Different products in the reaction of the alcohols with cyclic and acyclic 1,3-dicarbonyl compounds: K5CoW12O40 as an electron transfer nano catalyst

Article abstract of DOI:10.1016/j.cclet.2012.10.011

K₅CoW₁₂O₄₀ was used as a highly effective catalyst for the benzylation of 1,3-dicarbonyl compounds. β-Keto enol ethers were obtained when cyclic 1,3-dicarbonyl compounds used in this conditions instead of linear ones. The present methodology offers a practical, simple, mild, environmentally friendly, and time-saving method for etherification. Very low loading of catalyst, ease of workup, ease of handling, and reusability of catalyst are other advantages of this catalyst.

Full text of DOI:10.1016/j.cclet.2012.10.011

Available online at www.sciencedirect.com
Chinese Chemical Letters 23 (2012) 1363–1366
www.elsevier.com/locate/cclet
Different products in the reaction of the alcohols with cyclic and
acyclic 1,3-dicarbonyl compounds: K5CoW12O40 as an electron
transfer nano catalyst
*
Ezzat Rafiee , Masoud Kahrizi, Mohammad Joshaghani
Department of Chemistry, Razi University, Kermanshah 67149, Iran
Received 20 August 2012
Available online 20 November 2012
Abstract
K5CoW12O40 was used as a highly effective catalyst for the benzylation of 1,3-dicarbonyl compounds. b-Keto enol ethers were
obtained when cyclic 1,3-dicarbonyl compounds used in this conditions instead of linear ones. The present methodology offers a
practical, simple, mild, environmentally friendly, and time-saving method for etherification. Very low loading of catalyst, ease of
workup, ease of handling, and reusability of catalyst are other advantages of this catalyst.
# 2012 Ezzat Rafiee. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
Keywords: Benzylation; Etherification; b-Keto enol ethers; Nano catalyst; Polyoxometalate
Nano-sized catalysts continue to attract interest for different researcher areas due to their different physical and
chemical properties when compare to bulk material. The extremely small size of the particles maximized the surface
area exposed to the reactant, allowing more reactions to occur at the same time, thus speeding up the process. Catalysis
by polyoxometalates and related compounds is a field of increasing importance as nano catalysts [1–3]. They are
promising candidates for catalyst design due to their controllable acid and redox properties and have been actually
utilized in some industrial processes [4]. As an interesting example, 12-tungstocobaltate(III), [CoIIIW12O40]5À, having
E0 = 1.07 V (against a normal hydrogen electrode) has been used as an oxidant both for organic and inorganic
substances [5,6]. It is apparently a perfect outer-sphere oxidant because the central CoIII atom is protected by a sheath
of chemically inert oxygen atoms, which protect the central ion from undesired inner-sphere substitution reactions.
The self-exchange rate between [CoIIIW12O40]5À/6À couples found extensive use in the catalytic application [6]. This
catalyst showed excellent reactivity in our previous works [7,8] as a heterogeneous catalyst.
During the course of our studies directed towards the development of practical and environmentally friendly
procedures for some important transformations, we developed the applicability of a recyclable heterogeneous catalyst,
potassium dodecatungstocobaltate trihydrate, K5CoW12O40Á3H2O (KCoW), for the condensation reaction of alcohols
with 1,3-dicarbonyl compound. This reaction offers different products related to the cyclic or linear dicarbonyl
compounds. In the presence of linear 1,3-dicarbonyl compound construction of carbon–carbon bonds was happened
which is a fundamental task in organic synthesis [9,10]. On the other hand, condensation of cyclic 1,3-dicarbonyl
compound with alcohols produce b-keto enol ethers which have been widely used as key intermediates in organic
* Corresponding author.
E-mail addresses: ezzat_rafiee@yahoo.com, e.rafiei@razi.ac.ir (E. Rafiee).
1001-8417/$ – see front matter # 2012 Ezzat Rafiee. Published by Elsevier B.V. on behalf of Chinese Chemical Society. All rights reserved.
http://dx.doi.org/10.1016/j.cclet.2012.10.011

1364
E. Rafiee et al. / Chinese Chemical Letters 23 (2012) 1363–1366
Table 1
Optimization of the conditions.a.
O
O
O
O
KCoW
OH
H3C
OCH2CH3
OCH2CH3
}
Solvent-free, 80 oC
Ph
Entry
Catalyst (g)
Time (min)
Yield (%)b
1
2
3
4
5
6
–
60
15
15
15
12
60
5
10
54
95
96
29
KCoW (0.04)
KCoW (0.06)
KCoW (0.08)
KCoW (0.1)
KCoW (0.08)c
a
Reaction conditions: benzyl alcohol (1 mmol), ethylacetoacetate (1 mmol), 80 8C, solvent-free conditions.
Isolated yield.
b
c
Reaction proceed at room temperature.
synthesis [11,12]. As a starting point for optimization of the reaction conditions, the reaction of benzyl alcohol and
ethylacetoacetate was chosen as a model reaction (Table 1). The substrates did not react in the absence of the catalyst
(entry 1). To investigate the effect of the catalyst loading, the model reaction was carried out in the presence of
different amounts of the catalyst. The best result was obtained in the presence of 0.08 g of KCoW, and the use of higher
amounts of the catalyst, were slightly decreased the reaction time (entries 2–5). Only 29% yield of the corresponding
product was obtained in the presence of KCoW at room temperature even after 60 min (entry 6).
Encouraged by these results, we turned our attention to various cyclic and linear 1,3-dicarbonyl compounds and
benzylic alcohols (Table 2). Reaction of the linear 1,3-dicarbonyls such as acetylacetone with different types of alcohols
proceeded smoothly providing 89–96% yields (entries 1–13). Employing 5,5-dimethylcyclohexane-1,3-dione
(dimedone), as a cyclic 1,3-dicarbonyl using the same reaction conditions dose not produced corresponding 2-
benzylic-1,3-dicarbonylcompound and an unknown compound was generatedas main product (entry 15). This compound
wasisolated bycolumnchromatographyandcharacterizedbyFTIR, NMR, andCHNtechniques.Theresultsindicatedthat
the unknown compound is the corresponding b-keto enol ethers (5a). Due to the wide range applications of b-keto enol
ethers as synthons in several key intermediate compounds, various alcohols were treated with dimedone as a cyclic 1,3-
dicarbonyl compound. Towards these studies, effect of the amount of the catalyst and reaction temperature was
investigated (entries 16–20). Only 10%ofcorrespondingproductwasobtainedatroomtemperature(entry20). The present
conversion with primary alcohols proceeded rapidly to form the products in high to excellent yields. tert-Butyl alcohol as
substrate affordedtraceamountofcorrespondingproductafter1.5 h(entry25).Moreamountofthecatalyst(0.2 g)wasnot
improved the yield of this reaction. It should be mentioned that no acetal adducts were detected, even in the case of linear
1,3-diketones for which it has been reported [12]. This protocol was then applied to secondary, vinyl or substituted benzyl
alcohols and benzhydril which are structurally different. Reactions with these type alcohols obtained lower yield. In order
to prove catalytic activity of KCoW via electron-transfer mechanism, a radical scavenger (acrylonitrile) was added to the
model reaction after 10 min (when about 50% of the corresponding product was obtained). After continuation of the
reaction until 25 min, yield of the product was 52% and it seems that the reaction was stopped by addition of radical
scavenger. More investigation about the mechanism of the catalyst is under investigation in our laboratory.
To check that is this catalyst completely heterogeneous or not, the model reaction (Scheme 1) was carried out for
15 min, until yield of the product was 60%. Then, the reaction was stopped, catalyst was recovered according to the
procedure and separated by centrifugation and the filtrate was stirred again, no improvement was observed even after
1 h. This observation confirmed that the reaction was catalyzed heterogeneously. In addition, the content of W into
filtrates was evaluated quantitatively by inductively coupled plasma atomic emission spectroscopy (ICP-AES), which
showed just trace amount of the W. The stability of the catalyst has been studied by running the reaction successively
with the same catalyst in the same reaction. Fig. 1 shows the results of this study. It is interesting to note that the
catalyst could be used several times in one reaction with a little change in its percentage of the yield. Possible
mechanisms for catalytic addition of different diketones to alcohols are proposed (Scheme 1, pathways 1 and 2). By
action of KCoW the alcohol was protonated to generate a stable carbocation after dehydration. This carbocation could

E. Rafiee et al. / Chinese Chemical Letters 23 (2012) 1363–1366
1365
Table 2
Benzylation of different 1,3-dicarbonyl compounds catalyzed by KCoW.
O
O
1a: R= PhCH2
1i: R= CH3(CH2)2
1j: R= CH3(CH2)5
1k: R= (CH3)2CH
1l: R= C6H11
2a: R'= OCH2CH3
2b: R'= OCH3
2c: R'= CH3
'
O
O
H C
3
R
1b: R= (Ph)2CH
2
'
H C
3
R
1c: R= 4-Cl-PhCH2
1d: R= 2-MeO-PhCH2
1e: R= (Ph)(CH3)CH
R
}
KCoW
O
1m: R= (CH3)3CH
O
3
ROH
1
1f: R= 4-MeO-PhCH2 1n: R= 4-NO2-PhCH2
Solvent-free, 80 oC
1o: R= 2-NO2-PhCH2
1p: R= Cinamyl
1g: R= CH3(CH2)3
1h: R=CH3CH2
4
R
O
O
5
Entry
Alcohol
Diketone
Product
Catalyst (g)
Time (min)
Yield (%)a
1
2
1a
1b
1b
1b
1a
1a
1c
1d
1d
1d
1e
1f
2a
2c
2b
2a
2c
2b
2a
2c
2b
2a
2c
2c
2c
2c
4
3aa
3bc
3bb
3ba
3ac
3ab
3ca
3dc
3db
3da
3ec
3fc
3gc
3pc
5a
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.04
0.06
0.08
0.1
15
95
20
89
3
15
96
4
15
91
5
20
92
6
15
95
7
20
93
8
15
98
9
15
90
10
11
12
13
14
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
15
95
15
90
15
94
15b
1g
1p
1a
1h
1h
1h
1h
1h
1i
25
15
95
30
89
4
5d
5d
25 (45)
25 (35)
25
45 (61)
60 (75)
95
4
4
5d
5d
4
25
95
10c
4
5d
5e
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
0.08
25
4
25
96
1j
4
5f
5g
30
94
1k
1l
4
70
75
4
5h
75
81
Traceb
1m
1a
1d
1n
1o
1p
4
5i
5a
90
4
30
89
4
5j
5k
35
84
4
45
91
4
5l
5m
70
82
4
35
86
a
Isolated yield.
b
c
More amount of the catalyst (0.2 g) was not improved the yield of this reaction.
Reaction proceeded at room temperature.
quickly combine with the employed 1,3-dicarbonyl compound to produce, after the release of H+, the final alkylated
product. Also, due to the resonance of oxygen with adjacent p-bonding electrons, cyclic 1,3-dicarbonyl compound as a
nucleophile combined with the stable carbocation to produce b-keto enol ethers.
KCoW catalyst were prepared and purified according to literature procedures [13]. For the benzylation of 1,3-
dicarbonyl compound, the catalyst (0.08 g), was added to a mixture of alcohol (1.0 mmol) and 1,3-dicarbonyl
compound (1.0 mmol) at 80 8C. The progress of the reaction was monitored by TLC. After completion of the reaction,
the mixture was diluted with acetonitrile, and then the catalyst was removed by centrifugation. Catalyst was collected
and treated with 1,2-dichloroethane for removing coke followed by vacuum drying and calcinations at 150 8C, 1 h for
reusing. The filtrate was concentrated and the product was purified by column chromatography on silica-gel using
EtOAc/hexane (1:3) as eluent. For the synthesis of b-keto enol ethers, a mixture of dimedone (1.0 mmol) and alcohol
(4 mL) was stirred well in the presence of the catalyst (0.08 g) at 80 8C. After complete consumption of the starting
materials as indicated by TLC, the reaction was filtered. The excess alcohol in the filtrate was removed by rotary

1366
E. Rafiee et al. / Chinese Chemical Letters 23 (2012) 1363–1366
o
oOH
OH
OH
Pathway 1
Pathway 2
R2
R1
R1
R1
R2
R2
o
o
KCoW
KCoW
KCoW
KCoW
O
O
R1
R1
R2
O
O
R3
R1
R4
R2
R1
R2
R2
O
OH
O
R4
R3
R3
4
5
..
HO
O
O
OH
O
O
O
KCoW
R4
Scheme 1. Proposed mechanisms for catalytic addition of different diketones to alcohols.
Fig. 1. Reusability of the KCoW in the reaction of benzyl alcohol (1 mmol), ethylacetoacetate (1 mmol), catalyst (0.08 g), 80 8C under solvent-free
conditions after 30 min.
evaporation and the crude purified by column chromatography over silica-gel (ethyl acetate/hexane, 1:4). All the
products were identified by comparing of their spectral data with those of the authentic samples [14–16].
Acknowledgment
The authors thank the Razi University Research Council for support of this work.
References
[1] T. Okuhara, Chem. Rev. 102 (2002) 3641.
[2] L.M. Yang, Z.K. Yin, L.Q. Wu, Chin. Chem. Lett. 23 (2012) 265.
[3] E. Rafiee, F. Khajooei Nejad, M. Joshaghani, Chin. Chem. Lett. 22 (2011) 288.
[4] M. Misono, Chem. Commun. (2001) 1141.
[5] C.W. Baker, V.E. Simmons, J. Am. Chem. Soc. 81 (1959) 4744.
[6] I.A. Winstock, Chem. Rev. 98 (1998) 113.
[7] E. Rafiee, A. Azad, A. Bioorg, Med. Chem. Lett. 17 (2007) 2756.
[8] E. Rafiee, Sh. Tangestaninejad, M.H. Habibi, et al. Bioorg. Med. Chem. Lett. 14 (2004) 3611.
[9] M. Yasuda, T. Somyo, A. Baba, Angew. Chem. Int. Ed. 45 (2006) 793.
[10] J. Kischel, K. Mertins, D. Michalik, et al. Adv. Synth. Catal. 349 (2007) 865.
[11] K. Funabiki, T. Komeda, Y. Kubota, et al. Tetrahedron 65 (2009) 7457.
[12] B. Das, K. Laxminarayana, B. Ravikanth, J. Mol. Catal. A: Chem. 271 (2007) 131.
[13] E. Rafiee, Sh. Tangestaninejad, M.H. Habibi, et al. Synth. Commun. 34 (2004) 3673.
[14] E. Rafiee, M. Khodayari, M. Joshaghani, Can. J. Chem. 89 (2011) 1533.
[15] A. Clerici, N. Pastorie, O. Porta, Tetrahedron 57 (2001) 217.
[16] Z.S. Qureshi, K.M. Deshmukh, P.J. Tambade, B.M. Bhanage, Tetrahedron Lett. 51 (2010) 724.