Reduction of ditellurides

Alkyl- and aryltellurols generated in situ by the well-established reduction of ditellurides with NaBH /EtOH add to terminal acetylenes, giving (Z)-vinylic tellurides. "" ... 

Useful systems for the reduction of ditellurides to the corresponding tellurolates are for-mamidine sulphinic acid (HN=C(NH2)S02H), TUDO, in 50% aqueous NaOH under phase transfer catalysis/ K0H/H20/DMS0/SnCl2 or K0H/H20/DMS0/N2H4. ... 

Compounds 15 can be prepared either by acylation of bis(o-aminophenyl) ditelluride (88KGS276 89KGS120) or by reduction of A -acyl-2-trichlorotelluro-benzenes with sodium sulfide (88MI1). 

Actually, the method of choice for the preparation of organyl tellurolate anions is the reduction of diorganyl ditellurides with reducing agents. 

Dialkyl ditellurides (general procedure A mixture of powdered Te (3.58 g, 28 mmol), Na chips (0.65 g, 28 mmol) and naphthalene (0.36 g, 2.8 mmol) in anhydrous THF (25 mL) is refluxed under Nj and stirred for 1 h. During this time all the sodium is consumed and the mixture turns a light brown colour. The solution is stirred for an additional 3 h to ensure the complete reduction of Te, the temperature is then lowered to 10°C and the alkyl hahde (28 mmol) is added dropwise for 30 min with stirring. After an additional hour of stirring at room temperature, the reaction mixture is filtered, the solvent evaporated and the residue distilled under vacuum, giving the pure ditelluride (R=Et (85%), n-Pr (90%), n-Bu (90%), MeOCHjCHj (60%)). 

The reduction of organyltellurium trichlorides, which are the primary products of several reactions involving tellurium tetrachloride (see Section 3.5.1), is a useful and general method for the preparation of diorganyl ditellurides. 

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. 

In neutral conditions, the reduction of the divinyl ditellurides to the corresponding dialkyl ditellurides occurs preferentially to the hydrotelluration reaction. 

The reagent is easily generated in situ by the previously described methods (methods A and B ), or by the reduction of diphenyl ditelluride with NaBH4 (method C). In the last... 

Sodium phenyl tellurolate is a milder reducing agent than phenyltellurol. The reduction of nitroarenes is therefore limited at the intermediate azoxy or azo compound stage, depending on the reaction temperature. The reagent is prepared in situ by treatment of diphenyl ditelluride with NaBH4 in ethanol in the presence of sodium hydroxide. Since the... 

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... 

The types of compound obtainable by the condensation of tellurium tetrachloride and mixed ethers may be exemplified by the case of phenetole. The primary product obtained in the reaction is p-phenetyl telluritriehloride (I), which on heating with more phenetole gives Ws-p-phenetyl telluridichloride (II). The latter also occurs if an excess of phenetole be used in the initial condensation. Reduction of the trichloride by aqueous potassium metabisulphite at 0° C. produces bis-p-phenetyl ditelluride (III). 

Reduction of the telluritrichloride by aqueous potassium metabisulphite gives p -diphenoxydiphenyl ditelluride,... 

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. 

Because of the very strong reducing properties of the ditelluride dianion, its reaction with 5,6,11,12-tetrachlorotetracene results, along with tetratel-luratetracene 44 (yield 20%), in the formation of products of a partial reduction of the tetrachlorotetracene (82MI3). 

Benzo-1,3-ditellurole 54 was prepared in 40-47% yield by reacting dibro-momethane with disodium benzene-o-ditellurolate in ethanol, the latter generated in situ through reduction of poly(o-phenylene)ditelluride 55 with NaBH4 (88KGS1144 91 MI 1). The polymeric ditelluride 55, in turn, was obtained from a two-step procedure starting with bis-(o-trimethylsilyl)-benzene. 

Reduction of cyclic tellurium oxychloride 22 with stoichiometric amounts of sodium borohydride or excess sodium bisulfite yields the telluride 56, whereas treatment with a great excess of sodium borohydride quantitatively leads to di-/>-methoxyphenyl ditelluride and alkyne 57 (Scheme 6) <1999OM803>. 

Dibenzyl ditellurium was obtained in 70% yield by alkylation of the electrochemically generated ditelluride dianion with benzyl chloride in acetonitrile3. The ultrasonically promoted electrochemical reduction of tellurium powder was performed in H-type cells with the compartments separated by glass frits. Acetonitrile served as solvent and tetrabutylammonium tetrafluoroborate or hexafluorophosphate as the supporting electrolyte. At potentials beyond -1.1 V the dark-red ditelluride dianion is formed in the cathode and in the central compartment3. 

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. 

A number of more esoteric methods have been devised for the reductive cleavage of Troc groups, which the synthetic community have been reluctant to adopt. These include cobalt(l) phthalocyanine25 and sodium 2-thiophenetellur-olate,2S9 which is readily obtained in a catalytic cycle by the sodium borohy-dride reduction of commercially available bis(2-thienyl)ditellurid. Electrolysis is a method that might be considered in special cases.260... 

One common present day preparation of dialkyl mono-and ditellurides involves the reaction of alkyl halides with alkali metal tellurides that have been prepared in situ. The reactions are commonly carried out in aqueous or nonaqueous solutions. Solutions of sodimn in liquid ammonia also provide a useful media for the reduction of tellurium. ... 

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. 

Tellurols are extremely air sensitive and are normally not isolated as they arc converted to diteliurides (equation 44). Nevertheless there are two reports on the isolation of alkanetellurols. - Benzene-tellurol may be prepared in situ by treatment of trimethylsilyl phenyl telluride (PhTeSiMea) with meth-anoP or CFsCOiH and also by reduction of diphenyl ditelluride with H3PO2 or NaBH4 and used as a reducing agent. ... 

The reaction of disodiuni ditelluride in dimethylformamide with 2-halonitrobenzene 9 results in a displacement of the halide, reduction of the nitro group and a ring-closure reaction which affords phenazine 10 (mp 251 253 °C) in 37% yield. ... 

---