Zeolitic imidazolate frameworks as heterogeneous catalysts for a one-pot P–C bond formation reaction via Knoevenagel condensation and phospha-Michael addition

Abstract

A sequential one-pot reaction to produce organophosphorus compounds via Knoevenagel condensation and phospha-Michael addition has been realised by utilising ZIF-8 as a heterogeneous catalyst. The combination of 2-methylimidazolate anions and Zn²⁺ cations in ZIF-8 is revealed to be effective for the efficient promotion of the one-pot reaction.

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Over the last several decades, great attention has been paid to green chemistry in order to attain sustainable development in the world. Green chemistry is a concept used to produce chemical products with a consideration for inherently environmentally and ecologically benign materials. Most fine chemicals are synthesised through multi-step reactions industrially; therefore, inevitable purification steps produce a large amount of chemical wastes. Moreover, the loss of products during each purification step often dramatically reduces the overall efficiency of the synthetic process. Along this line, the use of one-pot chemical synthesis processes has been recognised as a promising approach to reduce the number of purification steps and improve overall efficiency.¹ However, the difficulties associated with the design of well-controlled isolated catalytic centres which are able to catalyse each reaction step efficiently, limit the wide range of general applications for one-pot chemical processes.

Metal–organic frameworks (MOFs) are organic–inorganic hybrid porous materials consisting of metal nodes and bridging organic linkers.² They possess well-defined framework structures and high surface areas derived from ordered pores. Furthermore, an appropriate choice of metal nodes and bridging organic linkers allows for accurate material design.³ These unique features make MOFs a special class of catalyst materials and stimulate our motivation to develop highly functional catalysts.

In the present communication, we report the potential catalytic performance of zeolitic imidazolate frameworks (ZIFs), which are a type of MOF material,⁴ in a one-pot reaction for the synthesis of organophosphorus compounds. Organophosphorus compounds have been widely used in the areas of industrial, agricultural and medicinal chemistry because such compounds containing P–C bonds often show attractive biological reactivities.⁵ Among the available methods for P–C bond formation, the phospha-Michael reaction is the most powerful way and is the primary method of choice.⁶ Recently, Hosseini-Sarvari et al. have reported that ZnO nanorods catalyse a one-pot Knoevenagel condensation and phospha-Michael addition reaction, in which the Lewis acid and base sites of the ZnO nanorods are considered to play a significant role in promoting the two step reaction.⁷ ZIFs are made up of Zn²⁺ cations as nodes and imidazole derivatives as linkers; therefore, it is anticipated to realise a bifunctional effect derived from two isolated active sites, i.e., unsaturated Zn²⁺ cations as Lewis acid sites and unsaturated N atoms in the imidazole derivatives as base sites. With these points in mind, ZIF materials have been prepared and applied to a one-pot Knoevenagel condensation and phospha-Michael addition reaction to produce an organophosphorus compound from benzaldehyde, malononitrile and diethyl phosphite (Scheme 1). In addition, the substrate scope of the ZIF-catalysed one-pot three component reaction and the recyclability of the ZIF catalyst have been investigated.

Scheme 1

A ZIF material named ZIF-8 that consists of 2-methylimidazole anions and Zn²⁺ cations was synthesised using a previously-reported method⁸ (see the Experimental section and Fig. S1–S4 in the ESI†). Firstly, the synthesised ZIF-8 was applied not to the one-pot reaction but to the respective catalytic reactions, the Knoevenagel condensation and the phospha-Michael addition, individually to evaluate its catalytic activities. Fig. 1a shows the results of the Knoevenagel condensation of benzaldehyde with malononitrile over ZIF-8. The reaction efficiently proceeded over ZIF-8 to give benzylidenemalononitrile with complete selectivity. Moreover, it was found from the inspection of the time course for the phospha-Michael addition shown in Fig. 1b that ZIF-8 also catalyses this reaction producing the corresponding diethyl (1-phenyl-2,2-dicyanoethyl)phosphonate from benzylidene-malononitrile and diethyl phosphite with high selectivity. These findings indicate that ZIF-8 holds potential for the catalysis of a one-pot reaction to produce organophosphorus compounds via a two-step Knoevenagel condensation and phospha-Michael addition.

Fig. 1 The results of (a) the Knoevenagel condensation and (b) the phospha-Michael addition using ZIF-8 as a catalyst. 1a, 2a and 3a represent benzaldehyde, benzylidenemalononitrile and diethyl (1-phenyl-2,2-dicyanoethyl) phosphonate, respectively. Reaction conditions: (a) catalyst (50 mg), benzaldehyde (1 mmol), malononitrile (3 mmol), 1,4-dioxane (4 mL), 323 K, in air; (b) catalyst (50 mg), benzylidenemalononitrile (1 mmol), diethyl phosphite (3 mmol), 1,4-dioxane (4 mL), 323 K, in air.

Next, ZIF-8 was applied to a one-pot sequential Knoevenagel condensation and phospha-Michael addition. The reaction was carried out at 323 K in the presence of benzaldehyde, malononitrile and diethyl phosphite as reactants and 1,4-dioxane as a solvent. As shown in Fig. 2, the benzaldehyde (1a) was consumed by ZIF-8, and diethyl (1-phenyl-2,2-dicyanoethyl)phosphonate (3a) was efficiently produced via the formation of the intermediate benzylidenemalononitrile (2a). The yield of 3a reached 98% after a 5 h period. Considering that this one-pot reaction does not take place in the absence of catalysts (Table 1, entry 12), it was found that ZIF-8 behaves as an effective catalyst to promote the one-pot Knoevenagel condensation and phospha-Michael addition reaction. The catalytic activity of ZIF-8 was then compared to those of conventional solid acid or base catalysts as well as MCM-41-NH₂, which is prepared by a post-synthetic modification of MCM-41 using 3-aminopropyltrimethoxysilane⁹ and contains aminopropyl groups that have already been reported to promote the phospha-Michael addition.¹⁰ The results clearly demonstrate the extremely high catalytic activity of ZIF-8 for this one-pot reaction (Table 1, entry 2–6). This would be due to the bifunctional effect between the Lewis acidity of the unsaturated Zn²⁺ cations and the basicity of the unsaturated N atoms of the 2-methylimidazolate units in ZIF-8. The former Knoevenagel condensation reaction is generally catalysed by base; therefore, the 2-methylimidazolate units in ZIF-8 will contribute the promotion of the reaction as base sites. On the other hand, Lewis acid sites are known to coordinate to the carbonyl oxygen atoms of aldehydes, and thus, activate the aldehydes in the Knoevenagel condensation.⁷ For the latter phospha-Michael addition, basic sites and Lewis acidic sites play a role in the deprotonation of the P–H bond in phosphites and the activation of the nitrile nitrogen atom of malonates, respectively.^6a The framework structure of ZIF-8, where basic and Lewis acidic sites exist close to each other, would enable it to activate reactive substrates and to facilitate the smooth flow of the following reactions, resulting in the efficient promotion of the one-pot reaction by ZIF-8.

Fig. 2 A time course plot of the one-pot sequential Knoevenagel condensation and phospha-Michael addition over ZIF-8. 1a, 2a and 3a represent benzaldehyde, benzylidenemalononitrile and diethyl (1-phenyl-2,2-dicyanoethyl) phosphonate, respectively. Reaction conditions: catalyst (50 mg), benzaldehyde (1 mmol), malononitrile (3 mmol), diethyl phosphite (3 mmol), 1,4-dioxane (4 mL), 323 K, in air.

Table 1 One-pot sequential Knoevenagel condensation and phospha-Michael addition using various catalysts^a

Entry	Catalyst	Conv. [%]	Yield [%]
			2a	3a
a Reaction conditions: catalyst (50 mg), benzaldehyde (1 mmol), malononitrile (3 mmol), diethyl phosphite (3 mmol), 1,4-dioxane (4 mL), 323 K, 5 h, in air. The progression of the reaction was monitored using gas chromatography and ^1H-NMR spectroscopy.
1	ZIF-8	100	2	98
2	ZnO	62	59	2
3	MgO	75	61	14
4	Al2O3	88	80	7
5	Hydrotalcite	100	70	26
6	MCM-41-NH2	81	63	15
7	ZIF-7	88	32	54
8	ZIF-9	81	45	36
9	ZIF-67	100	34	66
10	IRMOF-3	18	11	5
11	HKUST-1	21	17	4
12	None	0	0	0

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So far, various types of ZIF materials have been developed and reported, such as ZIF-7 formed by bridging benzimidazolate anions and Zn²⁺ cations, ZIF-9 formed by benzimidazolate anions and Co²⁺ cations and ZIF-67 formed by 2-methylimidazolate anions and Co²⁺ cations.^4b,d These three ZIFs were also prepared and used in the one-pot reaction. The formation of the ZIF materials was confirmed by XRD, TGA and FE-SEM measurements (Fig. S5–S7†). Among the four types of ZIF materials, ZIF-8 gave the best catalytic performance for this one-pot reaction (Table 1, entries 1 and 7–9). Also, ZIF-8 and ZIF-67 were found to show higher activities than ZIF-7 and ZIF-9. In consideration of the fact that 2-methylimidazole has a stronger Lewis basicity than benzimidazole,¹¹ the base strength of ZIF materials would have a strong influence on their catalytic performances. Moreover, the catalytic activity of ZIF-8 was higher than that of IRMOF-3, which consists of 2-amino-1,4-benzenedicarboxylate linkers and Zn-oxo clusters¹² (Table 1, entry 10). Although HKUST-1 prepared by using 1,3,5-benzenetricarboxylic acid and copper(II) nitrate has already been proven to be an effective catalyst for the Knoevenagel condensation in some papers,¹³ the one-pot reaction did not proceed efficiently under our experimental conditions (Table 1, entry 11). These results suggest that the combination of 2-methylimidazolate anions and Zn²⁺ cations is beneficial for promoting the one-pot reaction.

Subsequently, the ZIF-8-catalysed one-pot three component reaction was extended to other substrates. The results are summarized in Table 2. Using differently-substituted aldehydes (Table 2, entries 2–5), the reaction proceeded smoothly on ZIF-8 to give the corresponding organophosphorus compounds (3b–e). Furthermore, ZIF-8 promoted the one-pot reaction in the presence of benzaldehyde, ethyl cyanoacetate and diethyl phosphite and produced 3f in a high yield, although the reaction time needed for reaction completion was prolonged because the rate of the former Knoevenagel condensation is generally determined by the pK_a values of the donor molecules.¹⁴

Table 2 One-pot sequential Knoevenagel condensation and phospha-Michael additions using various substrates catalysed by ZIF-8^a

Entry	R^1	R^2	Product	Time [h]	Conv. [%]	Yield [%]
a Reaction conditions: catalyst (50 mg), aldehyde (1 mmol), active methylene compound (3 mmol), diethyl phosphite (3 mmol), 1,4-dioxane (4 mL), 323 K, in air.
1	H	CN	3a	5	100	98
2	CH3	CN	3b	5	100	100
3	OH	CN	3c	5	100	99
4	NO2	CN	3d	5	100	99
5	OCH3	CN	3e	5	100	99
6	H	COOC2H5	3f	10	100	98

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Finally, recycling experiments were performed for the evaluation of the recyclability of ZIF-8. The spent catalyst was recovered by filtration after cooling to room temperature, washed several times with methanol, dried at 333 K in air and then reused in the next run. The results shown in Fig. 3a reveal that ZIF-8 can be recycled for at least 3 cycles without significant loss of its reactivity and selectivity. Moreover, almost no change was observed in the XRD pattern of ZIF-8 after the reaction (Fig. 3b), indicating that ZIF-8 behaves as a heterogeneous catalyst while retaining its framework structure during the reaction.

Fig. 3 (a) Recycling experiments for the one-pot sequential Knoevenagel condensation and phospha-Michael addition using ZIF-8 and (b) the XRD patterns of ZIF-8 before and after the one-pot reaction.

Conclusions

In summary, we investigated the potential heterogeneous catalytic activity of ZIF-8 for a one-pot P–C bond formation reaction via a Knoevenagel condensation and phospha-Michael addition. The one-pot reaction efficiently proceeded on ZIF-8 to give the corresponding organophosphorus compounds from aldehydes, active methylene compounds and diethyl phosphite with extremely high selectivity. The framework structure of ZIF-8, which includes both base sites derived from 2-methylimidazolate anions and Lewis acid sites derived from Zn²⁺ cations, was found to be beneficial for promoting the one-pot reaction. Moreover, ZIF-8 was able to be reused several times without a significant loss of its selectivity and reactivity.

Experimental section

Preparation of ZIF-8

A mixture of 2-methylimidazole (1.32 g), zinc nitrate tetrahydrate (Zn(NO₃)₂·4H₂O, 0.60 g) and methanol (44 mL) was vigorously stirred at room temperature for 24 h. The generated precipitate was collected by centrifugation, washed repeatedly with methanol and dried at 333 K for 12 h.⁸

Characterisation results

The XRD pattern of the synthesised ZIF-8 matched well with that previously reported,⁸ indicating the successful formation of the ZIF-8 crystal (Fig. S1a†). Moreover, as shown in Fig. S1b,† the N₂ adsorption–desorption isotherm of the ZIF-8 traced the typical type I isotherm with a steep increase in the amount of adsorbed N₂ at very low pressures (P/P₀ < 0.01). This finding suggests that ZIF-8 possesses a microporous structure. The BET area of ZIF-8 was determined to be 1716 m² g⁻¹ from a BET-method-based calculation on the N₂ adsorption isotherm data. This high BET area is associated with micropores. In the FT-IR spectrum, ZIF-8 did not exhibit a broad absorption band between 3400–2200 cm⁻¹ attributable to N–H⋯N hydrogen bonds and a band at 1843 cm⁻¹ attributable to the resonance between the N–H⋯N “out of plane” bending and the N–H stretching vibrations (Fig. S2†), which are typically observed for 2-methylimidazole. These results indicate that the 2-methylimidazole units within the ZIF-8 were deprotonated.¹⁵ The thermal gravimetric analysis (TGA) curve of ZIF-8 observed in air showed a small weight-loss step from 150 °C corresponding to the removal of guest molecules from the cavities or of unreacted species and then reached a plateau up at 450 °C, followed by a significant weight-loss step of about 50% attributed to thermal decomposition (Fig. S3†). Therefore, it was found from the TGA result that ZIF-8 contains a small amount of impurity and has a high thermal stability up to 450 °C. Furthermore, SEM imaging demonstrated the formation of ZIF-8 particles of about 20 μm in size (Fig. S4†).

Acknowledgements

The present work is financially supported by the “ACCEL Project” from the Japan Science and Technology Agency, by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology of Japan (no. 25410241) and by the Global Research Program of the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology (MEST), Korea (grant number: 2010-00339). T.T. thanks the JSPS for the Research Fellowship for Young Scientists.

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Footnote

† Electronic supplementary information (ESI) available: Experimental procedures; XRD pattern, N₂ adsorption–desorption isotherm, FT-IR spectrum, TGA data and SEM image of XIF-8; XRD patterns, TGA data and SEM images of ZIF-7, ZIF-9 and ZIF-67. See DOI: 10.1039/c5ra02410b

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