Allylic rearrangement



An allylic rearrangement or allylic shift is an nucleophilic substitution.

In reaction conditions that favor a nucleophile Y. This type of process is called an SN1' substitution.

Alternatively, it is possible for nucleophile to attack directly at the allylic position, displacing the leaving group in a single step, in a process referred to as SN2' substitution. This is likely in cases when the allyl compound is unhindered, and a strong sodium hydroxide gives a mixture of 2-buten-1-ol and 1-buten-3-ol:

Nevertheless, the product in which the OH group is on the primary atom is minor. In the substitution of 1-chloro-3-methyl-2-butene, the tertiary 2-methyl-3-buten-2-ol is produced in a yield of 85%, while that for the primary 3-methyl-2-buten-1-ol is 15%.

In one nucleophile attacks not directly at the electrophilic site but in a conjugate addition over the double bond:

Scope

The synthetic utility can be extended to substitutions over butadiene bonds:[1]

Reaction in Hunig's base

In the first step of this conjugate addition reaction.

In one study[2] the allylic shift was applied twice in a ring system:

In this reaction sequence a Jacobson epoxidation adds an methylmagnesium bromide expulsing the pyrazole group.

An SN2' reaction should explain the outcome of the reaction of an methyllithium [3]:

In this reaction one equivalent of acetylene is lost.

Examples of allylic shifts:

SN2' reduction

In one adaptation called a SN2' reduction a formal Taxol total synthesis (ring C):[4]

The Whiting reaction forming dienes.

Electrophilic allyl shifts

Allyl shifts can also take place with Prins reaction):[5][6]

References

  1. ^ Molecular yardsticks. Synthesis of extended equilibrium transfer alkylating cross-link reagents and their use in the formation of macrocyclesStephen J. Brocchini, Martin Eberle, and Richard G. Lawton J. Am. Chem. Soc.; 1988; 110(15) pp 5211 - 5212; doi:10.1021/ja00223a061
  2. ^ Double Lawton SN2'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
  3. ^ 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
  4. ^ Synthetic Studies on Taxol: Highly Stereoselective Construction of the Taxol C-Ring via SN2' 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. ^ The active catalyst system in this reaction is a combination of a stereoisomer
 
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