Cyclohexene allylic bromination

The products of allylic bromination reactions are useful for conversion into dienes by dehydrohalogenation with base. Cyclohexene can be converted into 1,3-cyclohexadiene, for example. 

Novel results were reported for allylic bromination. In radical bromination of cyclohexene in CCI4 under light the selectivity of substitution over addition was shown to be controlled by bromine concentration.304 Substitution via the corresponding allyl radical, while relatively slow, is irreversible and fast enough to maintain the concentration of bromine at a sufficiently low level to prevent significant addition. The reaction of two strained alkenes, fZ)-1,2-dimethyl-1,2-di-ferf-butylethylene and the -isomer (14), leads to the corresponding bromosubstituted product, instead of addition 305... 

Allylic bromination normally uses NBS as bromine itself would add to the alkene. Thus cyclohexene gives the dibromide 18 with Br2 but the allylic bromide with NBS. Bromine radicals abstract one of the marked H atoms from 19 and the intermediate allylic radical 21 is delocalised so we don t know which end of the allylic system5 ends up attached to Br. 

Fig. 1.24. Mechanism for the allylic bromination of cyclohexene according to the Wohl-Ziegler process. 

Reaction with olefins. Wohl1 in 1919 briefly examined the reaction of tetra-methylethylene with NBA and tentatively suggested that the reaction involved allylic bromination. Wolfe and Awang2 have examined the reaction of NBA with olefins and find that the reaction does not involve allylic bromination but furnishes 2-bromo-N-bromoacetimidates, a new class of compounds. Thus the reaction of cyclohexene (1)... 

The reagent has been shown to be effective for allylic bromination, the reaction being markedly catalyzed by light. Thus bromination of cyclohexene was effected under illumination with a 275-watt Sylvania sunlamp. The reaction can be carried... 

PROBLEM 10.4 Assume that A-bromosuccinimide serves as a source of Br2, and I write equations for the propagation steps in the formation of 3-bromocyclohex- I ene by allylic bromination of cyclohexene. J... 

The following reaction with cyclohexene is another example of allylic bromination with NBS ... 

Consider the allylic bromination of cyclohexene labeled at C3 with Neglecting stereoisomers, what products would you expect from this reaction ... 

Racemic validatol (7) and deoxyvalidatol (8) have been synthesized by the procedure outlined in Scheme 3 Racemic cyclitols have also been prepared from cyclohexene by repeated sequential allylic bromination and treatment with sodium benzyloxide to give a mixture of di(benzyloxy)cyclohexenes the 3,4-diether (9) then being hydroxylated by standard procedures leading to the tetraol derivatives (10) (the 1,2,4/3-, 1,3/2,4-, and 1,4/2,3-isomers), from which racemic dihydroconduritols could be obtained. ... 

The allylic bromination of cyclohexene was successfully done using co(polyethylene-N-bromomaleimide). When polymeric AT-bromosuccin-imide ( -NBS) was used for bromination of cumene, products other than those of benzylic bromination were also formed (Scheme 12-10) (Yaroslavsky et aL, 1970a,b). The change in mechanism has been attributed to the polar environment provided by neighboring succinimide units in (p)-NBS. Polymeric A/-chloromaleimide, synthesized by Yaroslavsky and Katchalski (1972), on reaction with ethylbenzene, also gave products due to aromatic substitution. 

In a subsequent report, Ziegler and co-workers systematically studied the halogenation of several allylic substrates using a variety of reagents including A -bromo and A -chloro pthalimide, chloramine-T, N-bromo and A -chlorosaccharin, A -chloro-A -benzoyl-p-toluenesulfonamide, N-chloro-di-/ -toluylsulfonamide, and most notably, iV-bromo and N-chlorosuccinimide. Successful allylic bromination was reported for several substrates, among which were the conversion of cyclohexene (7) into bromocyclohexene (8) and the conversion of phenyl propene 9 into cinnamyl bromide (10). 

Another method for preparing alkyl halides from alkenes is by reaction with jV-brotnosuccinimide (abbreviated NBS) in the presence of light to give products resulting from substitution of hydrogen by bromine at the allylic position—the position next to the double bond. Cyclohexene, for example, gives 3-bromo-cyclohexene. 

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. 

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