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Comparison of mesoporous silica and alumina supports for palladium-catalyzed carbon–carbon coupling reactions: unexpected high acceleration by supported cetyltrimethylammonium bromide
Microporous and Mesoporous Materials 44±45 (2001) 517±522
www.elsevier.nl/locate/micromeso
Comparison of mesoporous silica and alumina supports for palladium-catalyzed carbon±carbon coupling reactions: unexpected high acceleration by supported cetyltrimethylammonium bromide E. Paetzold a,*, G. Oehme a, H. Fuhrmann a, M. Richter b, R. Eckelt b, M.-M. Pohl b, H. Kosslick b a
Institut f ur Organische Katalyseforschung, Buchbinderstrasse 5/6, D-18055 Rostock, Germany Institut f ur Angewandte Chemie, Richard-Willstatter-Strasse 12, D-12489 Berlin, Germany
b
Received 26 May 2000; accepted 31 August 2000
Abstract The coupling reaction of p-iodoanisole and phenylboronic acid occurs with high yields in aqueous medium in the presence of palladium complexes with water-soluble phosphine ligands on dierent mesoporous inorganic supports. Silica and alumina supports were similarly prepared and functionalized with sulfonate groups. The catalyst was immobilized by ion exchange. A novel approach to the amphiphilization of the supports by cetyltrimethylammonium bromide was applied to modify phase transfer steps favorably and to embed the catalyst in this bipolar surface layer. The reaction is accelerated and conversions with these embedded catalysts exceed those in homogeneous systems. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon±carbon coupling; Suzuki reaction; Two-phase system; Water-soluble phosphine catalyst; Mesoporous supports
1. Introduction Carbon±carbon bond formation is a general aim in the transition metal catalyzed organic synthetic chemistry [1]. The reaction of organoboron compounds with aryl, alkenyl and alkynyl halides by means of palladium compounds as catalysts is a modern coupling process (Suzuki reaction) [2]. In contrast to modi®ed water-soluble palladium catalysts, typical reaction components like aryl,
*
Corresponding author.
allyl or benzyl halogenides and boronic acids as well as their coupling products are very sparingly soluble in water. Hence, most of the syntheses are realized in organic solvents. Only few papers deal with the reaction in aqueous two-phase systems [3± 7], allowing the readily recycling of the water phase which contains the palladium catalyst. To overcome these problems, the Suzuki-type coupling has been performed in an aqueous micellar medium. However, the precipitation of the diaryl products could not be avoided. Therefore, a two-phase system consisting of water and toluene was used [8]. Several suitable phase transfer reagents have been added to promote the transport
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of the water-soluble palladium catalyst into the reactant-containing toluene phase. An accelerating eect on the reaction was found in dependence on the structure of the added amphiphiles [9]. However, the phase separation after reaction became dicult because of the amphiphilization of the interface. Certain improvements could be achieved by the utilization of a new type of tri-phase catalyst [10±12], however, for practical purposes it is preferable to carry out the reaction heterogeneously. It could be shown in a preceding paper [13] that anchoring of the palladium phosphine complex is favorably possible on mesoporous MCM-41 material after appropriate functionalization of the support. Principally, mesoporous alumina should be also applicable for this purpose. The use of alumina may be of advantage due to higher stability in aqueous basic media. Phase transfer processes are part of the catalyst preparation involving the anchoring of the palladium phosphine complex on the solid surface. The approach presented in this paper is the amphiphilization of the liquid±solid interphase by adsorption of a surfactant that simultaneously serves as phase transfer agent in two-phase Suzuki-type reactions. Results will be given for the reaction between phenylboronic acid and p-iodoanisole using cetyltrimethylammonium bromide for amphiphilization. 2. Experimental The chemicals were purchased from Aldrich, Fluka or Lancaster, except the water-soluble phosphine Ph2 P(CH2 )4 SO3 K which was prepared in our laboratory by reaction of potassium diphenylphosphide with 1,4-butanesultone [15]. All catalytic reactions were performed in oxygen-free solvents under argon atmosphere. The 1 H NMR and 13 C NMR spectra were recorded on a AC250 or ARX300 Bruker spectrometer using CDCl3 as solvent. The mass spectrometry was performed on an AMD 402/3 Intectra device (EI 70 eV). Nitrogen adsorption isotherms were recorded on an ASAP 2010 sorption system (Micromeritics) at 196°C. Combined TG/DTA measurements were performed on a STA 92 ther-
moanalyzer (SETARAM). XPS spectra were recorded on an ESCALAB IXL spectrometer (FISONS), and TEM images were obtained on a Philips CM 20 electron microscope at an acceleration voltage of 200 kV enabling also simultaneous EDX analysis of the samples. 2.1. Syntheses of mesoporous supports Al-MCM-41 samples of various Al content were synthesized hydrothermally from a reaction gel of the composition 13Na2 O : (0.4±14)Al2 O3 : 96SiO2 : 30TEAOH : 14CTACl : 1600H2 O using silica sol, sodium aluminate solution, sodium hydroxide, tetraethylammonium hydroxide as components and cetyltrimethylammonium bromide as template. Details of the synthesis and characterization of Al-MCM-41 samples have been reported recently [14]. The solid products were recovered, washed until neutrality, dried and calcined at 600°C for several hours. The mesoporous structured alumina was synthesized from gels containing Al alkoxides, appropriate pore regulating reagents [16] and a polar solvent. Hydrolysis and condensation were initiated by addition of water. After ®ltration, washing and drying, the pore size regulating agent was removed by calcination at 550°C in air. Nitrogen isotherms of the synthesized alumina are of the type IV shape (Fig. 1a) as is typical for mesoporous materials. Maximum surface areas of 700 m2 g 1 are obtained. After calcination at 550°C the alumina reveals uniform mesopores (Fig. 1b). with an average radius of 15 A The X-ray diraction line at 2H values <5, as characteristic for the MCM-41 type materials, was not found. Also X-ray diraction lines of other crystalline phases are absent up to a temperature of 1000°C. This indicates that mesoporous alumina synthesized in that way is amorphous. TG/ DTA measurements of calcined samples con®rm the stability of the mesoporous material. No thermal eects indicating structural changes are seen upon heating up to 800°C. The TEM micrographs show that the materials contain uniform mesopores but, in contrast to MCM-41 [17], these mesopores are not regularly arranged.
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(5% in water), again washed with water and dried at 80°C. 2.3. Anchoring of the Pd complex 2.3.1. Sulfonated amphiphilized support 1.00 g of the sulfonated support was mixed with an aqueous solution of 1.1 mmol (400 mg) cetyltrimethylammonium bromide (CTAB) in 10 ml water and stirred overnight. The amphiphilized support was separated from the water phase and washed with water until no bromide was found in the ®ltrate. The resulting material was dried in vacuum at 80°C.
Fig. 1. (a) N2 adsorption isotherm of mesoporous alumina, (b) Pore size distribution of mesoporous alumina.
2.2. Functionalization of the support For silylation, the mesoporous material was activated at 200°C in air for 1 h in order to remove adsorbed water. Then 2 g of the dehydrated alumina was silylated in excess with 6 g of a solution containing 4 or 6 g of 3-mercapto-propyltriethoxysilane (MPTS) in 30 ml n-heptane. Silylation was carried out for 10 min in Te¯on autoclaves under microwave irradiation (CEM 2000) at a power of 300 W. The mercaptopropyl silylated product was recovered and washed with n-hexane to remove the excess of the silylation agent. The anchored mercaptane was oxidized to the corresponding sulfonic acid with hydrogen peroxide (30 wt.% in water) at room temperature for 24 h. Additionally, the alkylsulfonated support was washed with water and ethanol and stirred with sulfuric acid
2.3.2. Preparation of immobilized catalysts (under argon) 1.00 g of a sulfonated (or sulfonated amphiphilized) support was mixed with an aqueous solution of 0.11 mmol (100 mg) PdCl2 [Ph2 P(CH2 )4 SO3 K]2 in 5 ml of water under stirring. The immobilization of the complex was indicated by the color change of the solution and the suspended support. The yellow solution became colorless and at the same time the white support became colored. After loading of the support the water was removed, the catalyst was washed with water and dried in vacuum at 80°C. 2.4. Reaction and analysis All preparations were performed under argon atmosphere. p-Iodoanisole (3.20 g, 13.5 mmol) was dissolved in 15 ml toluene and phenylboronic acid (1.83 g, 15 mmol) was dissolved in 15 ml ethanol using two separate Schlenk tubes. Sodium carbonate decahydrate (11.6 g, 45 mmol) and cetyltrimethylammonium bromide (1.27 g, 3.38 mmol) were placed in a 100 ml water jacketed ¯ask and dispersed in 15 ml water. The solutions of p-iodoanisole and phenyl boronic acid were added and the ®rst sample was taken. Then the reaction mixture was heated up to 78°C and the complex PdCl2 [Ph2 P(CH2 )4 SO3 K]2 (9 mg, 0.01 mmol) or 100 mg of the heterogenized catalyst was added under vigorous stirring (>1000 rpm). Samples of
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0.2 mL were taken after 10, 20, 30, 45, 60, 90, 120, 180, . . . min, diluted with 0.8 ml toluene, dried with sodium sulfate, and ®nally analyzed by GLC (capillary HP1; program: 2 min at 50°C then 10 K min 1 up to 260°C). After the completion of the reaction the mixture was extracted with 20 ml toluene, the organic phase was dried with sodium sulfate, the solvent evaporated and the residue dissolved in 10 ml hexane. The product was puri®ed by column chromatography (silica gel, eluent e.g. n-heptane/ethyl acetate 7=1) and analytically characterized by 1 H NMR, 13 C NMR and GLC-MS.
3. Results and discussion According to TEM images mesoporous alumina loaded with the amphiphilized palladium complex has the same morphology as the unloaded support. It consists of randomly distributed meso EDX analysis pores with an average size of 30 A. shows that palladium is nearly homogeneously distributed in the sample. Only some particles were found to be Pd free. The palladium concentration was 2 mol%. After reaction the removed catalyst was completely covered by precipitated sodium carbonate decahydrate. In this case no higher resolved images could be recorded. XPS measurements showed that the amphiphilized fresh catalysts contain Pd(II) giving rise to a signal at 336.7 eV.
The modi®ed supports containing the immobilized transition metal complexes have been applied in the following catalytic C±C coupling reaction of the Suzuki-type (Scheme 1) in a two-phase aqueous system. Usually, the PdCl2 [Ph2 P(CH2 )4 SO3 K]2 complex or the in situ formed complex derived from (NH4 )2 PdCl4 and 2Ph2 P(CH2 )4 SO3 K were used. The desirable product is a substituted biaryl 3 and the selectivity of the reaction is de®ned as 3=3 4 100 (in %). Addition of a surfactant, e.g. CTAB, accelerated the reaction and enhanced the degree of conversion. Moreover, the selectivity was very high and the byproduct 4 was <0.5%. This should be a phase transfer eect but we found a dependence on the structure of the surfactant and observed the best results with cationic compounds forming micelles [9]. Especially, the amount of phosphine plays an important role in the in situ formation of the precatalyst. More than two moles of the phosphine per mole of palladium inhibit the reaction. Consequently, all catalytic experiments were carried out with a phosphine/palladium ratio not higher than two. Fig. 2 displays a comparison of the homogeneously catalyzed Suzuki-type reaction with and without surfactant with the palladium complex anchored by ionic exchange on sulfonated SiO2 (MCM-41). The amounts of palladium complex are comparable in all systems. The eect of the phase transfer reagent is quite clear with respect to rate and conversion but the immobilized palladium complex was active after an induction period
Scheme 1. 15 mmol PhB(OH)2 ; 13.5 mmol p-iodoanisole; toluene/ethanol/water (15=15=15 (ml)), 0.01 mmol PdCl2 [Ph2 P(CH2 )4 SO3 K]2 ; substrate/catalyst 1350; 45 mmol Na2 CO3 10H2 O; up to 24 h, 78°C, conversion is related to p-iodoanisole.
E. Paetzold et al. / Microporous and Mesoporous Materials 44±45 (2001) 517±522
Fig. 2. Suzuki reaction with p-tolylboronic acid and p-iodoanisole catalyzed with 100 mg supported Pd catalyst on MCM41 in comparison to the homogeneously catalyzed reaction (homogeneous experiment: p-iodoanisole/CTAB 4).
of about 60 min. and attained more than 90% of conversion after 240 min. No induction period was observed, however, with the reused supported catalyst separated from the reaction mixture by ®ltration. The reaction started immediately with high rate and high conversion. The reason of this eect could be (i) that the active Pd(0) species, characterized by an XPS signal at 335.1 eV, has already been formed in the pores and/or (ii) that the accessibility of the catalyst is facilitated due to the mechanical destruction of the support by stirring. In a next step both supports were amphiphilized by careful stirring overnight in a dispersion of CTAB. Thereafter, the solid phase was washed with water until it was free of bromide. This procedure corresponds to the formation of admicelles [12,18]. Typically, the CTAB loading amounted from 0.6 to 0.8 mmol g 1 . As a characteristic feature we found much less bromide (<0.03 mmol g 1 ) by elemental analysis than nitrogen on the supports indicating a linking by ionic exchange with the sulfonate groups. The PdCl2 [Ph2 P(CH2 )4 SO3 K]2 complex used as precatalyst in the Suzuki-type reaction could be embedded in the amphiphilized supports very easily by stirring in an aqueous solution of the complex. After about 60 min stirring, the yellow brownish color of the aqueous phase vanished and the support became colored.
521
Fig. 3. Comparison of MCM-41 and mesoporous alumina supports in the Suzuki reaction (condition see Fig. 2) (homogeneous experiment: p-iodoanisole/CTAB 100).
Fig. 3 contains the average of four selected experiments with loaded SiO2 (MCM-41) in the twophase Suzuki reaction. Rate and conversion are very high and also the reproducibility is excellent. Both, activity and selectivity (with respect to the coupling product 3) exceed those values obtained in experiments with the same amount of the homogeneously dispersed palladium complex and CTAB. In comparison, a similar structured and loaded alumina support yielded only 80% conversion. Both supports had a very small particle size and could not be ®ltered easily. For recycling experiments, the aqueous phase was separated and used a second time adding new educts and a new toluene phase. The base concentration was sucient for at least two cycles. The reaction could be repeated, however, the reused catalyst revealed a distinctly lower activity. This is in contrast to the results obtained with the palladium complex anchored by ion exchange (Fig. 2). Probably, part of the adsorbed CTAB was lost due to transfer into the toluene phase and/or the embedded palladium complex was deactivated by formation of metal particles. The elucidation of the problem will be the topic of future work. The higher catalytic performance of silica in comparison with alumina as support is not yet quite understood. The pore structure is quite similar and also the amount of adsorbed amphiphile is comparable. Perhaps the dierent Lewis and Br onsted acidity are responsible for the dierent behavior. Future investigations are under way.
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4. Conclusions
References
Two dierent types of mesoporous supports have been synthesized by a template method consisting of mainly silica (MCM-41) or alumina. The sulfonated MCM-41 support was loaded with a palladium (II) complex and used in a Suzuki-type C±C coupling reaction. An induction period was observed when using the catalyst for the ®rst time, whereas in a second run the same catalyst was active from the beginning. Probably, the active catalyst is formed in a prereaction step. In the repeating run the activity was enhanced, but is still lower than in a homogeneous two-phase system in presence of CTAB as phase transfer reagent. As a consequence, both sulfonated mesoporous supports were loaded with CTAB ®rst. Then the palladium complex was embedded into the amphiphilized surface. Especially, activity and conversion of the reaction on the MCM-41 supported catalyst were higher than in the homogeneous system. This points to a real micellar eect occurring in the new tri-phase catalyst.
[1] F. Diederich, P.J. Stang (Eds.), Metal-Catalyzed CrossCoupling Reactions, Wiley-VCH, Weinheim, 1998. [2] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457. [3] A.L. Casalnuovo, J.C. Calabrese, J. Am. Chem. Soc. 112 (1990) 4324. [4] T.I. Wallow, B.M. Novak, J. Am. Chem. Soc. 113 (1991) 7411. [5] J.P. Genet, A. Linquist, E. Blart, V. Mouries, M. Savignac, M. Vaultier, Tetrahedron Lett. 36 (1995) 1443. [6] M. Beller, J.G.E. Krauter, A. Zapf, Angew. Chem. 109 (1997) 793, Angew. Chem. Int. Ed. 36 (1997) 772. [7] H. Dibowski, Dissertation, Techn. Universitat M unchen, 1997. [8] G. Oehme, I. Grassert, E. Paetzold, R. Meisel, K. Drexler, H. Fuhrmann, Coord. Chem. Rev. 185±186 (1999) 585. [9] E. Paetzold, G. Oehme, J. Mol. Catal. 152 (2000) 69. [10] S.L. Regen, Angew. Chem. 91 (1979) 465, Angew. Chem. Int. Ed. 18 (1979) 421. [11] H.N. Flach, I. Grassert, G. Oehme, Macromol. Chem. Phys. 195 (1994) 3289. [12] H.N. Flach, I. Grassert, G. Oehme, M. Capka, Colloid. Polym. Sci. 274 (1996) 261. [13] H. Kosslick, I. M onnich, E. Paetzold, H. Fuhrmann, R. Fricke, D. M uller, G. Oehme, Proc. Second Int. Symp. Mesoporous Molecular Sieves, Quebec, Canada, 2000, Micropor. Mesopor. Mater. 44±45 (2001) 537, this volume. [14] H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Fricke, Appl. Catal. A: General 184 (1999) 49. [15] E. Paetzold, A. Kinting, G. Oehme, J. Prakt. Chem. 329 (1987) 725. [16] H. Kosslick, R. Eckelt, D. M uller, M.-M. Pohl, M. Richter, R. Fricke, Proc. Int. Conf. Adv. Mater., ``Materials Week'', Munich, 25±28 Sept. 2000. [17] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, Mobil Oil Corp. US Patent 5,098,684, 1992. [18] R. Sharma (Ed.), Surfactant Adsorption and Surface Solubilization, ACS Symposium Series 165, American Chemical Society, Washington, DC, 1995.
Acknowledgements The authors thank Mrs. Dr. C. Fischer, Prof. M. Michalik and Mrs. B. Harzfeld for assistance in analytical characterization and Mrs. R uckert for synthetic work. We also thank the Fonds der Chemischen Industrie and the Federal Ministry of Education and Science, BMBF (project 13 N 7131/ 0) for ®nancial support.
www.elsevier.nl/locate/micromeso
Comparison of mesoporous silica and alumina supports for palladium-catalyzed carbon±carbon coupling reactions: unexpected high acceleration by supported cetyltrimethylammonium bromide E. Paetzold a,*, G. Oehme a, H. Fuhrmann a, M. Richter b, R. Eckelt b, M.-M. Pohl b, H. Kosslick b a
Institut f ur Organische Katalyseforschung, Buchbinderstrasse 5/6, D-18055 Rostock, Germany Institut f ur Angewandte Chemie, Richard-Willstatter-Strasse 12, D-12489 Berlin, Germany
b
Received 26 May 2000; accepted 31 August 2000
Abstract The coupling reaction of p-iodoanisole and phenylboronic acid occurs with high yields in aqueous medium in the presence of palladium complexes with water-soluble phosphine ligands on dierent mesoporous inorganic supports. Silica and alumina supports were similarly prepared and functionalized with sulfonate groups. The catalyst was immobilized by ion exchange. A novel approach to the amphiphilization of the supports by cetyltrimethylammonium bromide was applied to modify phase transfer steps favorably and to embed the catalyst in this bipolar surface layer. The reaction is accelerated and conversions with these embedded catalysts exceed those in homogeneous systems. Ó 2001 Elsevier Science B.V. All rights reserved. Keywords: Carbon±carbon coupling; Suzuki reaction; Two-phase system; Water-soluble phosphine catalyst; Mesoporous supports
1. Introduction Carbon±carbon bond formation is a general aim in the transition metal catalyzed organic synthetic chemistry [1]. The reaction of organoboron compounds with aryl, alkenyl and alkynyl halides by means of palladium compounds as catalysts is a modern coupling process (Suzuki reaction) [2]. In contrast to modi®ed water-soluble palladium catalysts, typical reaction components like aryl,
*
Corresponding author.
allyl or benzyl halogenides and boronic acids as well as their coupling products are very sparingly soluble in water. Hence, most of the syntheses are realized in organic solvents. Only few papers deal with the reaction in aqueous two-phase systems [3± 7], allowing the readily recycling of the water phase which contains the palladium catalyst. To overcome these problems, the Suzuki-type coupling has been performed in an aqueous micellar medium. However, the precipitation of the diaryl products could not be avoided. Therefore, a two-phase system consisting of water and toluene was used [8]. Several suitable phase transfer reagents have been added to promote the transport
1387-1811/01/$ - see front matter Ó 2001 Elsevier Science B.V. All rights reserved. PII: S 1 3 8 7 - 1 8 1 1 ( 0 1 ) 0 0 2 2 9 - 3
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of the water-soluble palladium catalyst into the reactant-containing toluene phase. An accelerating eect on the reaction was found in dependence on the structure of the added amphiphiles [9]. However, the phase separation after reaction became dicult because of the amphiphilization of the interface. Certain improvements could be achieved by the utilization of a new type of tri-phase catalyst [10±12], however, for practical purposes it is preferable to carry out the reaction heterogeneously. It could be shown in a preceding paper [13] that anchoring of the palladium phosphine complex is favorably possible on mesoporous MCM-41 material after appropriate functionalization of the support. Principally, mesoporous alumina should be also applicable for this purpose. The use of alumina may be of advantage due to higher stability in aqueous basic media. Phase transfer processes are part of the catalyst preparation involving the anchoring of the palladium phosphine complex on the solid surface. The approach presented in this paper is the amphiphilization of the liquid±solid interphase by adsorption of a surfactant that simultaneously serves as phase transfer agent in two-phase Suzuki-type reactions. Results will be given for the reaction between phenylboronic acid and p-iodoanisole using cetyltrimethylammonium bromide for amphiphilization. 2. Experimental The chemicals were purchased from Aldrich, Fluka or Lancaster, except the water-soluble phosphine Ph2 P(CH2 )4 SO3 K which was prepared in our laboratory by reaction of potassium diphenylphosphide with 1,4-butanesultone [15]. All catalytic reactions were performed in oxygen-free solvents under argon atmosphere. The 1 H NMR and 13 C NMR spectra were recorded on a AC250 or ARX300 Bruker spectrometer using CDCl3 as solvent. The mass spectrometry was performed on an AMD 402/3 Intectra device (EI 70 eV). Nitrogen adsorption isotherms were recorded on an ASAP 2010 sorption system (Micromeritics) at 196°C. Combined TG/DTA measurements were performed on a STA 92 ther-
moanalyzer (SETARAM). XPS spectra were recorded on an ESCALAB IXL spectrometer (FISONS), and TEM images were obtained on a Philips CM 20 electron microscope at an acceleration voltage of 200 kV enabling also simultaneous EDX analysis of the samples. 2.1. Syntheses of mesoporous supports Al-MCM-41 samples of various Al content were synthesized hydrothermally from a reaction gel of the composition 13Na2 O : (0.4±14)Al2 O3 : 96SiO2 : 30TEAOH : 14CTACl : 1600H2 O using silica sol, sodium aluminate solution, sodium hydroxide, tetraethylammonium hydroxide as components and cetyltrimethylammonium bromide as template. Details of the synthesis and characterization of Al-MCM-41 samples have been reported recently [14]. The solid products were recovered, washed until neutrality, dried and calcined at 600°C for several hours. The mesoporous structured alumina was synthesized from gels containing Al alkoxides, appropriate pore regulating reagents [16] and a polar solvent. Hydrolysis and condensation were initiated by addition of water. After ®ltration, washing and drying, the pore size regulating agent was removed by calcination at 550°C in air. Nitrogen isotherms of the synthesized alumina are of the type IV shape (Fig. 1a) as is typical for mesoporous materials. Maximum surface areas of 700 m2 g 1 are obtained. After calcination at 550°C the alumina reveals uniform mesopores (Fig. 1b). with an average radius of 15 A The X-ray diraction line at 2H values <5, as characteristic for the MCM-41 type materials, was not found. Also X-ray diraction lines of other crystalline phases are absent up to a temperature of 1000°C. This indicates that mesoporous alumina synthesized in that way is amorphous. TG/ DTA measurements of calcined samples con®rm the stability of the mesoporous material. No thermal eects indicating structural changes are seen upon heating up to 800°C. The TEM micrographs show that the materials contain uniform mesopores but, in contrast to MCM-41 [17], these mesopores are not regularly arranged.
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(5% in water), again washed with water and dried at 80°C. 2.3. Anchoring of the Pd complex 2.3.1. Sulfonated amphiphilized support 1.00 g of the sulfonated support was mixed with an aqueous solution of 1.1 mmol (400 mg) cetyltrimethylammonium bromide (CTAB) in 10 ml water and stirred overnight. The amphiphilized support was separated from the water phase and washed with water until no bromide was found in the ®ltrate. The resulting material was dried in vacuum at 80°C.
Fig. 1. (a) N2 adsorption isotherm of mesoporous alumina, (b) Pore size distribution of mesoporous alumina.
2.2. Functionalization of the support For silylation, the mesoporous material was activated at 200°C in air for 1 h in order to remove adsorbed water. Then 2 g of the dehydrated alumina was silylated in excess with 6 g of a solution containing 4 or 6 g of 3-mercapto-propyltriethoxysilane (MPTS) in 30 ml n-heptane. Silylation was carried out for 10 min in Te¯on autoclaves under microwave irradiation (CEM 2000) at a power of 300 W. The mercaptopropyl silylated product was recovered and washed with n-hexane to remove the excess of the silylation agent. The anchored mercaptane was oxidized to the corresponding sulfonic acid with hydrogen peroxide (30 wt.% in water) at room temperature for 24 h. Additionally, the alkylsulfonated support was washed with water and ethanol and stirred with sulfuric acid
2.3.2. Preparation of immobilized catalysts (under argon) 1.00 g of a sulfonated (or sulfonated amphiphilized) support was mixed with an aqueous solution of 0.11 mmol (100 mg) PdCl2 [Ph2 P(CH2 )4 SO3 K]2 in 5 ml of water under stirring. The immobilization of the complex was indicated by the color change of the solution and the suspended support. The yellow solution became colorless and at the same time the white support became colored. After loading of the support the water was removed, the catalyst was washed with water and dried in vacuum at 80°C. 2.4. Reaction and analysis All preparations were performed under argon atmosphere. p-Iodoanisole (3.20 g, 13.5 mmol) was dissolved in 15 ml toluene and phenylboronic acid (1.83 g, 15 mmol) was dissolved in 15 ml ethanol using two separate Schlenk tubes. Sodium carbonate decahydrate (11.6 g, 45 mmol) and cetyltrimethylammonium bromide (1.27 g, 3.38 mmol) were placed in a 100 ml water jacketed ¯ask and dispersed in 15 ml water. The solutions of p-iodoanisole and phenyl boronic acid were added and the ®rst sample was taken. Then the reaction mixture was heated up to 78°C and the complex PdCl2 [Ph2 P(CH2 )4 SO3 K]2 (9 mg, 0.01 mmol) or 100 mg of the heterogenized catalyst was added under vigorous stirring (>1000 rpm). Samples of
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0.2 mL were taken after 10, 20, 30, 45, 60, 90, 120, 180, . . . min, diluted with 0.8 ml toluene, dried with sodium sulfate, and ®nally analyzed by GLC (capillary HP1; program: 2 min at 50°C then 10 K min 1 up to 260°C). After the completion of the reaction the mixture was extracted with 20 ml toluene, the organic phase was dried with sodium sulfate, the solvent evaporated and the residue dissolved in 10 ml hexane. The product was puri®ed by column chromatography (silica gel, eluent e.g. n-heptane/ethyl acetate 7=1) and analytically characterized by 1 H NMR, 13 C NMR and GLC-MS.
3. Results and discussion According to TEM images mesoporous alumina loaded with the amphiphilized palladium complex has the same morphology as the unloaded support. It consists of randomly distributed meso EDX analysis pores with an average size of 30 A. shows that palladium is nearly homogeneously distributed in the sample. Only some particles were found to be Pd free. The palladium concentration was 2 mol%. After reaction the removed catalyst was completely covered by precipitated sodium carbonate decahydrate. In this case no higher resolved images could be recorded. XPS measurements showed that the amphiphilized fresh catalysts contain Pd(II) giving rise to a signal at 336.7 eV.
The modi®ed supports containing the immobilized transition metal complexes have been applied in the following catalytic C±C coupling reaction of the Suzuki-type (Scheme 1) in a two-phase aqueous system. Usually, the PdCl2 [Ph2 P(CH2 )4 SO3 K]2 complex or the in situ formed complex derived from (NH4 )2 PdCl4 and 2Ph2 P(CH2 )4 SO3 K were used. The desirable product is a substituted biaryl 3 and the selectivity of the reaction is de®ned as 3=3 4 100 (in %). Addition of a surfactant, e.g. CTAB, accelerated the reaction and enhanced the degree of conversion. Moreover, the selectivity was very high and the byproduct 4 was <0.5%. This should be a phase transfer eect but we found a dependence on the structure of the surfactant and observed the best results with cationic compounds forming micelles [9]. Especially, the amount of phosphine plays an important role in the in situ formation of the precatalyst. More than two moles of the phosphine per mole of palladium inhibit the reaction. Consequently, all catalytic experiments were carried out with a phosphine/palladium ratio not higher than two. Fig. 2 displays a comparison of the homogeneously catalyzed Suzuki-type reaction with and without surfactant with the palladium complex anchored by ionic exchange on sulfonated SiO2 (MCM-41). The amounts of palladium complex are comparable in all systems. The eect of the phase transfer reagent is quite clear with respect to rate and conversion but the immobilized palladium complex was active after an induction period
Scheme 1. 15 mmol PhB(OH)2 ; 13.5 mmol p-iodoanisole; toluene/ethanol/water (15=15=15 (ml)), 0.01 mmol PdCl2 [Ph2 P(CH2 )4 SO3 K]2 ; substrate/catalyst 1350; 45 mmol Na2 CO3 10H2 O; up to 24 h, 78°C, conversion is related to p-iodoanisole.
E. Paetzold et al. / Microporous and Mesoporous Materials 44±45 (2001) 517±522
Fig. 2. Suzuki reaction with p-tolylboronic acid and p-iodoanisole catalyzed with 100 mg supported Pd catalyst on MCM41 in comparison to the homogeneously catalyzed reaction (homogeneous experiment: p-iodoanisole/CTAB 4).
of about 60 min. and attained more than 90% of conversion after 240 min. No induction period was observed, however, with the reused supported catalyst separated from the reaction mixture by ®ltration. The reaction started immediately with high rate and high conversion. The reason of this eect could be (i) that the active Pd(0) species, characterized by an XPS signal at 335.1 eV, has already been formed in the pores and/or (ii) that the accessibility of the catalyst is facilitated due to the mechanical destruction of the support by stirring. In a next step both supports were amphiphilized by careful stirring overnight in a dispersion of CTAB. Thereafter, the solid phase was washed with water until it was free of bromide. This procedure corresponds to the formation of admicelles [12,18]. Typically, the CTAB loading amounted from 0.6 to 0.8 mmol g 1 . As a characteristic feature we found much less bromide (<0.03 mmol g 1 ) by elemental analysis than nitrogen on the supports indicating a linking by ionic exchange with the sulfonate groups. The PdCl2 [Ph2 P(CH2 )4 SO3 K]2 complex used as precatalyst in the Suzuki-type reaction could be embedded in the amphiphilized supports very easily by stirring in an aqueous solution of the complex. After about 60 min stirring, the yellow brownish color of the aqueous phase vanished and the support became colored.
521
Fig. 3. Comparison of MCM-41 and mesoporous alumina supports in the Suzuki reaction (condition see Fig. 2) (homogeneous experiment: p-iodoanisole/CTAB 100).
Fig. 3 contains the average of four selected experiments with loaded SiO2 (MCM-41) in the twophase Suzuki reaction. Rate and conversion are very high and also the reproducibility is excellent. Both, activity and selectivity (with respect to the coupling product 3) exceed those values obtained in experiments with the same amount of the homogeneously dispersed palladium complex and CTAB. In comparison, a similar structured and loaded alumina support yielded only 80% conversion. Both supports had a very small particle size and could not be ®ltered easily. For recycling experiments, the aqueous phase was separated and used a second time adding new educts and a new toluene phase. The base concentration was sucient for at least two cycles. The reaction could be repeated, however, the reused catalyst revealed a distinctly lower activity. This is in contrast to the results obtained with the palladium complex anchored by ion exchange (Fig. 2). Probably, part of the adsorbed CTAB was lost due to transfer into the toluene phase and/or the embedded palladium complex was deactivated by formation of metal particles. The elucidation of the problem will be the topic of future work. The higher catalytic performance of silica in comparison with alumina as support is not yet quite understood. The pore structure is quite similar and also the amount of adsorbed amphiphile is comparable. Perhaps the dierent Lewis and Br onsted acidity are responsible for the dierent behavior. Future investigations are under way.
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4. Conclusions
References
Two dierent types of mesoporous supports have been synthesized by a template method consisting of mainly silica (MCM-41) or alumina. The sulfonated MCM-41 support was loaded with a palladium (II) complex and used in a Suzuki-type C±C coupling reaction. An induction period was observed when using the catalyst for the ®rst time, whereas in a second run the same catalyst was active from the beginning. Probably, the active catalyst is formed in a prereaction step. In the repeating run the activity was enhanced, but is still lower than in a homogeneous two-phase system in presence of CTAB as phase transfer reagent. As a consequence, both sulfonated mesoporous supports were loaded with CTAB ®rst. Then the palladium complex was embedded into the amphiphilized surface. Especially, activity and conversion of the reaction on the MCM-41 supported catalyst were higher than in the homogeneous system. This points to a real micellar eect occurring in the new tri-phase catalyst.
[1] F. Diederich, P.J. Stang (Eds.), Metal-Catalyzed CrossCoupling Reactions, Wiley-VCH, Weinheim, 1998. [2] N. Miyaura, A. Suzuki, Chem. Rev. 95 (1995) 2457. [3] A.L. Casalnuovo, J.C. Calabrese, J. Am. Chem. Soc. 112 (1990) 4324. [4] T.I. Wallow, B.M. Novak, J. Am. Chem. Soc. 113 (1991) 7411. [5] J.P. Genet, A. Linquist, E. Blart, V. Mouries, M. Savignac, M. Vaultier, Tetrahedron Lett. 36 (1995) 1443. [6] M. Beller, J.G.E. Krauter, A. Zapf, Angew. Chem. 109 (1997) 793, Angew. Chem. Int. Ed. 36 (1997) 772. [7] H. Dibowski, Dissertation, Techn. Universitat M unchen, 1997. [8] G. Oehme, I. Grassert, E. Paetzold, R. Meisel, K. Drexler, H. Fuhrmann, Coord. Chem. Rev. 185±186 (1999) 585. [9] E. Paetzold, G. Oehme, J. Mol. Catal. 152 (2000) 69. [10] S.L. Regen, Angew. Chem. 91 (1979) 465, Angew. Chem. Int. Ed. 18 (1979) 421. [11] H.N. Flach, I. Grassert, G. Oehme, Macromol. Chem. Phys. 195 (1994) 3289. [12] H.N. Flach, I. Grassert, G. Oehme, M. Capka, Colloid. Polym. Sci. 274 (1996) 261. [13] H. Kosslick, I. M onnich, E. Paetzold, H. Fuhrmann, R. Fricke, D. M uller, G. Oehme, Proc. Second Int. Symp. Mesoporous Molecular Sieves, Quebec, Canada, 2000, Micropor. Mesopor. Mater. 44±45 (2001) 537, this volume. [14] H. Kosslick, G. Lischke, B. Parlitz, W. Storek, R. Fricke, Appl. Catal. A: General 184 (1999) 49. [15] E. Paetzold, A. Kinting, G. Oehme, J. Prakt. Chem. 329 (1987) 725. [16] H. Kosslick, R. Eckelt, D. M uller, M.-M. Pohl, M. Richter, R. Fricke, Proc. Int. Conf. Adv. Mater., ``Materials Week'', Munich, 25±28 Sept. 2000. [17] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, Mobil Oil Corp. US Patent 5,098,684, 1992. [18] R. Sharma (Ed.), Surfactant Adsorption and Surface Solubilization, ACS Symposium Series 165, American Chemical Society, Washington, DC, 1995.
Acknowledgements The authors thank Mrs. Dr. C. Fischer, Prof. M. Michalik and Mrs. B. Harzfeld for assistance in analytical characterization and Mrs. R uckert for synthetic work. We also thank the Fonds der Chemischen Industrie and the Federal Ministry of Education and Science, BMBF (project 13 N 7131/ 0) for ®nancial support.