Expanding applications of zeolite imidazolate frameworks in catalysis: synthesis of quinazolines using ZIF-67 as an efficient heterogeneous catalyst

Abstract

A cobalt zeolite imidazolate framework (ZIF-67) was successfully synthesized and characterized by several techniques. The ZIF-67 was used as an efficient heterogeneous catalyst for the cyclization reaction of 2-aminobenzoketones and benzylamine derivatives to form quinazoline products. The optimal conditions involved the use of TBHP oxidant in toluene solvent at 80 °C. Remarkably, the ZIF-67 catalyst exhibited better performance in the cyclization reaction than common cobalt salts such as Co(NO₃)₂, CoCl₂, and Co(OAc)₂ and other Co-MOFs such as ZIF-9, Co-MOF-74, and Co₂(BDC)₂(DABCO). In addition, the cyclization reaction could only proceed in the presence of the solid Co-ZIF catalyst and there was no contribution from leached active sites present in the solution. The catalyst could be recovered and reused several times without a significant degradation in catalytic activity.

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1. Introduction

Quinazoline derivatives are widely distributed in many natural products and synthetic pharmaceutical chemicals and their synthesis has attracted significant attention as they possess a wide range of biological and pharmacological activities.^1–3 These structures have been conventionally prepared by the Bischler cyclization between dicarbonyls and diamines.⁴ Previously, Fu and co-workers demonstrated a simple and efficient, ligand-free CuI-catalyzed condensation between substituted (2-bromophenyl)methylamines and amides to achieve quinazoline derivatives.⁵ Yu and co-workers employed CuCl/DABCO/4-HO-TEMPO as the catalyst for the aerobic oxidative synthesis of 2-substituted quinazolines from the one-pot reaction of aldehydes with 2-aminobenzylamines.⁶ Buchwald and co-workers reported a palladium-catalyzed N-monoarylation of amidines leading to a one-pot synthesis of quinazoline derivatives.⁷ Zhang and co-workers developed an efficient Cu(OTf)₂-catalyzed synthesis of quinazolines from amidines.² The synthesis of 2-phenylquinazolines was also obtained from 2-aminobenzophenones and benzylamines catalyzed by ceric ammonium nitrate (CAN)-TBHP in acetonitrile.⁸ The quinazoline synthesis protocols have recently been improved by using heterogeneous catalyst systems, including iron oxide (γ-Fe₂O₃) nanoparticles,⁹ and polymer-supported bi-metallic platinum/iridium (Pt/Ir) alloyed nanoclusters and 5,5′,6,6′-tetrahydroxy-3,3,3′,3′-tetramethyl-1,1′-spiro-bisindane.¹⁰ Although interesting results have been achieved, developing an efficient heterogeneous catalyst system for the quinazoline synthesis still remains to be explored.

Metal–organic frameworks (MOFs) are extended porous materials constructed from metal ions or metallic clusters and polyfunctional organic linkers.^11,12 MOFs offer potential applications in many fields, including gas storage media, separations, chemical sensors, thin film devices, optics, drug carriers, biomedical imaging, and catalysis.^13–20 Zeolite imidazolate frameworks (ZIFs) have emerged as a new subclass of MOFs, combining advantages from both zeolites and conventional MOFs.^21,22 MOFs have been investigated as catalysts or catalyst supports for many organic transformations,^23–25 including both carbon–carbon,^26–29 carbon-heteroatom forming reactions,^30–37 and cyclization.³⁷ As compared to conventional MOFs, reports on catalytic studies of ZIFs for organic transformations have been limited in the literature. These works include ZIF-8 as catalyst for the transesterification,³⁸ the synthesis of styrene carbonate from carbon dioxide and styrene oxide,³⁹ the Knoevenagel condensation,⁴⁰ the gas-phase ethylene and cyclohexene hydrogenation,⁴¹ and the esterification of glycerol with oleic acid;⁴² ZIF-9 as catalyst for the Knoevenagel condensation,⁴³ the oxidation of aromatic oxygenates,⁴⁴ and the hydrogen production from NaBH₄ hydrolysis;⁴⁵ ZIF-67 as catalyst for the synthesis of ethyl methyl carbonate from dimethyl carbonate and diethyl carbonate and;⁴⁶ and ZIF-68 as catalyst for the chemical fixation of carbon dioxide.⁴⁷ To the best of our knowledge, ZIF-67 has not been previously used as redox catalysts for organic transformations in the literature. Herein, we report the oxidative synthesis of 2-phenylquinazolines using ZIF-67 as an efficient heterogeneous catalyst. High activity and reaction yields were achieved while the ZIF-67 catalyst could be reused several times without a significant degradation in activity.

2. Experimental

2.1. Synthesis of the zeolitic imidazolate framework ZIF-67

In a typical preparation,⁴⁸ a mixture of 2-methylimidazole (3.284 g, 40 mmol) and triethylamine (5.6 mL, 40 mmol) was dissolved in water (10 mL). A solution of Co(NO₃)₂·6H₂O (0.73 g, 2.5 mmol) in water (10 mL) was then added, and the resulting solution was stirred at room temperature for 10 min. The purple precipitates were separated by centrifugation, washed with water (3 × 10 mL) and methanol (3 × 10 mL) at room temperature. The material was evacuated under vacuum at 150 °C for 6 h, yielding 0.314 g of ZIF-67, Co(2-MeIm)₂, in the form of purple crystals (56% yield).

2.2. Catalytic studies

In a typical experiment, a pre-determined amount of ZIF-67 was added to the flask containing a mixture of 2-aminoacetophenone (122 μL, 1 mmol), benzylamine (164 μL, 1.5 mmol), tert-butyl hydroperoxide (685 μL, 5 mmol), and diphenyl ether (100 μL) as internal standard in toluene (5 mL). The catalyst loading was calculated based on the molar ratio of cobalt/2-aminoacetophenone. The reaction mixture was stirred at 80 °C for 180 min. The reaction conversion was monitored by withdrawing aliquots from the reaction mixture at different time intervals, quenching with an aqueous KOH solution (5%, 1 mL), drying over anhydrous Na₂SO₄, analyzing by GC with reference to diphenyl ether. To investigate the recyclability of ZIF-67, the catalyst was filtered from the reaction mixture after the experiment, washed with copious amounts of toluene and methanol, dried under vacuum for 4 h, and reused if necessary. For the leaching test, a catalytic reaction was stopped after 30 min, analyzed by GC. Solid catalyst was removed by hot filtration. The reaction solution was then stirred for a further 150 min. Reaction progress, if any, was monitored by GC as previously described.

3. Results and discussion

The characterization results XRD, SEM, TEM, TGA, FT-IR, AAS, H₂-TPR, and nitrogen physisorption measurements are in good agreement with reported studies (Fig. S1–S7†).⁴⁸ In particular, SEM and TEM revealed the size of ZIF-67 was about 100–150 nm (Fig. S2 and S3†). Results from isotherm confirmed the permanent porosity of ZIF-67 with surface area of 601 m² g⁻¹ (BET) or 857 m² g⁻¹ (Langmuir). Additionally, thermal gravimetric analysis of activated ZIF-67 showed a high thermal stability (>300 °C) while the FT-IR exhibited the presence of imidazolate linker in the ZIF-67 structure. Furthermore, elemental analysis of ZIF-67 with ICP-MS indicated a cobalt loading of 4.32 mmol g⁻¹ which is comparable to theoretical value of 4.48 mmol g⁻¹.

In optimization studies, ZIF-67 was used as catalyst for the cyclization reaction of 2-aminoacetophenone and benzylamine (Scheme 1). Initial studies addressed the effect of temperatures on the conversion of 2-aminoacetophenone to 2-phenylquinazoline. The cyclization reaction was carried out in toluene at 3 mol% ZIF-67 catalyst for 180 min, using 1.5 equivalents of benzylamine and 5 equivalents of tert-butyl hydroperoxide 70% in water as the oxidant. It was observed that the cyclization reaction could not occur at room temperature, with no trace amount of product after 180 min. The reaction proceeded with difficulty at 60 °C, though 58% conversion was observed. As expected, increasing the temperature to 70 °C led to a significant enhancement in the reaction rate, affording 71% conversion. It was found that the reaction carried out at 80 °C proceeded at high rate and 97% conversion was obtained (Fig. 1). In the first example of the heterogeneous cyclization reaction of 2-aminoacetophenone and benzylamine using iron oxide (γ-Fe₂O₃) nanoparticles as catalyst, Burri and co-workers carried the transformation at 85 °C.⁹ Other approaches to achieve quinazoline derivatives previously required higher reaction temperature.^2,7,49,50

Scheme 1 Cyclization reaction of 2-aminoacetophenone and benzylamine using ZIF-67 catalyst.

Fig. 1 Conversion of the synthesis quinazolines of as a function of time at different temperatures using ZIF-67 catalyst, TBHP oxidant, and toluene solvent.

Different percentages of catalyst of 1 mol%, 3 mol%, and 5 mol% were tested, respectively to investigate the optimal amount of catalyst ZIF-67 for the cyclization reaction. It was found that at 1 mol% ZIF-67, reaction could proceed to 71% conversion after 180 min. Increasing the catalyst loading to 3 mol% led to 97% conversion. However, using more than 3 mol% catalyst was found to be unnecessary as the reaction conversion did not remarkably increase. It should be noted that no reaction occurred in the absence of the ZIF-67 (Fig. 2).

Fig. 2 Conversion of the synthesis quinazolines of as a function of time at different catalyst amount using ZIF-67 catalyst, TBHP oxidant at 80 °C in toluene.

Various different solvents including n-heptane, toluene, p-xylen, 1,4-dioxane, dibutyl ether, chlorobenzene, acetonitrile, and N,N-dimethylacetamide (DMAc) were investigated. Interestingly, it was found that among these solvents, toluene was the most effective for the cyclization transformation using ZIF-67 catalyst (Fig. 3). Particularly, solvents such as 1,4-dioxane, acetonitrile, and DMAc were found to be not suitable with less than 10% conversion. Moderate reaction conversions were obtained in p-xylene, n-heptane, dibutyl ether and chlorobenzene with about 50–60%. It is likely that coordinating solvent with substrates or metal complexes inhibited the reaction efficiency. Furthermore, the impact of the reagent molar ratio was also explored with 1.0, 1.25, and 1.5 equivalents of benzylamine, respectively. The experimental results indicated that using less than 1.5 equivalents of benzylamine resulted in a significant drop in the reaction conversion, with 82% and 71% conversions being detected for the case 1.25 and 1 equivalents of benzylamine, respectively (Fig. 4).

Fig. 3 Conversion of the synthesis quinazolines of as a function of time in different solvents amount using ZIF-67 catalyst, TBHP oxidant at 80 °C.

Fig. 4 Conversion of the synthesis quinazolines of as a function of time at different 2-aminoacetophenone : benzylamine molar ratio using ZIF-67 catalyst, TBHP oxidant at 80 °C in toluene solvent.

The effect of different oxidants on the reaction conversion was then examined. It was found that all di-tert-butyl peroxide, hydrogen peroxide, and Na₂S₂O₈ were ineffective for the transformation with less than 5% conversions (Fig. 5). The cyclization reaction using tert-butyl peroxybenzoate provided 69% conversion. Several previous studies also confirmed the efficiency of TBHP oxidant for oxidative cross coupling reactions under MOFs catalysis.^{28b,29b,52,53} In addition, decreasing the oxidant concentration led to a significant drop in the reaction rate, affording 84% and 55% conversions for the reaction using 04 equivalents and 03 equivalents of the oxidant, respectively (Fig. 6). It should be noted that no reaction occurred in the absence of tert-butyl hydroperoxide, indicating the importance of the oxidant for the transformation. Product formed under optimal condition was isolated by flash column chromatography. Corresponding to 97% conversion, isolated yield of 87% was obtained. This indicated the excellent selectivity of the reaction under optimal protocol.

Fig. 5 Conversion of the synthesis quinazolines of as a function of time with different oxidants using ZIF-67 catalyst at 80 °C in toluene solvent.

Fig. 6 Conversion of the synthesis quinazolines of as a function of time with different TBHP equivalents using ZIF-67 catalyst at 80 °C in toluene solvent.

To further gain the reaction mechanistic insights, (2,2,6,6-tetramethylpiperidin-1-yl)oxidanyl (TEMPO) as the antioxidant was added to the reaction mixture after the first 30 min. The mixture was stirred for an additional 150 min at 80 °C with aliquots being sampled at different time intervals. It was observed that the presence of TEMPO in the reaction mixture significantly slowed down the reaction rate. Similar results were obtained when adding 0.5 mol% of phloroglucinol antioxidant to the reaction mixture after the first 30 min reaction time (Fig. 7). It is likely that the formation of iminium radical is involved in reaction mechanism. With respect to reaction active sites, the size of the pore according to the ZIF-67 structure is about 11–14 Å while the kinetic diameters of aromatic reactions and product are calculated to be 6 Å and 11 Å, respectively.⁵⁴ It was reported that that pore size of ZIF changes and fluctuates upon the temperature and guest molecules.^48,55 Thus, further studies are needed to confirm whether reactions occurred inside or outside the cavities at reaction temperature.

Fig. 7 Conversion of the synthesis quinazolines of as a function of time with adding radical trapping reagents using ZIF-67 catalyst at 80°C in toluene solvent.

To confirm the exceptional catalytic activity of the ZIF-67, other homogeneous and heterogeneous cobalt-based catalysts were tested under optimal conditions. It was observed that the reaction using Co(NO₃)₂ catalyst afforded only 35% conversion while 72% conversion was obtained when CoCl₂ was employed. The reaction conversion could be improved to 89% by using Co(OAc)₂ as catalyst. In addition, the linker to synthesize ZIF-67, was completely inactive for the cyclization transformation, with no trace amount of product being detected (Fig. 8). The result ruled out the Brønsted active site of the catalyst. Although ZIF-9 contained cobalt sites, this Co-ZIF offered lower catalytic activity than ZIF-67. This could be rationalized based on the difference in the particle size of the two Co-ZIFs since the oxidation state of Co sites in these 2 ZIFs are identical. Indeed, the ZIF-9 possessed an average size of approximately ten fold bigger than ZIF-67.⁵¹ Co-MOF-74, which contains large pore size and aperture offered only 16% conversion. The other Co-MOF, Co₂(BDC)₂DABCO, offered moderate activity for the transformation with 79% conversion (Fig. 9). It is likely that the carboxylate linkers negatively affect to reactivity of Co complexes as compared to the imidazolate linkers.

Fig. 8 Conversion of the synthesis quinazolines of as a function of time with different cobalt salt catalysts using TBHP oxidant at 80 °C in toluene solvent.

Fig. 9 Conversion of the synthesis quinazolines of as a function of time with different Co-MOFs catalysts using TBHP oxidant at 80 °C in toluene solvent.

A control experiment was performed using a simple filtration during the course of the reaction to confirm the reaction heterogeneity. The cyclization reaction was carried out under optimal condition with toluene solvent at 80 °C for 180 min, using 1.5 equivalents of benzylamine and 5 equivalents of tert-butyl hydroperoxide, and 3 mol% ZIF-67 catalyst. After the first 30 min reaction time with 34% conversion, the ZIF-67 catalyst was removed from the reaction mixture by hot filtration. The liquid phase was then transferred to a new reactor vessel, and stirred for an additional 150 min at 80 °C with aliquots being sampled at different time intervals. It was found that almost no further conversion was observed for the cyclization reaction after the ZIF-67 catalyst was separated from the reaction mixture (Fig. 10). Furthermore, ICP-MS of filtrate indicate the 0.2 ppm% of Co. These data would confirm that the cyclization reaction of 2-aminoacetophenone and benzylamine could only proceed in the presence of the solid ZIF-67 catalyst, and no contribution from catalytically active cobalt species soluble in the solution was detected.

Fig. 10 Leaching test.

The ability to recover and reuse the ZIF-67 catalyst in the cyclization reaction of 2-aminoacetophenone and benzylamine was explored. In particular, catalyst was repeatedly separated from the reaction mixture after the reaction by hot filtration, washed with copious amounts of toluene and methanol to remove any physisorbed reagents, dried under vacuum for 4 h and then reusing it for the next runs. It was observed that the ZIF-67 catalyst could be recovered and reused several times without a significant degradation in catalytic activity. Indeed, a conversion of 98% was still achieved in the 5^th run (Fig. 11). The FT-IR spectra of the reused ZIF-67 after the 5^th run exhibited a similar absorption as compared to those of the fresh catalyst (Fig. 12). The XRD result of the recovered ZIF-67 after the 5^th run indicated that the crystallinity of the ZIF-based catalyst could be maintained during the course of the transformation, though slight difference in the diffractogram was detected (Fig. 13). This minor changes probably dues to the flexible behavior in ZIF-67 structure and the non-isotropy during fresh sample preparation.

Fig. 11 Catalyst recycling studies.

Fig. 12 FT-IR spectra of the fresh (a) and reused (b) ZIF-67 catalyst.

Fig. 13 X-ray powder diffractograms of the fresh (a) and reused (b) ZIF-67 catalyst.

The condition generality was tested by extending cyclization reaction of with various coupling partners (Table 1). Products were isolated using flash chromatography and structural analysis was done by NMR. With respect to ortho-amino benzoketones, both 2-aminoacetophenone and 2-aminobenzophenone are reactive and excellent yields were achieved (entries 1 and 2). Substrate with substituent on the benzene rings was cyclized in 78% isolated yield (entry 3). With respect to amine counterpart, cyclization of benzylamines with electron donating group, methyl, or electron withdrawing group, chloro, is possible and products were obtained in 75% and 83% yields, respectively.

Table 1 Reaction scope with respect to coupling partners

Entry	2-Aminobenzoketones	Benzylamines	Products	Yields
1				87
2				89
3				78
4				83
5				75

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4. Conclusions

In summary, the zeolite imidazolate framework ZIF-67 was synthesized and fully was characterized. The ZIF-67 exhibited exceptionally high catalytic activity for the cyclization reaction of 2-aminobenzoketones and benzylamines forming 2-arylquinazolines. Optimal condition involved the use of TBHP (5 equiv.), ZIF-67 (3 mol%) in toluene solvent at 80 °C in 3 hours. Excellent isolated yields of reaction scope were observed. Leaching test revealed no contribution from leached active sites present in the solution. Furthermore, the catalyst could be recovered and reused several times without a significant degradation in catalytic activity. The ZIF-67 catalyst exhibited better performance than common cobalt salts such as Co(NO₃)₂, CoCl₂, and Co(OAc)₂ and Co-MOFs such as ZIF-9, Co-MOF-74, and Co₂(BDC)₂(DABCO). Current research in our laboratory has been directed to the application of several ZIFs as catalysts for a wide range of organic transformations.

Acknowledgements

The Viet Nam National Foundation for Science and Technology Development – NAFOSTED is acknowledged for financial support through project code 104.01-2014.76.

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4ra16168h

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