Probing radical pathways in electrophilic addition of halogens: Classical vs. bridged intermediates

We examine radical mediated pathways in electrophilic addition to the simplest alkene, ethylene, where the structure of the radical intermediate has been extensively debated. Starting from the pi-complex with a dihalogen, C2H4 center dot center dot center dot I-2, isolated in an inert matrix, we initiate reaction by photolytically cleaving the I-2 bond. We succeed in trapping and spectroscopically interrogating the symmetrically bridged radical complex, which calculations confirm is the global minimum on the C2H5I Potential Energy Surface (PES). Consistent with the participation of a bridged intermediate, radical addition preferentially but not exclusively yields the anti-stereoisomer of the 1,2-diiodoethane product. (C) 2012 Published by Elsevier B.V.