NBS brominates alkenes at the allylic position via a radical chain mechanism
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Chemical literature and reference sources report that N-bromosuccinimide (NBS) brominates alkenes at the allylic position through a radical chain mechanism known as the Wohl-Ziegler reaction.
Radical Allylic Bromination (Wohl-Ziegler Reaction)
Preparation of Bromine (low concentration)
NBS (N-bromosuccinimide) is the most commonly used reagent to produce low concentrations of bromine. When suspended in tetrachloride (CCl4), NBS reacts with trace amounts of HBr to produce a low enough concentration of bromine to facilitate the allylic bromination reaction. Allylic Bromination Mechanism
Step 1: Initiation
Once the pre-initiation step involving NBS produces small quantities of Br2, the bromine molecules are homolytically cleaved by light to produce bromine radicals. Step 2: Propagation
One bromine radical produced by homolytic cleavage in the initiation step removes an allylic hydrogen of the alkene molecule. A radical intermediate is generated, which is stabilized by resonance. The stability provided by delocalization of the radical in the alkene intermediate is the reason that substitution at the allylic position is favored over competing reactions such as addition at the double bond. The intermediate radical then reacts with a Br2 molecule to generate the allylic bromide product and regenerate the bromine radical, which continues the radical chain mechanism.
to the suggestion of a radical-chain mechanism, as outlined at the beginning of this section … type of halogenation occurs by a radical chain mechanism which we shall illustrate here … N-bromosuccinimide (NBS),100 e.g. equation (198). The reaction is believed to be a radical chain bromination
The hydrogens bonded to the aromatic ring (referred to as phenyl hydrogens above) have relatively high bond dissociation energies and are not substituted. C6H5CH2CH3 + Cl2 → C6H5CHClCH3 + HCl
Since carbon-carbon double bonds add chlorine and bromine rapidly in liquid phase solutions, free radical substitution reactions of alkenes by these halogens must be carried out in the gas phase, or by other halogenating reagents. One such reagent is N-bromosuccinimide (NBS), shown in the second equation below. By using NBS as a brominating agent, allylic brominations are readily achieved in the liquid phase. The covalent bond homolyses that define the bond dissociation energies listed above may are described by the general equation:
R3C-H + energy → R3C· + H·
C-H bond dissociation energies are commonly interpreted in terms of radical stability. However, this interpretation has been questioned by Gronert. Most importantly, in terms of the selectivity of free radical reactions, it is the energies of the bonds that matter, and not why they are what they are. Contributors
- William Reusch, Professor Emeritus (Michigan State U.), Virtual Textbook of Organic Chemistry
Synthesis of some C-8-modified 3-deoxy-beta-D-manno-2-octulosonic acid analogs as inhibitors of CMP-Kdo synthetase. Selective C-8 modifications of 2,6-anhydro-3-deoxy-D-glycero-D-talo-octonic acid ("2,3-dideoxy-beta-D-manno-2-octulosonic acid", 1a) were effected via the protected 8-hydroxy derivatives 2d and 2e. Swern oxidation of 2d and 2e gave the aldehydes 3a and 3b, respectively. Compounds 3a and 3b were converted into the oxime 13b and the O-methyloxime 13c derivatives, respectively. Methodology was developed for selective cleavage of the protecting groups of 13b and 13c to give the deprotected oxime 12m and the deprotected O-methyloxime 12n, respectively. Side chain-extended products were prepared from the aldehyde 3a utilizing Wittig methodology. The branched chain allylic amine 12p was prepared from 3a in a sequence the keys steps of which were preparation of the methyl ketone 19a using LiCuMe2, followed by Swern oxidation, methylenation of 19a using CH2I2-Zn-TiCl4 to give the alkene 19b, followed by Wohl-Ziegler bromination of 19b to give the allylic bromide 19c, and conversion of the latter to the allylic azide 19d.
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