Amines functionalized C₆₀ as solid base catalysts for Knoevenagel condensation with high activity and stability

Abstract

Four kinds of amines functionalized C₆₀ were prepared through an amination reaction of C₆₀ with organic amines. Their surface structures and basic properties were studied by Fourier transform infrared spectrometry (FTIR), X-ray photoelectron spectroscopy (XPS) and temperature-programmed desorption (TPD) with CO₂ as the probe molecule. When used as solid base catalysts, N-aminoethylpiperazine functionalized C₆₀ (C₆₀-AEP) showed the best catalytic activity in the Knoevenagel condensation reaction due to the highest basic strength. In addition, the C₆₀-AEP catalyst also showed impressive catalytic stability. All these features endowed C₆₀-AEP with characteristics of both homogeneous and heterogeneous catalysts. More interestingly, chiral L-lysine functionalized C₆₀ (C₆₀-L-L) can be used for asymmetric intermolecular aldol reactions.

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1. Introduction

Basic-type catalysts play a decisive role in a large number of chemical processes.¹ Various solid base catalytic systems have been developed in the literature, such as layered double hydroxides,² zeolites,³ oxides,⁴ porous metal–carboxylates,⁵ graphene based materials,⁶ carbon nitride,⁷ and so on. Typically, organic bases immobilized on different supports, for example, amino-grafting onto silica (NH₂-SiO₂)⁸ and amino-grafting onto metal–organic frameworks (NH₂-MOF),⁹ have been actively investigated due to their unique textural and physicochemical properties. However, most of these solid base catalysts suffer from deactivation due to the instability of the supports, or their preparation methods are complicated.¹⁰ It is very desirable to develop a convenient and effective route to prepare highly active and stable heterogeneous base catalysts.

Compared to silica, zeolite and MOF, carbon-based materials are more suitable as supports for amino grafting due to their chemical stability. The most common carbon supports are carbon nanotubes and graphene. Fullerenes are also one kind of carbon materials, which are aromatic molecules containing double bonds, so they can undergo a wide variety of reactions characteristic of alkenes.¹¹ Over the last two decades, a great diversity of reactions had been developed to functionalize C₆₀ or C₇₀ for organic reactions.¹² Recently, our group reported that C₆₀ fullerenol, a simple derivative of C₆₀, was an outstanding catalyst for Henry reaction, aldol reaction and the cycloaddition between CO₂ and epoxides.¹³ We envision that the amines functionalized C₆₀ may also be used as effective heterogeneous base catalysts.

In this manuscript, we report four kinds of amines functionalized C₆₀ through simple amination reaction of C₆₀ with organic amines. Fourier transform infrared spectrometer (FTIR), X-ray photoelectron spectroscopy (XPS) and elemental analysis indicated the presence of abundant amine groups. When used as solid base catalysts, they showed excellent catalytic activity and stability in Knoevenagel condensation reaction. More interesting, chiral L-lysine functionalized C₆₀ can be used for asymmetric intermolecular aldol reactions.

2. Experimental section

2.1 Materials

All solvents were purchased from Sinopharm Chemical Reagent Beijing Co., Ltd. Organic amines were purchased from Adama's Reagent Co., Ltd. C₆₀ was purchased from Suzhou Dade Carbon Nanotechnology Co., Ltd. All the reagents used for Knoevenagel Condensation reaction were purchased from Alfa Aesar. All the reagents were used without further purification.

2.2 Catalyst preparation

The ethanediamine (EDA) functionalized C₆₀ were synthesized by following the reported procedure.¹⁴ 200 mg of C₆₀ (2.8 mmol) was added into a round bottom flask which contained 6 mL of ethanediamine, the mixture was stirred at 50 °C for 24 h, then 20 mL of toluene was added, the solid was collected by centrifugation, and washed three times with tetrahydrofuran. The product was dried at 60 °C for 12 h. The N-aminoethylpiperazine (AEP) functionalized C₆₀ and diethylenetriamine (DETA) functionalized C₆₀ were prepared in the same way except that the N-aminoethylpiperazine or diethylenetriamine was used instead of ethanediamine. The synthesis of L-lysine (L-L) functionalized C₆₀ was different as L-lysine was not liquid and there was acid site in it. The preparation method was as follows. 108 mg of C₆₀ (0.15 mmol) was dissolved in 50 mL of toluene, 500 mg of L-lysine (25 equiv.) and 150 mg of NaOH (25 equiv.) was added, the mixture was stirred at 110 °C for 6 h, and the toluene became colorless. After that, the toluene was removed, and the residue solid was dissolved with water, and precipitated by adding 5 times volume ethanol. This process repeated 5 times. The product was dried at 60 °C for 12 h. The N-aminoethylpiperazine functionalized carbon nanotube was prepared in the similar method with that of C₆₀. 100 mg of multi-wall carbon nanotube or few-wall carbon nanotube was added into a round bottom flask which contained 6 mL of N-aminoethylpiperazine. The mixture was stirred at 50 °C for 72 h, after that 20 mL of toluene was added. The solid was collected by centrifugation, and washed three times with tetrahydrofuran. The product was dried at 60 °C for 12 h.

2.3 Catalyst characterizations

X-ray diffraction (XRD) pattern was recorded on a Rigaku model D/MAX-2500V system (Cu Kα radiation) at 40 kV and 200 mA. Fourier transform infrared spectrum was performed on a TENSOR-27 type infrared spectrometer. X-ray photoelectron spectroscopy data were obtained with an ESCALab220i-XL electron spectrometer from VG Scientific using 300 W Al Kα radiation. Elemental analysis was carried out on a Flash EA 1112 type elemental analyzer. Thermogravimetric (TG) analysis was conducted on a SHIMADZU DTG-60 thermogravimetric analyzer. Temperature-programmed desorption (TPD) experiments were carried out on a Micromeritics Auto Chem II 2920 type instrument. Circular dichroism (CD) spectrophotometry was collected on a J815 type spectropolarimeter.

2.4 Catalytic activity test

2.4.1 Knoevenagel condensation reaction. Benzaldehyde (1.0 mmol), ethyl isocyanoacetate (1.2 mmol), para-xylene (1 mmol, as internal standard), ethanol (3 mL) and catalyst (30 mg) were added into a glass reactor. The temperature was kept at 60 °C. The products were analyzed by GC (SHIMADZU, GC-2010 Plus) and GC-MS (SHIMADZU, GCMS-QP2010s). To study the reusability of the catalyst, the catalyst was washed one time with ethanol before next cycle.

2.4.2 Asymmetric aldol reaction. 4-Nitrobenzaldehyde (0.5 mmol), cyclohexanone (3.0 mmol) and catalyst (100 mg of C₆₀-L-L) were added into a glass reactor which contained 1 mL of water. The mixture was stirred at room temperature for stated time. Ethyl acetate was then added, and the organic solvent was evaporated under vacuum. The crude aldol product was purified by silica-gel column chromatography. The ee value of the aldol product was determined by chiral-phase HPLC (SHIMADZU, LC-15C) analysis.

3. Results and discussion

As shown in Scheme 1, four kinds of amines functionalized C₆₀ were prepared through amination reaction of C₆₀ with N-aminoethylpiperazine (AEP), diethylenetriamine (DETA), ethanediamine (EDA) and L-lysine (L-L). The corresponding products were denoted as C₆₀-AEP, C₆₀-DETA, C₆₀-EDA and C₆₀-L-L, respectively. During the amination reaction, organic amines acted as nucleophilic reagents reacted with C₆₀ to obtain the respective amine functionalized C₆₀ according to the literatures.¹⁴ The amines functionalized C₆₀ were multi-addition products, and elemental analysis was difficult to reconcile with rational formulae.¹⁴ The average number of organic amines attached to one C₆₀ molecule was determined by the mass ratio of carbon and nitrogen, and the results were listed in Table S1 (ESI†).

Scheme 1 Preparation of amines functionalized C₆₀.

X-ray diffraction analyses showed that C₆₀ was crystalline, but amines functionalized C₆₀ were amorphous (Fig. S1†). It meant that the crystal structure of C₆₀ was destroyed after nucleophilic addition, and the products were not physical mixture of C₆₀ and organic amines. In order to confirm that organic amines were attached onto C₆₀, Fourier transform infrared spectrometer (FTIR) analyses were conducted (Fig. 1). The bands at around 3400 cm⁻¹ and 1630 cm⁻¹, 2920 cm⁻¹ and 1460 cm⁻¹, 1110 cm⁻¹ and 1040 cm⁻¹ can be assigned to amine groups (–NH–),¹⁵ alkyl groups (–CH₂–)¹⁶ and C–N groups,¹⁴ respectively. The spectrum of C₆₀-L-L showed a COO⁻ peak at 1580 cm⁻¹, suggesting the existence of L-lysine.¹⁶ These peaks cannot be found in the spectrum of C₆₀ (Fig. S2†), it indicates the presence of organic amines.

Fig. 1 FTIR spectra of amines functionalized C₆₀.

X-ray photoelectron spectroscopy was further used to characterize the nitrogen forms in amines functionalized C₆₀. The spectra (Fig. 2) showed a small difference among these samples, indicating the chemical environments of nitrogen in these samples were different. The peak with a binding energy at around 398.8 eV could be assigned to amine.¹⁷ C₆₀-EDA exhibited the lowest binding energy (398.6 eV), C₆₀-AEP and C₆₀-DETA showed a slightly higher value (398.8 eV), which could be attributed to the influence of alkyl groups. C₆₀-L-L showed the highest value among these samples (398.9 eV), which may be due to the electron withdrawing effect of carboxyl group. All these results indicated that organic amines were successfully attached to C₆₀. The C1s spectra of C₆₀ before and after the functionalization were nearly the same, which meant the electron structure of C was not changed greatly (Fig. S3†). C₆₀ might act as a carrier for organic amines.

Fig. 2 XPS spectra of the N1s of amines functionalized C₆₀.

The thermal stability of amines functionalized C₆₀ was analysed by thermogravimetric analysis. As shown in Fig. 3a, the weight loss profiles of C₆₀-AEP, C₆₀-DETA and C₆₀-EDA were similar. They all displayed a rapid weight loss at around 175 °C, which can be attributed to the decomposition of amines functionalized C₆₀. This result also indicated that amines functionalized C₆₀ were thermally stable at below 170 °C, ensuing their applications in a variety of catalytic reactions.

Fig. 3 (a) TG curves and (b) carbon dioxide TPD spectra of amines functionalized C₆₀.

In order to test the basicity of amines functionalized C₆₀ catalysts, temperature-programmed desorption with CO₂ as probe molecule was conducted. As shown in Fig. 3b, CO₂ desorption bands were observed between 100 °C and 250 °C for all amines functionalized C₆₀ catalysts. C₆₀-AEP displayed two bands concentrated at 147 °C and 206 °C, corresponding to secondary amines and tertiary amines, respectively. While the maximum desorption temperature for C₆₀-DETA, C₆₀-EDA and C₆₀-L-L were 140 °C, 157 °C and 167 °C, respectively. Typically, the desorption temperature is an indication of the basic site strength.¹⁸ Higher desorption temperature suggests stronger basicity. Therefore, C₆₀-AEP exhibited the highest basicity among these four kinds of amines functionalized C₆₀ catalysts, which was in agreement with the total amounts of adsorbed CO₂ (Table S2†).

Organic amines as non-ionic bases can be the catalysts for many reactions, such as esterification reaction,¹⁹ Baylis–Hillman reaction,²⁰ aldol reaction,²¹ Knoevenagel condensation, and Henry reaction.²² The amines functionalized C₆₀ have amine groups, which are similar to organic amines. We expect that they would be advantageous for catalysis. Knoevenagel condensation of benzaldehyde and ethyl cyanoacetate was then chosen as model reaction for testing their catalytic activities. As shown in Table 1, high yields were obtained when DMF and ethanol were used as solvents (Table 1, entries 1–6). Polar solvent was favorable for the reaction, so ethanol was used as solvent in the following reactions. Four kinds of amines functionalized C₆₀ showed different catalytic activity under the same condition. C₆₀-AEP and C₆₀-EDA exhibited the highest catalytic activity (Table 1, entries 6 and 8), while C₆₀-L-L showed the lowest activity (Table 1, entry 9). The catalytic activity results were consistent with their basic density.

Table 1 Catalytic activity of amines functionalized C₆₀ for Knoevenagel condensation in different solvents^a

Entry	Solvent	Catalyst	Yield (%)
a Reaction conditions: amines functionalized C60 (30 mg), benzaldehyde (1.0 mmol), ethyl cyanoacetate (1.2 mmol), solvent (3 mL), 60 °C, 1 h.
1	THF	C60-AEP	11
2	DMF	C60-AEP	97
3	Toluene	C60-AEP	13
4	Acetonitrile	C60-AEP	27
5	Isopropanol	C60-AEP	79
6	Ethanol	C60-AEP	99
7	Ethanol	C60-DETA	95
8	Ethanol	C60-EDA	99
9	Ethanol	C60-L-L	78

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Knoevenagel condensation between various aldehyde derivatives and ethyl cyanoacetates were tested. As shown in Table 2, excellent yields were obtained with C₆₀-AEP as catalyst when benzaldehyde derivatives reacted with ethyl cyanoacetate (Table 2, entries 1–4). For cyclohexanone derivatives (Table 2, entries 5–6), alkyl aldehyde (Table 2, entry 7) and alkenyl aldehyde (Table 2, entry 8), C₆₀-AEP also showed high activity. In addition, other kinds of substrates such as dimethyl malonate and 2,4-pentanedione could be activated as well (Table 2, entries 9–10).

Table 2 Knoevenagel condensation with C₆₀-AEP as the catalyst^a

Entry	Reagent	Reagent	Product	Time (h)	Yield (%)
a Reaction conditions: C60-AEP (30 mg), aldehyde (1.0 mmol), ethyl cyanoacetate (1.2 mmol, entries 1–8), dimethyl malonate (2.0 mmol, entry 9), diacetone (2.0 mmol, entry 10), ethanol (3.0 mL), 60 °C.
1				1	98
2				1	99
3				2	99
4				1	98
5				1	93
6				1	94
7				1	97
8				1	99
9				10	87
10				10	85

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The impressive tolerance of functional groups and the excellent catalytic activity shown in Table 2 confirmed that C₆₀-AEP was a highly active and effective catalyst for Knoevenagel condensation. In addition, due to the insolubility of C₆₀-AEP in ethanol, C₆₀-AEP can be separated easily from the reaction solution through centrifugation. As shown in Fig. 4, no obvious decrease of yield was observed after 14 cycles, indicating the catalyst was very stable. FTIR spectrum of C₆₀-AEP after being used for 14 times was nearly the same as that of the fresh one (Fig. S4†).

Fig. 4 Cycle performance of the prepared C₆₀-AEP catalyst.

With abundant amine groups, C₆₀-AEP was dispersed well in reaction system and it showed high activity like homogeneous catalyst. At the same time, due to the insolubility of C₆₀-AEP in ethanol, it can be recycled as heterogeneous catalyst. These two features enabled C₆₀-AEP with characteristics of both homogeneous and heterogeneous catalyst.

The amines functionalized C₆₀ fullerene can be advantageous for catalysis, we think that carbon nanotube may also work. The catalytic activity of N-aminoethylpiperazine functionalized few-wall carbon nanotube was higher than that of multi-wall carbon nanotube, however, all the yields were low. Elemental analysis showed that the nitrogen content in N-aminoethylpiperazine functionalized carbon nanotube was far lower than that of C₆₀-AEP (Table S3†). We supposed that carbon nanotube was more difficult to be functionalized by amines compared to C₆₀ fullerene.

Amino acids have been used as catalysts for asymmetric intermolecular aldol reactions.²³ L-Lysine is a chiral amino acid, we envision that L-lysine functionalized C₆₀ (C₆₀-L-L) may also be a chiral catalyst for asymmetric aldol reaction. In order to confirm this, circular dichroism spectrum was first conducted. The result showed C₆₀-L-L possessed CD peaks (Fig. S5†). When it was used as catalyst for asymmetric aldol reaction between cyclohexanone and 4-nitrobenzaldehyde (Scheme 2), 98% yield with ee value of 13% was obtained. Although the observed ee value was low at present, this chiral amino acid functionalized C₆₀ may provide an effective way for preparation of heterogeneous chiral catalysts.

Scheme 2 Asymmetric aldol reaction with C₆₀-L-L as catalysts.

4. Conclusions

In summary, four kinds of amines functionalized C₆₀ were prepared through an amination reaction of C₆₀ with organic amines. When used as solid base catalysts, N-aminoethylpiperazine functionalized C₆₀ (C₆₀-AEP) showed best catalytic activity in Knoevenagel condensation reaction due to the highest basic strength. In addition, no obvious decrease of yield was observed after 14 times, indicating the impressive catalytic stability of C₆₀-AEP catalyst. All these features enabled C₆₀-AEP with characteristics of both homogeneous and heterogeneous catalyst. More interesting, chiral L-lysine functionalized C₆₀ (C₆₀-L-L) can be used for asymmetric intermolecular aldol reaction.

Acknowledgements

We thank the National Natural Science Foundation of China (NSFC 21273244, 21333009, and 21121063) and the Chinese Academy of Sciences (KJCX2-YW-N41) for financial support.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available: Experimental details, XRD analysis, elemental analysis, FT-IR spectra of C₆₀-AEP after being reused 14 times, CD spectrum of C₆₀-L-L and CO₂ TPD analysis. See DOI: 10.1039/c5ra16011a

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