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Epoxidation of alkenes catalyzed by cobalt(II) calix[4]pyrrole
Catalysis Communications 8 (2007) 310–314 www.elsevier.com/locate/catcom
Epoxidation of alkenes catalyzed by cobalt(II) calix[4]pyrrole Pongchart Buranaprasertsuk a, Yupa Tangsakol b, Warinthorn Chavasiri b
a,*
a Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Received 29 March 2006; received in revised form 5 June 2006; accepted 12 June 2006 Available online 27 June 2006
Abstract The new developed epoxidation system utilizing cobalt(II) calix[4]pyrrole as a catalyst in the aldehyde/oxygen as an oxidant was explored. Six calix[4]pyrrole ligands were synthesized and the well characterized ligands were complexed with Co(II) salts. It was disclosed that Co(II) meso-tetrakis (4-methoxyphenyl)-tetramethyl calix[4]pyrrole revealed the best catalytic performance to provide the corresponding epoxide in high yields with excellent selectivity under mild conditions. In addition, the stereoselectivity and regioselectivity studies of the system using cis- and trans-stilbenes and 4-vinylcyclohexene, as chemical probes were thoroughly investigated. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Cobalt(II)calix[4]pyrrole
1. Introduction The oxidation of unfunctionalized as well as funtionalized olefins is still one of current interest and intensive researches in organic synthesis [1,2]. The main reason stems from the fact that epoxides are widely utilized as intermediates in the laboratory and chemical manufacturing and served as important building blocks for a variety of chemical compounds. Therefore the development of efficient and selective methods for the preparation of epoxides is always called for. Generally, epoxidation is mainly carried out with various peracids and peroxide such as m-chloroperbenzoic acid (MCPBA) [3,4] and hydrogen-peroxide (H2O2) [5–7], respectively. These two types of reagents commonly used to transform alkenes to epoxides, however, suffered from the disadvantage of producing of unwanted by products. All possible efforts have been studied to develop the epoxidation of alkenes under neutral conditions to avoid the undesirable decomposition or rearrangement of the formed epoxides by the co-produced carboxylic acid [8]. *
Corresponding author. Tel.: +66 2 218 7625; fax: +66 2 218 7598. E-mail address: [email protected] (W. Chavasiri).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.022
This is the breakthrough that the epoxidation of olefins is a particular challenging problems in organic synthesis leading to the great interest in new and more efficient catalytic versions for requiring high yield, high selectivity, low cost, safety and simplicity. Cobalt(II) schiff base complexes [9– 14], cobalt(II) complex [15], cobalt-containing polyoxotungstate [16] and Co(thd)2 [17] are amay those examples to be effective catalysts to furnish epoxides. We disclose that cobalt(II) calix[4]pyrrole catalysts in the presence of certain aldehydes and O2 exhibit an interesting chemoselectivity during the oxidation of alkenes at room temperature. Calix[4]pyrroles (meso-octaalkylporphyrinogens) are stable tetrapyrrolic macrocycles first synthesized in the 19th century by Baeyer via acid-catalyzed condensation of pyrrole with acetone in the presence of methanesulfonic acid as a catalyst to produce meso-octamethyl calix[4]pyrrole 1 [18,19]. The discovery of these species acting as receptors for anions and netural molecules has been utilized in the production of fluorescent, colorimetric, and electrochemical sensors for anions, in addition to new solid supports capable of separating mixtures of anions [20]. Eventhough calix[4]pyrroles could be synthesized according to the procedure reported in the literature with high yield [21–23], there was no report cited in the chemical
P. Buranaprasertsuk et al. / Catalysis Communications 8 (2007) 310–314
literature to employ these cobalt(II) calix[4]pyrrole complexes in the epoxidation reaction. Initial efforts focused on the screening of six calix[4]pyrroles (Fig. 1).
2. Experimental 2.1. General preparation of complexes Ligand 4 was prepared by slowly adding methanesulfonic acid (7% mol) to a solution of 4-methoxyacetophenone 15.02 g (0.1 mol), pyrrole 7.0 mL (0.1 mol) and ethanol 50 mL. The mixture was allowed to reflux for 4 h and then cooled. The brown solid was filtered off, washed with several portions of ethanol and dried at room temperature, the brown solid 19.82 g (99% yield) was obtained; m.p. 121–122 °C; Rf 0.43 (hexane:ethyl acetate 8:2); IR (KBr, cm 1): 3431 (s), 2970 (s), 2832 (m), 1608 (s), 1456 (s), and 1250 (s); 1H NMR (CDCl3)d (ppm): 7.53 (4H, s), 6.92–7.15 (8H, m), 6.50–6.85 (8H, m), 5.66–5.92 (8H, m), 3.78 (12H, m) and 1.85 (12H, s); 13C NMR (CDCl3)d (ppm): 157.5 (4C), 139.4 (4C), 136.8 (8C), 128.3 (8C), 113.0 (8C), 105.5 (8C), 55.0 (4C), 43.6 (4C) and 17.8 (4C). To a solution of 4 5.57 g (7 mmol) in tetrahydrofuran (THF) 40 mL was added butyl lithium (BuLi) 2.27 mL (28 mmol). The reaction mixture was refluxed under stirring for 2 h. The solvent was evaporated in vacuo and the residue was washed with dry hexane to afford the brown solid 5.67 g (76% yield); m.p. 112 °C; Rf 0.45 (hexane:ethyl acetate 8:2); IR (KBr, cm 1): 3090 (w), 2631 (w), 1607 (w), 1456 (m) and 1250 (m); 1H NMR (CDCl3)d (ppm): 6.93– 7.15 (8H, m), 6.64–6.78 (8H, m), 5.78–5.90 (8H, m), 3.77 (12H, m), 4.68 (16H, br, THF), 1.86 (12H, s) and 1.75 (16H, br, THF). Cobalt (II) chloride 0.79 g (3.3 mmol) and polyanion 4.43 g (4 mmol) were dissolved in toluene 40 mL. The reaction mixture was stirred at room temperature for 48 h. After the reaction completed, the brown solid was filtered off and washed with several portions of toluene. The filtrate was collected and kept in refrigerator for 24 h. The solvent was evaporated in vacuo to gave brown solid 3.99 g (87% yield); m.p. 116–117 °C, Rf 0.43 (hexane:ethyl acetate 8:2), IR (KBr, cm 1): 2959 (w), 1724 (s), 1672 (m), 1597 (s) and 1254 (s).
R2 R1
R2
N H NH
R2 R1
cis-Stilbene oxide: colorless liquid, Rf 0.31 (hexane:CHCl3 8:2); 1H NMR (CDCl3)d (ppm): 7.11–7.37 (m, 10H) and 4.37 (s, 2H). trans-Stilbene oxide: white solid, m.p 69–70 °C, Rf 0.36 (hexane:CHCl3 8:2); 1H NMR (CDCl3)d (ppm): 7.24–7.40 (m, 10H) and 3.92 (s, 2H). 2.3. General procedure for epoxidation reaction The epoxidation was carried out in toluene containing alkene, 2-ethylbutyraldehyde under an oxygen atmosphere, and cobalt(II) calix[4]pyrrole complex at room temperature (30 °C). The mixture was stirred for 24 h. After the reaction was finished, 1 mL of the reaction mixture was taken and extracted with diethyl ether containing saturated NaHCO3. The combined extracts were washed with brine. The organic layer was dried over anhydrous Na2SO4 and analyzed by gas chromatograph with the addition of an exact amount of an appropriate internal standard. 3. Results and discussion Since there has been no report concerning the use of this class of catalyst for alkene epoxidation, a search for efficient ligands coordinating to cobalt was scrutinized. Six cobalt calix[4]pyrroles 1–6 were investigated and the findings are intimated in Table 1. It could be noticed that most cobalt calix[4]pyrrole complexes could catalyze the cyclohexene epoxidation smoothly leading to the desired product cyclohexene oxide Table 1 The epoxidation of cyclohexene catalyzed by various cobalt(II) calix[4]pyrrole complexes Entry
Co(II) complex
% of cydohexene oxide
1 2 3 4 5 6
1 2 3 4 5 6
44 57 74 86 60 16
Reaction conditions: cyclohexene (5 mmol), catalyst (0.05 mmol), toluene (36 mL), O2 and 2-ethylbutyraldehyde (10 mmol), reaction time (24 h).
R1
2 R 1 , R 2 = Et
N H NH
HN H N
2.2. General preparation of epoxide
1 R 1 , R 2 = Me
R1
HN H N
4 R 1 = Me, R 2 = 4-methoxyphenyl 5 R 1 = Me, R 2 = phenyl 6 R 1 , R 2 = phenyl
R2
311
3 Fig. 1. Six synthesized of calix[4]pyrroles.
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with a small amount of cyclohexenone. None of cyclohexenol was detected. Considering the variation of ligands, 4methoxyphenyl group was found to be a good ligating agent that could assist the catalytic activation process. Several solvents including THF, DMF, acetonitrile, toluene, dichloromethane, pyridine and 1,2-dichloroethane were chosen to observe the catalytic potentials of cobalt(II) calix[4]pyrrole complex 4 in cyclohexene epoxidation. It was found that acetonitrile and toluene gave superior yield to other mixed solvents. Various oxidants including H2O2, tert-butyl-hydroperoxide (TBHP), m-CPBA and 2-ethylbutyraldehyde/O2 were investigated in this epoxidation reaction. H2O2 and TBHP were found not to be good oxidants under this particular condition. The use of 2-ethylbutyraldehyde/O2 was noticed to be an efficient oxidant affecting the epoxidation of cyclohexene. This developed epoxidation system was further explored for a variety of alkenes as presented in Table 2. The epoxidation of endocyclic alkene such as cyclohexene proceeded smoothly to form cyclohexene oxide 86% yield (entry 1). 1-Dodecene, an instance of aliphatic terminal alkenes could also be converted to 1-dodecene oxide; however to achieve the high yield of the desired product, about twice of the amount of 2-ethylbutyraldehyde required for cyclohexene was necessary (entry 2). This study indicated that terminal monosubstituted alkene could transform to epoxide more slowly than endocyclic disubstituted alkene. Moderate yield of styrene oxide was obtained (entry 3) when styrene was used as a substrate. When the amount of oxidant was increased, the total amount of the desired product was increased. Similar results were observed for the reactions of a-methylstyrene to yield a-methylstyrene oxide (entry 4). The epoxidation of R-(+)- and S-( )-limonenes furnished diastereomers mainly derived from the internal cyclic alkene epoxidation of each enantiomer (entries 5 and 6). These results should also be noted that the endocyclic double bond could be epoxidized more readily than exocyclic double bond. The
explanation for this could be stemmed from the electronrich trisubstituted double bond was more active than the other. The reaction of a- and c-terpinenes clearly demonstrated that the aromatization was more prevailed than other chemical processes (entries 7 and 8) according to the stability and the existence of aromatic compound.
Table 2 The epoxidation of alkenes catalyzed by Co(II) calix[4]pyrrole 4 Entry
Substrate
Product (%) 86
1 O
2
O
3
O
34, 68a
4
O
52, 76a
5
O
93
6
O
82
7
36
8
43
Reaction conditions: alkene (5 mmol), catalyst (0.05 mmol), toluene (36 mL), O2 and 2-ethylbutyraldehyde (10 mmol), reaction time (24 h). a 2-Ethylbutyraldehyde (20 mmol) was used.
70 60
%edixope
50 40
cyclohexene oxide 1-dodecene oxide 1-methyl cyclohexene oxide
30 20 10 0 0
90
180
270
39, 74a
360
450
time (min)
Fig. 2. Comparative kinetic study on the reaction rate of the epoxidation of cyclohexene, 1-dodecene and 1-methylcyclohexene.
P. Buranaprasertsuk et al. / Catalysis Communications 8 (2007) 310–314
O complex 4
+
2-ethylbutyraldehyde toluene, O 2, RT, 24hr
O 86%
14%
Scheme 1. Epoxidation of 4-vinylcyclohexene.
The kinetic study of the epoxidation reaction of cyclohexene, 1-dodecene and 1-methylcyclohexene catalyzed by cobalt complex 4 was compared (Fig. 2). The rate of the epoxidation of 1-methylcyclohexene was found to be faster than those of cyclohexene and 1-dodecene, respectively. These could be explained in terms of the more electron rich substrates could proceed more prevailed than the less ones. This observation strongly implied that the active site of catalyst should be electrophilic in character. The details of regioselectivity study were observed from the epoxidation of 4-vinylcyclohexene. The isolation of epoxide isomers from the epoxidation reaction of 4-vinylcyclohexene catalyzed by cobalt(II) complex of 4 yielded 1,2-oxide (86% composition) and 7,8-oxide (14% composition) (Scheme 1). The stereoselectivity study of the reaction was carefully examined employing cis- and trans-stilbenes. The detection
313
of cis- and trans-epoxide could be accomplished by 1H NMR spectroscopy [24]. These observed chemical shifts of the epoxidized products were in good agreement with those obtained from authentic synthesized samples. The more important point derived from this experimental fact was that the epoxidation of trans-stilbene provided solely trans-epoxide whereas the products achieved from the epoxidation of cis-stilbene was a mixture of cis- and trans-stilbene oxides in comparable amount (1:1). This result provided a useful information for mechanistic interpretation (Fig. 3). The mechanism that has been proposed for the metal complex-catalyzed oxygenation of substrates by O2 and aldehydes. In this mechanism, the metal complex is assumed to play two roles. First, the metal complex reacts with the aldehyde to generate an acyl radical (RC(O)). The acyl radical then reacts with dioxygen to give an acylperoxy radical (RC(O)OO). The acylperoxy radical acts as a carrier in a chain mechanism by reacting with another aldehyde molecule to give the peroxyacid, thereby generating another acyl radical. Oxygenation of substrate is assumed by this mechanism to occur via reactive highvalent metal oxo intermediates, which are produced by the reaction of the peroxyacid with the metal catalysts and which then react with the olefin in a fashion analogous to that observed previously for metal complex-catalyzed reactions of peroxy acids with olefins. 4. Conclusion
O
O RC
RC H O
H
In summary, six cobalt(II) calix[4]pyrrole complexes were screened for their capability to use as a catalyst in the epoxidation reaction. Cobalt(II) calix[4]pyrrole omplex 4 exhibited impressive catalytic activity for epoxidation of alkene. According to the stereoselectivity investigation, the epoxidation of trans-stilbene gave only trans-stilbene oxide. In addition, the regioselectivity of 4-vinylcyclohexene gave 1,2-oxide as major more substituted alkenes could undergo the epoxidation reaction more effectively. This also implied that the active species responsible for the epoxidation should be an electrophilic species.
O RC O O
O2
RC O RC O O
+
O RC O OH
RC HO
O RC O OH (L n)C oII
Acknowledgements O RC O O
O RC O O C oLn
(L n) C oII
O RC OH
O
Sincere thanks are extended to Natural Products Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University for the support of chemicals and laboratory facilities and to Graduate school Chulalongkorn University for the financial support. References
IV
(L n)C o O
Fig. 3. Proposed mechanism for cobalt(II) calix[4]pyrrole catalyzed epoxidation of alkenes.
[1] K. Wenissermel, H.J. Arpe, Industrielle Organische Chemie, Tokyo Kagakudouninn, Tokyo, 1996. [2] K.A. Jorgenson, Chem. Rev. 89 (1989) 431. [3] M. Kodera, H. Shimakoshi, K. Kano, Chem. Commun. (1996) 1737– 1738.
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[4] W. Nam, J.Y. Ryu, I. Kim, C. Kim, Tetrahedron Lett. 43 (2002) 5487–5490. [5] B.S. Lane, K. Burgess, J. Am. Chem. Soc. 123 (2001) 2933–2934. [6] M.C. White, A.G. Doyle, E.N. Jacobsen, J. Am. Chem. Soc. 123 (2001) 7194–7195. [7] W. Nam, S-Y. Oh, Y.J. Sun, J. Kim, W-K. Kim, S.K. Woo, W. Shin, J. Org. Chem. 68 (2003) 7903–7906. [8] F. Fringuelli, R. Germani, F. Pizzo, G. Savelli, Tetrahedron Lett. 30 (1989) 1427. [9] M.M. Reddy, T. Punniyawurthy, J. Iqbal, Tetrahedron Lett. 36 (1995) 159–162. [10] J. Estrada, I. Fernandez, J. Pedro, X. Ottenwaelder, R. Ruiz, Y. Journaux, Tetrahedron Lett. 38 (1997) 2377–2380. [11] R.I. Kureshy, N.H. Khan, S.H.R. Abdi, A.K. Bhatt, P. Iyer, J. Mol. Catal. A 121 (1997) 25–31. [12] T. Josep, D.P. Sawant, C.S. Gopinath, S.B. Halligudi, J. Mol. Catal. A 184 (2002) 289–299. [13] P. Oliverira, A.M. Ramos, I. Fonseca, A. Botelho do Rego, J. Vital, Catal. Today 103 (2005) 67–77.
[14] T. Toshihiro, H. Eiichiro, Y. Kiyotaka, T. Mukaiyama, Chem. Lett. 10 (1992) 2077–2080. [15] R. Chakrabarty, B.K. Das, J. Mol. Catal. A 223 (2002) 39–44. [16] S.K. Choi, H.J. Lee, H. Kim, W. Nam, Bull. Korean Chem. Soc. 23 (2002) 1039–1041. [17] K.J. O’ Connor, S.J. Wey, C.J. Burrows, Tetrahedron Lett. 33 (1992) 1001–1004. [18] P. Rothemund, C.L. Gage, J. Am. Chem. Soc. 77 (1955) 3340–3342. [19] S.D. Angelis, E. Solari, C. Floriani, A. Chiesi-Villa, C. Rizzoli, J. Am. Chem. Soc. 116 (1994) 5702–5713. [20] P.A. Gale, J.L. Sessler, V. Kral, J. Am. Chem. Soc. 118 (1996) 5140– 5141. [21] J.L. Sessler, P.J. Anzenbacher, H. Miyaji, K. Jursikova, E.R. Bleasdale, P.A. Gale, Ind. Eng. Chem. Res. 39 (2000) 3471–3478. [22] D. Jacoby, C. Floriani, A. Chiesi-Villa, C. Rizzoli, J. Am. Chem. Soc. 115 (1993) 3595–3602. [23] J. Jubb, D. Jacoby, C. Floriani, A. Chiesi-Villa, C. Rizzoli, Inorg. Chem. 31 (1992) 1306–1308. [24] D.Y. Curtin, D.B. Kellom, J. Am. Chem. Soc. 75 (1953) 6011–6018.
Epoxidation of alkenes catalyzed by cobalt(II) calix[4]pyrrole Pongchart Buranaprasertsuk a, Yupa Tangsakol b, Warinthorn Chavasiri b
a,*
a Department of Chemistry, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand Program of Petrochemistry and Polymer Science, Faculty of Science, Chulalongkorn University, Bangkok 10330, Thailand
Received 29 March 2006; received in revised form 5 June 2006; accepted 12 June 2006 Available online 27 June 2006
Abstract The new developed epoxidation system utilizing cobalt(II) calix[4]pyrrole as a catalyst in the aldehyde/oxygen as an oxidant was explored. Six calix[4]pyrrole ligands were synthesized and the well characterized ligands were complexed with Co(II) salts. It was disclosed that Co(II) meso-tetrakis (4-methoxyphenyl)-tetramethyl calix[4]pyrrole revealed the best catalytic performance to provide the corresponding epoxide in high yields with excellent selectivity under mild conditions. In addition, the stereoselectivity and regioselectivity studies of the system using cis- and trans-stilbenes and 4-vinylcyclohexene, as chemical probes were thoroughly investigated. Ó 2006 Elsevier B.V. All rights reserved. Keywords: Epoxidation; Cobalt(II)calix[4]pyrrole
1. Introduction The oxidation of unfunctionalized as well as funtionalized olefins is still one of current interest and intensive researches in organic synthesis [1,2]. The main reason stems from the fact that epoxides are widely utilized as intermediates in the laboratory and chemical manufacturing and served as important building blocks for a variety of chemical compounds. Therefore the development of efficient and selective methods for the preparation of epoxides is always called for. Generally, epoxidation is mainly carried out with various peracids and peroxide such as m-chloroperbenzoic acid (MCPBA) [3,4] and hydrogen-peroxide (H2O2) [5–7], respectively. These two types of reagents commonly used to transform alkenes to epoxides, however, suffered from the disadvantage of producing of unwanted by products. All possible efforts have been studied to develop the epoxidation of alkenes under neutral conditions to avoid the undesirable decomposition or rearrangement of the formed epoxides by the co-produced carboxylic acid [8]. *
Corresponding author. Tel.: +66 2 218 7625; fax: +66 2 218 7598. E-mail address: [email protected] (W. Chavasiri).
1566-7367/$ - see front matter Ó 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2006.06.022
This is the breakthrough that the epoxidation of olefins is a particular challenging problems in organic synthesis leading to the great interest in new and more efficient catalytic versions for requiring high yield, high selectivity, low cost, safety and simplicity. Cobalt(II) schiff base complexes [9– 14], cobalt(II) complex [15], cobalt-containing polyoxotungstate [16] and Co(thd)2 [17] are amay those examples to be effective catalysts to furnish epoxides. We disclose that cobalt(II) calix[4]pyrrole catalysts in the presence of certain aldehydes and O2 exhibit an interesting chemoselectivity during the oxidation of alkenes at room temperature. Calix[4]pyrroles (meso-octaalkylporphyrinogens) are stable tetrapyrrolic macrocycles first synthesized in the 19th century by Baeyer via acid-catalyzed condensation of pyrrole with acetone in the presence of methanesulfonic acid as a catalyst to produce meso-octamethyl calix[4]pyrrole 1 [18,19]. The discovery of these species acting as receptors for anions and netural molecules has been utilized in the production of fluorescent, colorimetric, and electrochemical sensors for anions, in addition to new solid supports capable of separating mixtures of anions [20]. Eventhough calix[4]pyrroles could be synthesized according to the procedure reported in the literature with high yield [21–23], there was no report cited in the chemical
P. Buranaprasertsuk et al. / Catalysis Communications 8 (2007) 310–314
literature to employ these cobalt(II) calix[4]pyrrole complexes in the epoxidation reaction. Initial efforts focused on the screening of six calix[4]pyrroles (Fig. 1).
2. Experimental 2.1. General preparation of complexes Ligand 4 was prepared by slowly adding methanesulfonic acid (7% mol) to a solution of 4-methoxyacetophenone 15.02 g (0.1 mol), pyrrole 7.0 mL (0.1 mol) and ethanol 50 mL. The mixture was allowed to reflux for 4 h and then cooled. The brown solid was filtered off, washed with several portions of ethanol and dried at room temperature, the brown solid 19.82 g (99% yield) was obtained; m.p. 121–122 °C; Rf 0.43 (hexane:ethyl acetate 8:2); IR (KBr, cm 1): 3431 (s), 2970 (s), 2832 (m), 1608 (s), 1456 (s), and 1250 (s); 1H NMR (CDCl3)d (ppm): 7.53 (4H, s), 6.92–7.15 (8H, m), 6.50–6.85 (8H, m), 5.66–5.92 (8H, m), 3.78 (12H, m) and 1.85 (12H, s); 13C NMR (CDCl3)d (ppm): 157.5 (4C), 139.4 (4C), 136.8 (8C), 128.3 (8C), 113.0 (8C), 105.5 (8C), 55.0 (4C), 43.6 (4C) and 17.8 (4C). To a solution of 4 5.57 g (7 mmol) in tetrahydrofuran (THF) 40 mL was added butyl lithium (BuLi) 2.27 mL (28 mmol). The reaction mixture was refluxed under stirring for 2 h. The solvent was evaporated in vacuo and the residue was washed with dry hexane to afford the brown solid 5.67 g (76% yield); m.p. 112 °C; Rf 0.45 (hexane:ethyl acetate 8:2); IR (KBr, cm 1): 3090 (w), 2631 (w), 1607 (w), 1456 (m) and 1250 (m); 1H NMR (CDCl3)d (ppm): 6.93– 7.15 (8H, m), 6.64–6.78 (8H, m), 5.78–5.90 (8H, m), 3.77 (12H, m), 4.68 (16H, br, THF), 1.86 (12H, s) and 1.75 (16H, br, THF). Cobalt (II) chloride 0.79 g (3.3 mmol) and polyanion 4.43 g (4 mmol) were dissolved in toluene 40 mL. The reaction mixture was stirred at room temperature for 48 h. After the reaction completed, the brown solid was filtered off and washed with several portions of toluene. The filtrate was collected and kept in refrigerator for 24 h. The solvent was evaporated in vacuo to gave brown solid 3.99 g (87% yield); m.p. 116–117 °C, Rf 0.43 (hexane:ethyl acetate 8:2), IR (KBr, cm 1): 2959 (w), 1724 (s), 1672 (m), 1597 (s) and 1254 (s).
R2 R1
R2
N H NH
R2 R1
cis-Stilbene oxide: colorless liquid, Rf 0.31 (hexane:CHCl3 8:2); 1H NMR (CDCl3)d (ppm): 7.11–7.37 (m, 10H) and 4.37 (s, 2H). trans-Stilbene oxide: white solid, m.p 69–70 °C, Rf 0.36 (hexane:CHCl3 8:2); 1H NMR (CDCl3)d (ppm): 7.24–7.40 (m, 10H) and 3.92 (s, 2H). 2.3. General procedure for epoxidation reaction The epoxidation was carried out in toluene containing alkene, 2-ethylbutyraldehyde under an oxygen atmosphere, and cobalt(II) calix[4]pyrrole complex at room temperature (30 °C). The mixture was stirred for 24 h. After the reaction was finished, 1 mL of the reaction mixture was taken and extracted with diethyl ether containing saturated NaHCO3. The combined extracts were washed with brine. The organic layer was dried over anhydrous Na2SO4 and analyzed by gas chromatograph with the addition of an exact amount of an appropriate internal standard. 3. Results and discussion Since there has been no report concerning the use of this class of catalyst for alkene epoxidation, a search for efficient ligands coordinating to cobalt was scrutinized. Six cobalt calix[4]pyrroles 1–6 were investigated and the findings are intimated in Table 1. It could be noticed that most cobalt calix[4]pyrrole complexes could catalyze the cyclohexene epoxidation smoothly leading to the desired product cyclohexene oxide Table 1 The epoxidation of cyclohexene catalyzed by various cobalt(II) calix[4]pyrrole complexes Entry
Co(II) complex
% of cydohexene oxide
1 2 3 4 5 6
1 2 3 4 5 6
44 57 74 86 60 16
Reaction conditions: cyclohexene (5 mmol), catalyst (0.05 mmol), toluene (36 mL), O2 and 2-ethylbutyraldehyde (10 mmol), reaction time (24 h).
R1
2 R 1 , R 2 = Et
N H NH
HN H N
2.2. General preparation of epoxide
1 R 1 , R 2 = Me
R1
HN H N
4 R 1 = Me, R 2 = 4-methoxyphenyl 5 R 1 = Me, R 2 = phenyl 6 R 1 , R 2 = phenyl
R2
311
3 Fig. 1. Six synthesized of calix[4]pyrroles.
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with a small amount of cyclohexenone. None of cyclohexenol was detected. Considering the variation of ligands, 4methoxyphenyl group was found to be a good ligating agent that could assist the catalytic activation process. Several solvents including THF, DMF, acetonitrile, toluene, dichloromethane, pyridine and 1,2-dichloroethane were chosen to observe the catalytic potentials of cobalt(II) calix[4]pyrrole complex 4 in cyclohexene epoxidation. It was found that acetonitrile and toluene gave superior yield to other mixed solvents. Various oxidants including H2O2, tert-butyl-hydroperoxide (TBHP), m-CPBA and 2-ethylbutyraldehyde/O2 were investigated in this epoxidation reaction. H2O2 and TBHP were found not to be good oxidants under this particular condition. The use of 2-ethylbutyraldehyde/O2 was noticed to be an efficient oxidant affecting the epoxidation of cyclohexene. This developed epoxidation system was further explored for a variety of alkenes as presented in Table 2. The epoxidation of endocyclic alkene such as cyclohexene proceeded smoothly to form cyclohexene oxide 86% yield (entry 1). 1-Dodecene, an instance of aliphatic terminal alkenes could also be converted to 1-dodecene oxide; however to achieve the high yield of the desired product, about twice of the amount of 2-ethylbutyraldehyde required for cyclohexene was necessary (entry 2). This study indicated that terminal monosubstituted alkene could transform to epoxide more slowly than endocyclic disubstituted alkene. Moderate yield of styrene oxide was obtained (entry 3) when styrene was used as a substrate. When the amount of oxidant was increased, the total amount of the desired product was increased. Similar results were observed for the reactions of a-methylstyrene to yield a-methylstyrene oxide (entry 4). The epoxidation of R-(+)- and S-( )-limonenes furnished diastereomers mainly derived from the internal cyclic alkene epoxidation of each enantiomer (entries 5 and 6). These results should also be noted that the endocyclic double bond could be epoxidized more readily than exocyclic double bond. The
explanation for this could be stemmed from the electronrich trisubstituted double bond was more active than the other. The reaction of a- and c-terpinenes clearly demonstrated that the aromatization was more prevailed than other chemical processes (entries 7 and 8) according to the stability and the existence of aromatic compound.
Table 2 The epoxidation of alkenes catalyzed by Co(II) calix[4]pyrrole 4 Entry
Substrate
Product (%) 86
1 O
2
O
3
O
34, 68a
4
O
52, 76a
5
O
93
6
O
82
7
36
8
43
Reaction conditions: alkene (5 mmol), catalyst (0.05 mmol), toluene (36 mL), O2 and 2-ethylbutyraldehyde (10 mmol), reaction time (24 h). a 2-Ethylbutyraldehyde (20 mmol) was used.
70 60
%edixope
50 40
cyclohexene oxide 1-dodecene oxide 1-methyl cyclohexene oxide
30 20 10 0 0
90
180
270
39, 74a
360
450
time (min)
Fig. 2. Comparative kinetic study on the reaction rate of the epoxidation of cyclohexene, 1-dodecene and 1-methylcyclohexene.
P. Buranaprasertsuk et al. / Catalysis Communications 8 (2007) 310–314
O complex 4
+
2-ethylbutyraldehyde toluene, O 2, RT, 24hr
O 86%
14%
Scheme 1. Epoxidation of 4-vinylcyclohexene.
The kinetic study of the epoxidation reaction of cyclohexene, 1-dodecene and 1-methylcyclohexene catalyzed by cobalt complex 4 was compared (Fig. 2). The rate of the epoxidation of 1-methylcyclohexene was found to be faster than those of cyclohexene and 1-dodecene, respectively. These could be explained in terms of the more electron rich substrates could proceed more prevailed than the less ones. This observation strongly implied that the active site of catalyst should be electrophilic in character. The details of regioselectivity study were observed from the epoxidation of 4-vinylcyclohexene. The isolation of epoxide isomers from the epoxidation reaction of 4-vinylcyclohexene catalyzed by cobalt(II) complex of 4 yielded 1,2-oxide (86% composition) and 7,8-oxide (14% composition) (Scheme 1). The stereoselectivity study of the reaction was carefully examined employing cis- and trans-stilbenes. The detection
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of cis- and trans-epoxide could be accomplished by 1H NMR spectroscopy [24]. These observed chemical shifts of the epoxidized products were in good agreement with those obtained from authentic synthesized samples. The more important point derived from this experimental fact was that the epoxidation of trans-stilbene provided solely trans-epoxide whereas the products achieved from the epoxidation of cis-stilbene was a mixture of cis- and trans-stilbene oxides in comparable amount (1:1). This result provided a useful information for mechanistic interpretation (Fig. 3). The mechanism that has been proposed for the metal complex-catalyzed oxygenation of substrates by O2 and aldehydes. In this mechanism, the metal complex is assumed to play two roles. First, the metal complex reacts with the aldehyde to generate an acyl radical (RC(O)). The acyl radical then reacts with dioxygen to give an acylperoxy radical (RC(O)OO). The acylperoxy radical acts as a carrier in a chain mechanism by reacting with another aldehyde molecule to give the peroxyacid, thereby generating another acyl radical. Oxygenation of substrate is assumed by this mechanism to occur via reactive highvalent metal oxo intermediates, which are produced by the reaction of the peroxyacid with the metal catalysts and which then react with the olefin in a fashion analogous to that observed previously for metal complex-catalyzed reactions of peroxy acids with olefins. 4. Conclusion
O
O RC
RC H O
H
In summary, six cobalt(II) calix[4]pyrrole complexes were screened for their capability to use as a catalyst in the epoxidation reaction. Cobalt(II) calix[4]pyrrole omplex 4 exhibited impressive catalytic activity for epoxidation of alkene. According to the stereoselectivity investigation, the epoxidation of trans-stilbene gave only trans-stilbene oxide. In addition, the regioselectivity of 4-vinylcyclohexene gave 1,2-oxide as major more substituted alkenes could undergo the epoxidation reaction more effectively. This also implied that the active species responsible for the epoxidation should be an electrophilic species.
O RC O O
O2
RC O RC O O
+
O RC O OH
RC HO
O RC O OH (L n)C oII
Acknowledgements O RC O O
O RC O O C oLn
(L n) C oII
O RC OH
O
Sincere thanks are extended to Natural Products Research Unit, Department of Chemistry, Faculty of Science, Chulalongkorn University for the support of chemicals and laboratory facilities and to Graduate school Chulalongkorn University for the financial support. References
IV
(L n)C o O
Fig. 3. Proposed mechanism for cobalt(II) calix[4]pyrrole catalyzed epoxidation of alkenes.
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