Allylic bromination of cyclohexene

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

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... 

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

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. 

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... 

Like allylic cations, allylic radicals are stabilized by resonance delocalization. For example, Mechanism 15-2 shows the mechanism of free-radical bromination of cyclohexene. Substitution occurs entirely at the allylic position, where abstraction of a hydrogen gives a resonance-stabilized allylic radical as the intermediate. 

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... 

We have seen (Section 4-13C) that bromination is highly selective, with only the most stable radical being formed. If there is an allylic position, the allylic radical is usually the most stable of the radicals that might be formed. For example, consider the free-radical bromination of cyclohexene. Under the right conditions, free-radical bromination of cyclohexene can give a good yield of 3-bromocyclohexene, where bromine has substituted for an allylic hydrogen on the carbon atom next to the double bond. 

Therefore, we expect bromination of cyclohexene to produce the allylic bromide ... 

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. 

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 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. 

In the first propagation step of the Wohl-Ziegler bromination, the bromine atom abstracts a hydrogen atom from the allylic position of the alkene and thereby initiates a substitution. This is not the only reaction mode conceivable under these conditions. As an alternative, the bromine atom could react with the C=C double bond and thereby start a radical addition to it (Figure 1.27). Such an addition is indeed observed when cyclohexene is reacted with a Br2/AIBN mixture. 

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)... 

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