Thallium(I) Bromide1


[7789-40-4]  · BrTl  · Thallium(I) Bromide  · (MW 284.28)

(Lewis acid; reagent for coupling and bromination)

Physical Data: mp 480 °C.

Solubility: sol 2360 parts water.

Form Supplied in: pale yellow crystalline powder; commercially available.

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.

The primary utility of thallium(I) bromide is in Ullmann-like coupling reactions of aryl and alkyl Grignard reagents. A very practical method for the synthesis of biaryls from aryl Grignards using TlBr as a reagent has been developed (eq 1).2 These reactions are operationally simple and proceed in good to high yields, thus providing a good alternative to similar coupling reactions mediated by other metals. The optimal ratio of thallium bromide to aryl Grignard reagent was found to be 1:1.5. Thallium bromide is regenerated during the coupling reaction. Coupling of two different aryl Grignards with thallium bromide provides all three possible biphenyls. The unsymmetrical biphenyl is generally the major product but the formation of multiple products makes this process of marginal utility. The coupling reaction of 3- and 4-substituted aryl Grignards provide the corresponding biphenyls in good yields. The only drawback to the reaction is that ortho-substituted aryl Grignards give no coupling products.

This reaction has been extended to secondary aliphatic bromides.3 The reactions proceed only in moderate yields (eq 2). Similar reactions with primary aliphatic Grignards give only traces of the coupled products.

Thallium(I) bromide serves as a catalyst for nuclear bromination of aromatic compounds (eq 3).4 For example, benzene can be converted to bromobenzene in moderate yields. Similarly, toluene reacts to produce a mixture of o- and p-bromotoluene, and the product resulting from benzylic bromination was not observed. These brominations proceed more effectively with TlIII salts as compared to TlI salts.

Triorganothallium derivatives are useful reagents in organic synthesis. The utility of this reagent in ketone synthesis has been reported.5 Thallium(I) bromide serves as a precursor for the preparation of triorganothallium reagents. Two methods for the preparation of this reagent are illustrated in eqs 4 and 5.5,6

Related Reagents.

Thallium(III) Acetate-Bromine.

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.
2. (a) McKillop, A.; Elsom, L. F.; Taylor, E. C. T 1970, 26, 4041. (b) Elsom, L. F.; McKillop, A.; Taylor, E. C. OS 1976, 55, 48.
3. McKillop, A.; Elsom, L. F.; Taylor, E. C. JACS 1968, 90, 2423.
4. Uemura, S.; Sohma, K.; Okano, M.; Ichikawa, K. BCJ 1971, 44, 2490.
5. Markó, I.; Southern, J. M. JOC 1990, 55, 3368.
6. Okhlobystin, O. Y.; Bilevitch, K. A.; Zakharkin, L. I. JOM 1964, 2, 281.

Mukund P. Sibi

North Dakota State University, Fargo, ND, USA

Nancy E. Carpenter

University of Minnesota, Morris, MN, USA

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