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The first synthesis of a cyclic carbonate from a ketal in SC-CO2
J. of Supercritical Fluids 25 (2003) 177 /182 www.elsevier.com/locate/supflu
The first synthesis of a cyclic carbonate from a ketal in SC-CO2 Michele Aresta , Angela Dibenedetto, Caterina Dileo, Immacolata Tommasi, Eliana Amodio Department of Chemistry, University of Bari, and Centro METEA, Via Celso Ulpiani 27, Bari 70126, Italy Received 12 November 2001; received in revised form 22 February 2002; accepted 11 April 2002
Abstract In this paper the synthesis of a cyclic organic carbonate by a halogen-free route is described. The ketal formed from cyclohexanone and 1,2-ethanediol was reacted with carbon dioxide, either in SC-CO2 or in organic solvents under CO2 pressure. Transition-metal complexes with functionalized fluorinated di-ketone ligands were used as catalyst. The synthetic methodology based on SC-CO2 is effective for the synthesis of ethylene carbonate. The use of methanol as cosolvent in SC-CO2 prevents the formation of the ethylene carbonate and only the alcoholysis of the ketal is observed. In organic solvents, no reaction takes place. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cyclic ketal; Cyclic organic carbonates; SC-CO2 reagent; SC-CO2 solvent
1 2CH3 OHCO O2 0 (CH3 O)2 COH2 O (2) 2
1. Introduction The synthesis of organic carbonates through halogen-free routes may avoid plant corrosion problems, while reducing the environmental burden [1]. A new approach replacing the old phosgenation (Eq. (1)), is based on the oxidative carbonylation of alcohols that uses CO/O2 (Eq. (2)). Such process has found industrial application in the synthesis of dimethylcarbonate [2 /11]. 2ROHCOCl2 0 (RO)2 CO2HCl
The carboxylation of epoxides is a second alternative to phosgene and is exploited at the industrial level [12,13] [Eq. (3)] for the synthesis of propylene carbonate or its polymer. (3)
(1)
Corresponding author. Tel.: /39-080-544-2430; fax: /39080-544-2429 E-mail address: [email protected] (M. Aresta).
More recently, the oxidative carboxylation of olefins [14,15] [Eq. (4)] or the carboxylation of alcohols [16] (Eq. (5)) have been investigated for their potential in the synthesis of cyclic and linear
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carbonates, respectively.
2.1. Synthesis of the cyclohexanone ethylene ketal (4)
2ROHCO2 0 (RO)2 COH2 O
(5)
Reaction (5) has not yet found an industrial exploitation because of the low turnover and decomposition of the catalyst [16]. Such limited activity may be caused by the formation of water that shifts to left reaction (5) or causes the deactivation of the catalyst. Therefore, synthetic pathways in which water is not formed [Eq. (4)] or is formed in a separate step [Eq. (6,7)] may represent a better alternative. Reactions (6) [17] and (7) [18], afford linear carbonates. They are characterized by very highpressure that rises cost and safety problems. (6)
In a flask equipped with Dean /Stark trap as segregate of water and a reflux condenser, 10.3 ml of cyclohexanone (99.38 mmol) were reacted with an excess of 1,2-ethanediol (6.7 ml, 119.7 mmol) in presence of 736 mg of anhydrous FeCl3 as catalyst in 30 ml of anhydrous benzene. The stirred mixture was heated to 367 /371 K, 0.5 ml of water was separated after 30 min in the Dean /Stark trap and the reaction was complete in 2 h. The reaction system was cooled to room temperature under a nitrogen flow. The product was washed with an aqueous solution of NaOH 32 mM (10 ml) and extracted with toluene (4 /20 ml). The toluene solution was dried overnight on anhydrous MgSO4 activated at 423 K for 24 h, then it was filtered and concentrated to 15 ml to afford the crude product in quantitative yield. By distillation under reduced pressure (3 /10 3 atm) of the crude product, a colorless oil was collected at 311 /314 K and characterized by GC-MS as pure cyclohexanone ethylene ketal (yield of pure product: 80%) (Eq. (8)).
2.2. Synthesis of CuL2 (7)
We report here the first example of synthesis of a cyclic carbonate from a ketal under mild conditions assisted by transition-metal catalysts.
2. Materials and methods CO2 (99.9999%, SIO) was dried before use. All solvents were dried [19]. GC analyses were carried out with a GC-MS Shimadzu QP5050 instrument.
CuCl2 ×/ 2H2O (103.8 mg, 0.6088 mmol) were dissolved in 2 ml of CH3OH and 316.6 mg of the ligand C11H7F4O2 (L) [20] (1.2177 mmol) dissolved in 10 ml of CH3OH were added. The resulting solution was stirred for 6 h at room temperature with continuous monitoring of the pH which lowered to ca. 2 because of the formation of HCl. The solvent was evaporated in vacuo at room temperature and the solid obtained was dissolved in THF (10 ml) and recrystallized by adding pentane (10 ml). The brown solid formed, was separated by filtration and the solution was dried in vacuo at room temperature to afford a green solid that was washed with pentane (6 ml) and dried in vacuo. Anal. Calcd for C22H14CuF8O4: C 47.36; H 2.53; Cu 11.4%. Found: C 47.0; H 2.9; Cu 11.34% .
M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182
179
2.5. Reaction of ketal(III) in SC-CO2
2.3. Synthesis of FeClL A colorless solution of 178.4 mg of FeCl2 ×/ 1.5THF [21] (0.7597 mmol) in 5 ml of ethanol was added to the pale yellow solution of 377.0 mg of ligand C11H7F4O2 (L) (1.519 mmol) in 13 ml of CH2Cl2. The resulting solution was stirred for 6 h at room temperature with continuous monitoring of the pH which lowered to ca. 3/4 because of the formation of HCl. The solvent was evaporated in vacuo at room temperature and the solid obtained was dissolved in THF (10 ml) and recrystallized by adding pentane (10 ml). The brown solid formed, was separated by filtration and the solution dried in vacuo at room temperature to afford a dark orange solid that was washed with pentane (6 ml) and dried in vacuo. Anal. Calcd for C11H7ClF4FeO2: C 49.61; H 4.89; Cl 8.61; Fe 13.57%. Found: C 48.9; H 5.03; Cl 8.51; Fe 12.82%.
2.4. Reaction of ketal(III) with CO2 in organic solvents A glass reactor containing 0.5 ml (3.614 mmol) of ketal, the catalyst (3.7 /10 2 mmol) and 5 ml of solvent (CH3CN or xylene) was introduced in a stainless steel autoclave, pressurized at 50 atm with CO2 and heated to 379 K. The reaction mixture was stirred for 5 h, then it was cooled to room temperature. The gas phase was bubbled slowly in the same solvent (CH3CN or xylene) at 273 K used for the reaction in order to collect the volatile phase. The GC-MS analysis of both the collected gas phase and the residue in the reactor did not show the presence of ethylene carbonate. The ketal was recovered in 98% yield.
All reactions in SC-CO2 were carried out using a SITEC apparatus equipped with a reactor and extractor, with fully automatized control of pressure and temperature. The apparatus can work in continuous or batch mode. In a typical trial, the ketal (0.5 ml, 3.614 mmol) was charged in a open glass basket and placed in the reactor and the catalyst was either added to the ketal, or suspended above it on a synthesized glass septum. The ratio ketal/catalyst was equal to 10009/10. The reaction was carried out for 8 h at 353 /373 K and a pressure of 100 /160 atm (see Table 1). The ketal and the catalyst revealed to be soluble in SC-CO2 under the reaction conditions, as monitored by withdrawing a sample in a IR-HP cell in series with the reactor, that showed a single phase. At the end of the reaction in batch, after cooling and depressurization of the reactor, the gas was collected at 250 K in THF and shown to contain traces of the carbonate. The ethylene carbonate, the unreacted ketal and cyclohexanone were recovered from the basket with the use of a solvent (see Table 1). The catalyst, if deposited on the glass synthered bed, did not contaminate the product [22]. No other products were observed besides the residual ketal, the cyclic carbonate and cyclohexanone. The mixture carbonate/ketal/cyclohexanone was separated by column chromatography by using a toluene/pentane (1:1 v/v) mixture to afford pure products. Table 1 Conditions of reaction and yield (8 h of reaction in any case) Catalyst
T (K)
P (bar)
TON
ZnCl2 ZnCl2 ZnCl2 ZnCl2 ZnCl2 CuL2 FeLCl FeCl2 FeCl2 ×/ THF
353 373 373 373 373 373 373 373 373
160 160 230 120 120 160 160 160 160
12 15 19 11 18 720 890 15 10
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2.6. Reaction of ketal(III) in SC-CO2 and CH3OH as cosolvent A glass reactor, charged under nitrogen with 0.5 ml of ketal (3.614 mmol), catalyst (ratio ketal/ catalyst /1000) and 0.5 ml of anhydrous CH3OH, was placed in the reactor basket of the SITEC apparatus and pressurized with SC-CO2 to 120 atm at 380 K. The stirred mixture was reacted for 2 h and, after cooling to room temperature, the gas phase was bubbled slowly in cold THF (250 K) to collect the volatile phase. The reactor was opened and the content analyzed. The GC-MS analysis of the volatile phase and the residue in the reactor did show the formation of XV (40%) and XVI (60%) (Eq. (10)) as the only products. The relative abundance of the two products depends on the time of reaction. The amount of XVI increases with time. Also varying the conditions of reaction (P / 160 /185 atm, T /350/380 K) the carbonate was not formed in the presence of methanol.
3. Discussion The use of SC-CO2 has been exploited so far essentially as solvent for extraction processes [23], polymerization [24], reactions [25], production of nano-particles [26] or for performing reactions like hydroformylation [27,28] etc. Only very rare examples of use of SC-CO2 as ‘solvent and reagent’ can be found in the literature. An interesting case is the synthesis of formic acid [22] from SC-CO2 and H2, and the synthesis of linear carbonates mentioned above [17,18]. As a matter of fact, the simultaneous use of CO2 as solvent and reagent would be very beneficial and produce positive results in terms of environmental impact and carbon recycling [30]. We discuss here the use of SC-CO2 as solvent and reagent in the synthesis of carbonates and describe the first example of a cyclic carbonate formation from a cyclic ketal under mild conditions. The reaction of cyclohexanone(I), a commercial product, with 1,2-ethane-diol(II) in presence of FeCl3 (see Section 2) affords, with almost quanti-
tative yield, the cyclic ketal(III), (Eq. (8)) (8)
III can be reacted with SC-CO2 by using ad hoc developed catalysts to afford recyclable I and ethylene carbonate(IV), (Eq. 9). (9)
The formation of IV takes place under much milder conditions (100 /160 atm and 379 K) than reported so far for similar systems [17,18]. The use of the correct metal catalyst is a crucial issue for the reaction to occur. We have tested several metal systems, either oxides [ZnO(V), Nb2O5(VI), ZrO2(VII), TiO2(VIII)], or metal halides [ZnCl2(IX), FeCl2(X)], or else metal complexes [FeCl2 ×/ 1.5THF] [21] (XI), CuL2 (XII), FeClL (XIII) (see Section 2). Only V, IX, XI, XII and XIII did show catalytic activity. The most active catalysts were XII and XIII, i.e. those bearing perfluoro alkyl groups, which are soluble in SCCO2 under the reaction conditions. Compounds IX and XI also had some catalytic activity, but 100 time lower than XII or XIII. Interestingly, complex XIII is active while the parent compound X is much less, confirming the role of ligands bearing perfluoroalkyl branches in enhancing the solubility of a metal system [29], and, thus, the reaction yield. We have also tested the use of toluene, CH3CN and methanol as co-solvents with SC-CO2. Toluene and CH3CN had no effect, while in the presence of methanol, reaction (9) did not take place, but an alcoholysis of III was observed, instead (Eq. 10).
M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182
(10)
181
treatment. We have shown that the synthesis of a cyclic carbonate can occur under quite mild conditions, supposed that an effective catalyst is used. Perfluoro diketones are good ligands for Zn, Fe and Cu and the relevant complexes are active in the catalytic formation of carbonate under aprotic conditions.
(11) Acknowledgements
XV and XVI have been isolated and characterized. Dimethylcarbonate, which could be formed from XVI and CO2 (Eq. 11), was not observed in the reaction conditions. It is worth to recall that 2,2-dimethoxypropane reacts with CO2 only at 2000 atm to afford acetone and DMC [18]. When the reaction of ketal(III) with CO2 was carried out in an organic solvent (CH3CN or xylene) under CO2 pressure (60 atm) at 379 K, the formation of carbonate was not observed. Conversely, if methanol was used as solvent, only XV and XVI were isolated, originated by protolytic cleavage of the ketal, as observed in SC-CO2. The solvent, thus, plays a very important role in the reaction. The mild conditions required for reaction 9 to take place in SC-conditions make it quite attractive for synthetic purposes, as the cyclic carbonate could be used in trans-esterification reaction for the production of other carbonates [30 /32]. The recovery and the reuse of keton(I) are good arguments for justifying further efforts for a better exploitation of the reaction.
4. Conclusions The reaction of a ketal in SC-carbon dioxide to afford a ketone and a cyclic carbonate is a particular case of reaction in which SC-CO2 acts as solvent and reagent. Such reaction is a quite useful tool for carrying out the synthesis of organic carbonates, cyclic or linear, avoiding the use of organic solvents and, thus, the waste solvent
The authors gratefully acknowledge Olga Burova and Maria Federova, for some experimental assistance. Financial support from MURST (Project no. 9803026360 and no. MM03027791) is gratefully acknowledged.
References [1] M. Aresta, E. Quaranta, Carbon dioxide, a potential substitute of Phosgene, Chem. Tech. 27 (1997) 32. [2] U. Romano, F. Vimercate, S. Rivetti, N. Di Muzio, US Patent 4,318,862, 1982 (ENIChem). [3] U. Romano, EP Patent 365,083, 1989 (ENIChem). [4] N. Di Muzio, C. Fusi, S. Rivetti, G. Sasselli, EP Patent 460,732, 1991 (ENIChem). [5] G. Paret, G. Donati, M. Ghirardini, EP Patent 460,735, 1991 (ENIChem). [6] D. Dreni, S. Rivetti, D. Delledonne, US Patent 5,322,958, 1994 (ENIChem) [7] S. Rivetti, U. Romano, G. Garone, M. Ghirardini, EP Patent 634,390, 1995 (ENIChem). [8] K. Nishihira, K. Mizutare, S. Tanaka, EP Patent 425,197, 1991 (Ube Ind. Ltd.). [9] T. Matsuzaki, T. Shimamura, S. Fujitsu, Y. Toriyahara, US Patent 5,292,916, 1994 (Ube Ind. Ltd.). [10] K. Nishihira, S. Tanaka, K. Kodama, T. Kaneko, T. Kawashita, Y. Nishida, T. Matsuzaki, K. Abe, US Patent 5,380,906, 1995 (Ube Ind. Ltd.). [11] S. Yoshida, S. Tanaka, EP Patent 655,433, 1995 (Ube Ind. Ltd.). [12] M. Aresta, A. Dibenedetto, Nb2O5 as catalyst in the fixation of carbon dioxide into epoxides to afford organic carbonates with retention of stereochemistry, 221st ACS National Meeting, San Diego, CA, 1 /5 April 2001, Organic Division, Abstract no. 220. [13] M. Aresta, A. Dibenedetto, Key issues in carbon dioxide utilisation as a building block for molecular organic compounds in the chemical industry, ACS Book on CO2 conversion and utilisation, 809 (2002) 54. [14] M. Aresta, A. Dibenedetto, I. Tommasi, Direct synthesis of organic carbonates by oxidative carboxylation of olefins
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M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182 catalysed by metal oxides: developing green chemistry based on carbon dioxide, Appl. Organometal. Chem. 14 (2000) 799. M. Aresta, A. Dibenedetto, Carbon Dioxide as Building Block for the Synthesis of Organic Carbonates: Behavior of Homogeneous and Heterogeneous Catalysts in the Oxidative Carboxylation of Olefins, J. Mol. Catal. A (2002), in press. D. Ballivet-Tkatchenko, O. Douteau, S. Stutzmann, Reactivity of carbon dioxide with n -butyl(phenoxy), (alkoxy)-, and (oxo)stannanes: insight into dimethyl carbonate synthesis, Organometallics 19 (2000) 45. N.S. Isaacs, B. O’Sullivan, High pressure routes to dimethyl carbonate from supercritical carbon dioxide, Tetrahedron 55 (1999) 11949. T. Sakakura, J.-C. Choi, Y. Saito, T. Sako, Synthesis of dimethyl carbonate from carbon dioxide: catalysis and mechanism, Polyhedron 19 (2000) 573. D.D. Perrin, W.L.F. Armarego, D.R. Perrin, Purification of Laboratory Chemicals, Pergamon Press, Oxford, England, 1986. M. Aresta, A. Dibenedetto, C. Dileo, V.L. Rusinov, V.N. Charushin, O. Burova, M. Federova, manuscript submitted. M. Aresta, C.F. Nobile, D. Petruzzelli, Iron(0) oxidation by hydrogen chloride in tetrahydrofuran: a simple way to anhydrous iron(II) chloride, Inorg. Chem. 16 (1977) 1817. See Vincenzo Nicastri Thesis, Department of Chemistry, University of Bari, Bari, Italy, 2000. G. Brunner, Gas Extraction, Springer, Berlin, 1994. S. Beuermann, M. Buback, V. El Rezzi, C. Isemer, M. Jurgens, Homogenous-phase free-radical polymerization in supercritical CO2, Proceedings of the 6th meeting on Supercritical fluids Chemistry and Materials, 1999, p. 325.
[25] W. Leitner, F. Loeker, Supercritical carbon dioxide as an innovative reaction medium for selective oxidation, Proceedings of the 6th meeting on Supercritical fluids Chemistry and Materials, 1999, p. 233. [26] E. Reverchon, Supercritical antisolvent precipitation of micro and nano-particles, J. Supercrit. Fluids 15 (1999) 1. [27] M. Sellin, I. Bach, D.J. Cole-Hamilton, Hydroformilation of alkene in supercritical carbon dioxide catalysed by rhodium trialkylphosphine complexes, Proceedings of the 6th meeting on Supercritical fluids Chemistry and Materials, 1999, p. 41. [28] A. Banet, I. Chadbond, B.T. Heaton, J.A. Iggo, R. Whyman, J. Xiao, Hydroformylation of higher olefins in supercritical CO2, Proceedings of the 6th meeting on Supercritical fluids Chemistry and Materials, 1999, p. 305. [29] P.G. Jessop, W. Leitner, Chemical Synthesis Using Supercritical Fluids, VCH, Wiley, 1999. [30] M. Aresta, A. Caroppo, A. Dibenedetto, Developing innovative synthetic methodologies based on carbon dioxide. Life Cycle Assessment (E-LCA) as a tool for the evaluation of the enviro-economic and energetic performance of new technologies: methanol and dimethylcarbonate as probe cases, 221th ACS National Meeting, San Diego, 2001, abstract no. 61. [31] M. Aresta, A. Dibenedetto, E. Quaranta, Reaction of aromatic diamines with diphenylcarbonate catalyzed by phosphorous acids: a new clean synthetic route to monoand di-carbamates, Tetrahedron 54 (1998) 14145. [32] M. Aresta, A. Dibenedetto, E. Quaranta, Selective carbomethoxylation of aromatic diamines with mixed acid diesters in the presence of phosphorous acids, Green Chemistry 1 (1999) 237.
The first synthesis of a cyclic carbonate from a ketal in SC-CO2 Michele Aresta , Angela Dibenedetto, Caterina Dileo, Immacolata Tommasi, Eliana Amodio Department of Chemistry, University of Bari, and Centro METEA, Via Celso Ulpiani 27, Bari 70126, Italy Received 12 November 2001; received in revised form 22 February 2002; accepted 11 April 2002
Abstract In this paper the synthesis of a cyclic organic carbonate by a halogen-free route is described. The ketal formed from cyclohexanone and 1,2-ethanediol was reacted with carbon dioxide, either in SC-CO2 or in organic solvents under CO2 pressure. Transition-metal complexes with functionalized fluorinated di-ketone ligands were used as catalyst. The synthetic methodology based on SC-CO2 is effective for the synthesis of ethylene carbonate. The use of methanol as cosolvent in SC-CO2 prevents the formation of the ethylene carbonate and only the alcoholysis of the ketal is observed. In organic solvents, no reaction takes place. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Cyclic ketal; Cyclic organic carbonates; SC-CO2 reagent; SC-CO2 solvent
1 2CH3 OHCO O2 0 (CH3 O)2 COH2 O (2) 2
1. Introduction The synthesis of organic carbonates through halogen-free routes may avoid plant corrosion problems, while reducing the environmental burden [1]. A new approach replacing the old phosgenation (Eq. (1)), is based on the oxidative carbonylation of alcohols that uses CO/O2 (Eq. (2)). Such process has found industrial application in the synthesis of dimethylcarbonate [2 /11]. 2ROHCOCl2 0 (RO)2 CO2HCl
The carboxylation of epoxides is a second alternative to phosgene and is exploited at the industrial level [12,13] [Eq. (3)] for the synthesis of propylene carbonate or its polymer. (3)
(1)
Corresponding author. Tel.: /39-080-544-2430; fax: /39080-544-2429 E-mail address: [email protected] (M. Aresta).
More recently, the oxidative carboxylation of olefins [14,15] [Eq. (4)] or the carboxylation of alcohols [16] (Eq. (5)) have been investigated for their potential in the synthesis of cyclic and linear
0896-8446/02/$ - see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 8 9 6 - 8 4 4 6 ( 0 2 ) 0 0 0 9 5 - 5
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M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182
carbonates, respectively.
2.1. Synthesis of the cyclohexanone ethylene ketal (4)
2ROHCO2 0 (RO)2 COH2 O
(5)
Reaction (5) has not yet found an industrial exploitation because of the low turnover and decomposition of the catalyst [16]. Such limited activity may be caused by the formation of water that shifts to left reaction (5) or causes the deactivation of the catalyst. Therefore, synthetic pathways in which water is not formed [Eq. (4)] or is formed in a separate step [Eq. (6,7)] may represent a better alternative. Reactions (6) [17] and (7) [18], afford linear carbonates. They are characterized by very highpressure that rises cost and safety problems. (6)
In a flask equipped with Dean /Stark trap as segregate of water and a reflux condenser, 10.3 ml of cyclohexanone (99.38 mmol) were reacted with an excess of 1,2-ethanediol (6.7 ml, 119.7 mmol) in presence of 736 mg of anhydrous FeCl3 as catalyst in 30 ml of anhydrous benzene. The stirred mixture was heated to 367 /371 K, 0.5 ml of water was separated after 30 min in the Dean /Stark trap and the reaction was complete in 2 h. The reaction system was cooled to room temperature under a nitrogen flow. The product was washed with an aqueous solution of NaOH 32 mM (10 ml) and extracted with toluene (4 /20 ml). The toluene solution was dried overnight on anhydrous MgSO4 activated at 423 K for 24 h, then it was filtered and concentrated to 15 ml to afford the crude product in quantitative yield. By distillation under reduced pressure (3 /10 3 atm) of the crude product, a colorless oil was collected at 311 /314 K and characterized by GC-MS as pure cyclohexanone ethylene ketal (yield of pure product: 80%) (Eq. (8)).
2.2. Synthesis of CuL2 (7)
We report here the first example of synthesis of a cyclic carbonate from a ketal under mild conditions assisted by transition-metal catalysts.
2. Materials and methods CO2 (99.9999%, SIO) was dried before use. All solvents were dried [19]. GC analyses were carried out with a GC-MS Shimadzu QP5050 instrument.
CuCl2 ×/ 2H2O (103.8 mg, 0.6088 mmol) were dissolved in 2 ml of CH3OH and 316.6 mg of the ligand C11H7F4O2 (L) [20] (1.2177 mmol) dissolved in 10 ml of CH3OH were added. The resulting solution was stirred for 6 h at room temperature with continuous monitoring of the pH which lowered to ca. 2 because of the formation of HCl. The solvent was evaporated in vacuo at room temperature and the solid obtained was dissolved in THF (10 ml) and recrystallized by adding pentane (10 ml). The brown solid formed, was separated by filtration and the solution was dried in vacuo at room temperature to afford a green solid that was washed with pentane (6 ml) and dried in vacuo. Anal. Calcd for C22H14CuF8O4: C 47.36; H 2.53; Cu 11.4%. Found: C 47.0; H 2.9; Cu 11.34% .
M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182
179
2.5. Reaction of ketal(III) in SC-CO2
2.3. Synthesis of FeClL A colorless solution of 178.4 mg of FeCl2 ×/ 1.5THF [21] (0.7597 mmol) in 5 ml of ethanol was added to the pale yellow solution of 377.0 mg of ligand C11H7F4O2 (L) (1.519 mmol) in 13 ml of CH2Cl2. The resulting solution was stirred for 6 h at room temperature with continuous monitoring of the pH which lowered to ca. 3/4 because of the formation of HCl. The solvent was evaporated in vacuo at room temperature and the solid obtained was dissolved in THF (10 ml) and recrystallized by adding pentane (10 ml). The brown solid formed, was separated by filtration and the solution dried in vacuo at room temperature to afford a dark orange solid that was washed with pentane (6 ml) and dried in vacuo. Anal. Calcd for C11H7ClF4FeO2: C 49.61; H 4.89; Cl 8.61; Fe 13.57%. Found: C 48.9; H 5.03; Cl 8.51; Fe 12.82%.
2.4. Reaction of ketal(III) with CO2 in organic solvents A glass reactor containing 0.5 ml (3.614 mmol) of ketal, the catalyst (3.7 /10 2 mmol) and 5 ml of solvent (CH3CN or xylene) was introduced in a stainless steel autoclave, pressurized at 50 atm with CO2 and heated to 379 K. The reaction mixture was stirred for 5 h, then it was cooled to room temperature. The gas phase was bubbled slowly in the same solvent (CH3CN or xylene) at 273 K used for the reaction in order to collect the volatile phase. The GC-MS analysis of both the collected gas phase and the residue in the reactor did not show the presence of ethylene carbonate. The ketal was recovered in 98% yield.
All reactions in SC-CO2 were carried out using a SITEC apparatus equipped with a reactor and extractor, with fully automatized control of pressure and temperature. The apparatus can work in continuous or batch mode. In a typical trial, the ketal (0.5 ml, 3.614 mmol) was charged in a open glass basket and placed in the reactor and the catalyst was either added to the ketal, or suspended above it on a synthesized glass septum. The ratio ketal/catalyst was equal to 10009/10. The reaction was carried out for 8 h at 353 /373 K and a pressure of 100 /160 atm (see Table 1). The ketal and the catalyst revealed to be soluble in SC-CO2 under the reaction conditions, as monitored by withdrawing a sample in a IR-HP cell in series with the reactor, that showed a single phase. At the end of the reaction in batch, after cooling and depressurization of the reactor, the gas was collected at 250 K in THF and shown to contain traces of the carbonate. The ethylene carbonate, the unreacted ketal and cyclohexanone were recovered from the basket with the use of a solvent (see Table 1). The catalyst, if deposited on the glass synthered bed, did not contaminate the product [22]. No other products were observed besides the residual ketal, the cyclic carbonate and cyclohexanone. The mixture carbonate/ketal/cyclohexanone was separated by column chromatography by using a toluene/pentane (1:1 v/v) mixture to afford pure products. Table 1 Conditions of reaction and yield (8 h of reaction in any case) Catalyst
T (K)
P (bar)
TON
ZnCl2 ZnCl2 ZnCl2 ZnCl2 ZnCl2 CuL2 FeLCl FeCl2 FeCl2 ×/ THF
353 373 373 373 373 373 373 373 373
160 160 230 120 120 160 160 160 160
12 15 19 11 18 720 890 15 10
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2.6. Reaction of ketal(III) in SC-CO2 and CH3OH as cosolvent A glass reactor, charged under nitrogen with 0.5 ml of ketal (3.614 mmol), catalyst (ratio ketal/ catalyst /1000) and 0.5 ml of anhydrous CH3OH, was placed in the reactor basket of the SITEC apparatus and pressurized with SC-CO2 to 120 atm at 380 K. The stirred mixture was reacted for 2 h and, after cooling to room temperature, the gas phase was bubbled slowly in cold THF (250 K) to collect the volatile phase. The reactor was opened and the content analyzed. The GC-MS analysis of the volatile phase and the residue in the reactor did show the formation of XV (40%) and XVI (60%) (Eq. (10)) as the only products. The relative abundance of the two products depends on the time of reaction. The amount of XVI increases with time. Also varying the conditions of reaction (P / 160 /185 atm, T /350/380 K) the carbonate was not formed in the presence of methanol.
3. Discussion The use of SC-CO2 has been exploited so far essentially as solvent for extraction processes [23], polymerization [24], reactions [25], production of nano-particles [26] or for performing reactions like hydroformylation [27,28] etc. Only very rare examples of use of SC-CO2 as ‘solvent and reagent’ can be found in the literature. An interesting case is the synthesis of formic acid [22] from SC-CO2 and H2, and the synthesis of linear carbonates mentioned above [17,18]. As a matter of fact, the simultaneous use of CO2 as solvent and reagent would be very beneficial and produce positive results in terms of environmental impact and carbon recycling [30]. We discuss here the use of SC-CO2 as solvent and reagent in the synthesis of carbonates and describe the first example of a cyclic carbonate formation from a cyclic ketal under mild conditions. The reaction of cyclohexanone(I), a commercial product, with 1,2-ethane-diol(II) in presence of FeCl3 (see Section 2) affords, with almost quanti-
tative yield, the cyclic ketal(III), (Eq. (8)) (8)
III can be reacted with SC-CO2 by using ad hoc developed catalysts to afford recyclable I and ethylene carbonate(IV), (Eq. 9). (9)
The formation of IV takes place under much milder conditions (100 /160 atm and 379 K) than reported so far for similar systems [17,18]. The use of the correct metal catalyst is a crucial issue for the reaction to occur. We have tested several metal systems, either oxides [ZnO(V), Nb2O5(VI), ZrO2(VII), TiO2(VIII)], or metal halides [ZnCl2(IX), FeCl2(X)], or else metal complexes [FeCl2 ×/ 1.5THF] [21] (XI), CuL2 (XII), FeClL (XIII) (see Section 2). Only V, IX, XI, XII and XIII did show catalytic activity. The most active catalysts were XII and XIII, i.e. those bearing perfluoro alkyl groups, which are soluble in SCCO2 under the reaction conditions. Compounds IX and XI also had some catalytic activity, but 100 time lower than XII or XIII. Interestingly, complex XIII is active while the parent compound X is much less, confirming the role of ligands bearing perfluoroalkyl branches in enhancing the solubility of a metal system [29], and, thus, the reaction yield. We have also tested the use of toluene, CH3CN and methanol as co-solvents with SC-CO2. Toluene and CH3CN had no effect, while in the presence of methanol, reaction (9) did not take place, but an alcoholysis of III was observed, instead (Eq. 10).
M. Aresta et al. / J. of Supercritical Fluids 25 (2003) 177 /182
(10)
181
treatment. We have shown that the synthesis of a cyclic carbonate can occur under quite mild conditions, supposed that an effective catalyst is used. Perfluoro diketones are good ligands for Zn, Fe and Cu and the relevant complexes are active in the catalytic formation of carbonate under aprotic conditions.
(11) Acknowledgements
XV and XVI have been isolated and characterized. Dimethylcarbonate, which could be formed from XVI and CO2 (Eq. 11), was not observed in the reaction conditions. It is worth to recall that 2,2-dimethoxypropane reacts with CO2 only at 2000 atm to afford acetone and DMC [18]. When the reaction of ketal(III) with CO2 was carried out in an organic solvent (CH3CN or xylene) under CO2 pressure (60 atm) at 379 K, the formation of carbonate was not observed. Conversely, if methanol was used as solvent, only XV and XVI were isolated, originated by protolytic cleavage of the ketal, as observed in SC-CO2. The solvent, thus, plays a very important role in the reaction. The mild conditions required for reaction 9 to take place in SC-conditions make it quite attractive for synthetic purposes, as the cyclic carbonate could be used in trans-esterification reaction for the production of other carbonates [30 /32]. The recovery and the reuse of keton(I) are good arguments for justifying further efforts for a better exploitation of the reaction.
4. Conclusions The reaction of a ketal in SC-carbon dioxide to afford a ketone and a cyclic carbonate is a particular case of reaction in which SC-CO2 acts as solvent and reagent. Such reaction is a quite useful tool for carrying out the synthesis of organic carbonates, cyclic or linear, avoiding the use of organic solvents and, thus, the waste solvent
The authors gratefully acknowledge Olga Burova and Maria Federova, for some experimental assistance. Financial support from MURST (Project no. 9803026360 and no. MM03027791) is gratefully acknowledged.
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