Mechanism

The reaction involves a free radical process (Scheme 2). The reaction is initiated by small amounts of Br radical. The role of NBS is to afford a constant low concentration of molecular bromine. Abstraction of an allylic or benzylic hydrogen by Br radical gives a resonance stabilized allyl or benzyl radical. The selective bromination occurs because the intermediate leading to the product is stabilized by resonance.

 

Scheme 2

α-Bromination of Carbonyl Derivatives

Ketones having enolizable hydrogen can be brominated at the α -position. The reaction probably involves an addition of Br₂ to the enol form of the carbonyl derivatives, and elimination of HBr generates the a -bromoketone (Scheme 3).

Scheme 3

Allylic Bromination of Unsaturated Acids, Esters, Aldehydes and Ketones

α,β -Unsaturated carbonyl compounds undergo reaction with NBS at the allylic C-H bond to give allylic brominated α,β -unsaturated carbonyl compound that can be used in the Reformatsky reaction. For example, ethyl crotonate can be transformed into ethyl 4-bromocronate using NBS

Scheme 4

in the presence of catalytic amount of AIBN in CCl₄ under reflux conditions (Scheme 4). Under similar conditions, testosterone acetate can be converted into 6-bromotesterone acetate in good yield (Scheme 5)

Scheme 5