Allylic Bromination of Alkenes

Why does bromination with NBS occur exclusively at an allylic position rather than elsewhere in the molecule The answer, once again, is found by looking at bond dissociation energies to see the relative stabilities of various kinds of radicals. 

There are three types of C-H bonds in cyclohexene, and Table 5.3 gives an idea of their relative strengths. Although a tyrpical seojndary alkyl C-H bond has a strength of about 400 kJ/mol (96 kcai/moi), and a typical vinylic C-H bond has a strength of 445 kJ/mol (106 kcal/mol), an allylic C-H bond has a strength of only about 360 kJ/mol (87 kcal/mol). An allylic radical is therefore more stable than a typical alkyl radical by about 40 kJ/mol (9 kcal/mol). 

We can thna expand the stability ordering to include vinylic and allylic radicals  

An orbital view of the allyl radical. The p orbital on the central carbon can overlap equally well with a p orbital on either neighboring carbon because the structure is electronically symmetrical. 

Since the allyl radical is electronically symmetrical, it can be drawn in either of two resonance forms—with the unpaired electron on the left and the double bond on the right, or with the unpaired electron on the right and the double bond on the left. Neither structure is correct by itself the true structure of the allyl radical is a resonance hybrid of the two. (You might want to review Sections 2.4-2.6 if you need to brush up on resonance.) As noted in Section 2.5, the greater the number of resonance forms, the greater the stability of a compound. An allyl radical, with two resonance forms, is therefore more stable than a typical alkyl radical, which has only a single structure. 

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The fact that the bromine concentration remains at veiy low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must... 

The mechanism of benzylic bromination is similar to that discussed in Section 10.4 for allylic bromination of alkenes. Abstraction of a benzylic hydrogen atom generates an intermediate benzylic radical, which reacts with Br2 to yield product and a Br- radical that cycles back into the reaction to carry on the chain. The Br2 necessary for reaction with the benzylic radical is produced by a concurrent reaction of HBr with NBS. 

Hydrogen atoms in the benzylic position can be replaced by elemental bromine as shown. This is not true for hydrogen atoms in the allylic position. The alkene reacts rapidly with molecular bromine via addition and allylic bromination is not observed (Figure 1.25, left). A chemoselective allylic bromination of alkenes succeeds only according to the Wohl-Ziegler process (Figure 1.25, right), that is, with A-bromosuccinimide (NBS). 

The other piece we need is an allylic bromide. Allylic bromides are formed by allylic bromination of alkenes (Section 6-6B). 

A thoughtful reader would have noticed that, while plenty of methods are available for the reductive transformation of functionalized moieties into the parent saturated fragments, we have not referred to the reverse synthetic transformations, namely oxidative transformations of the C-H bond in hydrocarbons. This is not a fortuitous omission. The point is that the introduction of functional substituents in an alkane fragment (in a real sequence, not in the course of retrosynthetic analysis) is a problem of formidable complexity. The nature of the difficulty is not the lack of appropriate reactions - they do exist, like the classical homolytic processes, chlorination, nitration, or oxidation. However, as is typical for organic molecules, there are many C-H bonds capable of participating in these reactions in an indiscriminate fashion and the result is a problem of selective functionalization at a chosen site of the saturated hydrocarbon. At the same time, it is comparatively easy to introduce, selectively, an additional functionality at the saturated center, provided some function is already present in the molecule. Examples of this type of non-isohypsic (oxidative) transformation are given by the allylic oxidation of alkenes by Se02 into respective a,/3-unsaturated aldehydes, or a-bromination of ketones or carboxylic acids, as well as allylic bromination of alkenes with NBS (Scheme 2.64). 

Radical substitution reactions, such as the light-induced chlorination of methane and the allylic bromination of alkenes with M-bromosuccinimide (Review Table 3, reaction 5), are also common. The key step in all these reactions is that a radical abstracts an atom from a neutral molecule, leaving a new radical. 

One of the complications in assessing the selectivity between atom abstraction and addition to an alkene is that one or the other might be reversible. The best known case where this appears is in two well-known reactions of bromine atoms. One of these is the allylic bromination of alkenes 7.16 7.18 using... 

The fact that the Br2 concentration remains at very low levels is important to the success of the allylic halogenation process. The allylic bromination of alkenes must compete with polar addition of bromine via a bromonium ion intermediate. The reactions differ in their dependence on bromine concentration. The allylic substitution is one-half order in bromine, whereas the addition reaction follows a first- or second-order dependence on [Br2] (see Section 5.3). Therefore a low concentration of Br2 favors substitution over addition. 

Allylic Bromination of Alkenes. The alcohols (1) were converted into the rearranged primary allylic bromides (2) via Sn2 displacement by treatment with NBS/Mc2S (eq 29). A well researched procedure for the allylic bromination of 1,5-cycloocta-diene has also appeared. NBS and water react with allylic ethers to regenerate alcohols. ... 

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