Diaryl tellurides diaryls

On treatment with an equivalent amount of (Et2N)3P, thiotelluroxide 67a is reduced to give telluroazocine 65 in almost quantitative yield, which can be converted into telluroxide 68 by the usual procedures for a diorganyl telluride to telluroxide transformation. It is worth noting that when treated with Lawesson reagent, the telluroxide 68 forms thiotelluroxide 67a, whereas diaryl telluroxides are reduced by this reagent to diaryl tellurides. 

Fig. 30 Correlation of resonance stabilization energy for diaryl telluride radical cations with Pa. Data from Ref. 113.

Fig. 34 Reactions of diaryl tellurides with peroxy radicals/ ...

A study of debrominations of vtc-dibromides promoted by diaryl tellurides and din-hexyl telluride has established several key features of the elimination process the highly stereoselective reactions of e/7f/tro-dibromides are much more rapid than for fhreo-dibromides, to form trans- and cw-alkenes, respectively the reaction is accelerated in a more polar solvent, and by electron-donating substituents on the diaryl telluride or carbocation stabilizing substituents on the carbons bearing bromine. Alternative mechanistic interpretations of the reaction, which is of first-order dependence on both telluride and vtc-dibromide, have been considered. These have included involvement of TeAr2 in nucleophilic attack on carbon (with displacement of Br and formation of a telluronium intermediate), nucleophilic attack on bromine (concerted E2- k debromination) and abstraction of Br+ from an intermediate carbocation. These alternatives have been discounted in favour of a bromonium ion model (Scheme 9) in which the role of TeArs is to abstract Br+ in competition with reversal of the preequilibrium bromonium ion formation. The insensitivity of reaction rate to added LiBr suggests that the bromonium ion is tightly paired with Br. ... 

Diorganyl tellurides have low molecular mass and are colourless or yellowish liquids with an unpleasant and penetrating odour. Dimethyl teUuride is a metabolite of tellurium and tellurium compounds in a variety of living organisms, including humans. Higher dialkyl tellurides and most diaryl tellurides are solids with low melting points (diphenyl telluride is a liquid). 

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. 

Since symmetrical dialkyl and diaryl tellurides are the most employed in organic synthesis, and often exhibit structural or biological interest, their preparation will be examined in detail, focusing on the methods and procedures that are considered as the most popular. 

Diaryl tellurides (generalprocedure). A mixture of powdered Te (0.128 g, 1.0 mmol), NaH (0.053 g, 2.2 mmol, 60% suspension in oil, washed with hexane) and dry DMF (3 mL) is heated at 140°C for 1 h. Within 0.5 h the initial deep red colour is lost, and a pale yellow snspension is obtained. After cooling at room temperature, the aryl iodide (2.0 mmol) in dry DMF (3 mL) is added and the mixture heated at 130°C for 24 h. After cooling at room temperatnre, the mixture is quenched with 10% aqueous NH4HSO4 (10 mL) and extracted with ether (10 mL). The ethereal extract is washed with HjO, dried (Na2S04) and evaporated, giving the teUuride, which is purified by column chromatography on SiOj (elnent/hexane). 

The Rongalite method (see Section 3.1.1.1b,i) can be successfully applied to the preparation of diaryl tellurides from aryl iodides. This method seems to be advantageous compared to the preceding one because of the higher yields and milder experimental conditions. 

Sodium telluride and sodinm 0,0-diethyl phosphorotellurolate, prepared respectively by the Te/NaBH4DMF method and by the reaction of elemental tellurium with NaH and 0,0-diethyl phosphate in ethanol, react with arenediazonium fluoroborates, giving good yields of diaryl tellurides. ... 

Another method that uses arenediazonium fluoroborates to prepare diaryl tellurides is the reaction with potassium tellurocyanate. Aryl teUurocyanides are postulated as intermediates. 

The reaction of diarylmercurials with elemental tellurium under heating, one of the oldest methods for the preparation of diaryl tellurides, " has been later employed in some specific cases. " ... 

On the basis that bis-organyl tellurides undergo Te/Li exchange by treatment with an organolithium reagent, if a thermodynamically more stable organolithium moiety is released,bis-(phenylethynyl) telluride has been employed as starting material for the synthesis of diaryl tellurides. ... 

The protocol shown in Section 3.1.2.7 is also usefnl to prepare unsymmetrical diaryl tellurides. ... 

Diaryltellurium dihalides (general procedure) The diaryl telluride dissolved in a small amount of benzene or CHCI3 is treated dropwise, while cooling (ice bath) and stirring, with a solution of an equimolar amount of SO2CI2, Br2 or I2 in the same solvent (in the case of SO2CI2, evolution of SO2 is observed). By addition of a large excess of petroleum ether at 30-60°C (50 mL for 1 mmol) the dihalide precipitates as a crystalline solid (the dichlorides are colourless, the dibromides yellow and the diiodides red). The yields are quantitative. 

Considering that diaryl tellurides debrominate vic-dibromides, forming diarylteUurium dibromides (see Section 4.1.12.1), which in tnm can be easily hydrolysed to the corresponding tellnroxides by aqueous bases, a catalytic procedure has been provided for the oxidation of thiocarbonyl compounds. 

Typical procedure A solution of 4-pentenoic acid (0.0500 g, 0.500 mmol), bis(p-chlorophenyl)tellurium diiodide (0.302 g, 0.500 mmol) and pyridine (0.040 g, 0.500 mmol) was heated at reflux in 20 mL of chloroform for 5 days. The reaction mixture was concentrated in vacuo, and the residue was purified via chromatography on silica gel eluted with 1 1 hexane/ethyl acetate to give 0.073 g (66%) of the iodolactone and 0.140 mg (80%) of diaryl telluride. 

The cross-coupling reaction of diaryl tellurides with alkenes in MeOH in the presence of Pd " catalyst, EtjN and AgOAc as oxidant, gives the corresponding aryl-substituted (Z)-aUcenes in good yields. ... 

An additional report describes Te/Zn exchange of diaryl tellurides and diaryl ditelluride by treatment with Zn under Ni catalysis." The resulting Zn derivatives submitted to trans-metallation with CuCN-2LiCl and subsequent allylation with aUyhc bromide, furnishes the expected product in high yield. 

It has been reported that substitution of selenium by tellurium in a series of diarylchalcogenides results in a pronounced increase of antioxidant activity. Therefore the development of new and potent antioxidants is an important goal. Diaryl tellurides exhibit potent glutathione-peroxidase-like activity. 

The most active diaryl telluride, bis(4-aminophenyl)telluride, demonstrated 348%, 530%, 995% and 900% of the catalytic activity of ebselen for the glutathione-dependent reduction of H2O2, f-butylhydroperoxide (TBH), cumene hydroperoxide and linoleic acid peroxide, respectively. ... 

Diaryldifluorotellurium compounds, e.g. 19, are formed in high yield by reaction of sulfur tetrafluoride with either diaryl tellurides or diaryl ditellurides.221... 

Aryl tellurides of the type RTeH are unknown at present. The diaryl tellurides are prepared by two methods. 

When the preparations are effected by use of the Grignard reagent it is customary to transform the crude diaryl telluride into the diaryl tellurium dibromide, then reduce the latter to obtain the pure diaryl telluride. 

The diaryl tellurides combine with halogens to form dihalides, R2TeX2, and with mercuric halides mercurihalides are produced, R2Te.HgX2. 

These products are formed by the direct combination of diaryl tellurides with alkyl halides. 

The interaction of diaryl tellurides and the esters of halogenated fatty acids, e.g. methyl bromacetate, gives diarvl-telluretin halide alkyl esters, e.g. (C6H5)2Te(Br).CH2.COOCH3. The halogen present may be chlorine, bromine or iodine, moist silver oxide replacing the bromine by the hydroxyl group. 

The synthesis of aromatic tellurides has been performed via the reaction of iodoarenes (0-, m- and p-iodonitrobenzene, and di- and tri-methyliodonitrobenzenes) with phenyltelluride in HMPA at 80-90 C, preferably in the presence of Cul. Yields range between 53 and 95%.218 Symmetrical diaryl tellurides (69 R = 3-Me, 4-Me, 2,4-Me2,2,4,6-Me3,4-MeO) were prepared in good yields from reaction of nonac-tivated aryl iodides with Na2Te in DMF at 60 "C.219 The mechanisms of these reactions, which are limited to the iodoarenes, have not been discussed. 

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