Aryltellurium trihalides

The reactivity of aryltellurium trihalides decreases on going from the chlorides to the iodides, the same trend occurring for hydrolysis. Aryltellurium trichlorides are very sensitive to water and moisture and are easily hydrolysed, the tribromides being more stable, while the triiodides are unaffected by cold water and can be prepared even by aqueous procedures. Diaryltellurium dihalides are stable in water, and ionic exchange reactions allow the conversion of dichlorides into dibromides and diiodides. 

Aryltellurium trihalides are sensitive to water and can be converted into products at different stages of hydrolysis. Oxohalides, aryltellurinic acids and aryltellurium anhydrides have been known for a long time, the oxohalides being the products of partial hydrolysis. 

In the presence of alcohols the addition of aryltellurium trihalides to alkenes gives rise to alkoxytelluration products. ° The addition is highly anfi-stereospecific and regiospe-cific for terminal alkenes, tellurium moieties attacking exclusively the terminal carbon. 

By addition of tellurium tetrahalides and aryltellurium trihalides to acetylenes... 

This class of organotellurium compounds is an important source of nucleophilic (Sections 9.13.3.1 and 9.13.3.2) or electrophilic (Sections 9.13.4.2.4 and 9.13.4.3) tellurium species. Diorganoditellurides can also behave as electrophiles, but in such cases normally one organotellurium group is lost. In this way, the corresponding organotellurium halides (Section 9.13.4.3) are prepared as source of electrophilic tellurium. In the past, the title compounds were prepared mainly by reduction of aryltellurium trihalides, in some cases under drastic reaction conditions5 9 11 12 (Scheme 11). 

Aryltellurinic acid anhydrides 9 are prepared by alkaline hydrolysis of aryltellurium trihalides 5, followed by acidification.84-86 The anhydrides 9 precipitate as white solids in high yields (Scheme 15). 

The aryltellurium trihalides are generally more stable than the alkyltcllurium trihalides (alkyltellurium trichlorides, produced by the addition of TcCI to olefins, easily liberate elemental tellurium). 

The conproportionation of diaryl ditellurides and aryltellurium trihalides, resulting in the formation of aryltellurenyl halides, has been investigated. The aryltellurenyl halides are, in general, unstable, and they disproportionate to diaryltellurium dihalides and elemental tellurium. ... 

Reduction of Organoselenium and -tellurium Halides and Oxides. TUD reduces aryltellurium trihalides to diaryl ditel-lurides (eq 2) and diorganyltellurium dihalides and telluroxides to diorganyl tellurides (eq 3) in high yield. The corresponding selenium compounds are reduced similarly. Some examples are given in Table 2. The reduction is performed by premixing the substrate and 2 N NaOH at room temperature for 15 min prior to the addition of TUD in petroleum ether. 

Tellurium tetrachloride and aryltellurium trichloride, as well as tellurium tetrabro-mide" and aryltellurium tribromides add to acetylenes to produce, respectively, 2-halovinyl tellurium trihalides and dihalides, which can be submitted to further manipulations. 

Aryltellurium trichlorides 5 (R = aryl, X = Cl) are usually yellow, very stable crystalline solids, with a slight odor of hydrochloric acid, which probably arises from the reaction of 5 with the air moisture. Contact of 5 with metallic spatulas, with moist solvents, or prolonged exposure to light must be avoided. These compounds, however, can be handled in the air with no risk of decomposition. Aryltellurium tribromides (5, R = aryl, X = Br) are yellow crystalline solids, and the triiodides (5, R = aryl, X = 1) are dark red solids. The aryltellurium tribromides and triiodides were less explored for synthetic purposes, in contrast to the aryltellurium trichlorides, which were frequently used in several synthetic transformations. The aliphatic tellurium trihalides are less stable than the aromatic ones and were much less studied and used for preparative purposes. 

The reactivity of aryltellurium trichlorides 116, the most synthetically used organotellurium trihalides, toward activated aromatics, alkenes, and alkynes is similar to the reactivity of tellurium tetrachloride toward the same substrates (Section 9.13.4.2.3). In Scheme 69, the principal features of the reactions involving aryltellurium trichlorides are presented. 

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