Dibenzo-18-crown-6

[14187-32-7]  · C20H24O6  · Dibenzo-18-crown-6  · (MW 360.41)

(solubilization and activation of a variety of potassium salts)

Physical Data: mp 162.5-163.5 °C. UV maxima: (MeOH solution) 223 nm (ε 17 500) and 275 nm (ε 5500). 1H NMR: (CDCl3) 6.8-7.0 ppm (8H multiplet) and 3.8-4.3 ppm (16H multiplet).

Form Supplied in: white fibrous needles.

Preparative Methods: dibenzo-18-crown-6 was first prepared in 39-48% yield from the reaction of catechol and bis(2-chloroethyl) ether using sodium hydroxide as the base and n-butanol as the solvent.1 The ditosylate of diethylene glycol has been substituted for the dichloride with accompanying yields of 32-35%.2

Handling, Storage, and Precautions: toxic; use in a well-ventilated fume hood.

Complexation With Metal Salts.

The cavity diameter of dibenzo-18-crown-6 is estimated to be 2.6-3.2 Å, which is ideal for complexing with a potassium cation (ionic diameter 2.66 Å).3 While the selectivity of dibenzo-18-crown-6 for potassium salts is well documented, this macrocyclic multidentate ligand will also effectively complex with other alkali metal cations.3 The solubility of potassium acetate in Acetonitrile is greater in the presence of dibenzo-18-crown-6 compared to 18-Crown-6.4 The solubility of potassium acetate in the presence of a variety of other ligands was also reported. Arguments related to cavity diameter, lipophilicity, and rigidity were advanced to explain, at least partially, the observed structure-solubilization order.

Reactions.

Halides.

The catalytic activity of a number of macrocyclic polydentate ligands in the reaction of 1-octyl bromide with Potassium Iodide to produce the 1-octyl iodide has been reported.5 Dibenzo-18-crown-6 appears to be less effective than Dicyclohexano-18-crown-6.5

Oxygen Anions.

The reaction of potassium acetate activated by a wide variety of macrocyclic polydentate ligands with benzyl chloride in acetonitrile indicates that dibenzo-18-crown-6 is somewhat less effective than 18-crown-6 or dicyclohexano-18-crown-6.4 At -45 °C in dry THF, sodium 3-(fluoren-9-ylidene)-2-phenylacrylate decarboxylated very slowly. In the presence of dibenzo-18-crown-6 the reaction proceeded at a rapid rate, indicating that the nature of the ion pair was critical in influencing the reaction rate profile.6

Dibenzo-18-crown-6 has been used in the reaction of an acetylated halo sugar with a series of alcohols catalyzed by Silver(I) Nitrate (Koenigs-Knorr reaction). The reaction proceeds in good yield with inversion of configuration.7,8

The rates of alkylation of potassium phenoxide with 1-bromobutane in dioxane at 25 °C in the presence of linear and cyclic polyether additives has been reported.9 Dicyclohexano-18-crown-6 was more effective than dibenzo-18-crown-6. A comparison has been made between the rates of reaction of t-butoxide in t-butanol with 2-nitrofluorobenzene and 4-nitrofluorobenzene where the counter cation was potassium and potassium/dibenzo-18-crown-6 and the corresponding reaction of potassium methoxide in methanol.10 Potassium methoxide was 30 times more reactive than Potassium t-Butoxide but in the presence of dibenzo-18-crown-6 the reverse was true.

Dibenzo-18-crown-6 is an effective phase-transfer catalyst in the reactions of 4-chloromethyl-1,3-dioxolane with mono- and dihydric phenols in the presence of metal hydroxides.11

The effect of dibenzo-18-crown-6 on the reaction of sodium 9-fluorenone oximate with Iodomethane in 33.5% acetonitrile and 66.5% t-butyl alcohol has been studied. The crown ether increased the fractional O-alkylation.12,13

Carbon Anions.

The effects of dibenzo-18-crown-6 on (a) the simultaneous base-catalyzed racemization, isotopic exchange, and isomerization of optically pure (-)-3-t-butyl-1-methylindene-1-h and its deuterated counterpart in the 1-position to 1-t-butyl-3-methylindene under a variety of conditions,14 (b) the isotopic exchange and racemization of (-)-4-biphenylylphenylmethoxydeuteromethane with potassium t-butoxide,15 (c) the stereochemistry accompanying the cleavage of (+)-4-phenyl-3,4-dimethyl-3-hexanol with potassium t-butoxide,12 and (d) the rates of isotopic exchange and racemization of (+)-2-methyl-2,3-dihydro-2-deuterobenzo[b]thiophene 1,1-dioxide with potassium methoxide12 have been reported.

The lithium, sodium, and potassium salts of 4H-cyclopenta[def]phenanthrene radical anion are considerably more stable in the presence of dibenzo-18-crown-6, especially when sodium or potassium is the counter cation.16 The presence of crown also affected the rate of decay of the radical anion.

The use of dibenzo-18-crown-6 as a liquid-liquid phase-transfer catalyst in the generation and reaction of carbanions and halocarbenes has been studied.17 Among the reactions studied were the alkylation of carbon acids, reactions involving trichloromethyl anions and dichloromethylene, reactions of carbanions with nitro compounds, and Darzens condensations.

Sulfur Anions.

The solid-liquid phase-transfer catalytic reaction between Benzyl Chloride and Potassium Thiocyanate produces a ratio of benzyl thiocyanate to benzyl isothiocyanate of 71:29 in the presence of dibenzo-18-crown-6.18

Reductions.

Studies related to the use of Sodium Borohydride in toluene in the reduction of a variety of ketones (acetophenone, cyclohexanone, methyl n-pentyl ketone, methyl isopropyl ketone) in the presence of equivalent amounts of dibenzo-18-crown-6, diglyme, or dimethoxyethane have been reported.19 In general, the crown was the most effective ligand. Potassium borohydride required longer reactions times.

Oxidations.

The preparation of N-(arylsulfonyl)sulfoximines by oxidation of the sulfilimines with Sodium Hypochlorite in an aqueous methylene chloride-ethyl acetate two-phase system has been reported.20

The reaction of Potassium Chromate with primary alkyl halides in HMPA containing dibenzo-18-crown-6 at 100 °C produces aldehydes in good yields.21 The chromate ion behaves as both a nucleophile and an oxidant.

The oxidation of 1-octene to n-heptanoic acid with a saturated aqueous Potassium Permanganate-benzene two-phase system proceeds in 80% yield at rt in the presence of dibenzo-18-crown-6.22

Related Reagents.

18-Crown-6; Dicyclohexano-18-crown-6.


1. Pedersen, C. J. OS 1972, 52, 66.
2. Ashby, J.; Hull, R.; Cooper, M. J.; Ramage, E. E. SC 1974, 4, 113.
3. Liotta, C. L. In Synthetic Multidentate Macrocyclic Compounds; Izatt, R. M.; Christensen, J. J., Eds.; Academic: New York, pp 111-205.
4. Knochel, A.; Oehler, J.; Rudolph, G. TL 1975, 3167.
5. Cinquini, M.; Montanari, F.; Rundo, P. CC 1975, 393.
6. Hunter, D. H.; Lee, W.; Sins, S. K. CC 1974, 1018.
7. Knochel, A.; Rudolph, G. TL 1974, 3739.
8. Knochel, A.; Rudolph, G.; Thiem, J. TL 1974, 551.
9. Thomassen, L. M.; Ellingsen, T.; Ugelstad, J. ACS 1971, 25, 3024.
10. Cima, F.; Biggi, G.; Pietra, F. JCS(P2) 1973, 2, 55.
11. Ufa Petroleum Institute. Translated from Zh. Prikl. Khim. 1990, 63, 1383.
12. Smith, S. G.; Milligan, D. V. JACS 1968, 90, 2393.
13. Smith, S. G.; Hanson, M. P. JOC 1971, 36, 1931.
14. Almy, J.; Garwood, D. C.; Cram, D. J. JACS, 1970, 92, 4321.
15. Roitman, J. N.; Cram, D. J. JACS 1971, 93, 2231.
16. Tabner, B. J.; Walker, T. JCS(P2) 1973, 1201.
17. Makosza, M.; Ludwikow, M. AG(E) 1974, 13, 665.
18. Dehmlow, E. V.; Torossian, G. O. ZN(B) 1990, 45b, 1091.
19. Matsuda, T.; Koida, K. BCJ 1973, 46, 2259.
20. Akutagawa, K.; Furukawa, N. JOC 1984, 49, 2282.
21. Cardillo, G.; Orena, M.; Sandri, S. CC 1976, 190.
22. Landini, D.; Montanari, F.; Pirisi, F. M. CC 1974, 879.

Charles L. Liotta

Georgia Institute of Technology, Atlanta, GA, USA



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