Allylic rearrangement

Scope

Allylic shifts become the dominant reaction pathway when there is substantial resistance to a normal (non-allylic) substitution. For nucleophilic substitution, such resistance is known when there is substantial steric hindrance at or around the leaving group, or if there is a geminal substituent destabilizing an accumulation of positive charge. The effects of substitution at the vinyl group are less clear. Metal complexes that admit an allyl ligand also catalyze allylic substitution, sometimes to rates exceeding direct substitution.

Although rarer still than SN', allylic shifts can occur vinylogously, as a "butadienylic shift":

S_N2' reduction

In SN2' reduction, a hydride allylically displaces a good leaving group in a formal organic reduction, similar to the Whiting diene synthesis. One example occurred in taxol total synthesis (ring C):

:[[Image:SN2reduction.png|400px|SN2 reduction]]

The hydride is lithium aluminium hydride and the leaving group a phosphonium salt; the allylic shift causes the exocyclic double bond in the product. Only when the cyclohexane ring is properly substituted will the proton add trans to the adjacent methyl group.

Electrophilic allyl shifts

Allyl shifts can also take place with electrophiles. In the example below the carbonyl group in benzaldehyde is activated by diboronic acid prior to reaction with the allyl alcohol (see: Prins reaction):

The active catalyst system in this reaction is a combination of a palladium pincer compound and p-toluenesulfonic acid, the reaction product is obtained as a single regioisomer and stereoisomer.

Examples

Repeated allylic shifts can "flip-flop" a double-bond between two possible locations:

An SN2' reaction should explain the outcome of the reaction of an aziridine carrying a methylene bromide group with methyllithium:

:[[File:AziridineAllylicRearrangement.png|400x400px|conversion of 1-alkyl-2-(bromomethyl)aziridines into 1-alkyl-2-(N-alkyl-N-ethylaminomethyl)aziridines]] In this reaction one equivalent of acetylene is lost.

Named reactions

• Ferrier rearrangement
• Meyer–Schuster rearrangement
• Mislow-Evans rearrangement

References

References

1. (1964-01-01). "Allylic reactions". John Wiley & Sons, Ltd..
2. Trost, Barry Martin. (1988-01-01). "Chemical Chameleons: Organosulfones as synthetic building blocks". Bulletin of the Chemical Society of Japan.
3. ''Molecular yardsticks. Synthesis of extended equilibrium transfer alkylating cross-link reagents and their use in the formation of macrocycles'' Stephen J. Brocchini, Martin Eberle, and Richard G. Lawton [[J. Am. Chem. Soc.]]; '''1988'''; 110(15) pp 5211 - 5212; {{doi. 10.1021/ja00223a061
4. ''Synthetic Studies on Taxol: Highly Stereoselective Construction of the Taxol C-Ring via S_N2' Reduction of an Allylic Phosphonium Salt'' Masayuki Utsugi, Masayuki Miyano, and Masahisa Nakada [[Org. Lett.]]; '''2006'''; 8(14) pp 2973 - 2976; (Letter) {{doi. 10.1021/ol0608606
5. ''Highly Selective and Robust Palladium-Catalyzed Carbon-Carbon Coupling between Allyl Alcohols and Aldehydes via Transient Allylboronic Acids'' Nicklas Selander, Sara Sebelius, Cesar Estay, Kálmán J. Szabó [[European Journal of Organic Chemistry]] Volume '''2006''', Issue 18 , Pages 4085 - 4087 {{doi. 10.1002/ejoc.200600530
6. ''Double Lawton S_N2' Addition to Epoxyvinyl Sulfones: Selective Construction of the Stereotetrads of Aplyronine A'' Ahmad El-Awa and Philip Fuchs [[Org. Lett.]]; '''2006'''; 8(14) pp 2905 - 2908; (Letter) {{doi. 10.1021/ol060530l
7. ''Highly unusual conversion of 1-alkyl-2-(bromomethyl)aziridines into 1-alkyl-2-(''N''-alkyl-''N''-ethylaminomethyl)aziridines using methyllithium'' Matthias D'hooghe and Norbert De Kimpe [[Chem. Commun.]], '''2007''', 1275 - 1277, {{doi. 10.1039/b616606g