Allylic bromination
المؤلف:
Jonathan Clayden , Nick Greeves , Stuart Warren
المصدر:
ORGANIC CHEMISTRY
الجزء والصفحة:
ص989-990
2025-07-29
395
Allylic bromination
Because radical brominations are so selective, they can be used successfully in the laboratory to make alkyl bromides. There are relatively few ways of functionalizing an unfunctionalized centre, and radical allylic bromination is one of the most effective. where we contrasted the radical reactivity of Br2 towards alkenes (leading to an allyl bromide by hydrogen abstraction) with its ionic reactivity (leading to addition of bromine across the alkene). We can now look in a little more detail at the selectivities involved. Here is a typical allylic bromination. NBS (N-bromosuccinimide) is used to form a small amount of Br2 and to keep the Br2 concentration low.
Photolysis of Br2 initiates the reaction, which then propagates as shown below. The mechanism also illustrates the first aspect of selectivity: only a (green) allylic H atom is abstracted because an allylic C–H bond is considerably weaker than a secondary C–H bond.
There is a problem with this reaction if bromine itself is used because an alternative radical addition reaction can compete with radical abstraction.
The fi rst step of this competing addition reaction is reversible; the reaction is driven forward by the participation of a second molecule of bromine that traps the product alkyl radical. This side reaction can be prevented if the concentration of Br2 in the reaction is kept very low, which is the role of NBS. The alternative competing polar addition of Br2 to the alkene is like wise prevented with the low bromine concentration provided by NBS, although the non-polar solvent CCl4 also disfavours the formation of the cationic bromonium ion intermediate. While radical halogenation of alkanes is used only rarely in the laboratory, radical allylic bromination of alkenes is a versatile and commonly used way of making allylic bromides. Nucleophilic substitution reactions can then be used to convert the bromide to other functional groups. For example, some chemists in Manchester needed to make the two diastereo isomers of 5-tert-butyl-cyclohex-2-en-1-ol to study their reactions with osmium tetroxide. tert-Butyl cyclohexene is readily available, so they used a radical allylic bromination to intro duce the functional group in the allylic position, which they converted to a hydroxyl group using aqueous base. Steric effects also play a role here in the regioselectivity of the reaction: only the less hindered allylic hydrogen atoms further from the t-butyl group are removed.
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