Potassium t-Butoxide-18-Crown-6


[865-47-4]  · C4H9KO  · Potassium t-Butoxide-18-Crown-6  · (MW 112.23) (18-crown-6)

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

(basic and especially nucleophilic crown ether complex of t-BuOK)

Preparative Method: prepared in situ by mixing the base and the crown ether in an appropriate solvent.

Handling, Storage, and Precautions: see 18-Crown-6 and Potassium t-Butoxide.

General Discussion.

The ability of 18-crown-6 to complex potassium cations is well known.1 When the crown ether is added to solutions of t-BuOK in nonpolar, aprotic solvents, where it exists primarily in the form of aggregated ion pairs,2 the equilibrium is shifted in the direction of ligand-separated ion pairs and dissociated t-butoxide anions.3 Thus in the presence of the crown ether, the basicity and especially the nucleophilicity of the t-butoxide anion are enhanced. Dipolar aprotic solvents such as DMSO, which strongly solvate potassium cations, also produce relatively high concentrations of t-butoxide anions.2 However, in such media these anions exhibit high basicity vis-à-vis nucleophilicity because solvent-assisted deprotonation reactions are favorable.4 18-Crown-6 is an expensive reagent, but it is useful in catalytic as well as stoichiometric amounts to activate t-BuOK. As already noted, the combination of this additive and a nonpolar solvent provides a relatively favorable medium for the t-butoxide ion to behave as a nucleophile.2 If an increase in the basicity of t-BuOK is all that is required, there still may be an advantage in using 18-crown-6 instead of DMSO, the most popular activator of the base (see Potassium t-Butoxide-Dimethyl Sulfoxide), for certain substrates. The recovery of relatively water-soluble reaction products from DMSO solutions is difficult, while the crown ether can usually be removed easily with silica gel. Also, DMSO solutions contain significant concentrations of DMSO-K+ which may cause unwanted side reactions.2

18-Crown-6 catalyzes the conversion of phosphonium salts to unstabilized phosphoranes with potassium t-butoxide in nonpolar solvents.5 The nature of the solvent has a remarkable effect on the stereochemistry of the subsequent Wittig reactions with aldehydes. The use of THF favors the production of cis-alkenes, while CH2Cl2 favors the formation of trans-isomers. When stabilized ylides are employed, trans-alkenes are favored in both solvents. Methylenations of 2-(dimethylaminomethyl)cycloalkanones occur in high yield using this procedure (eq 1).5c

Dihalotoluenes yield free halophenyl carbenes when treated with t-BuOK in benzene in the presence of 18-crown-6, but halophenyl carbenoids are produced in the absence of the crown ether.6 Primary and other alkyl halides are readily converted into alkenes with t-BuOK in petroleum ether solvents with a catalytic amount of 18-crown-6.7 This procedure is particularly useful for primary halides (eq 2), which yield alkyl t-butyl ethers under the usual conditions.7 Alkynes are produced in excellent yields from 1,1- and 1,2-dihalides (eq 3) under similar conditions.8

Benzyl t-butyl ether is obtained in good yield upon treatment of Benzyl Chloride with t-BuOK in the presence of 5-10 mol % 18-crown-6 (eq 4).3 The enhanced nucleophilicity of the t-butoxide ion in the presence of 18-crown-6 is evident, because in its absence the reaction is extremely slow. If t-BuOK is used in dipolar aprotic solvents (e.g. DMSO), stilbene, which is derived from initial deprotonation of benzyl chloride, is the major reaction product.

18-Crown-6 improves yields in the t-BuOK-catalyzed oxidations of nitriles to carboxylic acids with loss of the cyano group (eq 5).9 N-Alkylations of aromatic nitrogen heterocycles are accomplished in high yields using crown ether-activated t-BuOK in ether or benzene (eq 6).10

Alkynyloxiranes undergo isomerization to furan derivatives when treated with t-BuOK in the presence of 18-crown-6 (eq 7).11 t-BuOK complexes of C2-symmetric chiral derivatives of 18-crown-6 are efficient catalysts for the asymmetric Michael addition of phenylacetic acid esters to Methyl Acrylate.12

1. Gokel, G. W.; Durst, H. D. S 1976, 168.
2. Pearson, D. E.; Buehler, C. A. CRV 1974, 74, 45.
3. DiBiase, S. A.; Gokel, G. W. JOC 1978, 43, 447.
4. Cram, D. J.; Mateos, J. C.; Hanck, F.; Langemann, A.; Kopecky, K. R.; Nielsen, W. D.; Allinger, J. JACS 1959, 81, 5774.
5. (a) Boden, R. M. S 1975, 784. (b) Dehmlow, E. V.; Barahona-Naranjo, S. JCR(M) 1981, 1748. (c) Le, N. A.; Jones, M., Jr.; Bickelhaupt, F.; deWolf, W. H. JACS 1989, 111, 8491.
6. Moss, R. A.; Lawrynowicz, W. JOC 1984, 49, 3828.
7. Dehmlow, E. V.; Lissel, M. S 1979, 372.
8. Dehmlow, E. V.; Lissel, M. LA 1980, 1.
9. DiBiase, S. A.; Wolak, R. P., Jr.; Dishong, D. M.; Gokel, G. W. JOC 1980, 45, 3630.
10. Guida, W. C.; Mathre, D. J. JOC 1980, 45, 3172.
11. Marshall, J. A.; DuBay, W. J. JOC 1991, 56, 1685.
12. Aoki, S.; Sasaki, S.; Koga, K. TL 1989, 30, 7229.

Drury Caine

The University of Alabama, Tuscaloosa, AL, USA

Copyright 1995-2000 by John Wiley & Sons, Ltd. All rights reserved.