Allylic Free-Radical Halogenation

Bromination with A-bromosuccinimide generally gives the same result as bromination with free bromine or hypobromous acid. The reaction is considered to proceed with a small concentration of free bromine and does not generate an appreciable concentration of acid. Conditions are therefore mild. In addition, A-bromosuccinimide has been used to brominate the allylic position of a, -unsaturated ketones in the presence of free-radical promoters or with irradiation, and thus gives access to dienones by dehydro-halogenation, for exaraple " ... 

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). 

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. " ... 

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 ... 

Under high temperature or UV light and in the gas phase, cyclohexene can undergo free radical substitution by halogens. A common reagent for allylic... 

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

Halogenations may also occur by a free-radical mechanism.121,218 Besides taking place in the gas phase, halogenation may follow a free-radical pathway in the liquid phase in nonpolar solvents. Radical halogenation is initiated by the alkene and favored by high alkene concentrations. It is usually retarded by oxygen and yields substitution products, mainly allylic halides. 

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. 

Although free-radical halogenation is a poor synthetic method in most cases, free-radical bromination of alkenes can be carried out in a highly selective manner. An allylic position is a carbon atom next to a carbon-carbon double bond. Allylic intermediates (cations, radicals, and anions) are stabilized by resonance with the double bond, allowing the charge or radical to be delocalized. The following bond dissociation enthalpies show that less energy is required to form a resonance-stabilized primary allylic radical than a typical secondary radical. 

The mechanism is similar to other free-radical halogenations. A bromine radical abstracts an allylic hydrogen atom to give a resonance-stabilized allylic radical. This radical reacts with Br2, regenerating a bromine radical that continues the chain reaction. 

Show how free-radical halogenation might be used for the synthesis of some alkyl halides, especially for making allylic and benzylic alkyl halides. 

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. 

If we wish to direct the attack of halogen to the alkyl portion of an alkene molecule, then, we choose conditions that are favorable for the free-radical reaction and unfavorable for the ionic reaction. Chemists of the Shell Development Company found that, at a temperature of 500-600°, a mixture of gaseous propylene and chlorine yields chiefly the substitution product, 3-chloro-l-propene, known as allyl chloride (CH2=CH—CH2— = allyl). Bromine behaves similarly. 

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. 

Examination of reactions that involve attack not only by halogen atoms but by other free radicals as well has shown that this is a general rule benzylic hydrogens are extremely easy to abstract and thus resemble allylic hydrogens. We can now expand the reactivity sequence of Sec. 6.22 ... 

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