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Investigation of catalytic activity of cobalt–Schiff base complex covalently linked to the polyoxometalate in the alkene and benzyl halide oxidation with hydrogen peroxide
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Catalysis Communications 9 (2008) 219–223 www.elsevier.com/locate/catcom
Investigation of catalytic activity of cobalt–Schiff base complex covalently linked to the polyoxometalate in the alkene and benzyl halide oxidation with hydrogen peroxide Valiollah Mirkhani a,*, Majid Moghadam a,b, Shahram Tangestaninejad a, Iraj Mohammadpoor-Baltork a, Nahid Rasouli a a
Chemistry Department, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran b Chemistry Department, Yasouj University, Yasouj 75914-353, Iran Received 24 April 2007; received in revised form 3 June 2007; accepted 6 June 2007 Available online 12 June 2007
Abstract The catalytic activity of a hybrid compound Co(salen)–POM (1) consisting of cobalt(salen) [salen = N,N 0 -bis(salicylidene)ethylenediamine] complex covalently linked to a Keggin type polyoxometalate (POM) was studied, for the first time, in the oxidation of various olefins in acetonitrile, using hydrogen peroxide as an oxygen source. The complex (1) can catalyze oxidation of various olefins including non-activated terminal olefins. The effect of other parameters such as solvent, oxidant and temperature were also investigated. The selective oxidation of benzyl halides to their corresponding carbonyl compounds by complex (1), as catalyst, was also examined at room temperature. 2007 Published by Elsevier B.V. Keywords: Co(salen); Olefin oxidation; Polyoxometalate; Benzyl halides; Hydrogen peroxide
1. Introduction Transition-metal complexes with Schiff base and porphyrin ligands have been extensively used as models for the heme containing cytochrome P-450 [1,2]. Cytochrome P-450 catalyzes a wide variety of reactions including oxygen transfer to heteroatoms, epoxidation of olefins, hydroxylation of aromatic hydrocarbons and oxidative degradation of chemically inert xenobiotics such as drugs and environmental contaminants [3]. On the other hand, metal complexes of salen and salophen ligands have been used as reagents and catalysts in many reactions including olefin epoxidation, alkane hydroxylation, Diels–Alder transformations, amine oxidation and medicinal studies as models for mimicking the superoxide dismutase [4–8]. Although transition-metal compounds such as metallopor*
Corresponding author. Tel.: +98 311 7932713; fax: +98 311 6689732. E-mail address: [email protected] (V. Mirkhani).
1566-7367/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.catcom.2007.06.003
phyrins [9], polyoxometalates [10], and Schiff base complexes [11] have been used as effective catalysts for epoxidation with hydrogen peroxide, these systems have some disadvantages [9,12,13]. In these contexts, effective catalysts for epoxidation of a wide range of olefins with hydrogen peroxide are still in demand. The versatility and accessibility of polyoxometalates have led to various applications in the fields of structural chemistry, surface science, medicine, electrochemistry and photochemistry [14]. An important advance in transition-metal oxide cluster chemistry is the study of lacunary polyoxometalates as precursors for various complexes (e.g., transition-metalsubstitutes polyoxometalates and organic–inorganic hybrid materials). Recently, numerous POM-based hybrid complexes have been reported [15–18]. Some applications of metal-organic-polyoxometalate hybrid compounds in catalysis are the use of [(PPh3)2Rh(CO)]x[XW12O40] for a combined hydroformylation–oxidation reaction [19] and the use of a Wilkinson’s type catalyst that is covalently
220
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
POM Si
Si
N
N Co
O
O O
+ oxidation products
1 or
or
R
R
H2O2, CH3CN X
O
R'
R' Scheme 1.
attached to a Keggin type polyoxometalate as an effective, recyclable hydrogenation catalyst [20]. The disadvantage of using cobalt–Schiff base complexes is that an aerated solution of Co(salen), for example, is irreversibly auto-oxidized within hours [21]. Recently, several cobalt(II)–Schiff base complexes with bulky substituents on the CH2–CH2 bridge and/or the phenyl rings has been prepared. These compounds are remarkably resistant to irreversible auto-oxidation in O2-saturated DMF [22]. Here, we report, for the first time, our results on the oxidation of olefins and benzyl halides with hydrogen peroxide catalyzed by Co(salen)– Polyoxometalate hybrid complex (Scheme 1) that previously synthesized and well-characterized by Neumann and coworkers [23]. The main goal of this work is to show the catalytic activity of Co(salen) complex, which is stabilized by hybridization with POM. Co(salen) undergoes auto-oxidation, but Co(salen)–POM is resistance toward oxidation and can be used as catalyst in the oxidation reactions. 2. Experimental All materials were commercial reagent grade and obtained from Merck. Alkenes were obtained from Merck or Fluka and were passed through a column containing active alumina to remove peroxidic impurities. Hydrogen peroxide (30%) was stored at 5 C and titrated with potassium permanganate 0.1 N [24]. The Co(salen)–POM hybrid complex [23] has been synthesized according to published procedure and its spectroscopic and analytic data are in according with the reported literature data. FT Infrared (FT-IR) spectra were obtained as potassium bromide pellets in the range of 400–4000 cm1 with a Nicolet Impact 400D instrument. Gas chromatography experiments (GC) were performed with a Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200 or Carbowax 20m. The electronic absorption spectra were recorded on a Varian Cary NIR. 1H NMR spectra were recorded on a Bruker-Arance AQS 300 MHz.
2.1. Catalytic oxidation of olefins by Co(salen)–POM All reactions were carried out at 60 C in a 25 mL glass reactor. The glass reactor was equipped with a reflux condenser and a gas inlet. In a typical experiment, 0.01 mmol of the catalyst (1), 3 mL of acetonitrile, 0.5 mmol of olefin and 3 mmol of hydrogen peroxide (30%) were mixed. The reaction mixture was stirred at 333 K. The reaction progress was monitored by GC. At the end of the reaction, the reaction mixture was diluted with water (20 mL) and the products and substrates were extracted with CH2Cl2 (2 · 10 mL), and purified on a silica-gel plate to obtain the pure product. The identities of the products were confirmed by IR and 1H NMR spectral data. 2.2. Oxidation of benzyl halides with H2O2 catalyzed by Co(salen)–POM The reactions were carried out in CH3CN, at room temperature with constant stirring and the composition of the reaction medium was 2 mmol of benzyl halide, 0.02 mmol of cobalt-salen-polyoxometalate as catalyst, 4 mmol of hydrogen peroxide in CH3CN (3 mL). Progress of the reaction was monitored by TLC. After the reaction was completed, the reaction products were extracted with ether (20 mL) and were purified by silica-gel plate. The identities of products were confirmed by IR spectral data. 3. Results and discussion 3.1. The effect of solvent on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM To find the optimized conditions in the oxidation of olefins with H2O2 catalyzed by Co(salen)–POM, styrene was used as the model substrate. The optimum condition used for the oxidation of styrene by this catalytic system, was catalyst, oxidant, and substrate in a molar ratio of 1:50:300 ratio, respectively. To obtain the appropriate solvent in oxidation of olefins with H2O2, catalyzed by
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
Co(salen)–POM, the oxidation of styrene was carried out in various solvent. Among the acetonitrile, methanol, dichloromethane, acetone, dimethylformamide and ethyl acetate, acetonitrile was chosen as the reaction medium, because a higher oxidation yield was observed (Table 1). 3.2. The effect of terminal oxidants on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM In the catalytic oxidation of alkenes the choice of oxygen donor and solvent is crucial. In this study, we examined different oxidants such as NaOCl, NaIO4, Urea–H2O2 (UHP), tert-BuOOH and H2O2 in the oxidation of styrene. The results showed that H2O2 is the best oxygen source (Table 2). 3.3. The effect of temperature on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM We investigated the effect of reaction temperature on the oxidation of styrene. At room temperature (25 C), the product yields were low and with increasing the reaction temperature to 60 C, both the conversion and selectivity increased. 3.4. Catalytic olefins oxidation by Co(salen)–POM Oxidation of various olefins was performed at 60 C in CH3CN. The results are shown in Table 3. It is clear that electron rich olefins are more reactive than electron poor ones. It was found that the alkenes were not oxidized in
Table 1 Effect of solvent on the oxidation of styrene by Co(salen)-polyoxometalatea Row
Solvent
Yielda (%)
Epoxide selectivity (%)
1 2 3 4 5 6
CH3CN CH3OH CH2Cl2 CH3COCH3 CH3COOEt DMF
85 29 18 10 14 44
18 13.4 4.5 3 5.5 11
a
Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 mL), T = 60 C, t = 6 h.
221
Table 3 Product selectivity (%) in the oxidation of olefins with H2O2 catalyzed by Co(salen)–POM catalysta Substrate
Productb
Conversion Selectivity TOF (%)c (%) (h1)
Styrene
Benzaldehyde Styrene epoxide
69 16
18
7.08
Cyclooctene
Cyclooctene epoxide
78
33
6.5
Cyclohexene
Cyclohexene epoxide 2-Cyclohexene-1-one 2-Cyclohexene-1-ol
22 14 55
24
7.5
a-Methylstyrene Acetophenone 95 a-Methylstyrene epoxide 4
4
8.25
>98
4.08
1-Octene
1-Octene epoxide
49
1-Hexene
1-Hexene epoxide
56
>98
4.6
1-Dodecene
1-Dodecene epoxide
18
>98
2
a Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 ml), T = 60 C, t = 6 h. b After 6 h of reaction. c Based on the starting alkene.
the absence of catalyst or oxidant. Oxidation of styrene carried out in the presence of only polyoxometalate and the result indicated that polyoxometalate was almost inactive. Recently, Mizuno and coworkers examined the epoxidation of olefins with hydrogen peroxide catalyzed by mono- and tri-vacant lacunary compounds such as [a-SiW11O39]8 and [a-SiW9O34]10 in acetonitrile and showed that these compounds were inactive [25]. In the case of styrene and a-methylstyrene the major products were benzaldehyde and acetophenone, respectively. Oxidation of cyclohexene was accompanied by allylic oxidation. The notable feature of the catalytic oxidation with Co(salen)–POM is that non-activated terminal olefins such as 1-octene, 1-hexene could be transformed to the corresponding epoxides in good yields. The Co(salen)–POM is a homogeneous catalyst and can not be recovered, but the main aspect of this catalyst is the stabilization of Co(salen) by covalent attachment to the polyoxometalate. While, unhybridized Co(salen) is autooxidized before it can be used as catalyst in the oxidation reactions. 3.5. Oxidation of benzyl halides with H2O2 catalyzed by Co(salen)–POM
Table 2 Effect of various oxidant on the oxidation of styrene by Co(salen)-polyoxometalatea Entry
Oxidant
Solvent
Yielda (%)
Epoxide selectivity (%)
1 2 3 4 5 6
H2O2 NaIO4 NaClO H2O2/Urea tert-BuOOH No oxidant
CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
85 38 11 8 54 Trace
18 12 4.3 2.4 14 –
a Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 mL) T = 60 C, t = 6 h.
Oxidation of primary and secondary benzyl halides with H2O2 catalyzed by Co(salen)–POM was also investigated. It was found that different primary benzyl halides gave the corresponding aldehydes in high yields at room temperature. Secondary benzyl halides converted to the corresponding ketones (Table 4). Blank experiment under the same conditions in the absence of catalyst or in the absence of oxidant were also performed and only small amounts of product was detected in the reaction mixture.
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V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
Table 4 Oxidation of primary and secondary benzyl halides with H2O2 catalyzed by Cobalt(salen)–POMa Entry
Substrate
Product
Time (h)
1
CH2Cl
Yielda (%)
2
90
2
95
2
93
2
94
24
60
CHO Cl
Cl 2
Cl
CH2Cl
Cl
Cl
CH2Cl
H3 C
CHO
3
CHO
4
CH2Br
CHO Cl
Cl 5
O2N
CH2Br
O2 N
CHO
6
Br
O
3
85
7
Cl
O
3.5
82
a
Reaction conditions: benzyl halide (2 mmol), H2O2 (4 mmol), catalyst (0.02 mmol), CH3CN (3 mL), T = 25 C, t = 2 h.
4. Conclusions
Acknowledgment
In this study, we have demonstrated for the first time, a hybrid of metallosalen and polyoxometalate complex (1) as catalyst for liquid phase oxidation with the environmentally friendly H2O2 as a sole oxidant. The complex (1) exhibited good catalytic activity in the oxidation of various olefins including non-reactive terminal olefins with hydrogen peroxide. It was found that different primary benzyl halides gave the corresponding aldehyde without further oxidation to carboxylic acids in high yields at room temperature. Secondary benzyl halides gave the corresponding ketones in high yields. Other applications of this new catalytic system are now in progress in our laboratory.
The support of this work by Center of Excellence of Chemistry of Isfahan University (CECIU) is acknowledged. References [1] T. Katsuki, J. Mol. Catal. A: Chem. 113 (1996) 87. [2] T. Katsuki, Coord. Chem. Rev. 140 (1995) 189. [3] P.R. Ortiz de Montellano, Cytochrome P-450, Structure, Mechanism and Biochemistry, 2nd ed., Plenum Press, New York, 1995. [4] W. Zhang, J.L. Loebach, S.R. Wilson, E.N. Jacobsen, J. Am. Chem. Soc. 112 (1990) 2801. [5] T. Hamada, R. Irie, J. Mihara, K. Hamachi, T. Katsuki, Tetrahedron 54 (1998) 10017.
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223 [6] Y.J. Hu, X.D. Hung, Z.J. Yao, Y.L. Wu, J. Org. Chem. 63 (1998) 2456. [7] V. Mirkhani, S. Tangestaninejad, M. Moghadam, M. Moghbel, Bioorg. Med. Chem. 12 (2004) 903. [8] T. Yamada, K. Imagawa, T. Nagata, T. Mukaiyama, Bull. Chem. Soc. Jpn. 67 (1994) 2248. [9] P. Battioni, J.P. Renaud, J.F. Bartoli, M. Reina-Artiles, M. Fort, D. Mansuy, J. Am. Chem. Soc. 110 (1988) 8462. [10] R. Neumann, M. Gara, J. Am. Chem. Soc. 117 (1995) 5066. [11] B.S. Lane, M. Vogt, V.J. DeRose, K. Burgess, J. Am. Chem. Soc. 124 (2002) 11946. [12] K. Chen, M. Costas, L. Que Jr., J. Chem. Soc., Dalton Trans. (2002) 672. [13] M.G. Clerici, G. Bellussi, U. Romano, J. Catal. 129 (1991) 159. [14] C.L. Hill, Chem. Rev. 98 (1998) 1. [15] Z.H. Peng, Angew. Chem., Int. Ed. Engl. 43 (2004) 930.
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[16] S. Reinoso, P. Vitoria, L. Lezama, A. Luque, J.M. Gutierrez-Zorrilla, Inorg. Chem. 42 (2003) 3709. [17] S.T. Zheng, J. Zhang, G.Y. Yang, Inorg. Chem. 44 (2005) 2426. [18] S. Reinoso, P. Vitoria, L.S. Felices, L. Lezama, J.M. GutierrezZorrilla, Inorg. Chem. 45 (2006) 108. [19] A.R. Siedle, C.G. Markell, P.A. Lyon, K.O. Hodgson, A.L. Roe, Inorg. Chem. 26 (1987) 219. [20] R. Neumann, I. Bar-Nahum, Chem. Commun. (2003) 2690. [21] T. Tsumaki, Bull. Chem. Soc. Jap. 13 (1946) 252. [22] H. Elias, A. Huber, L. Mu¨ller, Eur. J. Inorg. Chem. (2005) 1459. [23] I. Bar-Nahum, H. Cohen, R. Neumann, Inorg. Chem. 42 (2003) 3677. [24] I.M. Kolthoff, E.B. Sandel, Textbook of Quantitative Inorganic Analysis, The Macmilan Co., New York, 1948. [25] N. Mizuno, K. Yamaguchi, K. Kamata, Coord. Chem . Rev. 249 (2005) 1944–1956.
Catalysis Communications 9 (2008) 219–223 www.elsevier.com/locate/catcom
Investigation of catalytic activity of cobalt–Schiff base complex covalently linked to the polyoxometalate in the alkene and benzyl halide oxidation with hydrogen peroxide Valiollah Mirkhani a,*, Majid Moghadam a,b, Shahram Tangestaninejad a, Iraj Mohammadpoor-Baltork a, Nahid Rasouli a a
Chemistry Department, Catalysis Division, University of Isfahan, Isfahan 81746-73441, Iran b Chemistry Department, Yasouj University, Yasouj 75914-353, Iran Received 24 April 2007; received in revised form 3 June 2007; accepted 6 June 2007 Available online 12 June 2007
Abstract The catalytic activity of a hybrid compound Co(salen)–POM (1) consisting of cobalt(salen) [salen = N,N 0 -bis(salicylidene)ethylenediamine] complex covalently linked to a Keggin type polyoxometalate (POM) was studied, for the first time, in the oxidation of various olefins in acetonitrile, using hydrogen peroxide as an oxygen source. The complex (1) can catalyze oxidation of various olefins including non-activated terminal olefins. The effect of other parameters such as solvent, oxidant and temperature were also investigated. The selective oxidation of benzyl halides to their corresponding carbonyl compounds by complex (1), as catalyst, was also examined at room temperature. 2007 Published by Elsevier B.V. Keywords: Co(salen); Olefin oxidation; Polyoxometalate; Benzyl halides; Hydrogen peroxide
1. Introduction Transition-metal complexes with Schiff base and porphyrin ligands have been extensively used as models for the heme containing cytochrome P-450 [1,2]. Cytochrome P-450 catalyzes a wide variety of reactions including oxygen transfer to heteroatoms, epoxidation of olefins, hydroxylation of aromatic hydrocarbons and oxidative degradation of chemically inert xenobiotics such as drugs and environmental contaminants [3]. On the other hand, metal complexes of salen and salophen ligands have been used as reagents and catalysts in many reactions including olefin epoxidation, alkane hydroxylation, Diels–Alder transformations, amine oxidation and medicinal studies as models for mimicking the superoxide dismutase [4–8]. Although transition-metal compounds such as metallopor*
Corresponding author. Tel.: +98 311 7932713; fax: +98 311 6689732. E-mail address: [email protected] (V. Mirkhani).
1566-7367/$ - see front matter 2007 Published by Elsevier B.V. doi:10.1016/j.catcom.2007.06.003
phyrins [9], polyoxometalates [10], and Schiff base complexes [11] have been used as effective catalysts for epoxidation with hydrogen peroxide, these systems have some disadvantages [9,12,13]. In these contexts, effective catalysts for epoxidation of a wide range of olefins with hydrogen peroxide are still in demand. The versatility and accessibility of polyoxometalates have led to various applications in the fields of structural chemistry, surface science, medicine, electrochemistry and photochemistry [14]. An important advance in transition-metal oxide cluster chemistry is the study of lacunary polyoxometalates as precursors for various complexes (e.g., transition-metalsubstitutes polyoxometalates and organic–inorganic hybrid materials). Recently, numerous POM-based hybrid complexes have been reported [15–18]. Some applications of metal-organic-polyoxometalate hybrid compounds in catalysis are the use of [(PPh3)2Rh(CO)]x[XW12O40] for a combined hydroformylation–oxidation reaction [19] and the use of a Wilkinson’s type catalyst that is covalently
220
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
POM Si
Si
N
N Co
O
O O
+ oxidation products
1 or
or
R
R
H2O2, CH3CN X
O
R'
R' Scheme 1.
attached to a Keggin type polyoxometalate as an effective, recyclable hydrogenation catalyst [20]. The disadvantage of using cobalt–Schiff base complexes is that an aerated solution of Co(salen), for example, is irreversibly auto-oxidized within hours [21]. Recently, several cobalt(II)–Schiff base complexes with bulky substituents on the CH2–CH2 bridge and/or the phenyl rings has been prepared. These compounds are remarkably resistant to irreversible auto-oxidation in O2-saturated DMF [22]. Here, we report, for the first time, our results on the oxidation of olefins and benzyl halides with hydrogen peroxide catalyzed by Co(salen)– Polyoxometalate hybrid complex (Scheme 1) that previously synthesized and well-characterized by Neumann and coworkers [23]. The main goal of this work is to show the catalytic activity of Co(salen) complex, which is stabilized by hybridization with POM. Co(salen) undergoes auto-oxidation, but Co(salen)–POM is resistance toward oxidation and can be used as catalyst in the oxidation reactions. 2. Experimental All materials were commercial reagent grade and obtained from Merck. Alkenes were obtained from Merck or Fluka and were passed through a column containing active alumina to remove peroxidic impurities. Hydrogen peroxide (30%) was stored at 5 C and titrated with potassium permanganate 0.1 N [24]. The Co(salen)–POM hybrid complex [23] has been synthesized according to published procedure and its spectroscopic and analytic data are in according with the reported literature data. FT Infrared (FT-IR) spectra were obtained as potassium bromide pellets in the range of 400–4000 cm1 with a Nicolet Impact 400D instrument. Gas chromatography experiments (GC) were performed with a Shimadzu GC-16A instrument using a 2 m column packed with silicon DC-200 or Carbowax 20m. The electronic absorption spectra were recorded on a Varian Cary NIR. 1H NMR spectra were recorded on a Bruker-Arance AQS 300 MHz.
2.1. Catalytic oxidation of olefins by Co(salen)–POM All reactions were carried out at 60 C in a 25 mL glass reactor. The glass reactor was equipped with a reflux condenser and a gas inlet. In a typical experiment, 0.01 mmol of the catalyst (1), 3 mL of acetonitrile, 0.5 mmol of olefin and 3 mmol of hydrogen peroxide (30%) were mixed. The reaction mixture was stirred at 333 K. The reaction progress was monitored by GC. At the end of the reaction, the reaction mixture was diluted with water (20 mL) and the products and substrates were extracted with CH2Cl2 (2 · 10 mL), and purified on a silica-gel plate to obtain the pure product. The identities of the products were confirmed by IR and 1H NMR spectral data. 2.2. Oxidation of benzyl halides with H2O2 catalyzed by Co(salen)–POM The reactions were carried out in CH3CN, at room temperature with constant stirring and the composition of the reaction medium was 2 mmol of benzyl halide, 0.02 mmol of cobalt-salen-polyoxometalate as catalyst, 4 mmol of hydrogen peroxide in CH3CN (3 mL). Progress of the reaction was monitored by TLC. After the reaction was completed, the reaction products were extracted with ether (20 mL) and were purified by silica-gel plate. The identities of products were confirmed by IR spectral data. 3. Results and discussion 3.1. The effect of solvent on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM To find the optimized conditions in the oxidation of olefins with H2O2 catalyzed by Co(salen)–POM, styrene was used as the model substrate. The optimum condition used for the oxidation of styrene by this catalytic system, was catalyst, oxidant, and substrate in a molar ratio of 1:50:300 ratio, respectively. To obtain the appropriate solvent in oxidation of olefins with H2O2, catalyzed by
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
Co(salen)–POM, the oxidation of styrene was carried out in various solvent. Among the acetonitrile, methanol, dichloromethane, acetone, dimethylformamide and ethyl acetate, acetonitrile was chosen as the reaction medium, because a higher oxidation yield was observed (Table 1). 3.2. The effect of terminal oxidants on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM In the catalytic oxidation of alkenes the choice of oxygen donor and solvent is crucial. In this study, we examined different oxidants such as NaOCl, NaIO4, Urea–H2O2 (UHP), tert-BuOOH and H2O2 in the oxidation of styrene. The results showed that H2O2 is the best oxygen source (Table 2). 3.3. The effect of temperature on the oxidation of styrene with H2O2 catalyzed by Co(salen)–POM We investigated the effect of reaction temperature on the oxidation of styrene. At room temperature (25 C), the product yields were low and with increasing the reaction temperature to 60 C, both the conversion and selectivity increased. 3.4. Catalytic olefins oxidation by Co(salen)–POM Oxidation of various olefins was performed at 60 C in CH3CN. The results are shown in Table 3. It is clear that electron rich olefins are more reactive than electron poor ones. It was found that the alkenes were not oxidized in
Table 1 Effect of solvent on the oxidation of styrene by Co(salen)-polyoxometalatea Row
Solvent
Yielda (%)
Epoxide selectivity (%)
1 2 3 4 5 6
CH3CN CH3OH CH2Cl2 CH3COCH3 CH3COOEt DMF
85 29 18 10 14 44
18 13.4 4.5 3 5.5 11
a
Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 mL), T = 60 C, t = 6 h.
221
Table 3 Product selectivity (%) in the oxidation of olefins with H2O2 catalyzed by Co(salen)–POM catalysta Substrate
Productb
Conversion Selectivity TOF (%)c (%) (h1)
Styrene
Benzaldehyde Styrene epoxide
69 16
18
7.08
Cyclooctene
Cyclooctene epoxide
78
33
6.5
Cyclohexene
Cyclohexene epoxide 2-Cyclohexene-1-one 2-Cyclohexene-1-ol
22 14 55
24
7.5
a-Methylstyrene Acetophenone 95 a-Methylstyrene epoxide 4
4
8.25
>98
4.08
1-Octene
1-Octene epoxide
49
1-Hexene
1-Hexene epoxide
56
>98
4.6
1-Dodecene
1-Dodecene epoxide
18
>98
2
a Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 ml), T = 60 C, t = 6 h. b After 6 h of reaction. c Based on the starting alkene.
the absence of catalyst or oxidant. Oxidation of styrene carried out in the presence of only polyoxometalate and the result indicated that polyoxometalate was almost inactive. Recently, Mizuno and coworkers examined the epoxidation of olefins with hydrogen peroxide catalyzed by mono- and tri-vacant lacunary compounds such as [a-SiW11O39]8 and [a-SiW9O34]10 in acetonitrile and showed that these compounds were inactive [25]. In the case of styrene and a-methylstyrene the major products were benzaldehyde and acetophenone, respectively. Oxidation of cyclohexene was accompanied by allylic oxidation. The notable feature of the catalytic oxidation with Co(salen)–POM is that non-activated terminal olefins such as 1-octene, 1-hexene could be transformed to the corresponding epoxides in good yields. The Co(salen)–POM is a homogeneous catalyst and can not be recovered, but the main aspect of this catalyst is the stabilization of Co(salen) by covalent attachment to the polyoxometalate. While, unhybridized Co(salen) is autooxidized before it can be used as catalyst in the oxidation reactions. 3.5. Oxidation of benzyl halides with H2O2 catalyzed by Co(salen)–POM
Table 2 Effect of various oxidant on the oxidation of styrene by Co(salen)-polyoxometalatea Entry
Oxidant
Solvent
Yielda (%)
Epoxide selectivity (%)
1 2 3 4 5 6
H2O2 NaIO4 NaClO H2O2/Urea tert-BuOOH No oxidant
CH3CN CH3CN CH3CN CH3CN CH3CN CH3CN
85 38 11 8 54 Trace
18 12 4.3 2.4 14 –
a Reaction conditions: Alkene (0.5 mmol), H2O2 (3 mmol), catalyst (0.01 mmol), CH3CN (3 mL) T = 60 C, t = 6 h.
Oxidation of primary and secondary benzyl halides with H2O2 catalyzed by Co(salen)–POM was also investigated. It was found that different primary benzyl halides gave the corresponding aldehydes in high yields at room temperature. Secondary benzyl halides converted to the corresponding ketones (Table 4). Blank experiment under the same conditions in the absence of catalyst or in the absence of oxidant were also performed and only small amounts of product was detected in the reaction mixture.
222
V. Mirkhani et al. / Catalysis Communications 9 (2008) 219–223
Table 4 Oxidation of primary and secondary benzyl halides with H2O2 catalyzed by Cobalt(salen)–POMa Entry
Substrate
Product
Time (h)
1
CH2Cl
Yielda (%)
2
90
2
95
2
93
2
94
24
60
CHO Cl
Cl 2
Cl
CH2Cl
Cl
Cl
CH2Cl
H3 C
CHO
3
CHO
4
CH2Br
CHO Cl
Cl 5
O2N
CH2Br
O2 N
CHO
6
Br
O
3
85
7
Cl
O
3.5
82
a
Reaction conditions: benzyl halide (2 mmol), H2O2 (4 mmol), catalyst (0.02 mmol), CH3CN (3 mL), T = 25 C, t = 2 h.
4. Conclusions
Acknowledgment
In this study, we have demonstrated for the first time, a hybrid of metallosalen and polyoxometalate complex (1) as catalyst for liquid phase oxidation with the environmentally friendly H2O2 as a sole oxidant. The complex (1) exhibited good catalytic activity in the oxidation of various olefins including non-reactive terminal olefins with hydrogen peroxide. It was found that different primary benzyl halides gave the corresponding aldehyde without further oxidation to carboxylic acids in high yields at room temperature. Secondary benzyl halides gave the corresponding ketones in high yields. Other applications of this new catalytic system are now in progress in our laboratory.
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