Halogenation, allylic

Of the reactions that involve carbon radicals, the most familiar are the chlorination and bromination of alkanes (Sections 4.15 through 4.19)  

Although alkenes typically react with chlorine and bromine by addition at room temperature and below (Section 6.14), substitution becomes competitive at higher temperatures, especially when the concentration of the halogen is low. When substitution does occur, it is highly selective for the allylic position. This forms the basis of an industrial preparation of allyl chloride  

The reaction proceeds by a free-radical chain mechanism, involving the following propagation steps  

Allyl chloride is quite reactive toward nucleophilic substitutions, especially those that proceed by the Sn2 mechanism, and is used as a starting material in the synthesis of a variety of drugs and agricultural and industrial chemicals. 

Allylic brominations are normally carried out using one of a number of specialized reagents developed for that purpose. A-Bromosuccinimide (NBS) is the most frequently used of these reagents. An alkene is dissolved in carbon tetrachloride, A-bromo-succinimide is added, and the reaction mixture is heated, illuminated with a sunlamp, or both. The products are an allylic halide and succinimide. 

Halogenation proceeds by the free-radical chain mechanism shown in Mechanism 10.2. 

A-Bromosuccinimide provides a low concentration of molecular bromine, which reacts with alkenes by a mechanism analogous to that of other free-radical halogenations. The 

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Although allylic brommations and chlormations offer a method for attaching a reactive functional group to a hydrocarbon framework we need to be aware of two important limitations For allylic halogenation to be effective m a particular synthesis... 

The fact that the bromine concentration remains at veiy low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must... 

A -Halogenated compounds such as iV-chlorotnfluoroacetamide, A -chloro-imidosulfuryl fluonde and N N dichlorotnfluoromethylamine add across C=C bonds to form saturated amides [14] tmidosulfury I fluorides [15] and amines [16], respectively Allylic halogenation also occurs with the use of A-bromo- or A-chIo roperfluoroamides The primary amine A,A-dichlorotrifluororaethylamine selectively affords 11 or 2 1 adducts with either tetrafluoroethylene or chlorotrifluoroethylene [16] (equation 7) The reaction mechanism is believed to involve thermal free radicals, with control achieved principally by reaction temperature The 1 1 adduct is formed even in the presence of a large excess of olefin... 

Interestingly, the photolysis of methyl 3-azido-2.6-difluorobenzoate (61) in diethylamine yields, in addition to the two expected 3//-azepines 62 and 63, a third azepine 64 formed by nucleophilic displacement of allylic fluorine from the 3-position by diethylamine.188 Displacement of allylic halogen from haloazepines has been noted previously.177... 

Sequential cross-coupling can be used also for synthesis of 1,1-disubstituted alkenes from 2,3-dibromopropene (Aldrich). The allylic halogen couples directly with a Grignard reagent vinylic coupling is slower and requires a catalyst (equation I). 

Allylic halogenation is a substitution reaction involving a free-radical mechanism. The general mechanism is in Figure 4-7. The final X cycles back to the beginning (shown with the Icirge curved arrow). 

Another example of allylic halogenation is shown in Figure 4-9. 

With Fe, the products are o-, p-, and some m-BrCftH4CH,. In light the product is benzyl bromide, PhCHjBr. Like allylic halogenation (Section 6.5), the latter reaction is a free-radical substitution ... 

Exchange between halides and organometallic compounds 4-1 Free-radical halogenation 4-2 Allylic halogenation... 

Bromo- and chloro-iodinanes (29 and 31) behave as free radical halogenating agents (79JA3060). They give photoinitiated benzylic halogenation of toluene or allylic halogenation of cyclohexene in high yield. Cyclic 10-C1-3 and 10-F-3 species have not yet been reported. 

Such rearrangements are common during attempted allylic halogen exchange reactions, especially when the stable trifluoromethyl group results.6,7... 

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