Hypervalent tellurium

In contrast to the cathodic reduction of organic tellurium compounds, few studies on their anodic oxidation have been performed. No paper has reported on the electrolytic reactions of fluorinated tellurides up to date, which is probably due to the difficulty of the preparation of the partially fluorinated tellurides as starting material. Quite recently, Fuchigami et al. have investigated the anodic behavior of 2,2,2-trifluoroethyl and difluoroethyl phenyl tellurides (8 and 9) [54]. The telluride 8 does not undergo an anodic a-substitution, which is totally different to the eases of the corresponding sulfide and selenide. Even in the presence of fluoride ions, the anodic methoxylation does not take place at all. Instead, a selective difluorination occurs at the tellurium atom effectively to provide the hypervalent tellurium derivative in good yield as shown in Scheme 6.12. 

The chemistry of tellurium-containing heterocycles is a growth point in heterocyclic chemistry. The tendency for tellurium to take up the Te(IV) state, especially when bonded to oxygen or the halogens, has resulted in the synthesis of new types of heterocycles containing hypervalent tellurium. 

For comparative purposes and in order to enhance our understanding of trends in chemical shifts, typical examples of hypervalent selenium and tellurium fluorides are provided in Scheme 7.20. 

As discussed in detail in Section 2.4, the pronounced tendency of tellurium to participate in both hypervalent bonding and secondary 5p2 —> 5a interactions can lead to the construction of polymeric telluride networks. 

Hypervalent Organic Derivatives of Tellurium and Selenium. D. Hellwinkel, Annals of the New York Academy of Sciences, 192, 158 (1972). 

HYPERVALENT FIVE-MEMBERED SELENIUM- AND TELLURIUM-NITROGEN HETEROCYCLES... 

Very little is known about these hypervalent selenium and tellurium compounds. 

Selenium(IV) as well as sulfur(fV) and tellurium(IV) tend to form a more or less distorted quasioctahedral (hypervalent) XY3+3E or XY4+2E coordination of six nearest neighbors in which there is a tendency to form stable trigonal pyramidal XY3+ subunits (X = Se, Y = Cl, Br, I E is a more or less stereochemically active electron pair). 

In contrast to sulfur, selenium and tellurium derivatives seem to show slightly different behavior. In particular, tellurium gives various stable hypervalent compounds (telluranes), and hence it has become one of the most useful elements as a model for investigation of sulfuranes and selenuranes [lb,c]. 

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