Allylic compounds Wohl-Ziegler reaction

The bromination of allylic compounds (Wohl-Ziegler reaction) is generally used in the conversion of aUcenes into conjugated dienes, which are obtained from the allylic bromides by base-catalyzed elimination (Scheme 4.22). 

Brominations are, in most cases, carried out by methods similar to those for the preparation of chlorine derivatives. Sampey (127) gives a history of the photobromination of benzene and toluene. Davis (123) made a thorough examination of the relative rates of bromination of the olefins, concentrating particularly on ethylene. An unusual method for producing bromine compounds is by the use of bromosuccinimide or related compounds. This method is called the Wohl-Ziegler reaction and causes allylic bromination. It has been the subject of a couple of reviews (124,1 0). 

Allylic bromination of glycals 10 and 18 was investigated under the usual Wohl-Ziegler conditions with /V-bromosuccinimide (NBS/CCLt/reflux) in the presence of azobi-sisobutyronitrile (AIBN) as initiator. Although both bromides were easily formed in this reaction, the nonfluorinated one appeared to be unstable and could not be isolated [70], The parent 10-CF3-16-bromo derivative 22 was also obtained in good yield (72%) (see Scheme 6.12). The compound could be purified by crystallization and stored for several weeks at 0°C. Clearly, the electron-withdrawing character of the CF3 group makes the allyl bromide less labile. 

---