Telluronium ion

Various sulfonium and carbosulfonium ions show remarkably enhanced reactivity upon superelectrophilic activation, similar to their oxygen analogs so do selenonium and telluronium ions. The alkylating ability of their trialkyl salts, for example, is greatly increased by protosolvation. 

A great number of trialkyl(aryl) selenonium and telluronium ions are known, and their synthesis does not require the use of strong electrophilic alkylating or arylating agents.268,269 The synthesis and transformations of triorganotellurium ions have been treated in recent reviews.270,271 However, acidic selenonium and telluronium ions can be obtained only under superacidic conditions. 

It has been rationalized that these reactions proceed via a cyclic telluronium ion intermediate which undergoes a nucleophilic attack by the chloride ion giving the products with trans configuration. The addition of 2-naphthyltellurium trichloride to a variety of acyclic and cyclic alkenes is completely anti or trans stereoselective, but tellurium tetrachloride usually gives mixtures arising from syn (cis) and anti (trans) addition representative results are presented in Tables 15 and 16. 

These results clearly indicate that in the tellurium tetrachloride addition to alkenes the main reaction pathway does not occur via a telluronium ion intermediate a radical addition pathway could explain the poor selectivity observed in many of these additions (see Tables 15 and 16). The strong effect of 4-benzoquinone on the syn/anti ratio supports the involvement of a radical pathway in chloro-telluration with tellurium tetrachloride. A possible radical chain reaction is shown87. 

The stereochemistry of the addition is anti, as expected by an attack of the acetate ion at an intermediate telluronium ion. 

TeCl4 has been found to display reactivity toward alkynes similar to that of p-methoxyphenyltellurium trichloride, whereas it reacts with aromatic and 3-hydro-xyalkynes by different mechanisms as shown by the characteristic stereochemistries of the products. The complete anti-stereospedfidty of the additions of TeCU to all propargyl alcohols studied is consistent with a cyclic chelated telluronium ion intermediate 87 in this reaction (Figure 3.8). Using ESI-MS and ESI-MS(/MS),... 

---