Stereochemistry of Halogen Azide Additions to Olefins. The Stability of Three-Membered Iodonium vs. Bromonium Ions

The reaction of iodine azide and bromine azide with olefins in polar solvents was studied with a view toward the stereochemistry of the addition. Specifically, cis-β-deuteriostyrene (6) was used as a substrate to elucidate the properties of three-membered ring iodonium and bromonium ion intermediates. Thus, addition of IN3 proceeded stereospecifically to 6 as evidenced by the exclusive formation of α-azido-trans-β-deuteriostyrene 10 on elimination of HI from the adduct. Though stereospecific anti addition of bromine azide was observed with cis- and trans-2-butene as well as with 2-cholestene, BrN3 addition to 6 led to a 1:1 mixture of cis- and trans-α-azido-β-deuteriostyrene, indicating the involvement of a benzylic cation. The free-radical addition of BrN3 to 6 indicates the reaction to be stereorandom and hence to involve no bridging of the intermediate radical by the azide group.