Carbon Dioxide1

O=C=O

[124-38-9]  · CO2  · Carbon Dioxide  · (MW 44.01)

(agent for the carboxylation of phenols,2 organometallic reagents3 and various carbanions, and for the preparation of dialkyl carbonates, urethanes, and ureas1)

Physical Data: bp -78.5 °C (sub); critical temp 31.06 °C; vapor pressure 6300 kPa at 25 °C; mp -56.6 °C (at 5.2 atm); density: solid: 1.56 g cm-3 (at -79 °C); liquid: 1.101 g cm-3 (at -37 °C); gas: 1.977 g L-1 (at 0 °C).

Solubility: 75.9 mL/100 mL in H2O at 20 °C and 101.3 kPa; 31 mL/100 mL in EtOH at 15 °C.

Form Supplied in: solid pellets (dry ice); compressed gas or liquid. Drying: pass gas through conc H2SO4 or a column of anhydrous calcium sulfate.

Handling, Storage, and Precautions: colorless, odorless gas which is dangerous to respiration at high concentrations; TLV = 5000 ppm.

Carboxylation of Phenoxides.

The reactions of sodium or potassium phenoxides with carbon dioxide under pressure (the Kolbe-Schmitt reaction) or of phenols with anhydrous Potassium Carbonate (Marasse modification) are a very important source of hydroxy aromatic acids.2 Carboxylation is favored at the ortho position, but para isomers may also be obtained at higher temperatures (eq 1). Depending on the reaction conditions, 2-naphthol may be converted into either the 1-carboxylic acid (<150 °C) or the 3-isomer (>200 °C) (eq 2). Carboxylation is retarded by electron withdrawing groups, but enhanced by electron releasing substituents. Resorcinol, for example, is carboxylated simply by bubbling CO2 through an alkaline solution (eq 3).4

Carboxylation of Organometallic Derivatives.

Carbon dioxide inserts readily into metal-carbon bonds with two possible outcomes (eq 4).5 By far the most familiar and common pathway is the formation of a new carbon-carbon bond leading to a carboxylic acid, as in the very familiar reactions6 of Grignard reagents and organolithium compounds (eqs 5 and 6),7,8 although care must be taken to use low temperatures and an excess of CO2 in order to avoid further reaction of the product with additional reagent.

In a few special cases, however, carbon-oxygen bond formation may take place (eq 4b), as in the reaction of the cobalt complex (1) to form (3) in addition to the expected (2) (eq 7).9 Treatment of (3) with Hydrogen Chloride afforded ethyl formate, while reaction with Iodomethane afforded ethyl acetate in high yield. Another example of carbon-oxygen bond formation was observed during the reaction of a methallyl palladium p-complex with CO2 (eq 8), although this could be suppressed by the addition of 1,2-Bis(diphenylphosphino)ethane (diphos).10

The reaction of diethylzinc with CO2 is very slow, but it may be accelerated by ligands such as N,N,N,N-Tetramethylethylenediamine or N-Methylimidazole.11 Vinyl and aryl copper reagents are similarly unreactive, but again they may be activated by the addition of suitable ligands, as in the stereoselective synthesis of b-substituted acrylic acids from alkynes (eq 9).12

The heteroatom facilitated lithiation of sp2 carbon, including the lithiation of heterocycles and the directed lithiation of aromatic compounds, is a powerful and flexible synthetic strategy.13 The intermediates react with CO2 in good to excellent yields to form carboxylic acids which, in many cases, could otherwise be obtained only with considerable difficulty. Carboxylation is strongly favored at C-2 in the common heterocycles: furan, pyrrole, thiophene, imidazole, oxazole, thiazole, pyridine, pyrimidine, benzofuran, indole, benzothiophene, and their derivatives.14 For benzenoid aromatic derivatives, reaction occurs at ortho (eqs 10 and 11)15,16 or benzylic sites (eq 12),17 but neighboring positions in a second aromatic ring may also participate (eqs 13 and 14).18

Carboxylation of Ketone Enolates and Other Carbanions.

Ketones may be converted into b-keto acids by treatment with CO2 in the presence of a variety of bases, e.g. potassium phenoxide,19 lithium 2,6-di-t-butyl-4-methylphenoxide,20 Triethylamine-Magnesium Iodide,21 and 1,8-Diazabicyclo[5.4.0]undec-7-ene,22 and with Methyl Magnesium Carbonate.23 Several urea-based magnesium complexes, inspired by the biotin carboxylases, have also been utilized.24 There has been considerable interest in the trapping of preformed enolates with CO2 (eqs 15 and 16),25 especially under conditions of kinetic control.26

Yields are only poor to fair in most cases, probably because of the competing formation of enol carbonates27 which decompose to the parent ketone on workup and/or the instability of the b-keto acid products. The successful trapping of enolates from Li/NH3 reduction of enones (eq 17)26,28 and n-butylthiomethylene ketones (eq 18)29 has been reported.

Carboxylation of Miscellaneous Carbanions.

In addition to ketones, various other carbanions have been carboxylated, including those from esters,30 dithianes,31 and aldimines,32 as have phosphoranes.33

Preparation of Dialkyl Carbonates, Urethanes, and Ureas.

Alcohols may be converted into symmetrical dialkyl carbonates by reaction with a combination of CO2, Triphenylphosphine, and Diethyl Azodicarboxylate,34 while cyclic carbonates may be obtained from the reaction of epoxides with CO2 with catalysis by a range of Lewis acids.35 Allylic epoxides react with CO2 and Palladium(II) Acetate plus Triisopropyl Phosphite to form cyclic carbonates with retention of configuration.36

Allylic and homoallylic alcohols may also be transformed into cyclic carbonates by conversion of the derived lithium alkoxides into hemicarbonates followed by in situ cyclization with iodine (eq 19).37 Amines have been converted into urethanes by a mixture of Copper(I) t-Butoxide, t-Butyl Isocyanide, and CO238 and into symmetrical ureas by treatment with Et3N, 1,3-Dicyclohexylcarbodiimide, and CO2.39

Related Reagents.

N,N-Carbonyldiimidazole; Diethyl Carbonate; Methyl Magnesium Carbonate; Methyl Chloroformate; Methyl Cyanoformate.


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2. Lindsey, A. S.; Jesky, H. CRV 1957, 57, 583.
3. Behr, A. Carbon Dioxide Activation by Metal Complexes; VCH: Weinheim, 1988.
4. Nierenstein, M.; Clibbens, D. A. OSC 1943, 2, 557.
5. Ref. 3, pp 25-67.
6. Kharasch, M. S.; Reinmuth, O. Grignard Reactions of Nonmetallic Substances; Prentice Hall: New York, 1954, pp 913-947.
7. Puntambeker, S. V.; Zoellner, E. A. OSC 1941, 1, 524.
8. Pearson, D. E.; Cowan, D. OSC 1973, 5, 890.
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13. (a) Beak, P.; Snieckus, V. ACR 1982, 15, 306. (b) Beak, P.; Meyers, A. I. ACR 1986, 19, 356.
14. Gschwend, H. W.; Rodriguez, H. R. OR 1979, 26, 1.
15. Snieckus, V. CRV 1990, 90, 879.
16. Trost, B. M.; Rivers, G. T.; Gold, J. M. JOC 1980, 45, 1835.
17. Loewenthal, H. J. E.; Schatzmiller, S. JCS(P1) 1976, 944.
18. Narasimhan, N. S.; Mali, R. S. Top. Curr. Chem. 1987, 138, 63.
19. Mori, H.; Yamamoto, H.; Kwan, T. CPB 1972, 20, 2440.
20. Corey, E. J.; Chen, R. H. K. JOC 1973, 38, 4086.
21. Tirpak, R. E.; Olsen, R. S.; Rathke, W. JOC 1985, 50, 4877.
22. Haruki, E.; Arakawa, M.; Matsumura, N.; Otsuji, Y.; Imoto, E. CL 1974, 427.
23. Stiles, M. JACS 1959, 81, 2598.
24. (a) Matsumura, N.; Sakaguchi, Y.; Ohba, T.; Inoue, H. CC 1980, 326. (b) Tsuda, T.; Chujo, Y.; Hayasaki, T.; Saegusa, T. CC 1979, 797. (c) Matsumura, N.; Ohba, T.; Inoue, H. BCJ 1982, 55, 3949. (d) Sakurai, H.; Shirahata, A.; Hosomi, A. TL 1980, 21, 1967. (e) Matsumura, N.; Asai, N.; Yoneda, S. CC 1983, 1487.
25. (a) Cardwell, H. M. E.; Cornforth, J. W.; Duff, S. R.; Holtermann, H.; Robinson, R. JCS 1953, 361. (b) Wenkert, E.; Jackson, B. G. JACS 1959, 81, 5601.
26. (a) Stork, G.; Rosen, P.; Goldman, N.; Coombs, R. V.; Tsuji, J. JACS 1965, 87, 275. (b) Caine, D OR 1976, 23, 1.
27. Caine, D. In Carbon-Carbon Bond Formation; Augustine, R. L., Ed.; Dekker: New York, 1979; Vol. 1, p 258, footnote 69.
28. (a) Spencer, T. A.; Weaver, T. D.; Villarica, R. M.; Friary, R. J.; Posler, J.; Schwartz, M. A. JOC 1968, 33, 712. (b) Spencer, T. A.; Friary, R. J.; Schmiegel, W. W.; Simeone, J. F.; Watt, D. S. JOC 1968, 33, 719. (c) Sharpless, K. B.; Snyder, T. E.; Spencer, T. A.; Maheshwari, K. K.; Guhn, G.; Clayton, R. B. JACS 1968, 90, 6874. (d) Czarny, M. R.; Maheshwari, K. K.; Nelson, J. A.; Spencer, T. A. JOC 1975, 40, 2079. (e) Afonso, A. JOC 1970, 35, 1949.
29. Coates, R. M.; Soweby, R. L. JACS 1971, 93, 1027.
30. Reiffers, S.; Wynberg, H.; Strating, J. TL 1971, 3001.
31. Corey, E. J.; Seebach, D. AG(E) 1965, 4, 1077.
32. (a) Walborsky, H. M.; Niznik, G. E. JACS 1969, 91, 7778. (b) Walborsky, H. M.; Morrison, W. H.; Niznik, G. E. JACS 1970, 92, 6675.
33. Bestmann, H. J.; Denzel, Th.; Salbaum, H. TL 1974, 1275.
34. Hoffman, W. A. JOC 1982, 47, 5209.
35. (a) Matsuda, H.; Ninagawa, A.; Nomura, R. CL 1979, 1261. (b) Ratzenhofer, M.; Kisch, H. AG(E) 1980, 19, 317.
36. Trost, B. M.; Angle, S. R. JACS 1985, 107, 6123.
37. (a) Cardillo, G.; Orena, M.; Porzi, G.; Sandri, S. CC 1981, 465. (b) Bartlett, P. A.; Meadows, J. D.; Brown, E. G.; Morimoto, A.; Jernstedt, K. K. JOC 1982, 47, 4013.
38. Tsuda, T.; Washita, H.; Watanabe, K.; Miwa, M.; Saegusa, T. CC 1978, 815.
39. Ogura, H.; Takeda, K.; Tokue, R.; Kobayashi, T. S 1978, 394.

Lewis N. Mander

The Australian National University, Canberra, Australia



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