20.23: Radical Substitution: Allylic Bromination
In organic synthesis, the formation of products can be altered by changing the reaction conditions. For example, a dibromo addition product is formed when propene is treated with bromine at room temperature. In contrast, propene undergoes allylic substitution in non-polar solvents at high temperatures to give 3-bromopropene. In order to avoid the addition reaction, the bromine concentration must be kept as low as possible throughout the reaction. This can be achieved using N-bromosuccinimide (NBS) as a reagent instead of molecular bromine.
Propene reacts with NBS in the presence of light or peroxide via radical substitution to form allyl bromide or 3-bromopropene. Similar to radical reactions, the allylic bromination mechanism involves three steps: initiation, propagation, and termination. In the initiation step, NBS undergoes homolytic cleavage of weak N–Br bonds in the presence of light or peroxide to form bromine radical. During the first propagation step, the generated bromine radical abstracts the allylic hydrogen to give resonance stabilized allylic radical and HBr. The formed HBr immediately reacts with NBS in an ionic reaction producing Br2, which participates in the second propagation step. Eventually, in the termination step, different radicals combine, which results in the formation of nonradical products leading to the termination of the reaction. Throughout the reaction, HBr and Br2 concentrations are kept at a minimum. Under these conditions, that is, in a non-polar solvent with a very low concentration of bromine, the ionic addition of Br2 does not successfully compete with radical bromination.
Radical bromination of allylic substituted alkenes forms a mixture of products. This is because of the resonance stabilization of formed allylic radical intermediate, which can abstract halogen from either site (Figure 1).