Symmetrical divinyl 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. 

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. °... 

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See also in sourсe #XX -- [ , ]

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