials were evidenced using TLC. Then another new reaction was set up with addition of nano-FGT which was stirred according to known procedures.²⁸ It is important to mention here that since the reaction is solvent free, reactants and magnetic nano-organocatalyst (nano-FGT) were mixed properly by using a glass rod into which a magnetic bit was added in order to maintain proper mixing during the reaction period, which in turn increases the contact between the surface of the catalyst and reactant molecules. Firstly, the reaction was carried out at room temperature, and to our delight it was found that after 12 h of continuous stirring the desired product was formed but the conversion was very poor, most of the starting materials remaining unreacted. This observation prompted us to apply thermal energy to the reaction. A number of reactions were set up in a pre-heated oil bath using a temperature-controlled magnetic stirrer at different temperatures ranging from 40 to 120 °C, the best result being obtained at 80 °C. However, no desired product was formed without addition of catalyst even at 80 °C after 24 h of stirring. All these observations proved the efficient role of catalyst in carrying out the reaction. The structure of the compound was identified by elemental and spectral analysis. The IR spectrum of product compound 15a showed absorption bands at 1453 and 1109 cm⁻¹ which are due to C–N and N–N band stretching, and the band at 1666 cm⁻¹ corresponds to carbonyl group of dimedone. In the ¹H NMR spectrum of compound 15a, eight aromatic protons appear at δ 8.29–7.24. The methine proton was observed as a singlet at δ 6.34. The four methylene protons of the dimedone residue appeared as an AB system at δ 3.35–3.13 and singlet at δ 2.26. The two methyl groups were observed as two singlets at δ 1.137 and δ 1.132. Next, the reaction conditions were optimized and, for this, various reaction parameters were examined.

Scheme 2 Synthesis of phthalazine derivatives by using nano-FGT.

Firstly we focused on the optimization of the amount of catalyst required to catalyze the reaction. It was discovered that 10 mg (6.32 mg of Fe, 11.31 mol%) of the catalyst was sufficient to furnish maximum yield within a very short period of time. Application of higher amount of catalyst, viz. 12 mg, showed no increase in the yield of desired product, neither did it result in decreased reaction time. On the other hand, reduction in the amount of catalyst below 10 mg produced lower yield within the same amount of time. The optimization plot of the reaction time and the amount of catalyst is shown in Fig. 6.

Fig. 6 Optimization of catalyst loading with respect to yield and time.

Encouraged by the success discussed above, we investigated the effect of various solvents on the pilot reaction at 80 °C using 10 mg nano-FGT as a catalyst. In the presence of solvents like water, ethanol, acetonitrile and THF, bis-adduct 15a′, 2,2′-(4-chlorophenylmethylene)-bis(3 hydroxy-5,5-dimethylcyclohex-2-enone) (see ESI†), was the major product instead of phthalazine derivative 15a. But under solvent-free conditions, the reaction proceeded very smoothly and afforded phthalazine derivative 15a as major product within a very short period of time (Fig. 7).

Fig. 7 Solvent standardization.

Now in order to determine whether the catalytic activity of nano-FGT in the above reaction was the best, we compared its catalytic activity with that of other homogeneous and heterogeneous catalysts (Scheme 3). The cases where FeCl₃, Fe₂SO₄ and Fe₃O₄ NPs were used as catalysts led to very much less conversion thereby leading to a poor yield of the desired product (Table 1). The fourth case where glutathione was used as the catalyst showed very good improvement in the result but the time required for the completion of the reaction was high, and in this case glutathione, being soluble in water, could not be recycled back. Application of nano-FGT however solved this problem of recyclability, and since it is attached to NPs which have higher surface/volume ratios, it shows better catalytic activity,⁹ which in turn reduces the time and increases the product yield.

Scheme 3 Catalyst screening.

Table 1 Standardization of catalyst^a

No.	Catalyst^b	Time	Yield^c (%)
a Reaction conditions: phthalic anhydride (1 mmol), hydrazinium hydroxide (1.2 mmol), dimedone (1 mmol) and 4-chlorobenzaldehyde (1 mmol), 80 °C, SFRC.b Amount of catalyst: 10 mg.c Isolated yield.
1	No catalyst	24 h	0
2	FeCl3	180 min	21
3	Fe2SO4	180 min	25
4	Fe3O4 NPs	180 min	53
5	Glutathione	120 min	78
6	Nano-FGT	20 min	97

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To explore the generality of the reaction, we extended the scope of the present protocol for the synthesis of various phthalazine derivatives (Scheme 4). The results of this study are shown in Table 2. As shown in Table 2, aromatic aldehydes carrying either electron donating or withdrawing substituents worked well, giving excellent yield of products (87–97%) with high purity. The reaction went smoothly with various active methylene compounds like dimedone (3), 1,3-cyclohexanedione (4), ethyl cyanoacetate (5a), and malononitrile (5b). To analyze the generality of our methodology, the said synthesis was also performed with different heteroaromatic and aliphatic aldehydes (15o, 15p and 18a) under similar conditions and we were delighted to see the formation of our desired product in good yields within similar time duration. As per a literature survey, very much less reported procedure for the synthesis of phthalazine derivatives showed good results with aliphatic and heteroaromatic aldehydes, but our methodology provides excellent results with both kinds of aldehydes (Table 2). Reaction was also tried with ethyl acetoacetate as active methylene compound but no desired product formed after 6 h of constant stirring; only starting materials were recovered. All the synthesized products were characterized from their analytical and spectroscopic data. Structure of compound 17a was again confirmed by single-crystal X-ray crystallography (Fig. 8).

Scheme 4 Synthesis of phthalazine derivatives.

Table 2 Synthesis of various phthalazine derivatives by using nano-FGT^a

No.	Aldehyde	Active methylene compound	Product	Time (min)	Yield^b (%)	Mp (°C)
a Reaction conditions: phthalic anhydride (1 mmol), hydrazinium hydroxide (1.2 mmol), active methylene compounds (1 mmol) and aldehydes (1 mmol), 80 °C, SFRC,10 mg of nano-FGT.b Isolated yield.
1	4-ClC6H5 (6a)	3	15a	20	97	261–262 (ref. 17)
2	4-CNC6H5 (6b)	3	15b	20	96	271–273 (ref. 25)
3	4-NO2C6H5 (6c)	3	15c	20	97	224–226 (ref. 17)
4	4-BrC6H5 (6d)	3	15d	25	96	265–267 (ref. 17)
5	4-FC6H5 (6e)	3	15e	20	95	219–220 (ref. 17)
6	3-ClC6H5 (6f)	3	15f	20	92	205–207 (ref. 25)
7	2-ClC6H5 (6g)	3	15g	23	91	266–268 (ref. 17)
8	3-NO2C6H5 (6h)	3	15h	24	92	269–270 (ref. 25)
9	3-BrC6H5 (6i)	3	15i	21	92	266–268 (ref. 25)
10	3-FC6H5 (6j)	3	15j	21	90	271–273 (ref. 25)
11	2-BrC6H5 (6k)	3	15k	22	91	262–265 (ref. 25)
12	C6H5 (6l)	3	15l	25	93	205–207 (ref. 17)
13	4-MeC6H5 (6m)	3	15m	25	87	228–230 (ref. 17)
14	4-MeOC6H5 (6n)	3	15n	20	89	220–222 (ref. 25)
15	2-C5NH4 (6o)	3	15o	30	88	227–230 (ref. 24)
16	C4H8 (6p)	3	15p	30	87	133–136 (ref. 22)
17	4-NO2C6H5 (6c)	4	16a	20	94	237–239
18	4-Cl,3-NO2C6H5 (6q)	5a	17aa	20	97	212–214
19	C4H8 (6p)	5b	18a	25	92	220–223

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Fig. 8 Molecular structure of compound 17a (CCDC 1438715).

The most probable mechanism of the said conversion is shown in Scheme 5. In the initial step nano-FGT facilitates the nucleophilic attack of hydrazinium hydroxide (2) to phthalic anhydride (1) leading to the formation of phthalhydrazide (7). On the other hand, aldehydes react with nano-FGT forming iminium intermediate (8).⁹ Then cyclic 1,3-diketones (3 and 4) form 9 which reacts with 8 and affords Knoevenagel product 11 (path a)⁹ where acyclic active methylene compounds (5a or 5b) generate Knoevenagel product 12 through carbanion formation (path b).²⁹ Phthalhydrazide (7) attacks these intermediates (11 and 12) under Michael fashion leading to the formation of other intermediates 13 and 14 which eventually undergo cyclization followed by dehydration to furnish our desired product.

Scheme 5 Plausible mechanism for the synthesis of 2H-indazolo[1,2-b]phthalazine-triones and 1H-pyrazolo[1,2-b]phthalazine-diones.

Following this, the possibility of recovery and reusability of the catalyst was investigated. After completion of the reaction, the catalyst was recovered from the reaction mixture simply by using an external magnet and washed with ethyl acetate and acetone and dried. The recovered catalyst was reused under the same reaction conditions, and it was found that the catalyst could be reused five times without any significant loss of the product yield. The reproducibility of the reaction was then investigated by performing five sets of reactions under the same conditions. Each set was repeated five times and the mean yield was calculated (95.6, 94.8, 94.2, 93.2 and 92.6%) by the standard deviation method; the error bars were also determined (Fig. 9). After carrying out the 5^th set of reactions, again ICP-OES analysis was performed and the amount of Fe found was 60.04%. This observation confirmed there is 3.19% leaching of the catalyst. Then, in order to confirm the structure of the reused (after 5 consecutive runs) catalyst, we performed powder XRD (Fig. 10a), SEM (Fig. 10b), TEM (Fig. 10c), FT-IR (Fig. SI 1b†) and TGA (Fig. 10d) analyses. The structure was very similar to that of freshly synthesized catalyst and it also remained well dispersed and retained its size (10–20 nm).

Fig. 9 Reproducibility chart.

Fig. 10 (a) Powder XRD pattern, (b) TEM image of nano-FGT at 50 nm, (c) SEM image of nano-FGT at 500 nm and (d) TGA curve of nano-FGT after the 5^th run.

Conclusion

In conclusion, herein we report a stable nano-organocatalyst-mediated solvent-free one-pot synthesis of 2H-indazolo[2,1-b]phthalazine-trione derivatives and 1H-pyrazolo[1,2-b]phthalazine-dione derivatives within a short period of time. The reaction went smoothly with various active methylene compounds and also with a diverse range of aldehydes (aromatic, aliphatic and heteroaromatic) showing the versatility of the methodology. After completion of the reaction, nano-FGT was easily recovered from the reaction mixture by a simple and convenient magnetic separation using an external magnet. This methodology also overcomes the disadvantages of formation of byproduct, low yields, use of toxic catalyst and use of hazardous organic solvent. Moreover, the catalyst can be recycled and reused for up to five consecutive runs without much of a decrease in catalytic activity.

Experimental

Phthalic anhydride (1 mmol) (1), hydrazinium hydroxide (1.2 mmol) (2), active methylene compounds (1 mmol) (3/4/5), aryl aldehydes (1 mmol) (6a–q) and nano-FGT (10 mg) were added to a round-bottom flask and since the reaction was under solvent-free conditions it was mixed properly by using a glass rod. Following this, a magnetic bit was added and the reaction mixture was set on a pre-heated oil bath at 80 °C accompanied with magnetic stirrer (model no. IKA C-MAG, HS4 Digital) and stirring was started and continued for the time mentioned in Table 2. During stirring a centrifugal force was generated, which mixed the reactants nicely thereby making a reasonable contact between the surface of the catalyst and the reactants. This fact was clearly explained from the observation that when no catalyst was added to the reaction mixture, no desired product was formed even after 24 h of stirring. After completion of the reaction (monitored by TLC), the reaction mixture was cooled to room temperature. 10 mL of ethyl acetate was added to the reaction mixture and nano-FGT attached to the magnetic bit was separated by another external magnet. After that, in order to reuse the catalyst, it was washed several times with ethyl acetate and acetone to remove any residual product and dried. The catalyst was then applied in subsequent reactions. The organic extract was then washed with water (3 × 10 mL), followed by brine and dried with anhydrous Na₂SO₄. The solvent was removed under vacuum and the crude reaction mixture was purified by column chromatography using ethyl acetate : hexane (4 : 6) to afford the pure products (15a–p, 16a, 17a, 18a).

Acknowledgements

We thank the Department of Chemistry, North-Eastern Hill University, Sophisticated Analytical and Instrumentation Facility (SAIF) of North-Eastern Hill University, Indian Association for the Cultivation of Science-Kolkata, IASST-Guwahati, IIT-Bombay and UGC for supporting this work under Special Assistance Programme (SAP). We also thank DST for financial support (sanctioned no. SERC/F/0293/2012-13) and DST-Purse. We are also grateful to NEHU-NON NET fellowship for their constant financial support.

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Footnote

† Electronic supplementary information (ESI) available. CCDC 1438715. For ESI and crystallographic data in CIF or other electronic format see DOI: 10.1039/c6ra06376d

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