Wagner–Meerwein rearrangements

Carbocation rearrangements involving migration of H or alkyl groups don’t just happen in NMR machines. They happen during normal reactions too. For example, acid-catalysed de hydration of the natural product camphenilol gives the alkene santene (a key component of the fragrance of sandalwood oil) in a reaction involving migration of a methyl group.

The mechanism shows why the rearrangement happens: the first-formed cation cannot eliminate H+ in an E1 reaction because loss of the only available proton would give a very strained bridgehead alkene (make a model and see!).

However, migration of a methyl group both stabilizes the cation—it becomes tertiary instead of secondary—and allows E1 elimination of H+ to take place to give a stable alkene.

The migration of an alkyl group to a cationic centre is known as a Wagner–Meerwein re arrangement or Wagner–Meerwein shift, and this migration is, of course, a synthetic manifestation of the rearrangement we have just been looking at in NMR spectra. Wagner–Meerwein shifts have been studied extensively in the class of natural products to which both of these natural products belong—terpenes. For the moment, though, we will just illustrate this type of reaction with one more example—another acid-catalysed dehydration, of isoborneol to give camphene.

This one seems much more complicated—but, in fact, only one alkyl migration is involved. To see what has happened, remember the ‘top tip’—number the carbons. You can number the starting material any way you choose—we’ve started with the gem-dimethyl group because it will be easy to spot in the product. The numbers just follow round the ring, with C8 being the methyl group attached to C5. Now for the hard bit—we need to work out which carbon in the starting material becomes which carbon in the product. The best thing is just to have a go—mistakes will soon become obvious and you can always try again.

• Use the substituents to help you—some will have changed, but most will be the same or similar, for example C1 is still easy to spot as the carbon carrying the dimethyl group.

 • Use connectivity to help you—again, a C–C bond or two may have broken or formed, but most of the C–C bonds in the starting material will be there in the product. C1 and C2 will probably still be next door to one another—C2 was a bridgehead carbon in the starting material, and there is a bridgehead C attached to C1 in the product; assume that’s C2.

• C3 and C4 were unsubstituted carbons in the starting material, and are identifi able in the product too. The other easily spotted atom is C7—an unsubstituted C attached to C2.

 • C5, C6, and C8 are harder. We can assume that C8 is the =CH2 carbon—it was a methyl group but perhaps has become involved in an elimination. C5 was attached to C1, C4, C6, and C8: one of the remaining carbons is attached to C1 and C8, so that seems more likely to be C5, which leaves C6 as the bridgehead, attached as before to C7 and C5. Now we have the whole picture and we can assess what has happened in the reaction— which old bonds have broken and which new bonds have formed.

Numbering the atoms this way identifi es the likely point of rearrangement—the only bond broken is between C4 and C5. Instead we have a new one between C4 and C6: C4 appears to have migrated from C5 to C6. Now for the mechanism. The fi rst step will, of course, be loss of water to generate a secondary cation at C6. The cation is next to a quaternary centre, and migration of any of three bonds could generate a more stable tertiary carbocation. But we know that the new bond in the prod uct is between C4 and C6, so let’s migrate carbon 4. Manipulating the diagrams a bit turns up a structure remarkably similar to our product, and all we need to do is lose a proton from C8.

Although migration of an alkyl group that forms part of a ring leads to much more significant changes in structure than simple migration of a methyl group, the reason why it happens is still just the same.

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الآخبار التكنلوجية

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المزيد

مواضيع ذات صلة

The Hammett relationship

Systematic structural variation

Crossover experiments

Labelling experiments reveal the fate of individual atoms

Be sure of the structure of the product

Variation in the structure of the aldehyde

formation of an ester intermediate

formation of an intermediate dimeric adduct

Be sure of the structure of the product

Variation in the structure of the aldehyde

Proposed mechanism C: formation of an ester intermediate

Proposed mechanism B: formation of an intermediate dimeric adduct