Allylic sources halogens

The amide N-H may also be halogenated, oxidized and nitrosated. A -Bromosuccinimide (NBS), like a number of other iV-halo compounds, readily undergoes a radical fission to give a bromine radical. This provides a useful reagent for radical bromination at, for example, allylic or benzylic positions. In the presence of acid, NBS is also a mild source of the halonium ion, which is used for the addition of hypobromous acid (Scheme 3.74) to alkenes or for the bromination of reactive aromatic rings. 

Under standard conditions (N-bromosuccinimide [NBS] and AIBN [azo-bis(isobutyronitrile)] in refluxing CCI4), the desired brominated poly(TMSP) products were formed in high yield. Nearly quantitative yields were obtained up to bromination levels of 50%, on the basis of one Br substitution per monomer unit, but unexpectedly, the maximum attainable bromination level was only 60%, even when a large excess of NBS was used (Table I). We attributed this curious behavior to the extreme steric crowding at the allylic methyl groups in the polymer structure. The accessibility of the allylic sites apparently is so limited that monobromination is the exclusive reaction, and the presence of the bulky halogen decreases the likelihood of bromination at adjacent allylic sites. Similar results were obtained when benzoyl peroxide was used as the free-radical source. 

Source C. Walling and W. Thaler, Positive Halogen Compounds. III. Allylic Chlorination with t-Butyl Hypochlorite. The Stereochemistry of Allylic Radicals, Journal of the American Chemical Society 83 3877-3884 (1961). With permission. 

Chlorination of alcohols by Thionyl Chloride the preparation of acyl chlorides from lactones and anhydrides, and the bromi-nation and iodination of aromatic rings by Benzyltrimethylammo-nium Tribromide and Benzyltrimethylammonium Dichloroio-date, respectively, are all effectively catalyzed by the presence of ZnCl2. In addition, ZnCl2 acts as a source of chloride for the halogenation of primary, secondary, and allylic alcohols using Triphenylphosphine-DiethylAzodicarboxylate. ... 

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