Wohl‐Ziegler Bromination

Abstract

This reaction is the bromination of unsaturated organic compounds at either allylic or benzylic position (or α‐position of the side chain of other aromatic compounds) using N ‐bromosuccinimide (NBS) as the bromination reagent. The bromination by N ‐bromoacetamide is simply referred to as the Wohl reaction, whereas the bromination by NBS is generally known as the Wohl–Ziegler bromination. Besides N ‐bromosuccinimide (NBS), several bromination reagents have been recommended for this reaction. The N ‐bromosuccinimide is an ideal bromination reagent for the allylic bromination. It has been reported that the Wohl–Ziegler bromination does not work for the conjugated olefins and 1,4‐dienes, which are the inhibitors for the Wohl–Ziegler bromination. Furthermore, the bromination of benzyl compounds, primary benzyl methyl ethers and p ‐nitrobenzyl methyl ether have been discussed. In addition, the electrophilic bromination on the activated aromatic compounds with NBS often occurs in the presence of an acidic catalyst. This reaction has wide application in organic synthesis for generating allylic and benzylic bromides.

Chlorinated Solvents: Their Advantages, Disadvantages, and Alternatives in Organic and Medicinal Chemistry

“…In general, superior selectivity of radical brominations is observed over chlorinations due to the Hammond postulate and also the inverse relationship between selectivity and reactivity . NBS in conjunction with a radical initiator remains the most popular and widespread method for radical bromination of benzylic C–H and allylic C–H groups, i.e., a Wohl–Ziegler type of reaction. − Due to the reactivity of halide radicals, solvents themselves must be unreactive, making solvent choice challenging and promoting CCl 4 to the position of go-to solvent. , CCl 4 was almost exclusively employed by Ziegler in the original studies of this reaction . Benzene is another popular choice, as it generally does not undergo radical attack in the presence of peroxides …”