Aryltellurium trichlorides

The tri- and dihalides are crystalline componnds. The chlorides are colourless (or yellow such as some aryltellurium trichlorides), the colour changing to orange and red (or deep red) for the bromides and iodides. 

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 trichlorides are highly soluble in methanol and ethanol but less soluble in benzene. Diaryltellurium dichlorides exhibit inverse solubilities, being more soluble in benzene than in methanol or ethanol. These properties allow an easy separation of diaryl tellurides from diaryl ditellurides (frequently formed as by-products in the preparation of tellurides) the mixture is treated with SOjClj and the obtained mixed di- and trichlorides are separated by the appropriate solvents, and reduced back into the pure tellurides and ditellurides. 

Aryltellurium trichlorides (see Sections 3.5.1.3 and 3.5.1.4) are easily reduced by means of several reducing agents. The most popular reduction procedures are described in sequence. 

This reagent, together with potassium hydrogen sulphite,has been used to reduce aryltellurium trichlorides since the early period of tellurium chemistry, and continues to find wide application. 

Diaryl ditellurides (general procedure) The aryltellurium trichloride is poured into 15 mol equiv of Na2S-9H20 heated (and melted) at 95-100°C in a beaker, with strong manual stirring. An exothermic reaction occurs. After 10 min at the same temperature, excess HjO is added. By cooling the ditelluride solidifies, and is collected by decantation or filtration. The yields are near to quantitative. 

Reduction of aryltellurium trichlorides with sodium ascorbate (typical procedure). p-Methoxyphenyltellurium trichloride (0.30 g, 0.88 mmol) was added to a stirred solution of sodium ascorbate (0.53 g, 2.7 mmol) in water/methanoEacetone (1 mL + 5 mL + 5 mL). After 5 h, water (50 mL) and CH2CI2 (50 mL) were added and the two phases separated. The organic phase was dried (CaCl2) and the solvent evaporated in vacuo. Flash chromatography (Si02 CH2CI2) yielded 0.20 g (97%) of bis(p-methoxyphenyl) ditelluride. 

Aryl boronic acids treated with TeCl4 generate aryltellurium trichlorides which are reduced to diaryl ditellurides without prior isolation. ... 

The condensation of tellurium tetrachloride with aromatic compounds is of general application and has long been used as an efficient route for the preparation of aryltellurium trichlorides. 

The above-described solventless procedure has recently been promoted to a general method to prepare aryltellurium trichloride. The crude trichlorides can be converted in situ into the corresponding aryl butyl tellurides by sequential treatment with aqueous sodium borohydride and n-butyl bromide (see Section 3.1.3.2). 

General procedure for preparation of aryltellurium trichlorides and aryl butyltellurides (typical procedure) " A one-necked, round-bottomed flask containing tellurium tetrachloride (5.38 g, 20 mmol) was heated to 120°C and the corresponding aromatic compound (20 mmol) was added at once, dissolving the tellurium tetrachloride promptly. An evolution of HCl was observed and a yellow solid was formed. The system was cooled to room temperature and the trichloride was used crude in the reduction/alkylation procedure. 

Arylmercury chlorides are valuable reagents for the preparation of aryltellurium trichlorides because they are used when the aromatic substrate is not sufficiently reactive for an easy direct condensation with tellurium tetrachloride. ... 

The formed mercury dichloride is separated as a crystalline complex with dioxane. Since the starting arylmercury chlorides are easily prepared from diazonium salts, this method allows the conversion of anilines into aryltellurium trichlorides. 

The ease of hydrolysis follows the order ArTeClj > ArTeBrj > ArTelj. Aryltellurium trichlorides and tribromides are converted into the oxohalides by simple treatment with cold water. The triiodides are unaffected under these conditions, and on treatment with boiling water give mixtures or undefined products in most cases. ... 

Aryltellurium triiodides can be prepared by treating aryltellurium trichlorides, tribro-inides, oxohalides or aryltellurinic anhydrides with aqueous sodium iodide. ... 

The addition of aryltellurium trichlorides to alkenes occurs with a high stereospecificity, giving exclusively anfi-2-chloroalkyltellurium dichlorides, " in contrast to the addition of tellurium tetrachloride which gives mixtures of syn and anti adducts... 

Aryltellurium trichlorides and tribromides condense with aromatic compounds bearing electron-donating groups, giving unsymmetrical diaryltellurium dihalides. This electrophilic substitution therefore parallels that described for tellurium tetrachloride. 

The reaction of aryltellurium trichlorides with aryl- or alkylmercury chlorides is a well-established and general method for the preparation of nnsymmetrical diaryl- or alkylaryl-tellnrinm dichlorides. ... 

Unsymmetrical diaryltellurium dichlorides (general procedure) The aryltellurium trichloride (20 mmol) and the aryhnercury chloride (20 mmol) are heated under reflux in dry dioxane (30 mL) for 1.5 h. The solution is cooled at 10°C and the HgCl2 dioxane precipitate so formed is fdtered off and washed with a small volume of cold dioxane. The dioxane solution is then ponred into 1% HCl (100 mL) with vigorous stirring. The formed oily product crystallizes by friction and is separated by filtration and then washed with cold EtOH. 

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. 

The addition of TeCl4, aryltellurium trichlorides and TeBr4 is stereoselective giving only Z addncts. 

The reaction of aryltellurium trichlorides with /,5-unsaturated carboxyhc acids gives rise to aryldichloroteUurium butyrolactones in high yields. - ... 

Aryltellurium trichlorides are suitable reagents for the telluroetherification of olefinic alcohols and allylphenols, exhibiting the advantages of stability and easy separability of the formed crystalline dichloroteUuro derivatives compared to the preceding tellurinyl acetate method. 

Olefmic benzyl ethers can also be submitted to ethercychzation with aryltellurium trichlorides. The yields and the reaction times are close to those observed for the cycliza-tion of the corresponding alcohols. The stereochemistry of the reaction is low. ... 

Synthesis of biaryls by Raney Ni-catalysed homocoupling of diaryltellurium dichlorides and aryltellurium trichlorides... 

The reactions are performed treating diarylteUurium dichlorides or aryltellurium trichlorides with excess olefin (5-10 mol equiv) in the presence of Pd(II) chloride-sodium acetate in refluxing acetic acid or acetonitrile. The yields are moderate to good with the dichlorides and poor with the trichlorides. Minor amounts of biaryls are the usual by-products beyond acetoxylated arylalkenes (formed by the addition of acetic acid to the olefins). 

Aryltellurium trichlorides and diaryltellurium dichlorides react with nickel tetracarbonyl in DMF, giving benzoic acids. Small amounts of diaryl tellnrides and diaryl ketones are by-products of these reactions. 

Detellurative carbonylation of aryltellurium trichloride NiiCO) method (typical procedure). Ni(CO)4 (2.6 mL, 20 mmol) is added to p-methoxyphenyltellurium trichloride (2.0 g,... 

Bromodetelluration of aryltellurium trichlorides (typical procedure) A mixture of p-methoxyphenyltellurium trichloride (0.68 g, 2 mmol) and bromine (0.64 g, 4 mmol) in MeCN (20 mL) is stirred at 35 5°C for 20 h. After work-up as described above, GLC analysis using ethyl cinnamate as the internal standard shows the presence of o,p-dibromo-p-methoxybenzene (69.5%) and a trace of p-methoxybromobenzene. 

Diaryltellnrinm dichlorides are unreactive by method A, and by method B furnish lower yields than aryltellurium trichlorides (eq. (7)). 

Reaction of a-dichloroaryltelluroketones (see Section 3.9.3.1) with equimolar amounts of chlorine in dichloromethane at room temperature leads to the corresponding a-chloroke-tones in good yields, accompanied by the corresponding aryltellurium trichlorides. ... 

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. 

One of the most studied and used reactions of tellurium tetrachloride 16 is the electrophilic aromatic substitution with activated aromatic hydrocarbons.5,9,11,12 Traditionally, the reaction is performed in carbon tetrachloride or chloroform under reflux.50-54 The aryltellurium trichlorides 5 are insoluble in these solvents and precipitate as crystalline solids. Usually, the reaction gives high yields. The /> ra-substituted aryltellurium trichlorides are formed. The reaction can take several hours under reflux to reach completion.5 It can be performed also in the absence of... 

---