Tellurols oxidation

The oxidative addition reaction of the type shown in Equation (9) were used to prepare some of the first selenolate and tellurolate complexes still used today. 

Only half of the tellurium of the starting ditelluride is converted into telluride. The other half, converted into tellurolate, can be oxidatively recovered as the starting ditelluride. 

Air is then bubbled through it to oxidize the lithium tellurolate. The red oily ditelluride that separates is extracted with ethyl ether (200 mL). The organic phase is washed with H2O (2X50 niL), dried (CaCl2) and evaporated to give 2.83 g of a viscous red oil. Distillation of 5.66 g of the crude material prepared in this manner affords 4.87 g of pure di-n-butyl ditelluride (89% based on Te b.p. 103-105°C/0.8 torr). 

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. ... 

Acidification of (35) gives the highly stable tellurol (Me3Si)3SiTeH as an air-sensitive wax, with pKa 7.3, which readily oxidizes to the ditelluride and gives base-free derivatives of the alkali metal with (Me3Si)2NM (M = Li, Na) or KOBu-t31. 

Organotellurols 1, the tellurium analogs of alcohols, constitute a class of organic tellurium compounds, which have to be prepared and used in situ due to their high succeptibility to oxidation. The participation of this class of compounds in several reactions has been proposed,5-12 but their real existence has never been demonstrated in such processes. A few tellurols exhibiting special structural features could be isolated and characterized (e.g., compound 10, Figure 2).14... 

Dihydrogen telluride is an acid in aqueous solution and is comparable in strength with phosphoric acid. Tellurols, especially arenetellurols, are expected to be at least as acidic as dihydrogen telluride. Therefore, tellurols should react easily with alkali metal hydroxides to form alkali metal tellurolates. Because tellurols are difficult to prepare, alkali metal tellurolates are best obtained by methods which avoid the tellurols. Sodium and lithium tellurolates are the most frequently used tellurolates. Although the tellurolates are not as sensitive to oxidation as the tellurols, tellurolates are almost always used in situ and are prepared and stored under nitrogen. Sodium benzenetellurolate was isolated as a moisture-and air-sensitive, grey powder. Its solutions in tetrahydrofuran or acetone were found to be stable for months when kept under nitrogen6. 

The tellurolates were either oxidized to ditellurium derivatives or alkylated to produce dialkyl tellurium derivatives1,2. 

Tellurolates were formed when bisfbenzoyl] tellurium compounds were reacted with primary amines, with secondary amines, or with potassium ethoxide in dichloromethane at — 30° under an atmosphere of argon. The tellurolates were oily substances that were too unstable thermally and towards oxygen for purification. They were alkylated to 7e-alkyl tellurolobenzoates or oxidized to ditellurium derivatives3. 

Benzenesulfonyl chloride oxidized the piperidinum tellurolate to the bis[2-methoxybenzoyl ditellurium3. 

Arenetellurolates are obtained when tellurium is reacted with aromatic lithium compounds in THF or diethyl ether (p. 154) or with aromatic Grignard reagents in THF. The arenetellurolates thus formed are hydrolyzed to tellurols. The tellurols are oxidized to the diaryl ditellurium products. 

Instead of air, potassium hexacyanoferrate(III) can also be used to oxidize tellurols to diaryl ditellurium products. This method produced the following (V-heterocyclic ditellurium compounds from the lithium tellurolates1. 

Dialkyl and diaryl ditellurium compounds are easily reduced to tellurols and tellurolates. Alkali metals in liquid ammonia or in an inert organic solvent, sodium borohydride in methanol, ethanol, alcohol/benzene, THF, DMF, or in a basic aqueous medium, lithium aluminum hydride in dioxane or THF/hexamethylphosphoric triamide, and thiourea dioxide in THF/50% aqueous sodium hydroxide have been used as reducing agents (p. 164). The tellurolates are easily oxidized in air. For this reason they are almost always used in situ. 

Oxidation of tellurolates generally produces diorgano ditellurium compounds (p. 264,270). However, the oxidation of ethynetellurolates led to the isolation of diethynyl tellurium products. When sodium ethynetellurolates were hydrolyzed with water in the presence of air, the diethynyl tellurium products were obtained in low yields. Oxidation by iodine in DMSO improved the yield to 70%4. The expected ditellurium compounds were postulated as unstable intermediates. 

Oxidation of the hydrolyzed tellurolates with air gave bis phenylethynyl] tellurium in 15% yield and bis[4-methylphenylethynyl] tellurium (m.p. 120°) in 14% yield1. 

Poly(l,4-phenylene Ditellurium)1 Under nitrogen, a solution of 1 mol of 1,4-dibromobenzene is cooled to — 15° with stirring, a solution of 22 mol of butyl lithium in hexane is added dropwise followed by 1 mol of finely ground tellurium, and the mixture is stirred at 20° until all of the tellurium has dissolved. To this mixture is added 2 mol of butyl lithium followed by 1 mol of powdered tellurium. When all of the tellurium has dissolved, 2 mol of water and then 2 mol of triethylaminc arc added. The resultant tellurolate is oxidized by addition of an aqueous solution of potassium hexacyanoferrate(Ill) yield 54% reddish-brown, amorphous powder. 

The oxidative addition of chalcogen-chalcogen bonds in RE—ER is widely applied in transition metal chemistry and is often a useful route for the preparation of thiolates, selenolates and, to a more limited extent, tellurolates. The oxidative addition of X2 = HO—OH and PhC(0)0—0C(0)Ph is also known and, in the case of Ptu complexes, gives predominantly trans products 47... 

The tellurides, Fc-Te-Fc and Fc-TeTe-Fc, where only obtained in 1987 via the solvated lithium ferrocenyl tellurolate, Fc-TeLi(thf) [102]. Oxidation by air produces Te2Fc2 which, in turn, may be cleaved by Fc-Li to give eventually TeFc2. 

The alkanetellurols are pale yellow liquids having obnoxious odors. Methanetellurolis spontaneously flammable in air and explodes upon contact with pure oxygen at 20° . It is easily oxidized to dimethyl ditellurium by mild oxidizing agents. Tellurols must be handled in an oxygen-free atmosphere at all times. Methanetellurol is stable at — 196°, but decomposes with deposition of tellurium at 20° . The decomposition seems to be catalyzed by stopcock grease. ... 

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