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Polystyrene encapsulation of manganese porphyrins: highly efficient catalysts for oxidation of olefins
Catalysis Communications 6 (2005) 125–129 www.elsevier.com/locate/catcom
Polystyrene encapsulation of manganese porphyrins: highly efficient catalysts for oxidation of olefins Rajan Naik *, Padmakar Joshi *, Shubhangi Umbarkar, Rajesh K. Deshpande Department of Organic Chemistry (Synthesis), National Chemical Laboratory, Pashan Road, Pune 411 008, India Received 9 August 2004; accepted 25 November 2004 Available online 25 December 2004
Abstract Metalloporphyrins (MPs) of manganese have been successfully encapsulated for the first time in polystyrene matrix. The manganese porphyrins have been anchored onto polymer on the basis of physical envelopment by the polystyrene fibers, rendering them highly dispersible in common organic solvents. These polystyrene supported catalysts are characterized by UV–Vis and diffuse reflectance FT-IR spectroscopy. These encapsulated catalysts (MCMPs) exhibit enhanced activity for oxidation of alkenes in the presence of NaIO4, KHSO5 and NaOCl as oxidants. These catalysts not only possess high turnover frequencies but they are found to be quite stable and could be recovered quantitatively by simple filtration and reused without loss of activity. 2004 Elsevier B.V. All rights reserved. Keywords: Metalloporphyrins; Immobilization; Microencapsulation; Polystyrene support; Catalase model; Turnover frequency
1. Introduction Synthetic metalloporphyrins (MPs) are of great interest, as oxidation catalysts because their structures resemble those of the Cytochrome P-450, natureÕs catalysts for oxidation of foreign organic compounds in our bodies. MPs are well known for their ability to catalyse selective oxidation processes with a variety of oxygen donors such as iodosylbenzene, peroxides, and dioxyen [1]. The high efficiency of some of these systems makes them potentially useful for the preparative oxidations in organic chemistry; however, recovery and instability are two of the major drawbacks of these expensive catalysts [2]. To overcome these drawbacks, an important goal in recent studies has been the development of oxidatively stable and catalytically active polymer supported MPs. Current interest in supported MPs is directed towards
*
Corresponding authors. Tel./fax: +91 20 25893153. E-mail address: [email protected] (R. Naik).
1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.11.010
developing oxidation catalysts that combine the versatility of homogeneous MPs with the advantages of heterogeneous systems [3,4], such as prevention of catalytic intermolecular self oxidation, dimerization of sterically unhindered metalloporphyrins and easy recovery and reuse of the catalyst [5]. Furthermore, heterogeneous catalysts have become an important attractive target to ‘‘clean technology’’ since they present the possibility to minimize the problem of industrial waste treatment and disposal [6]. It is anticipated that immobilization will stabilize and/or modify the catalytic performance by influencing the chemo selectivity, regioselectivity and shapeselectivity of the reaction. A variety of supports have been tested, including inorganic carriers such as molecular sieves [7], silica [8], zeolites [9], clays [10], as well as organic polymers [11,12] and resins [13]. Some polymer supported MPs have been reported recently [14,15] and are used for oxidations of olefins. Anchoring of MPs on polymer supports appears to provide a method of obtaining catalysts, which are
126
R. Naik et al. / Catalysis Communications 6 (2005) 125–129
easier to handle, and possibly exhibiting an improved catalytic activity because of the support environment. These immobilized systems have obvious advantages because the catalysts are more easily separated from products and recycled which is especially important when dealing with fairly expensive MPs. Most of the synthetic MPs are used with various exogenous oxidants such as iodosylbenzene [16], alkyl hydroperoxides [17], hydrogen peroxide [18], oxone [19] and aqueous hypochlorite [20] that do not require a reducing agent. Micro encapsulation [21] (see Supplementary data sheet) is a classic method for immobilizing catalysts onto polymers on the basis of physical envelopment by the polymers and the catalysts are firmly anchored through the electronic interactions between the p electrons of the benzene rings of the polystyrene-based polymers and vacant orbital of catalysts. In our previous communication [22], we have reported the unprecedented synthesis of polystyrene supported metallophthalocynines as a catalase model system by using this micro encapsulation technique. Following the same methodology, we now report here the synthesis of polystyrene supported MPs Ômicroencapsulated metalloporphyrinsÕ (MCMPs), which can be easily prepared and found to be quite stable and effective in the oxidation of alkenes. By applying micro encapsulation technique, Mn(III) complexes of mesotetraphenylporphyrin (MnTPP)Cl and meso-tetrakis (2,6-dichlorophenyl)porphyrin Mn(TDCPP)Cl have been successfully anchored on polystyrene and they were characterized by UV–Vis and FT-IR spectroscopy. Using these catalysts the oxidations of styrene and a-methyl styrene were carried out with NaIO4, KHSO5 and NaOCl as oxidants.
2.2. Catalyst preparation 2.2.1. Method of encapsulation of manganese porphyrins (MnPs) in polystyrene matrix Polystyrene (5 g) was dissolved in 50 ml of CHCl3 at 50 C. MnP (0.5 g) was added and the dark colored solution was stirred for 1 h. Cooling of the solution to 0 C and further addition of 50 ml of methanol (drop by drop) separates out a thick, highly viscous mass, which on drying gave a hard material (MCMnP), that was crushed manually. By applying this technique, Mn(TPP)Cl and Mn(TDCPP)Cl were anchored onto three varieties of polystyrenes having different average molecular weights. 2.3. Characterization of the catalyst These encapsulated catalysts were characterized by UV–Vis and diffuse reflectance FT-IR spectroscopy. Fig. 1 shows the comparison of diffuse reflectance FTIR spectra of polystyrene, MCMn(TDCPP)Cl and Mn(TDCPP)Cl in the region 700–1400 cm 1. The UV–Vis spectrum of MCMn(TDCPP)Cl is compared with that of Mn(TDCPP)Cl in the region 200–700 nm. (Fig. 2). The comparisons of the spectra for MCMn(TPP)Cl and Mn(TPP)Cl are given in the Supplementary data sheet. Percentage loading was determined by increase in weight and confirmed by atomic absorption spectroscopy (AAS) as well as isolating unencapsulated MPs. 2.4. Catalytic oxidations 2.4.1. Oxidation with NaIO4 A mixture of styrene (0.416 g, 4 mmol), MCMn(TDCPP)Cl [(0.198 g, 0.5 mol% Mn(TDCPP)Cl], 759.9
2. Experimental 3.2 3.0 2.8 2.6
All chemicals were used as received from different commercial sources (Merck, Aldrich, Fluka) without further purification. Meso-tetraphenylporphyrin (TPP), and meso-tetrakis (2,6-dichloro tetraphenyl)porphyrin (TDCPP) were synthesized by the method reported by us [23] and their metallation was performed as per known procedure [24]. The purity was checked by comparing the UV–Vis and NMR spectra. Three different varieties of polystyrenes having average molecular weights of 40,000, 200,000 and 230,000 were obtained from Thermax India Ltd. Diffuse reflectance FT-IR spectra were recorded using Shimadzu 8300 instrument with liquid nitrogen cooled MCTD. The UV–Vis spectra were recorded on UV-1601PC Shimadzu UV–Vis spectrophotometer.
Absorbance
2.1. Materials
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8
1028.0 715.5
906.5
1271.0
1068.5
1314.0 1375.2
1180.4 964.4
840.9
1110.9 1222.8
Polystyrene
727.2 783.1 906.5
1028.0 1012.6 1074.3 964.4
810.1
1375.2
1191.9 1153.4
1334.6
885.3 1012.6
783.1 1076.2
727.2
MCMn(TDCPP)Cl 1334.6 1191.9 1205.4 1153.4
0.6 0.4 0.2 0.0
1307.6
1384.0
1255.4
885.3 858.3
Mn(TDCPP)Cl
700
800
900
1000
1100
1200
1300
1400
-1
Wavenumber (cm )
Fig. 1. Diffuse reflectance FT-IR spectrum MCMn(TDCPP)Cl and Mn(TDCPP)Cl.
of
Polystyrene,
R. Naik et al. / Catalysis Communications 6 (2005) 125–129
127
100
Conversion (%)
80
60
40
NaIO4 / MCMn(TPP)Cl KHSO5/ MCMn(TPP)Cl NaOCl / MCMn(TPP)Cl NaIO4 / MCMn(TDCPP)Cl KHSO5/ MCMn(TDCPP)Cl NaOCl / MCMn(TDCPP)Cl
20
Fig. 2. UV–Vis spectrum of Mn(TDCPP)Cl (blue) and MCMn(TDCPP)Cl (black) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
imidazole (0.055 g, 0.8 mmol), NaIO4 solution (1.75 g in 20 ml H2O) and acetonitrile (40 ml) was refluxed for 2.5 h. Reaction was monitored by TLC. When all starting material has vanished, it was cooled and organic layer was separated. Aqueous layer was extracted with dichloromethane (2 · 50 ml). Combined organic extract was dried (Na2SO4) and concentrated to 10 ml. Hexane (10 ml) was added to it and precipitated catalyst was filtered. The catalyst was washed with 10 ml of dichloromethane:hexane mixture (1:1), and combined filtrate was evaporated to get a liquid. It was purified by column chromatography (silica gel) to get styrene oxide as a single major product, which was characterized by spectroscopy. (IR, NMR, Mass). Yield 95%. 2.4.2. Oxidation with KHSO5 To a stirred mixture of styrene (0.416 g, 4 mmol), pyridine (0.143 g, 2 mmol), MCMn(TDCPP)Cl [0.198 g, 0.5 mol% Mn(TDCPP)Cl], cetrimide (0.022 g) and dichloromethane (10 ml) was added a buffered solution of KHSO5 [2.15 g (7 mmol) of KHSO5 was dissolved in 15 ml of buffer prepared from 7.5 ml 0.5 M KH2PO4 and 7.5 ml 0.5 M Na2HPO4]. It was stirred at room temperature for 3 h. The reaction was monitored by TLC. The organic layer was separated and the aqueous layer was extracted with dichloromethane (2 · 50 ml). Combined organic layer was worked up as described before and the catalyst was recovered. Crude product was purified by column chromatography (silica gel) to get styrene oxide, which was characterized by spectroscopy. Yield 84%. 2.4.3. Oxidation with NaOCl A mixture of styrene (0.416 g, 4 mmol), imidazole (0.08 g,12 mmol), MCMn(TDCPP)Cl [(0.198 g, 0.5 mol% Mn(TDCPP)Cl)], cetrimide (0.022 g) and dichloromethane (10 ml) was stirred at room temperature for 10 min. A buffered solution of NaOCl (13 ml of 0.5 M, buffered to pH 9.5 using NaHCO3) was added to it
0 0
1
2
3
Time (hours)
Fig. 3. Comparative study using different catalysts/oxidants in oxidation of styrene.
and the reaction mixture was stirred at room temperature for 2.75 h (Fig. 3). The reaction was monitored by TLC and worked up as stated earlier to get styrene oxide as a major product. Yield 80%.
3. Results and discussions Polystyrene having average molecular weight 40,000 shows less loading as compared to polystyrene having higher molecular weights. Considering the higher loading and batch-to-batch consistency in the oxidation reactions, polystyrene having average molecular weight of 2,00,000 was found to be ideal for encapsulation (Table 1). The diffuse reflectance FT-IR spectrum of MCMn(TDCPP)Cl is in excellent agreement with that of neat Mn(TDCPP)Cl with additional peaks due to polystyrene network (Fig. 1). The spectrum of MCMn(TDCPP)Cl exhibit absorptions at 906.5 and 1028.0 cm 1 which are typical of polystyrene. Similarly, absorptions at 1012.6, 1191.9 and 1334.6 cm 1 are identical with that of free Mn(TDCPP)Cl. The UV–Vis spectrum of MCMn(TDCPP)Cl totally resembles with the spectrum of neat Mn(TDCPP)Cl (Fig. 2). Similar spectral behavior is also observed in case of MCMn(TPP)Cl and Mn(TPP)Cl (see Supplementary data sheet). This suggests that MPs are structurally unchanged and uniformly distributed in the polystyrene matrix, which proves that MPs preserve their identity after encapsulation, and expected to retain catalytic activity during oxidations. Using these catalysts the catalytic oxidations with various oxidants in presence of axial ligands have been carried out. These oxidations result in excellent yields with high turnover frequencies as depicted in Table 2. The encapsulated catalysts (MCMPs) were found to be superior to the corresponding ‘‘neat’’ MPs in terms of enhanced activity and improved stability. This is pos-
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Table 1 Epoxidation of styrene with NaIO4 using MCMPs Catalysta
Polymer
Oxidant.
Conversiond (%)
Time (h)
MCMn(TDCPP)Cl
P1a
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
94 88 73 99 92 80 98 90 79
3.8 3.2 3.0 3.5 3.0 2.8 3.4 2.8 2.6
P2b
P3c
a b c d
Polystyrene having average molecular weight of 40,000. Polystyrene having average molecular weight of 200,000. Polystyrene having average molecular weight of 230,000. Based on GC analysis.
sibly due to uniform dispersion and site isolation of metalloporphyrins in the polymer matrix. The availability of chemically stable Mn(TDCPP)Cl and the use of imidazole or pyridine as axial ligand which are entirely soluble in organic phase, allowed us to optimize the conditions for oxidation of olefins. In the present study, we observed the effect of imidazole in the oxidations using NaIO4 and NaOCl while pyridine is used in the reactions with KHSO5. Fig. 3 represents a comparative study for catalytic oxidations of styrene using both the MCMPs with different oxidants. The use of MCMn(TPP)Cl and MCMn(TDCPP)Cl as catalysts for the epoxidation of styrene and a-methyl styrene with NaIO4 as oxidant has been studied as depicted in Table 1. The reactions have been carried out
in the mixture of acetonitrile and water with imidazole as an axial ligand at reflux temperature. In all the four cases almost quantitative conversions were obtained with high TOF values to produce epoxides in excellent yields. Lower TOF values were obtained only in the case of neat Mn(TPP)Cl however neat Mn(TDCPP)Cl gave higher TOF values (Table 2), due to more stability and higher catalytic activity due to chlorine substituents, which stabilize a high oxidation state of the metal and increases its oxidation potential. The oxidations of styrene and a-methyl styrene with buffered mixture of water/dichloromethane in presence of cetrimide as a phase transfer agent and KHSO5 as oxidant were carried out as given in Table 2. As an aqueous solution of KHSO5 is very acidic, a solution of this oxidant in phosphate buffer of pH 7 was used in order to avoid fast acid catalyzed opening of the oxirane ring. For enhancement of the rate of reaction, pyridine was used as an axial ligand. As expected, high TOF values were obtained for MCMn(TDCPP)Cl than MCMn(TPP)Cl. The oxidation reaction in case of styrene with MCMn(TPP)Cl stops after 2.5 h undergoing a conversion of 68%, might be due to catalyst deactivation, while the reaction with MCMn(TDCPP)Cl gave 92% conversion after 3 h (Fig. 3). Epoxidations using MCMn(TPP)Cl and MCMn(TDCPP)Cl were performed in presence of imidazole as an axial ligand, a phase transfer catalyst cetrimide and 0.5 M NaOCl aqueous solution at pH 9.5, maintained by the addition of NaHCO3. Very low conversions were obtained in case of Mn(TPP)Cl and
Table 2 Catalytic oxidation of olefins with various oxidants using MCMPs Entry
Catalysta
Substrate
Oxidantb,c,d
Conversione (%)
Epoxide yieldsf (%)
Time (h)
TOFg (h 1)
TOFh (h 1)
1
MCMn(TPP)Cl
Styrene
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
98 68 42 97 62 38 99 92 80 98 89 78
94 62 37 92 54 30 95 84 80 94 86 74
2.8 2.5 1.0 3.5 2.8 3.5 3.5 3.0 2.8 6.5 3.0 3.5
71 54 84 55 44 22 57 61 57 30 59 43
60 46 – 43 31 – 50 56 23 27 54 21
a-methyl styrene
2
MCMn(TDCPP)Cl
Styrene
a-methyl styrene
a
Encapsulated Mn catalyst with polystyrene having average mol. wt. of 200,000. A mixture of styrene (4 mmol), encapsulated catalyst (0.5 mol% of MP), imidazole (0.8 mmol), NaIO4 (1.75 g, 8 mmol in 20 ml H2O), CH3CN (40 ml), under reflux. c A mixture of styrene (4 mmol), encapsulated catalyst (5 mol% of MP), pyridine (2 mmol), cetrimide (0.022 g), KHSO5 (7 mmol, in 15 ml buffer with a mixture of 7.5 ml. of 0.5 M KH2PO4 and 7.5 ml of 0.5 M Na2HPO4), dichloromethane (10 ml) stirred at room temperature. d A mixture of styrene (4 mmol) encapsulated catalyst (5 mol% of MP), imidazole (12 mmol), cetrimide (0.022 g), NaOCl (13 ml of 0.5 M, 6.5 mmol, buffered with NaHCO3 at pH 9.5), dichloromethane (10 ml) stirred at room temperature. e The conversions are obtained by GC analysis. f Isolated yields. g Turnover frequencies (TOFs) using MCMPs, defined as moles of olefin reacted per moles of metal per hour. h Turnover frequencies (TOFs) using neat MPs. b
R. Naik et al. / Catalysis Communications 6 (2005) 125–129 Table 3 Oxidation of styrene using recycled catalysts with NaIO4, KHSO5 and NaOCl as oxidants Entry
Catalyst
Oxidant.
Conversion (%) after 1st recycle
Conversion (%) after 2nd recycle
1
MCMn(TPP)Cl
2
MCMn(TDCPP)Cl
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
97 55 23 98 90 79
95 32 Degraded 96 88 77
MCMn(TPP)Cl catalysts since they were susceptible to self-oxidation. Both Mn(TDCPP)Cl and MC(MnTDCPP)Cl were stable to our oxidation conditions and gave good conversions (Table 2). The NaOCl oxidation of styrene using MCMn(TPP)Cl stops after 1 h giving conversion of only 42% most probably due to catalyst degradation but with MCMn(TDCPP)Cl it gave a conversion of 80% after 2.75 h. (Fig. 3). The reusability of the MCMPs was studied by comparing repetitive oxidations (Table 3). In NaIO4 oxidations both the encapsulated catalysts can be recycled at least two times without leaching, however, in case of KHSO5 and NaOCl oxidations MCMn(TDCPP)Cl catalysts are stable at least for two cycles but a partial leaching followed by degradation was observed in case of MCMn(TPP)Cl in the first recycle itself. The higher TOF values for MCMPs than the free MPs (Table 2) indicate their higher activity towards catalytic oxidations. Similarly, the catalyst Mn(TDCPP)Cl was found to be more stable towards oxidative degradation, may be perhaps due to the steric protection of the metal center caused by bulky and electron withdrawing chlorine substituents in the ortho positions of the phenyl rings. Considering very high TOF values for the oxidations with NaIO4, this was found to be the method of choice for catalytic oxidations. 4. Conclusion This communication provides an efficient and easy method for encapsulation of manganese porphyrins in polystyrenes, in general, which gives stable, reusable and very effective catalysts for the oxidation of alkenes in presence of NaIO4, KHSO5 and NaOCl. These MCMnPs exhibit higher catalytic activity and are found to be more stable than free MnPs due to site isolation and high dispersion in the organic phase. Because of more bulky and electronegative chlorine substituents at the ortho positions in the phenyl rings, the Mn(TDCPP)Cl catalysts are found to be more stable than Mn(TPP)Cl, and show better catalytic activity towards oxidations of olefins.
129
Acknowledgements We thank Dr. K.N. Ganesh for his encouragement. We thank Thermax (India) Ltd. for the supply of polystyrenes. We are also thankful to DST, New Delhi, for financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/j.catcom. 2004.11.010.
References [1] T. Mlodnicka, B.R. James, in: F. Montanari, I. Casella (Eds.), Metalloporphyrin Catalyzed Oxidations, Kluwer Academic publishers, Dordrecht, 1994. [2] B. Meunier, Chem. Rev. 92 (1992) 1411–1456. [3] R.A. Sheldon (Ed.), Metalloporphyrins in Catalytic Oxidations, Marcel Dekker Inc., New York, 1994 (Chapter 11). [4] D. Mansuy, Coord. Chem. Rev. 125 (1993) 129–141. [5] D.R. Leanord, J.R. Lindsay Smith, J. Chem. Soc. Perkin Trans. 2 (1990) 1917–1923. [6] R.A. Sheldon, Chemtech (1991) 566–576. [7] Z. Li, C. Xia, X. Zhang, J. Mol. Catal. A 185 (2002) 47–56. [8] F.S. Vinhado, C.M.C. Prado-Manso, H.C. Sacco, Y. Iamamoto, J. Mol. Catal. A 174 (2001) 279–288. [9] I.L. Viana Rosa, C.M.C.P. Manso, O.A. Serra, Y. Iamamoto, J. Mol. Catal. A 160 (2000) 199–208. [10] F. Bedioui, Coord. Chem. Rev. 144 (1995) 39–68. [11] P.R. Cooke, J.R. Lindsay Smith, J. Chem. Soc. Perkin Trans. I (1994) 1913–1923. [12] M.V. Vinodu, M. Padmanabhan, Proc. Indian Acad. Sci. (Chem. Sci.) 110 (1998) 461–470. [13] S. Campestrini, B. Meunier, Inorg. Chem. 31 (1992) 1999– 2006. [14] C. Du, Z. Li, X. Wen, J. Wu, X. Yu, M. Yang, R. Xie, J. Mol. Catal. A 216 (2004) 7–12. [15] M. Moghadam, S. Tangestaninejad, M.H. Habibi, V. Mirkhani, J. Mol. Catal. A 217 (2004) 9–12. [16] J.T. Groves, T.E. Nemo, R.S. Myers, J. Am. Chem. Soc. 101 (1979) 1032–1033. [17] Q. Zhang, Y. Wang, S. Itsuki, T. Shishido, K. Takehira, J. Mol. Catal. A 188 (2002) 189–200. [18] G. Traylor, S. Tsuchiya, Y. Byun, C. Kim, J. Am. Chem. Soc. 115 (1993) 2775–2781. [19] P. Hoffmann, G. Labat, A. Robert, B. Meunier, Terahedron Lett. 31 (1990) 1991–1994. [20] B. Meunier, E. Guilmet, M.E. de Carvalho, R. Poilblanc, J. Am. Chem. Soc. 106 (1994) 6668–6676. [21] S. Kobayashi, S. Nagayoma, J. Am. Chem. Soc. 120 (1998) 2985– 2986. [22] R.H. Naik, P.L. Joshi, R.K. Deshpande, Catal. Commun. 5 (2004) 195–198. [23] R.H. Naik, P.L. Joshi, S.P.K. Vakil, R.K. Deshpande, Tetrahedron 59 (2003) 2207–2213. [24] A.D. Adler, F. Kampas, K.J. Kim, J. Inorg. Nucl. Chem. 32 (1970) 2443–2445.
Polystyrene encapsulation of manganese porphyrins: highly efficient catalysts for oxidation of olefins Rajan Naik *, Padmakar Joshi *, Shubhangi Umbarkar, Rajesh K. Deshpande Department of Organic Chemistry (Synthesis), National Chemical Laboratory, Pashan Road, Pune 411 008, India Received 9 August 2004; accepted 25 November 2004 Available online 25 December 2004
Abstract Metalloporphyrins (MPs) of manganese have been successfully encapsulated for the first time in polystyrene matrix. The manganese porphyrins have been anchored onto polymer on the basis of physical envelopment by the polystyrene fibers, rendering them highly dispersible in common organic solvents. These polystyrene supported catalysts are characterized by UV–Vis and diffuse reflectance FT-IR spectroscopy. These encapsulated catalysts (MCMPs) exhibit enhanced activity for oxidation of alkenes in the presence of NaIO4, KHSO5 and NaOCl as oxidants. These catalysts not only possess high turnover frequencies but they are found to be quite stable and could be recovered quantitatively by simple filtration and reused without loss of activity. 2004 Elsevier B.V. All rights reserved. Keywords: Metalloporphyrins; Immobilization; Microencapsulation; Polystyrene support; Catalase model; Turnover frequency
1. Introduction Synthetic metalloporphyrins (MPs) are of great interest, as oxidation catalysts because their structures resemble those of the Cytochrome P-450, natureÕs catalysts for oxidation of foreign organic compounds in our bodies. MPs are well known for their ability to catalyse selective oxidation processes with a variety of oxygen donors such as iodosylbenzene, peroxides, and dioxyen [1]. The high efficiency of some of these systems makes them potentially useful for the preparative oxidations in organic chemistry; however, recovery and instability are two of the major drawbacks of these expensive catalysts [2]. To overcome these drawbacks, an important goal in recent studies has been the development of oxidatively stable and catalytically active polymer supported MPs. Current interest in supported MPs is directed towards
*
Corresponding authors. Tel./fax: +91 20 25893153. E-mail address: [email protected] (R. Naik).
1566-7367/$ - see front matter 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.catcom.2004.11.010
developing oxidation catalysts that combine the versatility of homogeneous MPs with the advantages of heterogeneous systems [3,4], such as prevention of catalytic intermolecular self oxidation, dimerization of sterically unhindered metalloporphyrins and easy recovery and reuse of the catalyst [5]. Furthermore, heterogeneous catalysts have become an important attractive target to ‘‘clean technology’’ since they present the possibility to minimize the problem of industrial waste treatment and disposal [6]. It is anticipated that immobilization will stabilize and/or modify the catalytic performance by influencing the chemo selectivity, regioselectivity and shapeselectivity of the reaction. A variety of supports have been tested, including inorganic carriers such as molecular sieves [7], silica [8], zeolites [9], clays [10], as well as organic polymers [11,12] and resins [13]. Some polymer supported MPs have been reported recently [14,15] and are used for oxidations of olefins. Anchoring of MPs on polymer supports appears to provide a method of obtaining catalysts, which are
126
R. Naik et al. / Catalysis Communications 6 (2005) 125–129
easier to handle, and possibly exhibiting an improved catalytic activity because of the support environment. These immobilized systems have obvious advantages because the catalysts are more easily separated from products and recycled which is especially important when dealing with fairly expensive MPs. Most of the synthetic MPs are used with various exogenous oxidants such as iodosylbenzene [16], alkyl hydroperoxides [17], hydrogen peroxide [18], oxone [19] and aqueous hypochlorite [20] that do not require a reducing agent. Micro encapsulation [21] (see Supplementary data sheet) is a classic method for immobilizing catalysts onto polymers on the basis of physical envelopment by the polymers and the catalysts are firmly anchored through the electronic interactions between the p electrons of the benzene rings of the polystyrene-based polymers and vacant orbital of catalysts. In our previous communication [22], we have reported the unprecedented synthesis of polystyrene supported metallophthalocynines as a catalase model system by using this micro encapsulation technique. Following the same methodology, we now report here the synthesis of polystyrene supported MPs Ômicroencapsulated metalloporphyrinsÕ (MCMPs), which can be easily prepared and found to be quite stable and effective in the oxidation of alkenes. By applying micro encapsulation technique, Mn(III) complexes of mesotetraphenylporphyrin (MnTPP)Cl and meso-tetrakis (2,6-dichlorophenyl)porphyrin Mn(TDCPP)Cl have been successfully anchored on polystyrene and they were characterized by UV–Vis and FT-IR spectroscopy. Using these catalysts the oxidations of styrene and a-methyl styrene were carried out with NaIO4, KHSO5 and NaOCl as oxidants.
2.2. Catalyst preparation 2.2.1. Method of encapsulation of manganese porphyrins (MnPs) in polystyrene matrix Polystyrene (5 g) was dissolved in 50 ml of CHCl3 at 50 C. MnP (0.5 g) was added and the dark colored solution was stirred for 1 h. Cooling of the solution to 0 C and further addition of 50 ml of methanol (drop by drop) separates out a thick, highly viscous mass, which on drying gave a hard material (MCMnP), that was crushed manually. By applying this technique, Mn(TPP)Cl and Mn(TDCPP)Cl were anchored onto three varieties of polystyrenes having different average molecular weights. 2.3. Characterization of the catalyst These encapsulated catalysts were characterized by UV–Vis and diffuse reflectance FT-IR spectroscopy. Fig. 1 shows the comparison of diffuse reflectance FTIR spectra of polystyrene, MCMn(TDCPP)Cl and Mn(TDCPP)Cl in the region 700–1400 cm 1. The UV–Vis spectrum of MCMn(TDCPP)Cl is compared with that of Mn(TDCPP)Cl in the region 200–700 nm. (Fig. 2). The comparisons of the spectra for MCMn(TPP)Cl and Mn(TPP)Cl are given in the Supplementary data sheet. Percentage loading was determined by increase in weight and confirmed by atomic absorption spectroscopy (AAS) as well as isolating unencapsulated MPs. 2.4. Catalytic oxidations 2.4.1. Oxidation with NaIO4 A mixture of styrene (0.416 g, 4 mmol), MCMn(TDCPP)Cl [(0.198 g, 0.5 mol% Mn(TDCPP)Cl], 759.9
2. Experimental 3.2 3.0 2.8 2.6
All chemicals were used as received from different commercial sources (Merck, Aldrich, Fluka) without further purification. Meso-tetraphenylporphyrin (TPP), and meso-tetrakis (2,6-dichloro tetraphenyl)porphyrin (TDCPP) were synthesized by the method reported by us [23] and their metallation was performed as per known procedure [24]. The purity was checked by comparing the UV–Vis and NMR spectra. Three different varieties of polystyrenes having average molecular weights of 40,000, 200,000 and 230,000 were obtained from Thermax India Ltd. Diffuse reflectance FT-IR spectra were recorded using Shimadzu 8300 instrument with liquid nitrogen cooled MCTD. The UV–Vis spectra were recorded on UV-1601PC Shimadzu UV–Vis spectrophotometer.
Absorbance
2.1. Materials
2.4 2.2 2.0 1.8 1.6 1.4 1.2 1.0 0.8
1028.0 715.5
906.5
1271.0
1068.5
1314.0 1375.2
1180.4 964.4
840.9
1110.9 1222.8
Polystyrene
727.2 783.1 906.5
1028.0 1012.6 1074.3 964.4
810.1
1375.2
1191.9 1153.4
1334.6
885.3 1012.6
783.1 1076.2
727.2
MCMn(TDCPP)Cl 1334.6 1191.9 1205.4 1153.4
0.6 0.4 0.2 0.0
1307.6
1384.0
1255.4
885.3 858.3
Mn(TDCPP)Cl
700
800
900
1000
1100
1200
1300
1400
-1
Wavenumber (cm )
Fig. 1. Diffuse reflectance FT-IR spectrum MCMn(TDCPP)Cl and Mn(TDCPP)Cl.
of
Polystyrene,
R. Naik et al. / Catalysis Communications 6 (2005) 125–129
127
100
Conversion (%)
80
60
40
NaIO4 / MCMn(TPP)Cl KHSO5/ MCMn(TPP)Cl NaOCl / MCMn(TPP)Cl NaIO4 / MCMn(TDCPP)Cl KHSO5/ MCMn(TDCPP)Cl NaOCl / MCMn(TDCPP)Cl
20
Fig. 2. UV–Vis spectrum of Mn(TDCPP)Cl (blue) and MCMn(TDCPP)Cl (black) (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.).
imidazole (0.055 g, 0.8 mmol), NaIO4 solution (1.75 g in 20 ml H2O) and acetonitrile (40 ml) was refluxed for 2.5 h. Reaction was monitored by TLC. When all starting material has vanished, it was cooled and organic layer was separated. Aqueous layer was extracted with dichloromethane (2 · 50 ml). Combined organic extract was dried (Na2SO4) and concentrated to 10 ml. Hexane (10 ml) was added to it and precipitated catalyst was filtered. The catalyst was washed with 10 ml of dichloromethane:hexane mixture (1:1), and combined filtrate was evaporated to get a liquid. It was purified by column chromatography (silica gel) to get styrene oxide as a single major product, which was characterized by spectroscopy. (IR, NMR, Mass). Yield 95%. 2.4.2. Oxidation with KHSO5 To a stirred mixture of styrene (0.416 g, 4 mmol), pyridine (0.143 g, 2 mmol), MCMn(TDCPP)Cl [0.198 g, 0.5 mol% Mn(TDCPP)Cl], cetrimide (0.022 g) and dichloromethane (10 ml) was added a buffered solution of KHSO5 [2.15 g (7 mmol) of KHSO5 was dissolved in 15 ml of buffer prepared from 7.5 ml 0.5 M KH2PO4 and 7.5 ml 0.5 M Na2HPO4]. It was stirred at room temperature for 3 h. The reaction was monitored by TLC. The organic layer was separated and the aqueous layer was extracted with dichloromethane (2 · 50 ml). Combined organic layer was worked up as described before and the catalyst was recovered. Crude product was purified by column chromatography (silica gel) to get styrene oxide, which was characterized by spectroscopy. Yield 84%. 2.4.3. Oxidation with NaOCl A mixture of styrene (0.416 g, 4 mmol), imidazole (0.08 g,12 mmol), MCMn(TDCPP)Cl [(0.198 g, 0.5 mol% Mn(TDCPP)Cl)], cetrimide (0.022 g) and dichloromethane (10 ml) was stirred at room temperature for 10 min. A buffered solution of NaOCl (13 ml of 0.5 M, buffered to pH 9.5 using NaHCO3) was added to it
0 0
1
2
3
Time (hours)
Fig. 3. Comparative study using different catalysts/oxidants in oxidation of styrene.
and the reaction mixture was stirred at room temperature for 2.75 h (Fig. 3). The reaction was monitored by TLC and worked up as stated earlier to get styrene oxide as a major product. Yield 80%.
3. Results and discussions Polystyrene having average molecular weight 40,000 shows less loading as compared to polystyrene having higher molecular weights. Considering the higher loading and batch-to-batch consistency in the oxidation reactions, polystyrene having average molecular weight of 2,00,000 was found to be ideal for encapsulation (Table 1). The diffuse reflectance FT-IR spectrum of MCMn(TDCPP)Cl is in excellent agreement with that of neat Mn(TDCPP)Cl with additional peaks due to polystyrene network (Fig. 1). The spectrum of MCMn(TDCPP)Cl exhibit absorptions at 906.5 and 1028.0 cm 1 which are typical of polystyrene. Similarly, absorptions at 1012.6, 1191.9 and 1334.6 cm 1 are identical with that of free Mn(TDCPP)Cl. The UV–Vis spectrum of MCMn(TDCPP)Cl totally resembles with the spectrum of neat Mn(TDCPP)Cl (Fig. 2). Similar spectral behavior is also observed in case of MCMn(TPP)Cl and Mn(TPP)Cl (see Supplementary data sheet). This suggests that MPs are structurally unchanged and uniformly distributed in the polystyrene matrix, which proves that MPs preserve their identity after encapsulation, and expected to retain catalytic activity during oxidations. Using these catalysts the catalytic oxidations with various oxidants in presence of axial ligands have been carried out. These oxidations result in excellent yields with high turnover frequencies as depicted in Table 2. The encapsulated catalysts (MCMPs) were found to be superior to the corresponding ‘‘neat’’ MPs in terms of enhanced activity and improved stability. This is pos-
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R. Naik et al. / Catalysis Communications 6 (2005) 125–129
Table 1 Epoxidation of styrene with NaIO4 using MCMPs Catalysta
Polymer
Oxidant.
Conversiond (%)
Time (h)
MCMn(TDCPP)Cl
P1a
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
94 88 73 99 92 80 98 90 79
3.8 3.2 3.0 3.5 3.0 2.8 3.4 2.8 2.6
P2b
P3c
a b c d
Polystyrene having average molecular weight of 40,000. Polystyrene having average molecular weight of 200,000. Polystyrene having average molecular weight of 230,000. Based on GC analysis.
sibly due to uniform dispersion and site isolation of metalloporphyrins in the polymer matrix. The availability of chemically stable Mn(TDCPP)Cl and the use of imidazole or pyridine as axial ligand which are entirely soluble in organic phase, allowed us to optimize the conditions for oxidation of olefins. In the present study, we observed the effect of imidazole in the oxidations using NaIO4 and NaOCl while pyridine is used in the reactions with KHSO5. Fig. 3 represents a comparative study for catalytic oxidations of styrene using both the MCMPs with different oxidants. The use of MCMn(TPP)Cl and MCMn(TDCPP)Cl as catalysts for the epoxidation of styrene and a-methyl styrene with NaIO4 as oxidant has been studied as depicted in Table 1. The reactions have been carried out
in the mixture of acetonitrile and water with imidazole as an axial ligand at reflux temperature. In all the four cases almost quantitative conversions were obtained with high TOF values to produce epoxides in excellent yields. Lower TOF values were obtained only in the case of neat Mn(TPP)Cl however neat Mn(TDCPP)Cl gave higher TOF values (Table 2), due to more stability and higher catalytic activity due to chlorine substituents, which stabilize a high oxidation state of the metal and increases its oxidation potential. The oxidations of styrene and a-methyl styrene with buffered mixture of water/dichloromethane in presence of cetrimide as a phase transfer agent and KHSO5 as oxidant were carried out as given in Table 2. As an aqueous solution of KHSO5 is very acidic, a solution of this oxidant in phosphate buffer of pH 7 was used in order to avoid fast acid catalyzed opening of the oxirane ring. For enhancement of the rate of reaction, pyridine was used as an axial ligand. As expected, high TOF values were obtained for MCMn(TDCPP)Cl than MCMn(TPP)Cl. The oxidation reaction in case of styrene with MCMn(TPP)Cl stops after 2.5 h undergoing a conversion of 68%, might be due to catalyst deactivation, while the reaction with MCMn(TDCPP)Cl gave 92% conversion after 3 h (Fig. 3). Epoxidations using MCMn(TPP)Cl and MCMn(TDCPP)Cl were performed in presence of imidazole as an axial ligand, a phase transfer catalyst cetrimide and 0.5 M NaOCl aqueous solution at pH 9.5, maintained by the addition of NaHCO3. Very low conversions were obtained in case of Mn(TPP)Cl and
Table 2 Catalytic oxidation of olefins with various oxidants using MCMPs Entry
Catalysta
Substrate
Oxidantb,c,d
Conversione (%)
Epoxide yieldsf (%)
Time (h)
TOFg (h 1)
TOFh (h 1)
1
MCMn(TPP)Cl
Styrene
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
98 68 42 97 62 38 99 92 80 98 89 78
94 62 37 92 54 30 95 84 80 94 86 74
2.8 2.5 1.0 3.5 2.8 3.5 3.5 3.0 2.8 6.5 3.0 3.5
71 54 84 55 44 22 57 61 57 30 59 43
60 46 – 43 31 – 50 56 23 27 54 21
a-methyl styrene
2
MCMn(TDCPP)Cl
Styrene
a-methyl styrene
a
Encapsulated Mn catalyst with polystyrene having average mol. wt. of 200,000. A mixture of styrene (4 mmol), encapsulated catalyst (0.5 mol% of MP), imidazole (0.8 mmol), NaIO4 (1.75 g, 8 mmol in 20 ml H2O), CH3CN (40 ml), under reflux. c A mixture of styrene (4 mmol), encapsulated catalyst (5 mol% of MP), pyridine (2 mmol), cetrimide (0.022 g), KHSO5 (7 mmol, in 15 ml buffer with a mixture of 7.5 ml. of 0.5 M KH2PO4 and 7.5 ml of 0.5 M Na2HPO4), dichloromethane (10 ml) stirred at room temperature. d A mixture of styrene (4 mmol) encapsulated catalyst (5 mol% of MP), imidazole (12 mmol), cetrimide (0.022 g), NaOCl (13 ml of 0.5 M, 6.5 mmol, buffered with NaHCO3 at pH 9.5), dichloromethane (10 ml) stirred at room temperature. e The conversions are obtained by GC analysis. f Isolated yields. g Turnover frequencies (TOFs) using MCMPs, defined as moles of olefin reacted per moles of metal per hour. h Turnover frequencies (TOFs) using neat MPs. b
R. Naik et al. / Catalysis Communications 6 (2005) 125–129 Table 3 Oxidation of styrene using recycled catalysts with NaIO4, KHSO5 and NaOCl as oxidants Entry
Catalyst
Oxidant.
Conversion (%) after 1st recycle
Conversion (%) after 2nd recycle
1
MCMn(TPP)Cl
2
MCMn(TDCPP)Cl
NaIO4 KHSO5 NaOCl NaIO4 KHSO5 NaOCl
97 55 23 98 90 79
95 32 Degraded 96 88 77
MCMn(TPP)Cl catalysts since they were susceptible to self-oxidation. Both Mn(TDCPP)Cl and MC(MnTDCPP)Cl were stable to our oxidation conditions and gave good conversions (Table 2). The NaOCl oxidation of styrene using MCMn(TPP)Cl stops after 1 h giving conversion of only 42% most probably due to catalyst degradation but with MCMn(TDCPP)Cl it gave a conversion of 80% after 2.75 h. (Fig. 3). The reusability of the MCMPs was studied by comparing repetitive oxidations (Table 3). In NaIO4 oxidations both the encapsulated catalysts can be recycled at least two times without leaching, however, in case of KHSO5 and NaOCl oxidations MCMn(TDCPP)Cl catalysts are stable at least for two cycles but a partial leaching followed by degradation was observed in case of MCMn(TPP)Cl in the first recycle itself. The higher TOF values for MCMPs than the free MPs (Table 2) indicate their higher activity towards catalytic oxidations. Similarly, the catalyst Mn(TDCPP)Cl was found to be more stable towards oxidative degradation, may be perhaps due to the steric protection of the metal center caused by bulky and electron withdrawing chlorine substituents in the ortho positions of the phenyl rings. Considering very high TOF values for the oxidations with NaIO4, this was found to be the method of choice for catalytic oxidations. 4. Conclusion This communication provides an efficient and easy method for encapsulation of manganese porphyrins in polystyrenes, in general, which gives stable, reusable and very effective catalysts for the oxidation of alkenes in presence of NaIO4, KHSO5 and NaOCl. These MCMnPs exhibit higher catalytic activity and are found to be more stable than free MnPs due to site isolation and high dispersion in the organic phase. Because of more bulky and electronegative chlorine substituents at the ortho positions in the phenyl rings, the Mn(TDCPP)Cl catalysts are found to be more stable than Mn(TPP)Cl, and show better catalytic activity towards oxidations of olefins.
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Acknowledgements We thank Dr. K.N. Ganesh for his encouragement. We thank Thermax (India) Ltd. for the supply of polystyrenes. We are also thankful to DST, New Delhi, for financial support.
Appendix A. Supplementary data Supplementary data associated with this article can be found, in the online version at doi:10.1016/j.catcom. 2004.11.010.
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