Aryl ditellurides

It has, however, also been reported that the reaction of aryl ditellurides with Ni°, Pd°, and Pt° may result in the cleavage of the carbon-chalcogen bond. The oxidative addition of... 

Aryl ditellurides are cleaved by samarium diiodide (Sml2) to generate the new nucleophilic species diiodosamarium aryl tellurolate which reacts smoothly with alkyl and acyl... 

Recently, a one-pot method for preparation of 2-aryl-1,5-benzotelluroazepines 60 has been developed based on the reaction of sodium 2-aminophenyltellurolate [from di(o-aminophenyl) ditelluride) with arylpropargyl aldehydes (99MI1). Considering the high affinity of supemucleophilic aryltellurolate anions to a triple bond, one may assume that at the first stage of this reaction arylvinyl tellurides 61 are formed. Cyclization of 61 spontaneously or on silica gel in a chromatographic column forms the heterocycles 60. 

Three main routes have been well established for the preparation of diorganyl ditellurides (1) The reaction of sodium ditelluride with alkylating or arylating agents. 

The low reactivity of aryl halides towards nucleophilic reagents makes their reaction with sodium ditelluride unattractive for the preparation of diaryl ditellurides. Low yields are obtained, like the similar reaction with sodium telluride (see Section 3.1.2.1). 

The direct oxidation of lithium aryl tellurolates into the ditellurides without previous aqueous work-up is exemplified by the preparation of di(2,4,6-tri-t-butylphenyl) ditel-luride. ... 

The arylation of sodium teUuride in NMP (see Section 3.1.2.1) followed by air oxidation gives rise to diaryl ditellurides in medium to good yields. 

Diaryl ditellurides from aryl boronic acids... 

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

Organyl tellurols are very unstable compounds owing to their extreme sensitivity to oxygen, giving the corresponding ditellurides. The first short-chain alkyltellurols (C1-C4) have been isolated as yellow liquids with an obnoxious odour, from the reaction of aluminium telluride and hydrogen telluride, respectively, with alcohols and aUcyl bromides. Aryltellurols seem not to have been isolated. As shown in Sections 3.1.3.2 and 3.2.2, aryl tellurolates are... 

Starting materials for the preparation of aryl tellurides and diaryl ditellurides and, consequently, aryltellurols find wide use in the manipulation of several organic functionalities. 

Addition of aryl tellurolates to acetylenic ketones (general procedure) NaBH4 (0.55 g, 15 mmol) is added in small portions, under N2, to a solution of the diaryl ditelluride (5 mmol) in EtOH (20 mL) at room temperature. A solution of the acetylenic ketone (10 mmol) in EtOH (10 mL) is then added to the reaction mixture, with stirring. The resulting precipitate of the 2-carbonylvinyl telluride is filtered off and recrystallized from EtOH. 

At present there are few examples of isolable, well-characterized sources of tellurolate anions (RTe-).1 Although insertion of elemental tellurium into reactive metal-carbon bonds has been known for many years, the resulting solutions contain a mixture of compounds in addition to the RTe- species of interest.2 Alkali metal phenyltellurolate salts, prepared via metal reduction of diphenyl ditelluride in liquid ammonia, were first isolated by Klar and co-workers.3 More recently Lange and Du Mont reported the synthesis of the bulky aryl tellurolate (THF)3Li[Te(2,4,6-f-Bu3C6H2)],4 and Sladky described the in situ formation of a bulky alkyl tellurolate via reaction of tellurium with LiC(SiMe3)3.5 Acidification of aryltellurolate anions affords thermally sensitive tellurols (RTeH) that are stable only below room temperature.6... 

By treating aryl magnesium bromides in ether solution with tellurium dibromide (in the case of diphenyl telluride the by-products consist of diphenyl ditelluride, diphenyl and tellurium). 

Unsymmetrical diaryl sulfides (60 R = 2-NO2, 4-NO2, 2,4-(N02)2 R = H, 4-NMe2) were obtained in 80-97% yields from the reaction under PTC conditions of nitro-activated aryl halides with arenethiolates generated in situ from the reduction of the corresponding diaryl disulfides with aminoiminomethanesul-finic acid (61).192 Arylthiolates carrying electron-withdrawing substituents were not sufficiently reactive. The reaction could also be applied to the synthesis of diaryl selenides, but not of ditellurides. 

In general, from among the protic solvents, only liquid ammonia (the first used)1 is particularly useful, and is still used more than any other solvent despite the low temperature at which reactions have to be carried out (b.p. -33 °C) and the fact that solubilities of some aromatic substrates and salts (M+Nu-) are poor. Ammonia has the added advantage of being easily purified by distillation, being an ideal system for production of solvated electrons, and has very low reactivity with basic nucleophiles and radical anions, and aryl radicals. Also, poor solubilities can sometimes be ameliorated by use of cosolvents such as THF. In addition it can be used as a solvent for the in situ reductive generation of nucleophiles such as ArSe- and ArTe- ions, e.g. the formation of PhTe- from diphenyl ditelluride (equation 16).54 55... 

NiBr2(2,2 -bipyridine) complex44. Aryl halides, such as 4-bromobenzophenone, or 2-bromoquinoline could be transformed into diaryl diselenides 20 or ditellurides 21 elec-trochemically using sacrificial selenium or tellurium cathodes as can be seen in equation 20. 

The diaryl or aryl alkyl tellurides are dense yellow oils or crystalline solids, which are easier to handle than the dialkyl tellurides of similar molecular weight. Some of the diaryl derivatives are almost odorless solids. The same comments are valid for the diorganoditellurides 4, which are dark red oils (aliphatic derivatives) and dark red solids (aromatic derivatives). It is recommended that solutions of tellurides or ditellurides should not be kept in contact with air, since an amorphous white solid will form after some time. For some compounds, this reaction with oxygen is very fast. Aliphatic derivatives are more air sensitive than the aromatic ones. In view of this fact, it is recommended to bubble nitrogen into the solutions while a column or thin-layer chromatographic separation is performed. Evaporation of the solvent, however, minimizes the air oxidation. Pure liquids or solids can be handled in air with no need for special precautions, but prolonged exposure to air and to ambient light should be avoided. 

The low reactivity of aryl halides makes the direct arylation of disodium ditelluride with these compounds difficult. Only a few aryl halides have been reacted with disodium ditelluride. The yields of diaryl ditellurium products were generally less than 40%. 

Diphenyl ditellurium (17% yield) and bis[l-naphthyl] ditellurium were isolated after disodium ditelluride and the aryl iodide were irradiated in a liquid ammonia medium with a UV-lamp. The reaction mixtures were oxidized with air2. 

The halogenolysis of dialkyl ditellurium compounds is a convenient method for the preparation of alkyl tellurium trihalides because dialkyl ditellurium compounds are easily accessible through alkylation of disodium ditelluride (p. 258). Diaryl ditellurium compounds generally obtained via reduction of organo tellurium trichlorides (p. 274), serve as starting materials for the syntheses of aryl tellurium tribromides and triiodides (Vol. IX, p. 1156) that are not obtainable from the hydrocarbons and tellurium telrabromide or tetraiodide. 

Diorganyl mono- and ditellurides are the best-known classes of organic tellurium compounds comprising symmetric and unsymmetric alkyl, aryl, and alkyl-aryl tellurides. In addition to acyclic tellurides, heterocychc aliphatic telluroethers as well as aromatic tellurophene-derivatives are known (for a review, see Ref 36). 

The reaction can be performed in different organic solvents. Currently ethanol seems to be the most common solvent. This method has been employed almost exclusively for the preparation of sodium alkyl-, alkenyl-, aryl-, and heteroaryl tellnrolates. Metallic lithium and sodium can also be used for the reduction of diorganyl ditellurides. 

Two substituted aryl organotellurium compounds and a tellurium-free analogue of one of these were evaluated for in vitro cytotoxicity using human promyelocytic (HL-60) cells as an experimental system [249]. Both tellurium-containing ditellurides (174, 175) were cytotoxic at concentrations as low as 5 pM. Data indicate that both compounds induce apoptosis in both a time and dose dependent manner however, 2,2 -dimethoxybiphenyl, structurally comparable to the first of these but without the tellurium bridge, did not produce apoptosis under the concentrations and time course studied. Therefore, the telluride moiety was apparently an important factor in the apoptotic effect. 

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