ORGANIC LETTERS
Cyclic Carbonate Formation from Carbon Dioxide and Oxiranes in Tetrabutylammonium Halides as Solvents and Catalysts
2002 Vol. 4, No. 15 2561-2563
Vincenzo Calo´,* Angelo Nacci, Antonio Monopoli, and Antonello Fanizzi Department of Chemistry, Istituto di Chimica dei Composti Organometallici (Sezione di Bari), Via Amendola 173, UniVersity of Bari, 70126-Bari, Italy
[email protected] Received May 15, 2002
ABSTRACT
Epoxides dissolved in molten tetralkylammonium salts bearing halides as counterions are converted into cyclic carbonates under atmospheric pressure of carbon dioxide. The reaction rate depends on the nucleophilicity of the halide ion as well as the structure of the cation.
The chemical fixation of CO2 onto organic compounds such as epoxides, which affords cyclic carbonates, is an important process (Scheme 1), as it allows the transformation of
Scheme 1
harmful waste such as CO2 into useful raw materials for engineering plastics, cosmetics, and polar solvents.1,2 Many inorganic and organic compounds including amines,3 phosphanes,3 organotin halides,4 alkali metal salts,5 transition (1) Behr, A. Carbon Dioxide ActiVation by Metal Complexes; VCH: New York, 1988. (2) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. ReV. 1996, 153, 155-174. (3) Brindo¨pke, G. German Pat. DE 3529263, 1987. (4) Baba, A.; Nozaki, T.; Matsuda, H. Bull. Chem. Soc. Jpn. 1987, 60, 1552-1554. (5) Kihara, N.; Hara, N.; Endo, T. J. Org. Chem. 1993, 58, 6198-6202. 10.1021/ol026189w CCC: $22.00 Published on Web 06/29/2002
© 2002 American Chemical Society
metal complexes,6-8 magnesium oxide,9 calcinated hydrotalcites,10 and phthalocyaninatoaluminum in supercritical carbon dioxide11 are known to catalyze activation of the CO2 molecule. More recently, propylene carbonate was synthesized in imidazolium tetrafluoroborate as solvent under electrochemical conditions.12,13 For these processes, high temperatures, high pressures of CO2, and toxic polar solvents such as DMF or CH2Cl2 have been thought to be necessary. However, under these reaction conditions, some inactive or polymerization-sensitive oxiranes can be hardly converted to the corresponding cyclic carbonates. We report a straightforward method for chemical fixation of CO2 onto epoxides by simply dissolving these compounds (6) Kruper, W. J.; Dellar, D. V. J. Org. Chem. 1995, 60, 725-727. (7) Magdesieva, T. V.; Milovanov, S. V.; Lokshin, B. V.; Klemenkova, Z. S.; Tomilova, L. G. Russ. Chem. Bull. 1998, 47, 2137-2145. (8) Kim, H. S.; Kim, J. J.; Lee, B. G.; Jung, O. S.; Jang, H. G.; Kang, S. O. Angew. Chem., Int. Ed. 2000, 39, 4096-4098. (9) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1997, 1129-1130. (10) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, H.; Kaneda, K. J. Am. Chem. Soc. 1999, 121, 4526-4527. (11) Lu, X. B.; Pan, Y. Z.; Ji, D. F.; He, R. Chin. Chem. Lett. 2000, 11, 589. (12) Deng, Y.; Peng, J. New J. Chem., 2001, 25, 639. (13) Yang, H.; Gu, Y.; Deng, Y.; Shi, F. Chem. Commun. 2002, 274.
Scheme 2
Table 1. Synthesis of Cyclic Carbonates in Tetralkylammonium Salts
solvents due to low solvation.16 If so, the substitution in the ammonium salt of a bromide ion by a more nucleophilic iodide ion would yield better results such as an increase of reaction rate.17-20 Indeed, by using a mixture of TBAB and tetrabutylammonium iodide (TBAI), the conversion rate increased as glycidyl methacrylate reacted fully in only 1 h (Table 1, run 2, method A). As a clean chemical process should require the lowest possible energy input and solvent quantity, we tested the CO2 fixation method by dissolving only 10% (w/w) of TBAI into liquid epoxides at 60 °C. Though the rates were lower, all of the epoxides reacted well to afford pure carbonates (method B).
a Isolated yields are average of at least two runs; >95% pure as judged by 1H NMR and GC. b As method A but with TBAB as solvent. c Catalyst 1. d Catalyst 2. e Catalyst 3.
in molten tetrabutylammonium bromide (TBAB) as solvent, in the presence of CO2 at atmospheric pressure. Once the reaction was complete, pure cyclic carbonates were isolated by vacuum distillation or extraction with ethyl acetate, in which the ionic liquid is insoluble. This procedure allowed the recycling of the ammonium salt.14 Polymerizationsensitive epoxides also reacted very well. For example, the glycidyl methacrylate was converted into the corresponding carbonate in 4 h without polymerization (Table 1, run 1). A plausible mechanism for this reaction is the ring opening of the epoxide by means of a nucleophilic attack by the bromide ion, which leads to an oxy anion species affording the corresponding cyclic carbonate after reaction with CO2 (Scheme 2). This is conceivable15 since the anions may have greatly enhanced nucleophilicity in the absence of other organic (14) General Procedure. Method A: 20 g of epoxide and 50 g of a mixture of TBAB and TBAI (1:1 w/w) were added to a Schlenk flask, stirred, and heated at 120 °C under CO2. The reaction pressure (1 bar CO2) was held constant by means of a CO2 reservoir connected to the flask. After the proper reaction time, the flask was cooled and the carbonate was distilled under reduced pressure or extracted with ethyl acetate, leaving the ammonium salts, which were further recycled. Method B: 3 g of TBAI dissolved in 30 g of epoxide were heated at 60 °C under CO2 as described above. The carbonate was distilled or extracted with ethyl acetate. (15) As a proof for this mechanism, we isolated, in some cases, a small quantity of bromohydrines. 2562
In addition to the anion effect, we tested a possible role exerted by the cation on this reaction. This was explored by using the salts 1 and 2 and the poly(N-methyl-4-vinylpyridinium) salt 3 as catalysts (10% w/w), which were found inefficient at 60 °C (Figure 1). Furthermore 1 and 3 decomposed by increasing the temperature (Table 1, entries 14-16).
Figure 1.
We believe that the better performance of tetralkylammonium salts as catalysts could be ascribed to the structural differences among [NBu4]+ and [bmim]+ (1-butyl-3-meth(16) Reichardt, C. SolVents and SolVent Effects in Organic Chemistry, 2nd ed.; VCH: New York, 1988. (17) Calo´, V.; Nacci, A.; Lopez, L.; Mannarini, N. Tetrahedron Lett. 2000, 41, 8973-8976. (18) Calo´, V.; Nacci, A.; Lopez, L.; Napola, A. Tetrahedron Lett. 2001, 42, 4701-4703. (19) Calo´, V.; Nacci, A.;. Monopoli, A.; Lopez, L.; di Cosmo, A. Tetrahedron 2001, 57, 6071-6077. (20) Calo´, V.; Nacci, A. Z. Naturforsch. 2001, 56a, 702-706. (21) Gannon, T. J.; Law, G.; Watson, P. R.; Carmichael, A. J.; Seddon, K. R. Langmuir 1999, 15, 8429-8434. Org. Lett., Vol. 4, No. 15, 2002
ylimidazolium) or pyridinium cations, which could influence the behavior of anions. Indeed, the bulkiness of the tetrahedral ammonium ion forces the bromide or iodide ion away from the cation, and this less electrostatic interaction would render these anions more nucleophilic. On the contrary, the planar cations 1-3, by binding the anion tightly,21 would decrease its availability for reaction with epoxides.
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Acknowledgment. This work has been supported, in part, by Ministero dell’Universita` e della Ricerca Scientifica e Tecnologica, Rome, and the University of Bari (National Project: “Stereoselezione in Sintesi Organica: Metodologie ed Applicazioni”). OL026189W
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