Tellurophenes

Tellurophenes are a family of organotellurium compounds derived formally from the parent compound tellurophene with the chemical formula C₄H₄Te.

Tetraphenyltellurophene was described in 1961 by Braye et al. The synthesis involved salt-metathesis reaction of 1,4-dilithiotetraphenylbutadiene with tellurium tetrachloride. Tetraphenyltellurophene is a yellow-orange solid with a melting point of 239-239.5 °C. The same compound was obtained from 1,4-diiodotetraphenylbutadiene and lithium telluride in 82% yield.

In 1966, Mack report a synthesis of the unsubstituted tellurophene through the reaction of sodium telluride with diacetylene . This method could be generalised to prepare 2,5-derivatives of tellurophene by selecting a suitably-substituted diacetylene precursor.

A one-pot procedure allows the synthesis of a variety of functionalized tellurophenes without the use of transition metals. This was done by reacting substituted 1,1-dibromo-1-en-3-ynes with telluride salts (Na₂Te/Na₂Se) under mild conditions. The telluride salts were synthesized through an earlier protocol, wherein Te/Se was reduced with sodium borohydride in ethanol. The synthesis of the 3-functionalized tellurophenes is as follows:

The reaction was highly influenced by the polarity of the solvent. Polar solvents such as water were thought to polarize the Te–H bond, thus increasing the negative charge on Te and making it more nucleophilic. To obtain a wider scope of the reaction, the authors used dimethylformamide (DMF) as the solvent since DMF not only has a higher dielectric constant (and therefore, higher polarity) than water, but also was found to be able to dissolve enynes better compared to water. Using a solvent combination of DMF and t-BuOH, the authors were able to synthesize 2,4-disubstituted tellurophenes at room temperature.

The copper-catalyzed cyclizations of [[chalcogenoenynes gives 3-substituted chalcogenophenes which could be further functionalized using boronic acids via palladium-catalyzed Suzuki coupling.

The geometry of tellurophene was first determined in 1973 through microwave spectroscopy, and has been further refined through X-ray diffraction studies. It has been found that the Te–C bond has a length of 2.046 Å, which is longer than that of selenophene. Further, the C–Te–C angle has been determined to be 82°, smaller than that found in selenophene, an observation attributed to the larger size of the tellurium atom. These findings are also consistent with the aromaticity of selenophene being greater than that of tellurophene; amongst its congeners, the order of decreasing aromaticity has been demonstrated to be: benzene > thiophene > selenophene > tellurophene > furan.

Halide-substituted tellurophenes participate in metal-catalyzed cross coupling reactions. Perfluoroaryl-substituted tellurophenes form by Stille coupling.