N-Bromosuccinimide (NBS) is a convenient source of bromine for radical substitution as well as electrophilic addition reaction. It is prepared from succinimide and bromine in the presence of NaOH solution. The colourless solid obtained is washed with water and recrystallized from hot water, and stored in a refrigerator and protected from moisture to avoid decomposition. NBS is easier and safer to handle compared to bromine.
5.1.1 Substitution Reactions
Allylic and Benzylic Brominations
Alkenes react with NBS in dry CCl4 under reflux conditions to give allyl bromide. The reaction is initiated by light or peroxide. Although a number of reagents are available for bromination of allylic C-H bond of alkenes, NBS is most commonly used. The reaction is called Wohl-Zigler bromination. For example, cyclohexene reacts with NBS to give 3-bromocyclohexene in the presence of catalytic amount AIBN (radical initiator).
Allylic methylene groups are prone to undergo reaction more readily than allylic methyl group due to stabilization of the radical (2>1). For example, 2-heptene can be selectively brominated at the secondary allylic carbon compared to the primary carbon.
The reaction conditions are compatible for the bromination of benzylic C-H bonds, which are important from synthetic and mechanistic standpoint. For examples, diphenylmethane reacts with NBS to give bromodiphenylmethane in 81% yield.
Similarly, 1-phenylbutane, 1-methylnaphthalene and 3-methylthiophene could be reacted with NBS to give the corresponding brominated products (Scheme 1).
Scheme 1