Radical intermediates in the Zn-promoted Barbier-type alkylation of diphenyl diselenide in aqueous medium

The zinc promoted Barbier-type reaction of alkyl halides and diphenyl diselenide in aqueous medium leads to high yields of mixed selenides even in acidic medium. Especially the efficient formation of t-butylphenylselenide cannot be explained by nucleophilic substitution and raises the question of an alternative reaction mechanism. Three suitable halide precursors of ‘radical clocks’ of increasing rate of unimolecular rearrangement were used in order to evidence possible radical intermediates. Only the fastest one, halomethyl cyclopropane led to linear, rearranged, mixed selenides in amounts increasing from the chloride to bromide and iodide as leaving group. The results indicate a nucleophilic substitution as the most important mechanism in the formation of mixed selenides competing with an alternative, radical based process. The latter becomes exclusive in the case of tertiary alkyl halides.