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Chitosan: A Natural Polymeric Support of Catalysts for the Synthesis of Fine Chemicals Franc¸ oise Quignard,† Agne`s Choplin,*,† and Alain Domard‡ Institut de Recherches sur la Catalyse-CNRS, 2 avenue A. Einstein, 69 626 Villeurbanne Cedex, France, and Laboratoire des Mate´ riaux Polyme` res et des Biomate´ riaux, UMR-CNRS 5627, Universite´ Cl. Bernard, ISTIL, 43 boulevard du 11 Novembre 1918, 69622 Villeurbanne Cedex, France Received July 3, 2000 The high surface hydrophilicity of chitosan is the property which has allowed the synthesis of an efficient, selective, and stable palladium supported aqueous phase catalyst for the reaction of allylic substitution of (E)-cinnamyl ethyl carbonate by morpholine. The adjustment of the water content of the catalytic solid and the addition of a cationic surfactant (CTAB) in the medium both contribute to improve drastically the efficiency of the catalyst while keeping the palladium leaching below 0.5%.
Introduction Chitosan is a copolymer of linked β,(1f4),2-amino-2deoxy-D-glucan and 2-acetamidodeoxy-D-glucan. The applications of chitosan are numerous and of wide scope, encompassing biology, medicine, and food industries.1,2 Yet, chitosan has so far captured little or no attention as a support nor as a supramolecular ligand for catalysts, although its physical and chemical properties are potentially well-adapted.3-7 Indeed, its low price and its easy and clean elimination from the metal entity by simple thermal treatment under oxygen are some of the advantages of chitosan as a support over inorganic oxides, for example. Chitosan bears hydroxyl and amino groups (Chart 1), which are excellent functional groups for the anchoring of a large variety of organometallic complexes which are precursors of heterogeneous molecular catalysts.8 These functional groups provide chitosan with a high surface hydrophilic character, which is necessary to heterogenize biphasic water/organic catalytic systems by the supported aqueous phase (SAP) methodology.9 Finally, the thermal stability of chitosan, although restricted to a temperature range below 150 °C,10 is compatible with many organic reactions of industrial interest for the synthesis of fine chemicals. We wish to report here the first example, to our knowledge, of the synthesis of a chitosan-supported aqueous-phase catalyst based on palladium and its application to the allylic substitution of † ‡
Institut de Recherches sur la Catalyse-CNRS. Universite´ Cl. Bernard.
(1) Roberts, G. A. F. Chitin Chemistry; MacMillan Press: London, 1992. (2) Domard, A.; Roberts, G. A. F.; Varum, K. M. Advances in Chitin Science; Jacques Andre´: Lyon, 1998. (3) Kise, H.; Hayakawa, A.; Noritomi, H. Biotechnol. Lett. 1987, 9, 543. (4) Kise, H.; Hayakawa, A.; Noritomi, H. J. Biotechnol. 1990, 14, 239. (5) Kise, H.; Hayakawa, A. Enzymol. Microb. Technol. 1991, 13, 548. (6) Chiessi, E.; Pispisa, B. J. Mol. Catal. 1994, 87, 177. (7) Isaeva, V.; Sharf, V.; Nifant′ev, N.; Chernetskii, V.; Dykh, Zh. In Studies in Surface Science and Catalysis; Delmon, B., Jacobs, P. A., Maggi, R., Martens, J. A., Grange, P., Poncelet, G., Eds.; Elsevier: Amsterdam, 1998; Vol. 118, p 237. (8) See for example: Choplin, A.; Quignard, F. Coord. Chem. Rev. 1998, 178-180, 1679 and references therein. (9) Arhancet, J. P.; Davis, M. E.; Merola, J. S.; Hanson, B. E. Nature 1989, 339, 454. (10) Demarger-Andre´, S.; Domard, A. Carbohydr. Polym. 1994, 23, 211.
Chart 1. Chemical Structure of Chitosan
(E)-cinnamyl ethyl carbonate by morpholine (eq 1). This
reaction, Trost-Tsuji type, belongs to one of the most important reactions of carbon-carbon and carbonheteroatom bond formation. It allows a considerable hydrocarbon chain lengthening in one step under very mild conditions, with very high regio- and sometimes enantioselectivities.11 The classical catalysts of this reaction are palladium(0) complexes stabilized by the appropriate phosphine ligands. Their heterogenization is a major challenge because their productivity is so low that their recycling becomes necessary for economical reasons (high price of palladium and sometimes of the ligands). The SAP methodology is well-suited to this case because it allows maintaining the coordination sphere of palladium practically intact, a condition which must be met to preserve activity and selectivity of the original catalyst. Experimental Section Products. Pd(OAc)2 (95% purity), benzonitrile (anhydrous, 99%), and morpholine (98%) were used as received from Aldrich. TPPTS (P(m-C6H4SO3Na)3‚3H2O) was a generous gift from Rhoˆne Poulenc. (E)-Cinnamyl ethyl carbonate was synthesized and purified according to a literature procedure.12 Chitosan, produced from squid bones (lot nο. 113 from France Chitine, Marseille, France) is characterized by a degree of acetylation of 2.6% as measured by 1H NMR spectroscopy13 and a degree of crystallinity of 40% as deduced from the X-ray pattern.14 Its BET surface (N2 isotherm) is equal to 1.41 m2/g (solid evacuated at 80 °C, 2 days, 10-4 Torr); its wetting volume (defined as the volume of H2O (11) Tsuji, J. Palladium Reagents and Catalysts; Wiley and Sons: New York, 1995. (12) Tsuji, J.; Saito, K.; Okumoto, H. J. Org. Chem. 1984, 49, 1341. (13) Hirai, A.; Odani, H.; Nakajima, A. Polym. Bull. 1991, 26, 17. (14) Piro, E.; Accominotti, M.; Domard, A. Langmuir 1997, 13, 1653.
10.1021/la000937d CCC: $19.00 © 2000 American Chemical Society Published on Web 10/21/2000
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which can be added to 1 g of solid before a drop of liquid is detected de visu) is equal to 7.3 mL H2O/g. All experiments are performed under strict exclusion of air using standard Schlenk tube techniques. Synthesis of the Catalyst. Pd(OAc)2 (42 mg, 0.187 mmol) and TPPTS‚3H2O (P(m-C6H4SO3Na)3‚3H2O, 586 mg, 0.946 mmol) are dissolved under magnetic stirring in degassed H2O (15 mL). This solution is maintained at 40 °C for 30 min to allow for completion of the reaction between the metal complex and the phosphines. It is then poured onto chitosan (2 g), pre-evacuated at room temperature (10-4 Torr, 16 h). This wet solid is stirred for 30 min at 40 °C and then evacuated (25 °C, 10-2 Torr, 16 h). The resulting lemon yellow solid (CHT-SAP-Pd) is characterized by a palladium content of 0.75 wt % (determined by ICP) and a residual water content close to 3 wt % (as determined by thermogravimetric analysis (TGA)). Catalytic Tests. A solution of (E)-cinnamyl ethyl carbonate (61.5 µL, 0.32 mmol) and morpholine (26.5 µL, 0.30 mmol) is prepared in benzonitrile (8 mL) in a Schlenk tube. The solid CHT-SAP-Pd (180 mg, 0.013 mmol Pd) is introduced in a roundbottomed flask, fitted with a condenser and a septum. The desired amount of water (360 mg for example, to obtain a solid characterized by a water content of 2.6 g H2O/g CHT) is then added with a syringe and the system left for equilibration for 30 min at room temperature. The solution of reactants is then introduced on the solid in the flask and the temperature raised to 50 °C by immersion in a preheated oil bath; this corresponds to time t ) 0 of the catalytic test. The course of the reaction is monitored by periodic samplings and quantitative analysis by gas chromatography (HP 5890, HP5 capillary column, Tinj ) 220 °C, Tdet ) 240 °C).
Results and Discussion Synthesis of the Catalyst. The method of synthesis of the chitosan-supported aqueous-phase palladium catalyst, CHT-SAP-Pd, is similar to that described previously with a mesoporous silica as the support.15 An aqueous solution of the catalyst precursor is prepared in situ from a palladium(II) complex, palladium acetate, and an excess of the water soluble trisulfonated triphenylphosphine, TPPTS. The reduction of Pd(II) to Pd(0) occurs readily, as demonstrated by 31P NMR spectroscopy; it leads to the formation of Pd(TPPTS)3, and part of the phosphine is simultaneously oxidized (OTPPTS)15 (eq 2). This reaction
Pd(OAc)2 + 4TPPTS 9 8 H O 2
Pd(TPPTS)3 + OTPPTS + 2AcOH (2) is close to completion before the solution is put into contact with chitosan. The method of impregnation, the so-called incipient wetness method, allows, in the case of inorganic oxides, for a good dispersion of the deposited complex over the whole surface of the solid.16 In the case of chitosan as the support, one admits that, owing to the swelling properties of chitosan in the presence of water, at least the amorphous portion of the solid is accessible to the catalyst solution.1,2 On the basis of 31P MAS NMR data of the related silica-supported catalyst,15 we consider that the removal of water under vacuum does not change the coordination sphere of palladium, although the precise nature of the chemical interaction between the complex and chitosan, if any, is so far not known. Indeed, one may reasonably consider the possibility of an interaction between the positively charged surface of chitosan and the sulfonyl groups of the phosphine ligands. Nevertheless, this interaction would be sufficiently remote from the (15) Dos Santos, S.; Tong, Y. Y.; Quignard, F.; Choplin, A.; Sinou, D.; Dutasta, J. P. Organometallics 1998, 17, 78. (16) Weitkamp, J. In Handbook of Heterogeneous Catalysis; Ertl, G., Kno¨zinger, H., Eds.; Wiley-VCH: Weinheim, 1997; Vol. 1, p 117.
Figure 1. Influence of the water content of CHT-SAP-Pd on its catalytic properties for reaction 1. g H2O/g CHT: ×, 0.36; [, 2.6; 9, 3.9; /, 6.5. Comparison with the same palladium catalytic system in PhCN/H2O (0) and on silica-SAP (0.04 g H2O/g silica) (4) (data from ref 15). Experimental conditions: T ) 50 °C; solvent, PhCN; [carbonate]/[morpholine]/[Pd] ) 25/ 30/1; [carbonate] ) 30 mmol‚L-1.
metal center to have only a weak influence (electronic effect) on the catalytic properties of the complex. Catalytic Properties of CHT-SAP-Pd. The solid CHT-SAP-Pd becomes significantly active only when at least 2 g of H2O are added per gram of CHT. Under these conditions, no more than 15% of 1 are converted after 2 h. After this time delay, the reaction does not progress anymore. The reaction rate increases sharply when the water content of the solid is further increased: thus 80% of 1 is converted after 5 h when 6.5 g of H2O is added per gram of CHT. Figure 1 includes for comparison the data obtained with the same catalytic system, that is, [Pd(OAc)2/5TPPTS] working either in the biphasic liquid medium H2O/PhCN (v/v ) 1/1) or on the surface of silicaSAP-Pd.15 All these experiments were performed under the same conditions of reactant concentration, temperature, and substrate/Pd ratio to allow for an easy and safe comparison of the catalytic properties. On one hand, the activity of the catalytic system [Pd(OAc)2/5TPPTS] is higher when supported on chitosan than when working under homogeneous biphasic conditions, but only when the amount of water on the CHT-SAP system is above 3.9 g H2O/g CHT. On the other hand, CHT-SAP-Pd is much less active than the silica-supported catalyst, and this, whatever the water content of the latter. Figure 1 shows indeed the activity of the poorest silica-SAP-Pd solid, which is the one which has a water amount close to 3 wt % or 0.04 g H2O/g silica. These observations may be rationalized on the basis of the simple scheme proposed by Davis et al.:9 the support of the catalytic system increases the interphase surface area between water and benzonitrile up to a value close to the surface area of the support, if one considers that water is immobilized as a thin film. Thus, the poor performance of chitosan as a support may be related to its very low specific surface area as compared to that of the mesoporous silica used here: 1.41 m2/g versus 185 m2/g. Nevertheless, these surface areas were determined from the isotherm of adsorption of liquid nitrogen using the BET method. Their values may be different if one considers the adsorption of water; more precisely, the specific surface area of chitosan is certainly larger due to swelling of chitosan. We observed a poor dispersion of the chitosan-supported catalyst in benzonitrile: indeed, the small powder particles show a
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Table 1. Amount of Palladium Leached in PhCN at the End of Each Catalytic Test (for Experimental Conditions See the Captions for Figures 1 and 2) m(H2O)/m(CHT) (g/g)
[CTAB] in PhCN (mmol/L)
Pd (ppm)
% Pda
2.6 3.9 6.5 2.6 6.5
0 0 0 4 4
