Zhiguo Zhang,
Jingwen Chen,
Zongbi Bao,
Ganggang Chang,
Huabin Xing and
Qilong Ren*
Key Laboratory of Biomass Chemical Engineering of Ministry of Education, College of Chemical and Biological Engineering, Zhejiang University, Zheda Road 38, Hangzhou 310027, China. E-mail: renql@zju.edu.cn; Fax: +86-571-87952375; Tel: +86-571-87951224
First published on 27th August 2015
Here we present a systematic investigation of the cyanosilylation of aldehydes with trimethylsilyl cyanide (TMSCN) by using metal–organic frameworks (MOFs) as catalysts. Four types of thermally stable MOFs (MIL-47 (V), MIL-53 (Al), MIL-101 (Cr), and UiO-66 (Zr)) constructed with the same organic linker, terephthalic acid, were studied, among which MIL-101 (Cr) exhibits the highest catalytic activity. Experimental results revealed that the catalytic activities are in close relation with the types of coordinatively unsaturated metal ions, pore sizes as well as solvents. Using MIL-101 (Cr) as the catalyst, both aliphatic and aromatic aldehydes were efficiently transformed to cyanohydrin trimethylsilyl ether, meanwhile significant size selectivities and electronic effects have also been observed. The solvent-free reaction conditions not only provide a high TON for MOF catalyzed cyanosilylation, but also render the current protocol more attractive to industrial applications.
In the past several decades, a variety of activators or promoters have been reported for this transformation.1a,1c,2b,3,4 In light of environmental benign pressure, organocatalysts have grown rapidly in promoting the cyanosilylation of carbonyl compounds with TMSCN.5 Although organocatalytic systems comply with some features of green chemistry, they are still encountered with tedious separation and recycle problems in practical applications. Therefore, a mild, efficient and environmental friendly synthetic method for cyanohydrin trimethylsilyl ethers is still highly desirable.
Metal–organic frameworks (MOFs) are a new generation of materials, which were constructed via the coordination of organic ligands with metal clusters.6 The unique properties of MOFs, such as porosity, high specific surface area, tunable pore sizes and diverse functionalizations have pointed toward their potential utility to be size- and shape-selective heterogeneous catalysts.7 The vacant coordination sites in MOFs can activate carbonyl compounds for nucleophilic addition in a manner similar to Lewis acids. Since the first example of MOFs catalyzed reaction was reported by Fujita et al.,8 dozens of papers concerning MOF-based heterogeneous catalysis have been thus far published.9 More specifically, Kaskel and co-workers10 demonstrated that pure MIL-101 (Cr) is an efficient catalyst for the cyanosilylation of benzaldehyde. Recently, Corma and coworkers11 selected several MOFs to catalyze the cyanosilylation of benzaldehyde with TMSCN, in which they demonstrated the differences in the catalytic performance of MOFs with their homogeneous counterparts and other conventional solid catalysts.
Interestingly, despite the progress on MOFs catalyzed reactions, there has been little focus on the investigation of reaction mechanisms and subsequent improvement of their catalytic performance. As part of our efforts to develop practically effective catalysts for cyanosilylations,5a we turned our attention to several typical MOFs with diverse structures and topologies by presenting their potential and limitations using cyanosilylation of aldehydes as a probe reaction. Herein, we report our preliminary results by means of a systematic study on kinetic profile, Lewis acidity effect, solvent effect, and substrate scope, etc.
The FT-IR spectra were recorded on a Nicolet 6700 FT-IR spectrometer in the range of 400–4000 cm−1 by using potassium bromide pellets. 1H NMR spectra were measured on a Bruker 400 MHz NMR spectrometer. The catalytic results were monitored by a gas chromatography (GC) on a SHIMADZU GC2010 Plus. Nitrogen adsorption and desorption isotherms were measured on a 3Flex instrument. The powder X-ray diffraction (XRD) pattern of MIL-101 (Cr) was obtained on a SHIMADZU XRD-6000 diffractometer with Cu Kα radiation. Following conditions were used: 40 kV, 40 mA, scan speed = 4 degree per min, increment = 0.02°.
Entry | Catalyst | Formula | Time (h) | Conv.b (%) |
---|---|---|---|---|
a Conditions: benzaldehyde (1.0 mmol), TMSCN (1.2 mmol), catalyst (1.0 mol%), rt, 3 h.b The conversion of benzaldehyde was determined by GC analysis using tridecane as the internal standard. | ||||
1 | MIL-47 (V) | VIVO[O2C–C6H4–CO2] | 3 | 46 |
2 | MIL-53 (Al) | Al(OH)[O2C–C6H4–CO2] | 3 | 26 |
3 | MIL-101 (Cr) | Cr3XO[O2C–C6H4–CO2]3 (X = F/OH) | 3 | 96 |
4 | UiO-66 (Zr) | Zr6O4(OH)4(CO2)12 | 3 | 68 |
5 | None | — | 3 | 19 |
Since these MOFs are constructed by the same organic linker but with different metal clusters, we had initially hypothesized that the reaction rate is mainly related with the nature of metal ions in the catalysts. The coordinatively unsaturated metal ions can act as Lewis acid sites and coordinate with the carbonyls. In order to obtain experimental evidences for such interactions, we applied FT-IR spectroscopic method using benzaldehyde as a probe molecule.16 After comparison of the frequency shift between pure benzaldehyde and the MOFs absorbed benzaldehyde in FT-IR spectrum, we found that all the CO stretching vibrations of the absorbed benzaldehyde are shifted to low-frequency. As shown in Table 2, MIL-53 (Al) (Table 2, entry 2) had a least influence on ν(C
O), indicating the weak interaction between benzaldehyde and MIL-53 (Al), which consequently resulted in lowest catalytic activity. The effect of MIL-47 (V) (Table 2, entry 1) on ν(C
O) is medium, and MIL-101 (Cr) (Table 2, entry 3) and UiO-66 (Zr) (Table 2, entry 4) had strong influence on ν(C
O). So, the interaction strength of benzaldehyde with the framework is in the following sequence: UiO-66 (Zr) ≈ MIL-101 (Cr) > MIL-47 (V) > MIL-53 (Al).
The larger low-frequency shift of ν(CO) in MIL-101 (Cr) and UiO-66 (Zr) absorbed benzaldehyde, indicating the strong interaction between the metal centres and the carbonyl oxygen atoms. From another point of view, it should be noted that, the pore size (30 to 40 Å)14b inside the framework of MIL-101 (Cr) is the largest amongst these four MOFs, which allows the easy diffusion and permeability of substrates to the exposed metal sites within the pores; while accessing to the internal surface of UiO-66 (Zr) is restricted by triangular windows with opening of 6 Å.15 Therefore, given that MIL-101 (Cr) and UiO-66 (Zr) have similar influence on ν(C
O), they showed different catalytic activities (Table 1, entry 3 and 4). Similarly, the low catalytic activities of MIL-47 (V) and MIL-53 (Al) might also be affected by their respective small pore sizes (10.5 × 11.0 Å and 8.5 × 8.5 Å).12,13 Furthermore, the deficiency of active Lewis acidic sites within MIL-47 (V) and MIL-53 (Al) is probably the main restriction on the catalytic performance. This study suggested that both active Lewis acid sites and pore sizes are crucial to the MOF catalyzed cyanosilylation. Taking advantage of both the biggest pore size and the strongest Lewis acidity among MOFs tested, MIL-101 (Cr) was selected to be the catalyst for further studies on cyanosilylation reaction.
With the optimal catalyst in hand, we then focused on optimizing the reaction conditions. First, different loadings of MIL-101 (Cr), 1.00 mol%, 0.55 mol%, 0.30 mol%, 0.25 mol% and 0.15 mol%, were employed to catalyze the cyanosilylation reaction of benzaldehyde. The reactions were carried out under the following conditions: 1.0 mmol of benzaldehyde, 1.2 mmol of TMSCN, and the chosen amount of MIL-101 (Cr), and the resulting mixture was stirred vigorously at room temperature under solvent free conditions. Aliquot of the reaction mixture was taken out to be analyzed by GC to measure the conversion of benzaldehyde. The conversion of benzaldehyde vs. time was plotted in Fig. 2. The catalyst loading of MIL-101 (Cr) can be reduced to 0.3 mol% without obvious influence on reaction outcome. In addition, 0.15 mol% of MIL-101 (Cr) is still sufficient to activate this transformation albeit at the expense of somewhat time elongation. Finally, 0.3 mol% of MIL-101 (Cr) was chosen for further studies (Table S1, ESI†).
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Fig. 2 Kinetic profiles for the cyanosilylation of benzaldehyde catalyzed by different amounts of MIL-101 (Cr). |
Entry | Solvent | Conv.b (%) |
---|---|---|
a Reaction conditions: benzaldehyde (1.0 mmol), TMSCN (1.2 mmol), catalyst (0.3 mol%), solvent (3 mL), rt, 4 h.b The conversion was determined by GC analysis using tridecane as the internal standard.c Using o-xylene as the internal standard for GC analysis. | ||
1 | Solvent free | 96 |
2 | Heptane | 87 |
3 | Acetonitrile | 9c |
4 | Dichloromethane | 11 |
5 | Tetrahydrofuran | Trace |
Entry | R | Yieldb (%) | TONc |
---|---|---|---|
a Reaction conditions: aldehyde (1.0 mmol), TMSCN (1.2 mmol), catalyst (0.3 mol%), rt, 4 h.b The yields were determined by 1H NMR spectroscopy.c TON = yield/(mol% of metal ions).d The amount of TMSCN was 3.0 mmol. | |||
1 | (CH3)2CH | 100 | 333 |
2 | n-C7H15 | 100 | 333 |
3 | cyclo-C6H11 | 100 | 333 |
4 | C6H5 | 96 | 320 |
5 | 2-NO2C6H4 | 100 | 333 |
6 | 2-ClC6H4 | 100 | 333 |
7 | 4-ClC6H4 | 97 | 323 |
8 | 3-FC6H4 | 100 | 333 |
9 | 4-FC6H4 | 97 | 323 |
10 | 4-CF3C6H4 | 99 | 330 |
11 | 3-OCH3C6H4 | 90 | 300 |
12 | 4-OCH3C6H4 | 68 | 227 |
13 | 2-Furanyl | 93 | 310 |
14 | 2-Thienyl | 79 | 263 |
15 | 1-Naphthyl | 64 | 213 |
16d | 9-Anthryl | 19 | 63 |
To further probe the size selectivity of MIL-101 (Cr), larger size of aromatic aldehydes i.e. 1-naphthal- (Table 4, entry 15) and 9-anthryl aldehydes (Table 4, entry 16) were used as substrates. It is with no surprise that significant size selectivity is observed with MIL-101 (Cr) and yields for both substrates decrease dramatically as compared with benzaldehyde in the following order: benzaldehyde > 1-naphthaldehyde > 9-anthraldehyde. The relative substrates dimension indicated that the pore windows of MIL-101 (Cr) are large enough to allow benzaldehyde (8.21 × 5.83 Å)17 to diffuse swiftly through the channels to reach the catalytic active centres. In contrast, a significant decrease in reaction rate was observed for larger size substrates. The yield for 1-naphthaldehyde (9.69 × 8.29 Å)17 and 9-anthraldehyde (10.88 × 8.60 Å)17 reduced to 64% and 19% under similar conditions, respectively. As evident for the above results, MIL-101 (Cr) demonstrated its size selectivity and applicable substrate dimensions.
It is worthy of noting that the TON of the cyanosilylation of benzaldehyde catalyzed by MIL-101 (Cr) under solvent free condition is 320 (refers to metal ions), which exhibits high catalytic performance in comparison to those protocols reported in literature (Table S2, ESI†). It suggests that this present protocol by combination of MIL-101 (Cr) as catalyst and solvent free conditions is advantageous in MOF-catalysed cyanosilylations.
Consequently, we shifted our attention to the reusability of this heterogeneous catalyst and a recycling experiment was carried out. After a reaction time of 4 h, the solid catalyst was recovered by centrifugation, then washed with ethanol and activated at 150 °C under vacuum for 12 h. The catalyst was introduced into the reaction system again. The conversion of benzaldehyde was analyzed by GC. After that, the procedure was repeated for another two times. The conversion of benzaldehyde in four consecutive runs was displayed in Fig. 4. There is only a slight decrease in the conversion of benzaldehyde in the latter three runs. Hence, MIL-101 (Cr) can be recycled and reused for several times in the cyanosilylation reaction.
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Fig. 4 Recycling test of the cyanosilylation of benzaldehyde catalyzed by MIL-101 (Cr). Reaction conditions: benzaldehyde (1.0 mmol), TMSCN (1.2 mmol), MIL-101 (Cr) (0.3 mol%), rt, 4 h. |
Based on the experimental results and previously reported results,17,18 a plausible reaction mechanism is proposed to illustrate the process of MIL-101 catalyzed cyanosilylation reaction. The labile water molecules in the channels of MIL-101 (Cr) were removed by heating to expose the unsaturated metal centres previously. The aldehydes were activated by the coordinatively unsaturated Cr centres to react with TMSCN (Scheme 1). The products were replaced by aldehydes, and the catalysts were continued to activate the aldehydes in the next catalytic cycle.
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Scheme 1 Proposed mechanism for the cyanosilylation reaction of carbonyl compounds catalyzed by MIL-101 (Cr). |
Footnote |
† Electronic supplementary information (ESI) available: Experimental procedure and characterization data. See DOI: 10.1039/c5ra13102b |
This journal is © The Royal Society of Chemistry 2015 |