TELLURIUM

DONALD C. DITTMER, SYRACUSE UNIVERSITY

The discovery of tellurium by Franz J. Müller in 1782 almost went unnoticed. He was examining the gold-containing ores of Transylvania, from one of which, aurum paradoxum, a supposedly pure sample of antimony (a neighbor of tellurium in the periodic table) had been obtained. After many experiments, the possibilities--antimony, bismuth, or an alloy of both--were eliminated. Müller called the new element, for that was what he had found, "metallum problematicum." He published his findings in an obscure journal, Physikalischen Arbeiten der einträchtigen Freunde in Wien, of which only two volumes were issued. The title of his work was equally obscure: "An Investigation of the Supposed Native Antimony from the Mariahilf Mine in the Facebaj Mountains Near Zalatna." In 1798, Müller's discovery was rescued from oblivion by Martin H. Klaproth, who examined the "problematical metal" and named it tellurium.

Elemental tellurium exists mainly in right- and left-handed helical chains of atoms, and reduction yields colored anions, Te_n^2–, in addition to colorless Te^2–. The first organic compound of tellurium, diethyl telluride, was reported in 1840 by Friedrich Wöhler. Since then, many chemists have investigated organic tellurium chemistry, with most activity being concentrated in the past 50 to 60 years.

GOLDEN TOUCH Gold telluride, or calaverite, is a rare mineral but an important source of gold. This photomicrograph shows calaverite crystals in a quartz matrix. LOU PERLOFF/PHOTO ATLAS OF MINERALS

Müller was made Baron von Reichenstein in 1795 for his skills in mining administration, and he no doubt would be interested in the roles that tellurium is playing in recent times, for example, in the conversion of solar energy into electricity via CdTe, in the possible coating of the highlands of the planet Venus, in an episode of the 1950s TV science fiction program "Space Patrol," in triggering and catalyzing chemical reactions, and in alloys. The retail cost of tellurium varies from about $60 to $560 per mole, depending on form and purity.

Elemental tellurium originally was put on the list of "extremely hazardous substances," but a review of the study by the Environmental Protection Agency indicated that sodium tellurate was used instead of tellurium. The LD₅₀ is now given as 5,000 mg per kg for mice. Ingestion of tellurium by humans causes garlic-like "tellurium breath" because of the formation of dimethyl telluride. Tellurium compounds in microgram amounts occur fairly widely in plants (for example, onions, peas, and tea leaves), and larger quantities (31–73 µg per g) are found in garlic buds.

My interest in tellurium came as the result of an attempt to prepare telluracyclobutene, a four-membered cyclic compound with one tellurium atom and a carbon-carbon double bond, to be used as an electron donor in semiconductor formation. Treatment of epichlorohydrin (chloromethyloxirane) with sulfide ion gave 3-hydroxythiacyclobutane, a precursor of thiacy-clobutene (which does form a semiconducting material), but treatment with telluride ion gave allyl alcohol and elemental tellurium. This represents a nucleophilic reduction in which the powerful nucleophilic telluride ions, Te^2– or Te_n^2–, are oxidized and the organic compound is formally reduced. The first nucleophilic reduction by telluride ion, reported by W. V. Farrar and J. Masson Gulland (J. Chem. Soc. 1945, 11–14), was the unexpected conversion of 1,2-dibromoethane to ethylene by sodium telluride, with elemental tellurium also being produced.

The driving force for these reactions is caused by the high nucleophilicity of Te^2– and the thermodynamic instability of Te^2– with respect to elemental tellurium. Nucleophilic attack by telluride ion on an organic compound with an electrophilic site or sites opens a pathway whereby oxidation to the element occurs with concomitant transformation of the original organic compound to a new compound.

My coworkers found that Sharpless-Katsuki asymmetric epoxidation of primary allylic alcohols; conversion of the alcohol function to an electrophilic site with a good leaving group, typically a tosylate or mesylate; followed by treatment with telluride ion obtained by reduction of elemental tellurium produce new chiral secondary and tertiary allylic alcohols [J. Org. Chem., 58, 718 (1993)]. Similar results for the synthesis of allylic amine derivatives have been obtained via O-tosylates of aziridinemethanols or oxazolidinonemethanols [J. Org. Chem., 62, 7920 (1997); Tetrahedron Lett., 40, 2255 (1999); and 42, 5789 (2001)].

Müller's discovery is an interesting element whose unique properties, which may be inferred from its position in the periodic table, make it useful in a variety of organic chemical transformations as well as in the construction of alloys and other solid-state materials such as semiconductors and solar cells.

Donald C. Dittmer is a professor emeritus at Syracuse University. His research interests include telluride-induced nucleophilic reductions.

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