[118-75-2]  · C6Cl4O2  · Chloranil  · (MW 245.89)

(oxidant, particularly useful for dehydrogenation to aromatic1,2 and a,b-unsaturated carbonyl2 compounds)

Alternate Names: 2,3,5,6-tetrachloro-2,5-cyclohexadiene-1,4-dione; 2,3,5,6-tetrachloro-p-benzoquinone.

Physical Data: mp 290 °C; sublimes: E0 742 mV (benzene).

Solubility: insol H2O; sol ether; slightly sol alcohol, chloroform, CS2, and light petroleum.

Form Supplied in: yellow solid.

Analysis of Reagent Purity: UV (lmax 290 [log ε 4.34, cyclohexane]).

Purification: sublimation or recrystallization from benzene or acetic acid.

Handling, Storage, and Precautions: potential skin irritant; this reagent should be handled in a fume hood.

Dehydrogenation Reactions.

Chloranil and other quinones of high oxidation potential (see 2,3-Dichloro-5,6-dicyano-1,4-benzoquinone) are powerful oxidants which have found extensive application in dehydrogenation reactions.1,2 Chloranil is particularly suitable for the aromatization of hydroaromatic substrates and has been used successfully in the synthesis of aromatic carbocycles3 and nitrogen4 and sulfur5 heterocycles. The sequential aromatization of Diels-Alder adducts is an important application (eq 1).6 Chloranil has also been used to facilitate the formation of a,p-dimethylstyrene from citral (eq 2), where its cheapness and efficacy makes it a preferred dehydrogenating reagent.7

Dehydrogenation reactions have been shown to proceed faster in polar solvents such as alcohols and DMF and are enhanced by electron donating substituents in the ring system to be oxidized.1

Although skeletal rearrangements rarely occur during dehydrogenation reactions with quinones, Wagner-Meerwein rearrangements leading to alkyl group migration have been reported in the aromatization of cycloalkanes possessing gem-dialkyl substituents.8 A somewhat more unusual oxidation of gem-dialkylpyrazolidines, which is markedly accelerated by the addition of chloranil, gives pyrazoles without rearrangement by C-C bond cleavage (eq 3).9 Chloranil is also an effective reagent for the dehydrogenation of N-substituted dihydroquinolines10 and -isoquinolines,11 where high yields of quinolinium and isoquinolinium salts are formed (eq 4).

While quinones do not usually react directly with furans, oxidation of the intermediate hydroquinone by the addition of a high potential quinone such as chloranil results in the formation of 2-furylquinones in reasonable yield (eq 5).12

Chloranil and other high potential quinones have been extensively used in the dehydrogenation of cyclic carbonyl compounds,2 particularly steroidal ketones.2,13 One reaction of importance is the formation of 4,6-dien-3-ones from 4-en-3-ones (eq 6).14 In certain instances, steroids may be fully aromatized using this procedure.15

Other Oxidation Reactions.

In addition to dehydrogenation, quinones with high E0 values effect various oxidation reactions.1 Chloranil in particular is an effective electron acceptor photosensitizer for the oxidation of toluenes to benzaldehydes, where other classical photosensitizers have been shown to fail.16

Miscellaneous Reactions.

Chloranil undergoes facile nucleophilic displacement reactions with primary and secondary amines, leading to 2,5-bis-amino derivatives (eq 7).17 Spectroscopic studies have demonstrated that with aliphatic amines at least, the reaction proceeds via a one-electron oxidation of the amine.18,19 Chloranil has also been shown to induce the dimerization of diaryldiazomethanes through a reaction believed to involve prior addition across the quinone carbonyl group.20

1. (a) Jackman, L. M. In Advances in Organic Chemistry, Methods and Results; Raphael, R. A., Ed.; Interscience: New York, 1960; Vol. 2, p 329. (b) Becker, H-D. In The Chemistry of the Quinonoid Compounds; Patai, S., Ed.; Wiley: New York, 1974; Part 2, Chapter 7.
2. Buckle, D. R.; Pinto, I. L. COS 1991, 7, 119.
3. (a) Kosak, A. I.; Palchak, R. J. F.; Steele, W. A.; Selwitz, C. M. JACS 1954, 76, 4450. (b) Noland, W. E.; Wann, S. R. JOC 1979, 44, 4402. (c) Carlin, R. B.; Moores, M. S. JACS 1962, 84, 4107.
4. (a) Vo-Quang, L.; Vo-Quang, Y. JHC 1982, 19, 145. (b) Landberg, B. E.; Lown, J. W. JCS(P1) 1975, 1326. (c) Huisgen, R.; Seidel, M.; Wallbillich, G.; Knupfer, H. T 1962, 17, 3.
5. (a) McIntosh, J. M.; Khalil, H. CJC 1975, 53, 209. (b) Tilak, B. D.; Desai, H. S.; Gupta, S. S. TL 1964, 1609.
6. Tominaga, Y.; Lee, M. L.; Castle, R. N. JHC 1981, 18, 967.
7. Barton, D. H. R.; Parekh, S. I. SC 1989, 19, 3353.
8. Fieser, L. F.; Fieser, M. FF 1967, 1, 125.
9. Le Fevre, G.; Hamelin, J. TL 1978, 4503.
10. Braude, E. A.; Hannah, J.; Linstead, R. JCS 1960, 3249.
11. Fryer, R. I.; Earley, J. V.; Evans, E.; Schneider, J.; Sternbach, L. H. JOC 1970, 35, 2455.
12. Bridson, J. N.; Bennett, S. M.; Butler, G. CC 1980, 413.
13. Walker, D.; Hiebert, J. D. CRV 1967, 67, 153.
14. Agnello, E. J.; Laubach, G. D. JACS 1960, 82, 4293.
15. Dannenberg, H.; Hebenbrock, K.-F. LA 1963, 662, 21.
16. Julliard, M.; Galadi, A.; Chanon, M. JPP 1990, 54, 79.
17. Buckley, D.; Henbest, H. B.; Slade, P. JCS 1957, 4891.
18. Yamaoka, T.; Nagakura, S. BCJ 1971, 44, 2971.
19. Oshima, T.; Nagai, T. BCJ 1989, 62, 2580.
20. Bourguinon, J.; Becue, C.; Queguiner, G. JCR(S) 1981, 104.

Derek R. Buckle

SmithKline Beecham Pharmaceuticals, Epsom, UK

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