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Debrominations of vic-Dibromides with Diorganotellurides. 1. Stereoselectivity, Relative Rates, and Mechanistic Implications.

Debrominations of vic-dibromides with diaryl tellurides 1-4 and di-n-hexyl telluride (9) are described. A mechanistic explanation of the debromination is offered which accounts for several key experimental observations: (1) the reaction is highly stereoselective with erythro-dibromides giving trans-olefins and threo-dibromides giving cis-olefins, (2) the reaction is accelerated by more electron-rich diorganotellurides, (3) the reaction is accelerated in a more polar solvent, (4) the reaction is accelerated by the addition of carbocation-stabilizing substituents to the carbons bearing the bromo substituents, and (5) erythro-dibromides are much more reactive than threo-dibromides. It is proposed that bromonium ion formation from the vic-dibromide is slow and rate-determining. Bromonium ion formation is followed by rapid scavenging of "Br(+)" by the diorganotelluride. The bromonium ion formation provides stereoselectivity and eclipsing interactions lower the reactivity of threo-dibromides. No intermediate species were observed by (1)H NMR.