Bonus Topic: Allylic Rearrangements

Allylic Rearrangements – Allylic Bromination With Rearrangement

• In allylic bromination reactions, the C-H bond of an allylic carbon breaks, and a new C-Br bond is formed.
• This reaction sometimes occurs with allylic rearrangement. That is, the new C-Br bond forms on the end carbon of the pi-bond, and the pi bond moves over such that the original sp³-hybridized C-H carbon becomes an alkenyl carbon.
• This can be favored in cases where a more substituted double bond results (i.e. Zaitsev’s rule)

Table of Contents

1. An Allylic  Bromination  That Gives  Two Products
2. “Product B” Is An Example Of Allylic Bromination Accompanied By Allylic Rearrangement
3. Why Might “Product B” Be Favored Here? Remember  Zaitsev’s Rule!  
4. Notes
5. (Advanced) References and Further Reading

1. An Allylic Bromination That Gives Two Products

Today’s topic flows right from the subject of the last post, on allylic bromination. It’s on allylic bromination reactions  that happen with a rearrangements.

In the last post on allylic bromination, the examples used were actually quite simple.

For example, if you take cyclopentene and treat with NBS and light (hv) in carbon tetrachloride solvent (CCl₄), you get this product:

There’s no other  possibility.

Now let’s extend the complexity of the substrate a little bit. Just replace one of the C–H bonds with a C–CH₃ bond, and do the exact same reaction.

Here, we get not one product, but two. And note how our major product is different!

What’s going on here?

2. “Product B” Is An Example Of Allylic Bromination Accompanied By Allylic Rearrangement

Let’s think about the mechanism of this reaction. What’s the first thing to happen (after initiation, of course)?

Removal of the weakest C-H bond by the bromine radical! This leaves us with an allylic radical, which can then react with Br₂ to give us product A.

Simple enough. But how do we explain the formation of product B?

Look again at the free radical that is produced. Notice anything special about it?

We can draw a resonance form!

This means that there are two carbons on this molecule which can potentially participate in free radical reactions. Therefore we can also draw a reaction mechanism which shows Br₂ reacting at this bottom (tertiary) carbon:

See how the tertiary radical forms a new π bond while the other π bond breaks? This leaves one of the electrons of the “top” alkene to form a new bond with bromine, giving us a new C-Br bond.

If you analyze the bonds that form and break in this reaction (always a useful exercise), note that this reaction has an extra pair of events – one C-C π bond breaks, and one C-C  π bond forms.

The net effect is that it looks like the  π bond has moved. This phenomenon is called “allylic rearrangement“.

Note: as commenter Keith helpfully points out, remember that resonance forms are “hybrids”. When drawing the mechanism, it’s best to show it all happening in one step (as in the middle drawing, above) rather than to draw the resonance form and then draw bromination.

3. Why Might “Product B” Be Favored Here? Remember  Zaitsev’s Rule!

Last question. Can you think of a reason why product B might be more favoured, especially under conditions of high temperature?

Think back to Zaitsev’s rule (if you’ve covered this) : the more substituted an alkene is, the more stable it is. 

(why? the reason is complex and usually not covered in introductory textbooks – it has to do with a phenomenon called “hyperconjugation”) .

The alkene in product A is what we’d call “di-substituted” – it is directly attached to two carbon atoms and two hydrogen atoms. The alkene in product B is “trisubstituted” – it is directly attached to three carbon atoms and one hydrogen atom. Therefore there is good reason to expect that product B will be a significant product in this case.

[I’m hedging on the exact ratio because I don’t have a literature reference. You shouldn’t completely believe me without firm data from a literature reference; I’ll try to dig one up].

Next Post: Free Radical Addition Of HBr To Alkenes

Notes

(Advanced) References and Further Reading

1. Brominations with N-Bromosuccinimide and Related Compounds. The Wohl-Ziegler Reaction.
   Carl Djerassi
   Chemical Reviews 1948, 43 (2), 271-317
   DOI: 1021/cr60135a004
   An old review by noted chemist Carl Djerassi, whose major contribution to global health was the development of norethindrone – the first female contraceptive. This review contains several examples of allylic systems that rearrange under free-radical conditions (bromination with NBS).
2. Substitution And Rearrangement Reactions Of Allylic Compounds
   R. H. DeWolfe and W. G. Young
   Chemical Reviews 1956, 56 (4), 753-901
   DOI: 10.1021/cr50010a002
   This early review mentions free-radical allylic rearrangements briefly – see pg. 761. Prof. W. G. Young helped build up the UCLA department of chemistry to what it is today, and there is a building named after him on the UCLA campus.
3. Cyclic Polyolefins. II. Synthesis of Cycloöctatetraene from Chloroprene
   Arthur C. Cope and William J. Bailey
   Journal of the American Chemical Society 1948, 70 (7), 2305-2309
   DOI: 1021/ja01187a002
   An early paper by Prof. Arthur C. Cope (of Cope rearrangement fame, and now the eponymous ACS Cope Scholar Award) on the synthesis of cyclooctatetraene. One of the steps involves bromination of 1,5-cyclooctadiene with NBS, which gives a monobromo compound. Cope mentions that they did not ascertain the exact location of the Br in the product and that 2 possible compounds can be formed (IIIa and IIIb), due to rearrangement.
4. THE PEROXIDE EFFECT IN THE ADDITION OF REAGENTS TO UNSATURATED COMPOUNDS. XIII. THE ADDITION OF HYDROGEN BROMIDE TO BUTADIENE
   M. S. KHARASCH, ELLY T. MARGOLIS, and FRANK R. MAYO
   The Journal of Organic Chemistry 1936, 01 (4), 393-404
   DOI: 10.1021/jo01233a008
   Another early paper by M. S. Kharasch in which he studies the addition of HBr to butadiene, in which he attempts to rigorously separate the two modes of addition – electrophilic vs. radical. There is an example of an ‘allylic rearrangement’ here – Prof. Kharasch shows that reaction of 3-bromo-1-butene with HBr under free-radical conditions (peroxides) can give 1,3-dibromobutane. This would be obtained via the intermediate allyl radical, which can add a bromine radical at either the 1- or 3- position.
5. Cyclobutane Derivatives. IV. Ziegler Bromination of Methylenecyclobutane
   Edwin R. Buchman and David R. Howton
   Journal of the American Chemical Society 1948, 70 (7), 2517-2520
   DOI: 1021/ja01187a063
   Ziegler reaction of methylenecyclobutane gives a mixture of “normal” and rearranged allylic bromination products (normal dominates).
6. CYCLOHEXENE-3,3,6,6-d4 A USEFUL COMPOUND FOR THE STUDY OF MECHANISM AND STRUCTURE
   Saul Wolfe and P. G. C. Campbell
   Can. J. Chem. 1965, 43 (5), 1184-1198
   DOI: 10.1139/v65-156
   Allylic bromides can easily undergo rearrangement – while the initial “normal” product might be formed in the reaction mixture, workup can often lead to scrambling. This study found that allylic bromination occurred “normally” but upon workup, rearrangement occurred to give the isomeric allyl bromide.