Allylic halogenation reactions mechanism

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. 

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... 

The radical addition of halogen to an alkene has been referred to briefly in Section 9.3.2. We saw an example of bromination of the double bond in cyclohexene as an unwanted side-reaction in some allylic substitution reactions. The mechanism is quite straightforward, and follows a sequence we should now be able to predict. 

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). 

Wohl in 1919 reported that A -bromoacetamide (CH CONHBr) induced allylic bromination. " Then iV-bromosuccinimide (30) was described in 1942 by Ziegler and co-workers to be useful in such free radical bromination reactions (equation 41), " and this widely utilized procedure is known as the Wohl-Ziegler reaction. In 1963 the mechanism of the reaction was proposed to involve halogen atoms in the hydrogen abstraction step " " " instead of succinimidyl radicals as had been commonly supposed. The halogen atom mechanism had previously been proposed by Gosselain et al. for reactions of yV-chlorosuccinimide. " ... 

We have further examined the reactions of thiolato-bridged diruthenium complexes with other unsaturated organic substrates. When the cationic Ru complex 4 is treated with cinnamyl alcohol in p-xylene, the allylated aromatic compound 18 is obtained in good yield (Equation 6). We assume a Tc-allyl intermediate because the reaction using l-phenylprop-2-en-l-ol gives, instead of cinnamyl alcohol, the same product 18 however, the detailed reaction mechanism is still obscure. This novel allylation reaction is halogen-free, and may replace the conventional Friedel-Crafts alkylation. 

This allylic bromination with NBS is analogous to the methane chlorination reaction discussed in Section 6.3 and occurs hy a similar radical chain reaction mechanism. As in methane halogenation, Br- radical abstracts an allylic hydrogen atom of the alkene, thereby forming an allylic radical plus HBr. This allylic radical then reacts with Br2 to yield the product and a Br-radical, which cycles hack into the first step and carries on the chain. The Br2 results from reaction of NBS with the HBr formed in the first step. 

The photochemistry of ir-allylpalladium complexes has been studied to a limited extent. Two basic reactions have been observed. Irradiation at 366 nm of ir-allylpalladium complexes produced 1,5-diene dimers, reportedly via a radical coupling mechanism.334 333 Similar irradiations in the presence of species capable of trapping the presumed allyl radical intermediate, such as BrCCb, BrCH2Ph or allyl bromide, now yield alkylated and halogenated allyls, in addition to 1,5-diene dimer. This reaction fails for simple alkyl or aryl halides due to the instability of the associated radical (equations 130 and 131 ).336... 

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