an efficient method for synthesis of acylals from aldehydes using multi-walled carbon nanotubes...

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Chinese Chemical Letters xxx (2014) xxx–xxx

G Model

CCLET 3067 1–5

Original article

An efficient method for synthesis of acylals from aldehydes usingmulti-walled carbon nanotubes functionalized with phosphonic acid(MWCNTs-C-PO3H2)

Farzaneh Dehghani a, Ali Reza Sardarian a,*, Mohammad Mehdi Doroodmand a,b

a Chemistry Department, College of Sciences, Shiraz University, Shiraz 71454, Iranb Nanotechnology Research Center, Shiraz University, Shiraz 71545, Iran

A R T I C L E I N F O

Article history:

Received 11 February 2014

Received in revised form 16 June 2014

Accepted 17 June 2014

Available online xxx

Keywords:

Acylal

MWCNTs-C-PO3H2

Heterogeneous catalyst

Reusable catalyst

Solvent-free condition

A B S T R A C T

MWCNTs-C-PO3H2 has been used as an efficient, heterogeneous and reusable nanocatalyst for synthesis

of acylals from aldehydes under solvent-free conditions at room temperature. A wide range of aldehydes

was studied and corresponding products were obtained in good to excellent yields in short reaction

times. Nanocatalyst can be easily recovered by centrifuge and reused for subsequent reactions for at least

five times without deterioration in catalytic activity. The major advantages of the present method are

high yields, short reaction time, recyclable catalyst and solvent-free reaction conditions at room

temperature.

� 2014 Ali Reza Sardarian. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights

reserved.

Contents lists available at ScienceDirect

Chinese Chemical Letters

jo u rn al h om epag e: ww w.els evier .c o m/lo cat e/cc le t

28293031323334353637383940414243444546

1. Introduction

Protection and deprotection of carbonyl groups are extremelyimportant steps in modern organic chemistry [1]. Acylal formationis one of the most useful methods to protect the carbonyl group ofaldehydes and ketones and has found wide application inmultistep syntheses due to acylal stability in neutral and basicmedia [2]. Also, the diacetates of a,b-unsaturated aldehydes areimportant starting materials for Diels–Alder reactions, sinceacylals have also been used as cross-linking agents for cellulosein cotton [3] and are useful intermediates in industries [4].

The conventional method for the preparation of 1,1-diacetatesinvolves the reaction of aldehydes with acetic anhydride usingLewis and Brønsted acids, such as FeCl3 [5], Cu(OTf)2 [6], Bi(OTf)3

[7], FeSO4 [8], N-bromosuccinimide [9], montmorillonite clay [10],zeolites [11], H6P2W18O62�24H2O [12], methanesulfonic acids [13],silica sulfuric acid (SSA) [14], P2O5/Al2O3 [15], silica-bondedS-sulfonic acid (SBSSA) [16], BEA-SO3H (zeolite beta (BEA)) [17],Fe3O4@SiO2/Schiff base complex of Cr (III) [18], silica-bonded

474849505152

* Corresponding author.

E-mail addresses: [email protected] (F. Dehghani),

[email protected] (A.R. Sardarian).

Please cite this article in press as: F. Dehghani, et al., An efficient mecarbon nanotubes functionalized with phosphonic acid (MWCNTsj.cclet.2014.07.005

http://dx.doi.org/10.1016/j.cclet.2014.07.005

1001-8417/� 2014 Ali Reza Sardarian. Published by Elsevier B.V. on behalf of Chinese

propyl-diethylene-triamine-N-sulfamic acid (SPDTSA) [19] andsulfonated rice husk ash (RHA-SO3H) [20].

The development of heterogeneous catalysts for organicsynthesis has become a major area of research. The potentialadvantages of these materials over homogeneous systems(simplified recovery, reusability and the potential for incorpo-ration in continuous reactors and microreactors) could lead tonovel, environmentally benign chemical procedures for academiaand industry [21].

Application of solid acids in organic transformations isimportant because they have many advantages including ease ofhandling, decreased reactor and plant corrosion problems andmore environmentally safe disposal [22–27].

Catalysis is currently recognized as a potential field ofapplication for carbon nanotubes (CNTs), and throughout the pastdecade, the number of publications on this subject has beenincreasing exponentially [28].

As a part of our program aiming at developing efficient andenvironmentally friendly heterogeneous catalysts for organicsynthesis, we have developed multi-walled carbon nanotubes(MWCNTs), functionalized with phosphonic acid (MWCNTs-C-PO3H2) as an efficient, heterogeneous and reusable nanocatalystfor acylation of phenolic hydroxyl groups, different classes ofalcohols, aromatic amines and thiols with acetic anhydrides undersolvent-free conditions [29]. In this paper, we report a facile and

thod for synthesis of acylals from aldehydes using multi-walled-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/

Chemical Society. All rights reserved.


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53 ef54 co55 ef

56 2.

57

58 da59 pu60 de61 sp62 1H63 de64 st65

66 m67 (S68 (A69 an

70 2.

71

72 �73 (074 (275 to76 in77 1378 w79 pu80 pr81 (U82

83 m84 w85 m86 re87 TL88 an89 1090 CH

919293949596979899100101102103104105

106

107108109110111112113114115116117118119120121122123124125126127128129130131132133134135136137138139

Scheme 1. Preparation of acylals under solvent-free condition at room temperature

using MWCNTs-C-PO3H2.

F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx2

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CCLET 3067 1–5

ficient method for the preparation of acylals under solvent-freenditions at room temperature using MWCNTs-C-PO3H2 as anficient, heterogeneous and reusable nanocatalyst (Scheme 1).

Experimental

All products were known and their physical and spectroscopicta were compared to those of authentic samples. Chemicals wererchased from Fluka or Merck. The purity of the products wastermined by TLC on silica gel polygram SIL G/UV 254 plates. NMRectra were recorded on a Bruker Avance DPX-250 (250 MHz for

NMR and 62.9 MHz for 13C NMR) spectrometer in pureuterated solvents with tetramethylsilane (TMS) as an internal

andard.The synthesized CNTs were characterized using some electron

icroscopic techniques such as scanning electron microscopyEM, XL-30 FEG SEM, Philips, 20 KV), atomic force microscopyFM, DME-SPM, version 2.0.0.9), and also thermogravimetricalysis.

1. Preparation of MWCNTs-C-PO3H2

According to Ref. [28], acetylene gas with a flow rate of50 mL min�1 was bubbled into a solution containing ferrocene.30 g), triphenylphosphine (3.0 g), thiophene (0.7 mL) in benzene5 mL). This was mixed with hydrogen and argon with flow rates

0.5 and 800 mL min�1, respectively, followed by introductionto a quartz tube passed through a 80 cm tubing furnace set at00 8C. The produced phosphor-doped carbon nanostructures

ere then directly purified from any amorphous carbon viarging oxygen and aerosols of hydrogen peroxide into theoduction line, followed by on-line activation using ultravioletV) and microwave irradiators.General procedure for preparation of 1,1-diacetates: to a

ixture of aldehyde (1 mmol) and acetic anhydride (3 mmol)as added MWCNTs-C-PO3H2 (0.1 mol%, 0.026 g). The reactionixture was stirred at room temperature for the appropriateaction time. After completion of the reaction, as indicated byC, the reaction mixture was diluted with hot CH2Cl2 (5 mL 2�)d the resultant mixture was stirred at room temperature for min. Then, the catalyst was separated by centrifuge and

2Cl2 was evaporated under reduced pressure. The crude

Fig. 1. Schematic of CVD-synthesized MWCNTs-C-PO3H2 catalyst (A),

Please cite this article in press as: F. Dehghani, et al., An efficient mcarbon nanotubes functionalized with phosphonic acid (MWCNTj.cclet.2014.07.005

product was purified by column chromatography on silica gelwith petroleum ether and ethyl acetate (80:20) to give the pureproduct.

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 addedMWCNTs-C-PO3H2 (0.1 mol%, 0.026 g). The reaction mixture wasstirred at room temperature for the appropriate reaction time.After completion of the reaction, as indicated by TLC, the reactionmixture was diluted with hot CH2Cl2 (5 mL 2�) and the resultantmixture was stirred at room temperature for 10 min. Then, thecatalyst was separated by centrifuge and CH2Cl2 was evaporatedunder reduced pressure. The crude product was concentrated andrecrystallized from CH2Cl2 to give the pure product in 92% yield asa light yellow solid, mp 124 8C.

3. Results and discussion

The CVD techniques were used for the synthesis of MWCNTs-C-PO3H2 (Fig. 1A). For this purpose, acetylene with 99% purity andferrocene were used as CNT precursor and catalyst. Ironnanoparticles were librated in situ from ferrocene and catalyzedMWCNTs formation in the presence of thiophene as sulfurprecursor for increasing the length of CNT.

Then phosphorous atom was doped on carbon nanostructuresby the reaction of MWCNTs with triphenylphosphineoxide at1300 8C under the atmosphere of hydrogen and argon in a quartztube. Then hydrogen peroxide oxidized the produced phosphor-doped carbon nanostructures to obtain MWCNs-C-PO3H2 and alsosimultaneously purified it from any amorphous carbon asgraphically shown in Fig. 1B. The MWCNTs-C-PO3H2 catalyst hasbeen fully characterized using some different microscopic andspectroscopic techniques [29].

Due to stability and mild acidity of MWCNTs-C-PO3H2, wedecided to investigate its capability for the preparation of acylals.Therefore we needed to determine the best reaction conditions forthis transformation.

For this purpose, the reaction of p-nitrobenzaldehyde (1 mmol)with acetic anhydride (3 mmol) in the presence of MWCNTs-C-PO3H2 was chosen as a model reaction. The model reaction wasinvestigated in various solvents such as H2O, EtOH, CH2Cl2, CHCl3,CH3CN and also under solvent-free condition at room temperaturein the presence of various amount of catalyst. The results aresummarized in Table 1.

As it is shown in Table 1, the yield of the reaction under solvent-free conditions in the presence of 0.1 mol% catalyst was the highestand the reaction time was the shortest. Aprotic solvents, such asCHCl3, CH2Cl2 and CH3CN, afforded the desired product in loweryields and longer reaction times (Table 1, entries 1–3). In proticsolvents, such as water and ethanol (Table 1, entries 4–5), thisprotection reaction proceeded with longer reaction times and very

schematic representing the CVD-synthesis of MWCNT-C-PO3H2 (B).

ethod for synthesis of acylals from aldehydes using multi-walleds-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/



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Table 1Conversion of p-nitrobenzaldehyde to the corresponding acylal in the presence of

MWCNTs-C-PO3H2 in different conditions.a

O

H

O2N

Ac2OReaction conditions

CH(OAc)2

O2N

Entry Solvent/catalyst (mol%) Time (min) Yields (%)b

1 CH2Cl2/0.1 20 87

2 CH3CN/0.1 20 75

3 CHCl3/0.1 20 75

4 C2H5OH/0.1 60 25

5 H2O/0.1 60 10

7 Solvent-free/0.1 5 92



a The reaction was carried out with 3 mmol of Ac2O at r.t.b Isolated yield.

Table 2Preparation of various acylals in theQ3 presence of MWCNTs-C-PO3H2 under solvent-free

Entry Aldehyde Product Tim

1 O

H

CH(OAc)2

2 O

H

NO2

CH(OAc)2

NO2

3 O

H

O2N

CH(OAc)2

O2N

4 O

H

Br

CH(OAc)2

Br

5 O

H

Cl

CH(OAc)2

Cl

1

6 O

H

Cl

CH(OAc)2

Cl

7 O

H

H3CO

CH(OAc)2

H3CO

8 O

H

OCH3

CH(OAc)2

OCH3

1

9cO

H

HO

CH(OAc)2

AcO

9

10 O

H

H3C

CH(OAc)2

H3C

F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx 3

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CCLET 3067 1–5


poor yields, which may be related to the instability of aceticanhydride and corresponding product in protic solvents.

Therefore, we employed the optimized conditions for theconversion of various aldehydes into the corresponding acylals.The results of the preparation of acylals from aromatic aldehydesin the presence of MWCNTs-C-PO3H2 at room temperature areshown in Table 2.

Aldehydes with electron-donating or electron-withdrawinggroups were converted into the corresponding acylals in highyields after short reaction times. The acid-sensitive substrate,thiophene-2-carbaldehyde, gave the expected acylal in 80% yieldwithout any by-product formation (Table 2, entry 11). We alsoinvestigated the reactions of 4-hydroxybenzaldehyde under theabove-mentioned conditions and observed after 90 min, both thecarbonyl and phenolic groups were acylated (Table 2, entries 9).Several aliphatic and aromatic ketones, including cyclohexanoneand acetophenone, were not reactive under the describedexperimental conditions even after 2 h (Table 2, entries 14 and 15).

conditions at room temperature.a

e (min) Yields (%)b Mp (8C) found (reported) [reference]

5 90 44–46 (45–46) [16]

5 92 64 (64–65) [16]

5 92 124 (124) [30]

5 90 90–91 (90–93) [16]

0 87 60 (58–60) [16]

5 91 82–84 (81–83) [16]

5 91 64–65 (65) [31]

0 85 75–76 (76) [31]

0 70 93–95 (94–96) [16]

5 90 80–81 (79–81) [16]




158

159 ar160 C-161 al162 of163 cy164

165 re166 3-167 M168 tu169 so170 re

171172173174175176177178179180181182183

Table 2 (Continued )

Entry Aldehyde Product Time (min) Yields (%)b Mp (8C) found (reported) [reference]

11 S

H

O S

CH(OAc)2

20 80 66–68 (67–68) [16]

12

H

O CH(OAc)2 5 87 101–103 (101) [31]

13 O

H

CH(OAc)2 5 88 83–85 (84) [31]

14 O 120

15 O 120

a Reaction conditions: aldehyde (1 mmol), acetic anhydride (3 mmol), catalyst (0.1 mol %), at r.t.b Isolated yield.c The reaction was carried out with 4 mmol of Ac2O at r.t.

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.

TaCo

E

1

2

3

4

5

6

7

F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx4

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CCLET 3067 1–5

Next, we studied the competitive acylation reactions ofomatic aldehydes in the presence of ketones using MWCNTs-PO3H2. Under these conditions exclusive acylation of thedehyde functions was observed. The chemoselective acylations

p-chlorobenzaldehyde in the presence of acetophenone andclohexanone are shown in Scheme 2.To show the advantage of MWCNTs-C-PO3H2 over some of the

ported catalysts in the literature, we compared the reaction ofnitrobenzaldehyde with acetic anhydride, in the presence ofWCNTs-C-PO3H2 and different catalysts reported in the litera-re (Table 3). As evident from the results, the required ratio for theme catalysts, used for this purpose, is a large amount and also thequired reaction times are much longer in comparison with

ble 3mparison between different acid catalysts with MWCNTs-C-PO3H2 in the reaction

ntry Catalyst (g) Molar ratio

Fe3O4@SiO2/Schiff base complex of Cr (III) (0.035) 1:3

BEA-SO3H (0.015) 1:15

Bi(OTf)3 (0.031) 1:3

P2O5/Al2O3 (0.05) 1:1

SBSSA (0.02) 1:15

H6P2W18O62�24H2O (0.1) 1:2

MWCNTs-C-PO3H2 (0.026) 1:3

Please cite this article in press as: F. Dehghani, et al., An efficient mcarbon nanotubes functionalized with phosphonic acid (MWCNTj.cclet.2014.07.005

MWCNTs-C-PO3H2, and also excess amounts of Ac2O are required.We also observed, some of these catalysts resulted in shorterreaction time in comparison with MWCNTs-C-PO3H2 (Table 3,entries 2 and 5).

For checking the reusability of the catalyst, we performed thereaction in the following way, the reaction of p-nitrobenzaldehyde(1 mmol) with acetic anhydride (3 mmol) in the presence ofMWCNTs-C-PO3H2 (0.1 mol%) was chosen in a 10 mmol scale. Aftercompletion of the reaction, hot CH2Cl2 (20 mL) was added and thecatalyst was recovered by centrifuging, dried under air and thenreused for the next cycle. The recovered catalyst was reused for fiveruns in the preparation of acylals from aldehyde and aceticanhydride. The results are summarized in Table 4.

of 3-nitrobenzaldehyde with acetic anhydride.

(aldehyde:aceticanhydride) Time (min) Yield (%) Ref.

20 92 [18]

4 85 [17]

270 88 [7]

45 90 [15]

2 87 [16]

30 93 [12]

5 92 This work

ethod for synthesis of acylals from aldehydes using multi-walleds-C-PO3H2), Chin. Chem. Lett. (2014), http://dx.doi.org/10.1016/



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Table 4Reusability of MWCNTs-C-PO3H2 in the reaction of p-nitrobenzaldehyde and acetic

anhydride.a

Runs Fresh 1 2 3 4 5

Yieldsb (%) 92 92 91 91 91 91

a Reaction conditions: MWCNTs-C-PO3H2 (0.1 mol %), p- nitrobenzaldehyde

(1 mmol), acetic anhydride (3 mmol), 5 min at r.t.b Isolated yields.

F. Dehghani et al. / Chinese Chemical Letters xxx (2014) xxx–xxx 5

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4. Conclusion

In conclusion, we report a mild and efficient method for thepreparation of 1,1-diacetates from aldehydes in the presence ofacetic anhydride under solvent-free conditions at room tempera-ture using MWCNTs-C-PO3H2. This method is selective for thepreparation of 1,1-diacetates from aldehydes in the presence ofketones. Good yields were obtained within short reaction times;reusability of the catalyst and mild conditions are some of thenotable features of this protocol.

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