H5CoW12O40@nanosilica: A green, bifunctional, heterogeneous nanocatalyst for one-pot three-component Mannich reaction in water

Article abstract of DOI:10.1007/s11164-013-1392-9

Abstract 12-Tungestocobaltic acid, H₅CoW₁₂O₄₀, was immobilized on nanosilica from rice husk ash to produce H₅CoW₁₂O₄₀@nanosilica as a bifunctional nanocatalyst. The TEM result showed that the particles are mostly spherical in shape, and that they have an average size of approximately 10 nm. The catalytic activity of this catalyst was investigated in Mannich reaction of various aldehydes, ketones, and amines in water. This rapid procedure afforded structurally diverse β-amino ketones in high to excellent yields. Short reaction time, simple work-up procedure, elimination of toxic solvents, and low catalyst loading are the advantages of this procedure.

Full text of DOI:10.1007/s11164-013-1392-9

Res Chem Intermed
DOI 10.1007/s11164-013-1392-9
H5CoW12O40@nanosilica: a green, bifunctional,
heterogeneous nanocatalyst for one-pot three-
component Mannich reaction in water
•
Ezzat Rafiee Masoud Kahrizi
Received: 1 September 2012 / Accepted: 4 September 2013
Ó Springer Science+Business Media Dordrecht 2013
Abstract 12-Tungestocobaltic acid, H5CoW12O40, was immobilized on nanosilica
from rice husk ash to produce H5CoW12O40@nanosilica as a bifunctional nano-
catalyst. The TEM result showed that the particles are mostly spherical in shape,
and that they have an average size of approximately 10 nm. The catalytic activity of
this catalyst was investigated in Mannich reaction of various aldehydes, ketones,
and amines in water. This rapid procedure afforded structurally diverse b-amino
ketones in high to excellent yields. Short reaction time, simple work-up procedure,
elimination of toxic solvents, and low catalyst loading are the advantages of this
procedure.
Keywords Rice husk ash Á 12-Tungestocobaltic acid Á Nanocatalyst Á
Mannich reaction Á b-Amino carbonyl compounds
Introduction
During the past decade, interest in the development of nanocatalysis has increased
in both homogeneous and heterogeneous catalysis. Since nanoparticles have a large
surface-to-volume ratio compared to bulk materials, they are attractive candidates
for use as catalysts. In recent years, heteropoly acids (HPAs) as important
nanocatalysts have received much attention in academic and industrial applications
because of their unique properties, which offer several advantages in terms of
catalytic performance, strong acid and redox sites ,and selectivity to particular
reaction products [1]. In spite of the merits of HPAs, the main disadvantages of
these nanocatalysts are low thermal stability, low surface area (1–10 m2/g), and
separation from reaction mixtures. Heterogenization of bulk HPAs on nanosupports
E. Rafiee (&) Á M. Kahrizi
Faculty of Science, Department of Chemistry, Razi University, 67149 Kermanshah, Iran
e-mail: ezzat_rafiee@yahoo.com
123

E. Rafiee, M. Kahrizi
is an interesting area of research from an industrial point of view. To achieve a
higher surface area and heterogeneous catalytic activity, HPA catalysts are often
supported on high surface area materials like nanosilica. In recent years, silica-
supported reagents are gaining considerable attention because of higher activity of
the catalyst due to the larger surface area and better selectivity. In addition, silica-
supported reagents have high mechanical and thermal stabilities, easiness in
handling, low toxicity, easy separation of the catalyst after completing the reaction,
and reusability of the catalyst. At present, nanoscale silica materials are prepared
using several methods, including vapor-phase reaction, sol–gel, and thermal
decomposition techniques [2–5]. However, the high cost of preparation has limited
their wide applications especially when tetraethyl orthosilicate was used as
precursor. The production of reactive nano-scale silica from rice husk (RH) is a
simple process compared to other conventional production techniques [6]. RH is a
by-product from rice mills that has been used as an energy source in many
industries, such as biomass power plants and rice mills. Burning rice husk generates
rice husk ash (RHA), which is rich in silica and could be an economically valuable
raw material for production of natural silica [7]. The small particle diameter and
high surface area of the ultra-fine silica powders give rise to many technological
applications [2, 8], especially as support to heterogenization of the catalyst [9].
12-Tungestocobaltic acid, H5CoW12O40 (CoW), is a bifunctional catalyst. This
acts as electron transfer and Brønsted acid catalyst. Immobilization of CoW on
silica nano-particles with high surface area improved the catalytic performance of
CoW in various organic reactions. This catalyst exhibited different characteristic
from the bulk CoW and pure nanosilica. These properties make it as an excellent
and versatile catalyst for various types of organic reactions like multicomponent
reactions (MCRs).
MCRs are very important and attractive subjects in organic synthesis due to the
formation of carbon–carbon or carbon–hetero atom bonds in one step [10, 11]. This
type of reaction allows the creation of several bonds in a single operation, and they are
attractive as one of the most powerful emerging synthetic tools for the creation of
molecular diversity and complexity. They also have considerable advantages in terms
of environmental sociability. b-Amino carbonyl compounds are very useful
compounds as building blocks in the synthesis of pharmaceuticals and natural
products [12] that are formed via the Mannich reaction [13–15]. The Mannich reaction
is one of the most important carbon–carbon bond-forming reactions in organic
synthesis for the preparation of secondary and tertiary amine derivatives [16]. Various
catalyst systems have been used in three-component Mannich reactions including
Lewis acids [17, 18], Brønsted acids [19], HPAs [20, 21], and organic catalysts [22].
Experimental
General
The reagents and solvents used in this work were obtained from Fluka, Aldrich, or
Merck, and were used without further purification. TEM analyses were performed
123

H5CoW12O40@nanosilica
1
using a TEM microscope (Philips CM 120 kV, The Netherlands). H NMR spectra
were recorded on a Bruker Avance 200 MHz NMR spectrometer with CDCl3 as
solvent and TMS as internal standard. Mass spectra were determined on an Agilent
6890 N GC–MS.
Preparation of catalyst
Nanosilica was prepared from RH according to our previously published paper [23].
The synthesis of CoW were started with the preparation K5CoW12O40, according to
the procedure described previously [24]. For the preparation of the
H5CoW12O40@nanosilica (CoW@SiO2) catalyst, 3.0 g of nanosilica was dispersed
in 30 mL water by sonication. The solution of CoW (2 g in 10 mL water) was added
and stirred overnight at room temperature. Finally, the solvent was removed using a
rotary evaporator, dried, and calcinated at 150 °C for 2 h.
General procedure for the synthesis of b-amino ketones
To a mixture of aldehyde (1.0 mmol), amine (1.0 mmol), and ketone (2.0 mmol), in
H2O (5.0 mL), 0.04 g of catalyst was added. The reaction mixture was vigorously
stirred. The progress of the reaction was monitored by TLC. The resulting solid
substance was filtered off and diethyl ether was added to dissolve the target product.
After filtration, the filtrate was concentrated to generate the crude product. The
crude products were purified by column chromatography on silica-gel using ethyl
acetate/hexane as eluent. Known products were identified by comparing their
spectral data with those of the authentic samples [21, 25, 26], and the
characterization data for new compounds are presented below:
1
See Table 2, entry 8 (below): H NMR (200 MHz; CDCl3, TMS): d 7.89–7.32
(5H, m, aromatic), 7.23–7.06 (5H, m, aromatic), 6.85–6.43 (4H, m, aromatic), 4.65
(1H, m, CH), 4.13 (1H, brs, NH), 3.38 (2H, m, C(O)CH2), 2.39 (2H, m, CH2), 1.22
(3H, t, J 6.8 Hz, CH3); MS m/z 329.43.
1
See Table 2, entry 9 (below): H NMR (200 MHz; CDCl3, TMS): d. 7.92–7.38
(5H, m, aromatic), 7.26–7.07 (5H, m, aromatic), 6.73–6.38 (4H, m, aromatic), 4.62
(1H, m, CH), 4.38 (2H, s, NH2), 4.07 (1H, brs, NH), 3.38 (2H, m, C(O)CH2); MS m/
z 316.16.
Results and discussion
In the present work, we immobilized CoW onto the nanosilica to synthesize
CoW@SiO2 as a highly efficient nanocatalyst. The catalytic activity was investigated
in the synthesis of b-amino ketones in aqueous media at room temperature.
First, three components reaction of benzaldehyde (1 mmol), aniline (1 mmol)
and cyclohexanone (1 mmol) as model substrates was performed in different
conditions (Table 1). Without addition of the catalyst, the reaction proceeds via
imine formation through the condensation of benzaldehyde and aniline, and no
123

E. Rafiee, M. Kahrizi
Table 1 Mannich-type reaction of benzaldehyde (1 mmol), aniline (1 mmol), and cyclohexanone
(1 mmol) as model substrates in the presence of various catalysts
Ph
O
CHO
NH
O
NH2
Catalyst
Ph
+
+
H2O (5 mL), R.T
Entry
Catalyst
–
Amount of the catalyst (g)
Time (min)
Yield (%)a
1
2
3
4
5
6
7
a
–
80
-(50)b
38 (49)b
(70) 94
95
K5CoW12O40
CoW/SiO2
0.04
0.04
0.04
0.02
0.06
0.08
80
(20) 30
20
CoW@SiO2
CoW@SiO2
CoW@SiO2
CoW@SiO2
(20) 45
(20) 25
(20) 30
(20) 61
(88) 95
(42) 79
Isolated yield
b
Results in the parenthesis refer to imine products
products from the Mannich-type reaction were observed (Table 1, entry 1). This
showed that a catalyst is essential in this reaction. K5CoW12O40 is the most
important salt of CoW that is used as an electron transfer catalyst in some organic
transformations [27, 28]. The poor catalytic performance of K5CoW12O40 in the
Mannich reaction indicated that this conversion is not proceeding via an electron
transfer mechanism (Table 1, entry 2). Silica- and nanosilica-supported CoW
accelerated the reaction, but CoW@SiO2 gave superior results in term of yield and
reaction time (Table 1, entry 4).
The structural features of CoW@SiO2 catalyst was investigated using TEM
techniques (Fig. 1). TEM observations indicated that CoW@SiO2 have a uniformity
and spherical morphology. The average particle size of catalyst was about 10 nm.
To study the effect of catalyst loading, the model reaction was performed in the
presence of different quantities of CoW@SiO2 catalyst (Table 1, entries 4–7). The
yield of product using 0.08 g of CoW@SiO2 was lower and the reaction time was
longer than when using 0.06 and 0.04 g. The use of larger amounts of catalyst had
no effect on the reaction rate. The use of 0.04 g of CoW@SiO2 is optimal to ensure
a high level of efficiency (Table 1, entry 4).
In order to prove that the reaction did not proceed through the electron transfer
mechanism, a radical scavenger (acrylonitrile) was added to the mixture of
benzaldehyde, aniline, and cyclohexanone in the presence of CoW@SiO2 (0.04 g)
after 8 min (when 50 % of the corresponding product was obtained). The addition
of a radical scavenger had no significant effect on the reaction progress (Fig. 2).
Therefore, it seems that the Mannich reaction proceeded via Brønsted acid
mechanism. More investigations are in progress in our laboratory.
As shown in Table 2, the good yields of b-amino ketones were obtained from a
variety of substituted aldehydes, amines, and ketones. When p-nitro-benzaldehyde
was used, the yield of reaction markedly decreased (Table 2, entry 4). This might be
123

H5CoW12O40@nanosilica
Fig. 1 TEM image of CoW@SiO2 catalyst
Fig. 2 Progress of the model reaction. Radical scavenger was added after 8 min
attributed to the electronic effects. The results showed that the electronic effect had
a significant effect on the reaction efficacy.
It should be noted that, under conventional high speed stirring conditions, the
aromatic aldehydes with electron-donating groups generally gave higher yield
(entries 3, 5, 6, 11). Moreover, substitution of cyclohexanone with acetophenone in
this reaction had no effect on reaction time and efficiency (Table 2, entries 3 and
11). To our surprise, n-butylamin afforded the corresponding product in good yield
(Table 2, entry 7), as previous approaches gave negative results, or very poor yields,
for similar reactions [25, 26].
A possible mechanism of the Mannich-type reaction in the presence of
CoW@SiO2 as Brønsted acid catalyst is proposed (Scheme 1). First, the acid
catalyst converts the aldehyde into a convenient electrophile via protonation of
123

E. Rafiee, M. Kahrizi
Table 2 Synthesis of b-amino ketones in the presence of CoW@SiO2
R2
R1
O
NH
R3
O
CoW@SiO2 (0.04 g)
R3
R1CHO
R2NH2
+
R4
+
R4
H2O, R.T
Entry R1
R2
R3
R4 Time (min) Yield (%)a Anti/syn ratiob
1
2
3
Ph
Ph
Ph
–(CH2)4–
–(CH2)4–
–(CH2)4–
20
30
30
95
90
95
64/36
65/35
62/38
4-Cl-C6H4
4-Cl-C6H4
4-MeO-
C6H4
4
5
4-NO2-C6H4
4-Cl-C6H4
Ph
–(CH2)4–
H
70
60
95
64/36
–
4-MeO-
C6H4
Ph 30
6
7
8
9
4-Me-C6H4
Ph
H
H
H
H
Ph 50
Ph 30
Ph 40
Ph 25
80
80
90
80
–
–
–
–
Ph
Ph
Ph
n-C4H9
2-Et-C6H4
2-NH2-
C6H4
10
11
4-Cl-C6H4
4-Cl-C6H4
4-Cl-C6H4
H
H
Ph 55
Ph 30
90
95
–
–
4-MeO-
C6H4
Reaction conditions : aldehyde (1 mmol), amine (1 mmol), ketone (2 mmol), solvent (H2O)
a
Isolated yield
Anti/syn-ratio measured by 1H NMR spectroscopy analysis of the crude reaction mixture
b
R2
H
O
C
OH
C
R2
N
N
H+(CoW@SiO2)
R1
H
H
R1
H
H
R1
O
OH
H+(CoW@SiO2)
R3
R3
R4
R2
R4
R2
R1
NH
O
NH
OH
-H+
R1
R4
R4
R3
R3
Scheme 1 Suggested mechanism for the synthesis of b-amino ketones in the presence of CoW@SiO2
carbonyl group, and then amine condenses with aldehyde to produce imine.
Protonation of the imine with the catalyst, and the attack of the enol derived from
the ketone to the protonated imine, leads to the formation of the Mannich product.
123

H5CoW12O40@nanosilica
In conclusion, CoW@SiO2 as a heterogeneous catalyst showed effective catalytic
activity in the synthesis of b-amino ketones via three-component Mannich reaction
in water. The good yields, mild conditions, low catalyst loading, and convenient
operations as well as no formation of by-products, such as aldol or di-aminated
products, made it believable that this protocol would play important roles in the
synthesis of natural products.
Acknowledgment The authors thank the Razi University Research Council for support of this work.
References
1. N. Mizuno, M. Misono, Chem. Rev. 199, 98 (1998)
2. L. Sun, K. Gong, Ind. Eng. Chem. Res. 5861, 40 (2001)
3. M. Tomozawa, D.L. Kim, V. Lou, J. Non-Cryst. Solids 102, 296 (2001)
4. P.A. Tanner, B. Yan, H. Zhang, J. Mater. Sci. 4325, 35 (2000)
5. G. Wu, J. Wang, J. Shen, T. Yang, Q. Zhang, B. Zhou, Z. Deng, F. Bin, D. Zhou, F. Zhang, J. Non-
Cryst. Solids 169, 275 (2000)
6. E. Rafiee, M. Khodayari, Sh. Shahebrahimi, M. Joshaghani, J. Mol. Catal. A. 204, 351 (2011)
7. A. Protor, J. Shultz, U. Kalapathy, Bioresour. Technol. 257, 73 (2000)
8. H.O. House, Modern Synthetic Research, 2nd edn. (Benjamin, Menlopark, 1972), p. 492
9. F.W. Chang, T.J. Hsiao, J.D. Shih, Ind. Eng. Chem. Res. 3838, 37 (1998)
10. W. Wei, W. Jian, L. Hao, Tetrahedron Lett. 7243, 45 (2004)
11. Y.Z. Hong, L. Hua, Sh. Huawu, Ultrason. Sonochem. 758, 16 (2009)
12. R. Muller, H. Goesmann, H. Waldmann, Angew. Chem. Int. Ed. 184, 38 (1999)
13. H. Ishitani, M. Ueno, S. Kobayashi, J. Am. Chem. Soc. 7153, 119 (1997)
14. A.G. Wenzel, E.N. Jacobsen, J. Am. Chem. Soc. 12964, 124 (2002)
15. N.S. Josephsohn, E.L. Carswell, M.L. Snapper, A.H. Hoveyda, Org. Lett. 2711, 7 (2005)
16. S. Kobayashi, H. Ishitani, Chem. Rev. 1069, 99 (1999)
17. C. Rondot, J. Zhu, Org. Lett. 1641, 7 (2005)
18. G. Pandey, R.P. Singh, A. Garg, V.K. Singh, Tetrahedron Lett. 2137, 46 (2005)
19. K. Manabe, Y. Mori, S. Kobayashi, Tetrahedron 2537, 57 (2001)
20. E. Rafiee, S. Eavani, Green Chem. 2116, 13 (2011)
21. E. Rafiee, S. Eavani, F. Khajooei Nejad, M. Joshaghani, Tetrahedron 6858, 66 (2010)
22. B.M. Trost, J. Jaratjaroonphong, V. Reutrakul, J. Am. Chem. Soc. 2778, 128 (2006)
23. E. Rafiee, Sh. Shahebrahimi, Chin. J. Catal. 1326, 33 (2012)
24. E. Rafiee, M. Khodayari, M. Kahrizi, R. Tayebee, J. Mol. Catal. A 121, 385 (2012)
25. N. Azizi, L. Torkiyan, M.R. Saidi, Org. Lett. 2079, 8 (2006)
26. W.B. Yi, C. Cai, J. Fluorine Chem. 1515, 127 (2006)
27. E. Rafiee, A. Azad, Bioorg. Med. Chem. Lett. 2756, 17 (2007)
28. E. Rafiee, A. Azad, Synth. Commun. 1127, 37 (2007)
123