Selenium halogen displacement

Selenium and tellurium reagents have been used for stereoselective halogenations of alkenes. For example, trans addition of benzeneselenenyl chloride to alkenes followed by the displacement of the seleno moie with chloride can lead to dr-1,2-dichlorides (equation 25). The addition of 2-naph-thyltellurium trichloride proceeds in an anti stereospecific manner (equation 26), whereas tellurium tetra-diloride gives a mixture of syn and anti adducts. The reaction of allyl esters with tellurium tetrachloride accompanies acyl migration to give the l-(trichloiottlluro)-3-chk>io adduct (54 equation 27). ... 

The nucleophilic displacement of active halogens in allylic halides to give allylic tellurides is effected by an even simpler reagent than either (300) or (301), i.e. Te -. Once formed, the allylic tellurides decompose spontaneously to allyl radicals which couple to form 1,5-dienes, a reaction in which tellurium appears to be far superior to selenium. 

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