Thallium(III) Acetate-Bromine1


[2570-63-0]  · C6H9O6Tl  · Thallium(III) Acetate-Bromine  · (MW 381.53)

[7726-95-6]  · Br2  · Thallium(III) Acetate-Bromine  · (MW 159.80)

(brominating agent)

Physical Data: see entries Thallium(III) Acetate and Bromine.

Form Supplied in: generated in situ by simple admixture of the two commercially available components.

Handling, Storage, and Precautions: all thallium compounds are extremely toxic to inhalation, skin contact, and ingestion. Toxicity is cumulative. Extreme caution should be used when handling these materials. Use in a fume hood.

General Discussion.

A mixture of thallium(III) acetate and bromine has been shown to be a superior reagent for the bromination of aromatic compounds (eq 1, Table 1).2 When care is taken to control the rate of addition of the bromine solution, use of this system results in good selectivity, giving the monobrominated product in very good yields. It has also been shown to be especially useful for the preparation of 9-bromoanthracene, 2-bromobiphenylene, and 2-bromofluorene, compounds otherwise difficult to obtain using conventional bromination methods. Although this methodology has been used successfully on a wide variety of aromatic substrates, it is not suitable for those with strongly deactivating groups on the aromatic ring; if a readily oxidized group is present (e.g. benzyl alcohol or benzaldehyde), oxidation at this site competes with the substitution reaction (see, however, entry 5 in Table 1). Earlier researchers postulated that the reason for the good regioselectivity was the formation of a TlIII-bromine-arene complex whose steric demands regulated the position of the bromine. However, further work3 demonstrated that excellent regioselectivity results even in the presence of competing steric requirements (eq 2). Thus it was concluded that this reaction provides a demonstration of remarkable electronic control, not steric governance.

Other thallium reagents have also been used synthetically in conjunction with elemental bromine. For example, there have been several examples of the use of thallium(I) reagents with bromine to affect a Hunsdieker reaction (eq 3).4 The transformation begins with the conversion of the carboxylic acid to the thallium salt, which is then treated with a solution of bromine in carbon tetrachloride to accomplish the bromodecarboxylation. The reaction is successful only for primary carboxylic acids. It is interesting to note that a thallium(III) species is involved in the reaction; it has been determined5 that 1.1 equiv of bromine is used to convert the thallium carboxylate to a TlIII carboxylate dibromide, which is then converted to the alkyl bromide product upon treatment with an additional 0.5 mol bromine.

Related Reagents.

Bromine; Thallium(III) Acetate; Thallium(I) Bromide; Thallium(I) Carbonate.

1. (a) McKillop, A. PAC 1975, 43, 463. (b) McKillop, A.; Taylor, E. C. In Comprehensive Organometallic Chemistry; Wilkinson, G.; Ed.; Pergamon: Oxford, 1982; Vol. 7, p 465. (c) McKillop, A.; Taylor, E. C. Chem. Br. 1973, 9, 4.
2. (a) McKillop, A.; Bromley, D.; Taylor, E. C. TL 1969, 1623. (b) McKillop, A.; Bromley, D.; Taylor, E. C. JOC 1972, 37, 88.
3. Erickson, K. L.; Barowsky, H. W. CC 1971, 1596.
4. McKillop, A.; Bromley, D.; Taylor, E. C. JOC 1969, 34, 1172.
5. Cambie, R. C.; Hayward, R. C.; Jurlina, J. L.; Rutledge, P. S.; Woodgate, P. D. JCS(P1) 1981, 2608.

Mukund P. Sibi

North Dakota State University, Fargo, ND, USA

Nancy E. Carpenter

University of Minnesota, Morris, MN, USA

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