Elimination in vicinal disubstituted substrates

Several methods achieving the debromination of v/c-dibromides by means of tellurium reagents are well established. These methods are particularly advantageous compared to the conventional ones in terms of the mildness of the experimental conditions, good yields, lack of important side reactions and inermess of several functionalities to the employed reagents. A relevant characteristic of these reactions is the high E2-type stereospecificity demonstrated by the formation of olefins with Z and E geometry from threo-and eryf/iro-dibromides, respectively. 

Method Diaryltellurium dibromides are formed together with the olefmic product. The starting tellurides can easily be recovered by reduction of the dibromides (see Section 3.1.4). 

Method B. This is an improved modification of method A, effecting the debromination in a catalytic cycle by means of potassium bisulphite and catalytic amounts of diaryl tel-luride. 

Di-(j -methoxyphenyl telluride) had at first been used but better results have been achieved with electron-rich diorganyl tellurides, such as (p-Me2NCgH4)2Te, (CgHi3)2Te, 2(PhTe)CgH4CH2NMe2 associated to different reducing agents, reduced glutathione (GSH) or sodium ascorbate.  

It was suggested that halogenation-dehalogenation with diaryltellurium derivatives is an equilibrium process since, at identical concentrations, debromination of erythro-l, 2-dibromo-l,2-diphenylethane with diaryItellurides andbromination of trani-stilbene with dibromodiarylteUurides gives identical product mixtures. 

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