2,3-sigmatropic rearrangement

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2,3-Sigmatropic rearrangements are a type of sigmatropic rearrangements and can be classified into two types. Rearrangements of allylic sulfoxides, amine oxides, selenoxides are neutral. Rearrangements of carbanions of allyl ethers are anionic. The general scheme for this kind of rearrangement is:

Atom Y may be sulfur or nitrogen, in which case such reaction is called a 2,3-Stevens rearrangement. If Y is oxygen then it is called 2,3-Wittig rearrangement.

Reaction may result in carbon-carbon bond formation. It can also be used for ring-expansion.

[edit] Stereoselectivity

2,3-Sigmatropic rearrangements can offer high stereoselectivity. At the newly formed double bond there is a strong preferrence for formation of the E-alkene or trans isomer product. The stereochemistry of the newly formed C-C bond is harder to predict. It can be inferred from the five-membered ring transition state. Generally, the E-alkene will favor the formation of anti product, while Z-alkene will favor formation of syn product.

Diastereoselectivity can be high for Z-alkene with alkynyl, alkenl, or aryl anion-stabilizing group. Diastereoselectivity is usually lower with E-alkenes. Hydrocarbon group will prefer exo orientation in the envelope-like transition state. Anion-stabilizing group will prefer the endo orientation in transition state.