b. Nanotechnology Research Center, Shiraz University, Shiraz 71545, Iran
Protection and deprotection of carbonyl groups are extremely important steps in modern organic chemistry . Acylal formation is one of the most useful methods to protect the carbonyl group of aldehydes and ketones and has found wide application in multistep syntheses due to acylal stability in neutral and basic media . Also,the diacetates of α,β-unsaturated aldehydes are important starting materials for Diels-Alder reactions,since acylals have also been used as cross-linking agents for cellulose in cotton  and are useful intermediates in industries .
The conventional method for the preparation of 1,1-diacetates involves the reaction of aldehydes with acetic anhydride using Lewis and Brønsted acids,such as FeCl3 ,Cu(OTf)2 ,Bi(OTf)3 ,FeSO4 ,N-bromosuccinimide ,montmorillonite clay , zeolites ,H6P2W18O62⋅24H2O ,methanesulfonic acids , silica sulfuric acid (SSA) ,P2O5/Al2O3 ,silica-bonded Ssulfonic acid (SBSSA) ,BEA-SO3H (zeolite beta (BEA)) , Fe3O4@SiO2/Schiff base complex of Cr (Ⅲ) ,silica-bonded propyl-diethylene-triamine-N-sulfamic acid (SPDTSA)  and sulfonated rice husk ash (RHA-SO3H) .
The development of heterogeneous catalysts for organic synthesis has become a major area of research. The potential advantages of these materials over homogeneous systems (simplified recovery,reusability and the potential for incorporation in continuous reactors and microreactors) could lead to novel,environmentally benign chemical procedures for academia and industry .
Application of solid acids in organic transformations is important because they have many advantages including ease of handling,decreased reactor and plant corrosion problems and more environmentally safe disposal [22, 23, 24, 25, 26, 27].
Catalysis is currently recognized as a potential field of application for carbon nanotubes (CNTs),and throughout the past decade,the number of publications on this subject has been increasing exponentially .
As a part of our program aiming at developing efficient and environmentally friendly heterogeneous catalysts for organic synthesis,we have developed multi-walled carbon nanotubes (MWCNTs),functionalized with phosphonic acid (MWCNTs-C-PO3H2) as an efficient,heterogeneous and reusable nanocatalyst for acylation of phenolic hydroxyl groups,different classes of alcohols,aromatic amines and thiols with acetic anhydrides under solvent-free conditions . In this paper,we report a facile and efficient method for the preparation of acylals under solvent-free conditions at room temperature using MWCNTs-C-PO3H2 as an efficient,heterogeneous and reusable nanocatalyst (Scheme 1).
|Scheme 1.Preparation of acylals under solvent-free condition at room temperature using MWCNTs-C-PO3H2.|
All products were known and their physical and spectroscopic data were compared to those of authentic samples. Chemicals were purchased from Fluka or Merck. The purity of the products was determined by TLC on silica gel polygram SIL G/UV 254 plates.NMR spectra were recorded on a Bruker Avance DPX-250 (250 MHz for 1H NMR and 62.9 MHz for 13C NMR) spectrometer in pure deuterated solvents with tetramethylsilane (TMS) as an internal standard.
The synthesized CNTs were characterized using some electron microscopic techniques such as scanning electron microscopy (SEM,XL-30 FEG SEM,Philips,20 KV),atomic force microscopy (AFM,DME-SPM,version 184.108.40.206),and also thermogravimetric analysis.
|Fig. 1.Schematic of CVD-synthesized MWCNTs-C-PO3H2 catalyst (A),schematic representing the CVD-synthesis of MWCNT-C-PO3H2 (B).|
According to Ref. ,acetylene gas with a flow rate of ~50 mL min-1 was bubbled into a solution containing ferrocene (0.30 g),triphenylphosphine (3.0 g),thiophene (0.7 mL) in benzene (25 mL). This was mixed with hydrogen and argon with flow rates to 0.5 and 800 mL min-1,respectively,followed by introduction into a quartz tube passed through a 80 cm tubing furnace set at 1300 ℃. The produced phosphor-doped carbon nanostructures were then directly purified from any amorphous carbon via purging oxygen and aerosols of hydrogen peroxide into the production line,followed by on-line activation using ultraviolet (UV) and microwave irradiators.
General procedure for preparation of 1,1-diacetates: To a mixture of aldehyde (1 mmol) and acetic anhydride (3 mmol) was added MWCNTs-C-PO3H2 (0.1 mol%,0.026 g). The reaction mixture was stirred at room temperature for the appropriate reaction time. After completion of the reaction,as indicated by TLC,the reaction mixture was diluted with hot CH2Cl2 (5 mL × 2) and the resultant mixture was stirred at room temperature for 10 min. Then,the catalyst was separated by centrifuge and CH2Cl2 was evaporated under reduced pressure. The crude product was purified by column chromatography on silica gel with petroleum ether and ethyl acetate (80:20) to give the pure product.
Typical procedure for preparation of 1,1-diacetate from 4- nitrobenzaldehyde: to a mixture of 4-nitrobenzaldehyde (0.151 g, 1 mmol) and acetic anhydride (0.28 mL,3 mmol) was added MWCNTs-C-PO3H2 (0.1 mol%,0.026 g). The reaction mixture was stirred at room temperature for the appropriate reaction time. After completion of the reaction,as indicated by TLC,the reaction mixture was diluted with hot CH2Cl2 (5 mL × 2) and the resultant mixture was stirred at room temperature for 10 min. Then,the catalyst was separated by centrifuge and CH2Cl2 was evaporated under reduced pressure. The crude product was concentrated and recrystallized from CH2Cl2 to give the pure product in 92% yield as a light yellow solid,mp 124 ℃. 3. Results and discussion
The CVD techniques were used for the synthesis of MWCNTs-CPO3H2 (Fig. 1A). For this purpose,acetylene with 99% purity and ferrocene were used as CNT precursor and catalyst. Iron nanoparticles were librated in situ from ferrocene and catalyzed MWCNTs formation in the presence of thiophene as sulfur precursor for increasing the length of CNT.
Then phosphorous atom was doped on carbon nanostructures by the reaction of MWCNTs with triphenylphosphineoxide at 1300 ℃ under the atmosphere of hydrogen and argon in a quartz tube. Then hydrogen peroxide oxidized the produced phosphordoped carbon nanostructures to obtain MWCNs-C-PO3H2 and also simultaneously purified it from any amorphous carbon as graphically shown in Fig. 1B. The MWCNTs-C-PO3H2 catalyst has been fully characterized using some different microscopic and spectroscopic techniques .
Due to stability and mild acidity of MWCNTs-C-PO3H2,we decided to investigate its capability for the preparation of acylals. Therefore we needed to determine the best reaction conditions for this transformation.
For this purpose,the reaction of p-nitrobenzaldehyde (1 mmol) with acetic anhydride (3 mmol) in the presence of MWCNTs-CPO3H2 was chosen as a model reaction. The model reaction was investigated in various solvents such as H2O,EtOH,CH2Cl2,CHCl3, CH3CN and also under solvent-free condition at room temperature in the presence of various amount of catalyst. The results are summarized in Table 1.
As it is shown in Table 1,the yield of the reaction under solventfree conditions in the presence of 0.1 mol% catalyst was the highest and the reaction time was the shortest. Aprotic solvents,such as CHCl3,CH2Cl2 and CH3CN,afforded the desired product in lower yields and longer reaction times (Table 1,entries 1-3). In protic solvents,such as water and ethanol (Table 1,entries 4-5),this protection reaction proceeded with longer reaction times and very poor yields,which may be related to the instability of acetic anhydride and corresponding product in protic solvents.
Therefore,we employed the optimized conditions for the conversion of various aldehydes into the corresponding acylals. The results of the preparation of acylals from aromatic aldehydes in the presence of MWCNTs-C-PO3H2 at room temperature are shown in Table 2.
Aldehydes with electron-donating or electron-withdrawing groups were converted into the corresponding acylals in high yields after short reaction times. The acid-sensitive substrate, thiophene-2-carbaldehyde,gave the expected acylal in 80% yield without any by-product formation (Table 2,entry 11). We also investigated the reactions of 4-hydroxybenzaldehyde under the above-mentioned conditions and observed after 90 min,both the carbonyl and phenolic groups were acylated (Table 2,entries 9). Several aliphatic and aromatic ketones,including cyclohexanone and acetophenone,were not reactive under the described experimental conditions even after 2 h (Table 2,entries 14 and 15).
Next,we studied the competitive acylation reactions of aromatic aldehydes in the presence of ketones using MWCNTs- C-PO3H2. Under these conditions exclusive acylation of the aldehyde functions was observed. The chemoselective acylations of p-chlorobenzaldehyde in the presence of acetophenone and cyclohexanone are shown in Scheme 2.
|Scheme 2.Competitive acylal formation of aldehydes in the presence of ketones using MWCNTs-C-PO3H2 under solvent-free conditions. Reaction conditions: substrate (1 mmol each),acetic anhydride (3 mmol),MWCNTs-C-PO3H2 (0.1 mol %),5 min at r.t.|
To show the advantage of MWCNTs-C-PO3H2 over some of the reported catalysts in the literature,we compared the reaction of 3- nitrobenzaldehyde with acetic anhydride,in the presence of MWCNTs-C-PO3H2 and different catalysts reported in the literature (Table 3). As evident from the results,the required ratio for the some catalysts,used for this purpose,is a large amount and also the required reaction times are much longer in comparison with MWCNTs-C-PO3H2,and also excess amounts of Ac2O are required. We also observed,some of these catalysts resulted in shorter reaction time in comparison with MWCNTs-C-PO3H2 (Table 3, entries 2 and 5).
For checking the reusability of the catalyst,we performed the reaction in the following way,the reaction of p-nitrobenzaldehyde (1 mmol) with acetic anhydride (3 mmol) in the presence of MWCNTs-C-PO3H2 (0.1 mol%) was chosen in a 10 mmol scale. After completion of the reaction,hot CH2Cl2 (20 mL) was added and the catalyst was recovered by centrifuging,dried under air and then reused for the next cycle. The recovered catalyst was reused for five runs in the preparation of acylals from aldehyde and acetic anhydride. The results are summarized in Table 4.
In conclusion,we report a mild and efficient method for the preparation of 1,1-diacetates from aldehydes in the presence of acetic anhydride under solvent-free conditions at room temperature using MWCNTs-C-PO3H2. This method is selective for the preparation of 1,1-diacetates from aldehydes in the presence of ketones. Good yields were obtained within short reaction times; reusability of the catalyst and mild conditions are some of the notable features of this protocol.
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