Preparation of a-methylene ketones

The reaction of TeCl4 with siloxycyclopropanes, followed by treatment with DMSO, or bases such TMEDA, EtjN and pyridine, gives a-methylene ketones via the intermediacy of jS-oxoalkyl- or bis(/l-oxoalkyl)tellurium trichlorides or dichlorides.  

Similar dehydrotrichlorostannation of j9-trichlorostannyl ketones by DMSO requires prolonged heating at 60°C. 

The inversion of the olefin configuration can be attained by employing a one-pot procedure involving a sequential. tyn-chlorotelluration/anh-dechlorotelluration  

Reaction of the appropriate alkene with 1 equiv of tellurium tetrachloride in chloroform or acetonitrile gives a 2-chloroalkyltellurium trichloride adduct in a high yield (see Section 3.5.1.2). Reduction of this adduct with aqueous sodium sulphide results in the instantaneous separation of elemental tellurium and formation of the starting alkenes which, in most cases, exhibit an inverted configuration (with relevant variations depending on the solvent and olefin used). The proposed mechanism involves an intermediate epitelluride. 

Inversion of olefins (typical procedure ( )-2-butene (0.038 g) is added to a solution of TeCl4 (0.220 g, freshly sublimed) in MeCN (5 mL). The mixture is stirred at 0°C for 3 h and then evaporated at room temperature under vacuum. NajS OHjO (14% aqueous solution, 10 mL) is added dropwise (through a septum), producing evolution of gas and precipitation of elemental Te. Analysis of the gaseous 2-butene shows an E Z ratio at 3 97. 

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