[7440-28-0] · Tl · Thallium · (MW 204.38)
(reducing agent, coupling agent)
Physical Data: mp 303 °C, bp 1437 °C, d 11.85 g cm-3.
Solubility: insol organic solvents.
Form Supplied in: bluish-white very soft metal, packaged under water; widely available.
Handling, Storage, and Precautions: oxidizes superficially in air, forming a coating of thallium oxide. 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.
Thallium is a very soft, abundant, and inexpensive metal. Like a variety of other metals, the utility of thallium in Ullmann-like coupling reactions has been explored. Inactivated thallium metal is very inefficient in coupling reactions of aryl halides. For example, reaction of bromobenzene with thallium under reflux produces <5% of biphenyl (eq 1).2 The use of mossy or pyrophoric thallium in these coupling reactions does not lead to improvement in yields of the biaryls.
The use of an activated form of thallium in the preparation of trialkylthalliums from alkyl iodides has been reported (eq 2).3 The activated thallium was prepared by reduction of thallium(I) chloride with Potassium in boiling m-xylene.
Metallic thallium is useful in converting aromatic nitro compounds to azoxy compounds (eq 3).4 In this reaction, thallium is believed to act as a reducing agent and thallium oxide is the byproduct.
The preparation of azoxy compounds by this route is very efficient and simple. The yields range from good to excellent and a variety of substrates can be used. Aromatic nitro compounds with electron-withdrawing substituents and substrates containing acidic hydrogens such as OH and NH2 fail to give azoxy compounds. Bromo- and iodo-substituted nitrobenzenes gave complex products. An interesting coupling reaction using 2,4-dichloronitrobenzene was also reported in this study.4 This reaction gives a high yield of 2,2´,4,4´-tetrachloroazobenzene as the product (eq 4).
Mukund P. Sibi
North Dakota State University, Fargo, ND, USA
Nancy E. Carpenter
University of Minnesota, Morris, MN, USA