Radical allylic halogenation

Physical Properties of Haloalkanes Preparation of Haloalkanes by Halogenation of Alkanes Mechanism of Halogenation of Alkanes Allylic Halogenation Radical Autoxidation Radical Addition of HBr to Alkenes... 

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... 

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). 

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. 

NBS contains a weak N- Br bond that is homolytically cleaved with light to generate a bromine radical, initiating an allylic halogenation reaction. Propagation then consists of the usual two steps of radical halogenation as shown in Mechanism 15.3. 

Trie allylic radical reacts with Brg in the second propagation step to form the product of allylic halogenation. Because the Br- radical formed in Step [3] is also a reactant in Step [2], Steps [2] and [3] repeatedly occur without the need for Step [1]. 

How can we account for the unusual reactivity of conjugated dienes In our discussion of halogenation of the simple alkenes (Sec. 3.27), we found that not only orientation but also relative reactivity was related to the stability of the free radical formed in the first step. On this basis alone, we might expect addition to a conjugated diene, which yields a stable allyl free radical, to occur faster than addition to a simple alkene. 

Nechvatal, A. Allylic halogenation. Advances in Free-Radical Chemistry (London) 1972, 4,175-201. 

Free-radical halogenation is most commonly applied to allylic or benzylic halo-genation, as the radicals formed at these positions are lowest in energy. Of course, in allylic halogenation, transposition of the double bond can easily occur. 

Allylic halogenation proceeds by a radical chain mechanism. 

Step 1 Allylic halogenation by a radical chain mechanism (Section 8.6A). 

There are some instances where the chlorination or bromination reaction can be used to good effect, however. Alkenes that have allylic hydrogens can sometimes be halogenated specifically at an allylic position in a process called allylic halogenation, another free radical chain reaction. For example, when low concentrations of bromine are photolyzed in the presence of the complicated cyclohexene shown in Figure 11.51, the product is exclusively brominated in one allylic position. 

Benzylie halogenations are similar to allylic halogenations (Seetion 13.2) in that they involve the formation of unusually stable radicals (Section 15.12A). 

In earlier years, radical allyl halogenations and selenium dioxide oxidation were quite popular, although in various ways problematic. Nowadays, catalytic hydroxy-lations or ene-reactions with singlet oxygen are the better accepted alternatives. 

In Summary Under radical conditions, alkenes containing allylic hydrogens enter into allylic halogenation. A particularly good reagent for allylic bromination is A-bromobutanimide (A-bromosuccinimide, NBS). 

---