OCCURRENCE OF SINGLE-ELECTRON TRANSFER DURING THE REDUCTION OF ALPHA-HALO SULFONES WITH ZINC

In order to study the frontier between inner sphere (polar) and outer sphere (monoelectronic transfer) mechanisms, the reduction of alpha-halo sulfones with zinc has been investigated using as starting materials 5-endo-[(1'-halo-1'-methylethyl)sulfonyl]-5-exo-cyano-2-norbornenes 1, efficient radical clock precursors. For the bromo as well as for the chloro sulfone, the tricyclic compound characteristic of the rearrangement of a radical intermediate has been detected: the yields are lower in methanol (1-2%) than in hexamethylphosphoramide (3-11%). These reductions could therefore proceed at least partly, via two monoelectronic transfers leading to the intermediate alpha-sulfonyl carbanion.