Divinylic tellurides

Finely symmetrical divinyl tellurides 135, especially useful in transmetallation reactions, have been prepared in situ by sequential reaction of an excess of the ylide 134 with TeCl4 and aldehydes (Scheme 36) [134]. These compounds whatever their geometry lead in presence of -BuLi to the formation of -a,/l-unsat-urated aldehydes. 

Elemental tellurium, treated with strong aqueous alkali, undergoes an oxido-reduction disproportionation giving the corresponding telluride ion which reacts with acetylenes to give divinylic tellurides in modest yields. ... 

Preparation of alkylvinyl tellurides by reaction of Te/KOH/alkyl iodides, SnCf in HMPA/H2O with pressurized acetylene. Methylvinyl telluride. 12.8 g of Te, 1.42 g of methyl iodide, 48.7 g KOH, 24.7 g SnCl2, 80 mL HMPA and 70 mL HjO were heated (105-115°C) in a rotating autoclave 1 L under acetylenic pressure (initial pressure 14 atm, residual pressure 7 atm, absorbed acetylene 0.8 mol) tor 5 h. The reaction mixture is evacuated at 1 mmHg. The fraction (10.5 g) collected into a cooled trap (-70°C) contains, according to GLC data, methylvinyl telluride (0.34 g, yield 20%), divinyl telluride (10 g, yield 55%) and dimethyl telluride (0.12 g, yield 8%) b.p. 115-116°C. 

Divinyl tellurides (typical procedure). Sodium borohydride (0.34 g, 9 mmol) was added in small portions to a stirred suspension of tellurium (0.383 g, 3 mmol) in ethanol (15 ml.)... 

Another application of the above hydrotellnration protocol involves the synthesis of unsymmetrical divinyl tellurides by the addition of sodinm vinyhc teUnrolates, generated... 

Sodium telluride, obtained by heating elemental tellurium with sodium hydride in DMF, reacts similarly, leading to symmetrical divinylic tellurides (compare the similar reaction with aryl iodides, see Section 3.1.2.1). 

Symmetrical divinyl tellurides VI have been prepared via a ylidation reaction involving the treatment of bis-phosphonium halotellurate I with excess base and then with aldehydes in THF at -1S°C (method c). °... 

Considering that the described reaction is feasible for both aromatic and aliphatic aldehydes, that the experimental procedure is very easy, that the yields, in spite of moderate, are not far from the theoretical, the described method is certainly a useful contribution for the synthesis of symmetrical divinyl tellurides. 

In the case of symmetrical divinyl tellurides, the displacement of both vinyl groups is achieved by employing 2 equiv of n-butyllithium. Aryl vinyl tellurides give a mixture of products, since both Ar-Te and vinyl-Te bonds are transmetallated on reaction with n-BuLi, leading to vinyl- and aryllithiums. The butyl vinyl tellurides give only the desired vinyl-lithiums. The reactions are stereospecific with retention of the C=C bond geometry. °... 

The (Z)-enynes are also obtained in good yields by applying the above Pd-catalysed process to (Z)-divinyl tellurides."... 

Despite the excess of alkyne used, the transfer of only one vinylic group was observed. A more general approach of this method involves the reaction of (Z,Z)- and (f ,fi)-divinylic tellurides with alkynes under nickel catalysis, to give (Z)- and ( )-enynes with complete retention of configuration."... 

Tellurophene is obtained in low yield (3%) by reaction of powdered tellurium with acetylene in a KOH-HMPTA-H2O system (HMPTA = hexamethylphosphorotriamide) <1989ZOR39>. The reaction proceeds through the intermediate formation of divinyl telluride. Heating divinyl telluride with acetylene at 420-450 °C affords tellurophene in 30% yield (Equation 11) <1990MOK1201>. 

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