Potassium Carbonate-18-Crown-6


[584-08-7]  · CK2O3  · Potassium Carbonate-18-Crown-6  · (MW 138.21) (18-crown-6)

[17455-13-9]  · C12H24O6  · Potassium Carbonate-18-Crown-6  · (MW 264.36)

(basic reagent combination that operates efficiently under catalytic two-phase conditions, yet is sufficiently gentle not to hydrolyze or otherwise destroy sensitive products;1,2 capable of promoting trans-alkene formation during Wittig condensations;3 heightens selectivity and reactivity of carbanions in nonpolar solvents4)

Physical Data: prepared in situ.

Solubility: while 18-crown-6 is soluble in aprotic solvents, potassium carbonate exhibits very limited solubility. As a consequence, it is believed that proton abstraction occurs on the surface of the solid carbonate.1 For this reason, it is particularly important to recognize that K2CO3 is a finely powdered material that exhibits no tendency to aggregate and form lumps.

Handling, Storage, and Precautions: this reagent combination eliminates the need to prepare anhydrous solvents and is convenient because the prior preparation of carbanions is unnecessary. Furthermore, hazardous and expensive ingredients are not involved.

Efficiency as a Base.

Phase-transfer catalysis of reactions by aqueous hydroxide in an inert organic medium is often disadvantaged by competing saponification if the starting materials are prone to hydrolysis. To regain efficiency, it is advantageous to turn instead to a liquid-solid two-phase system constituted of Potassium Carbonate and an aprotic solvent, such as CH2Cl2 or benzene, to which 18-Crown-6 is added to facilitate transport of the conjugate base into the organic phase. Under these conditions, Diethyl Malonate readily undergoes highly selective monoalkylation (eq 1).1 Other notably sensitive esters respond well to this rather mild base (eqs 2 and 3),1 despite the fact that somewhat higher temperatures are sometimes required.

In addition to the Darzens reaction, it is also possible to prepare dibromocarbene, ethers by the Williamson synthesis, phosphonates by Michaelis-Becker alkylation, and alkenes by the Wittig reaction. In this last process, the conditions are sufficiently mild that solvent effects can be utilized to control product stereochemistry.3 When nonstabilized ylides are involved, the use of THF leads predominantly to the cis-alkenic isomer in a manner akin to salt-free conditions (eq 4). In CH2Cl2 solution the product distribution is reversed. If the ylide is stabilized, either solvent leads to the trans-alkene.

Dendritic polymers having an average molecular weight in excess of 100 000 have been produced by initiating polycondensation with the title reagent system.5

Methylation and Silylation Reactions.

Neat dimethyl carbonate can supplant Dimethyl Sulfate as methylating agent when reaction is promoted by potassium carbonate and 18-crown-6 (eq 5).6 The same catalyst system appears ideally suited to the methylation of carboxylic acids and phenols with methyl trichloroacetate (eqs 6 and 7).7,8 Since the byproducts are CHCl3 and CO2, product isolation is conveniently realized. Primary and allylic trichloroacetates can be substituted with equal success. An added major advantage of this technology is its lack of dependence on stoichiometric base.

The companion reagent trimethylsilyl trichloroacetate has been developed into a convenient tool for the salt-free silylation of compounds carrying an acidic hydrogen (carboxylic acids, phenols, amides, thiols, terminal alkynes, etc). As before, a catalytic amount of potassium carbonate/18-crown-6 gives rise to CO2 and CHCl3 in addition to product (eq 8).9 When aldehydes or ketones are involved, trimethylsilyl trichloromethyl carbinols are formed efficiently (eq 9).

Intramolecular Wadsworth-Emmons Cyclizations.

Attempts to prepare bicyclo[3.3.0]oct-1-en-3-one and derivatives thereof by intramolecular cyclization have been fraught with complications because of the sensitivity of both the starting material and product to basic conditions. This problem has been resolved by making recourse to potassium carbonate and 18-crown-6 in benzene or toluene at 60 °C (eq 10).2 The gentle nature of this reagent has proven highly serviceable in a number of related ring closures (eqs 11 and 12),10,11 including macrocyclizations (eq 13).12

Formation of Carbonate Esters.

Historically, it has not been practical to prepare organic carbonates from inorganic carbonate. However, the advent of solid-liquid phase techniques has opened the door to striking developments in this area. A catalyst other than potassium carbonate is necessary for acceleration and the realization of high-yield conversions. Suitable candidates are Tri-n-butylchlorostannane,13 hexabutyldistannoxane,13 hexabutyldistannathiane (eq 14),13 potassium hydrogen carbonate,14 and quaternary ammonium salts.15 Polycarbonates have similarly been produced from a,a-dibromo-p-xylene.13

Cyclic carbonates are likewise readily available via the reaction of oxiranes with Carbon Dioxide under phase-transfer catalysis of potassium carbonate/18-crown-6 (eq 15).16 Homopolymerization is not a serious complication as long as the crown ether is present.

1. Fedorynski, M.; Wojciechowski, K.; Matacz, Z.; Makosza, M. JOC 1978, 43, 4682.
2. Aristoff, P. A. SC 1983, 13, 145.
3. Boden, R. M. S 1975, 784.
4. Renga, J. M.; Wang, P.-C. SC 1984, 14, 69.
5. (a) Uhrich, K. E.; Hawker, C. J.; Fréchet, J. M. J.; Turner, S. R. Macromolecules 1992, 25, 4583. (b) Spindler, R.; Fréchet, J. M. J. JCS(P1) 1993, 913.
6. Lissel, M. LA 1987, 77.
7. Renga, J. M.; Wang, P.-C. SC 1984, 14, 77.
8. Renga, J. M.; Wang, P.-C. SC 1984, 14, 69.
9. Renga, J. M.; Wang, P.-C. TL 1985, 26, 1175.
10. Aristoff, P. A. JOC 1981, 46, 1954.
11. Dauben, W. G.; Walker, D. M. TL 1982, 23, 711.
12. Nicolaou, K. C.; Seitz, S. P.; Pavia, M. R. JACS 1982, 104, 2030.
13. Fujinami, T.; Sato, S.; Sakai, S. CL 1981, 749.
14. Lissel, M.; Dehmlow, E. V. CB 1981, 114, 1210.
15. Cella, J. A.; Bacon, S. W. JOC 1984, 49, 1122.
16. Rokicki, G.; Kuran, W.; Pogorzelska-Marciniak, B. M 1984, 115, 205.

Leo A. Paquette

The Ohio State University, Columbus, OH, USA

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