2,3-Sigmatropic rearrangements are a type ofsigmatropic rearrangements and can be classified into two types. Rearrangements ofallylicsulfoxides, amineoxides,selenoxides areneutral. Rearrangements ofcarbanions of allylethers areanionic.[1] The general scheme for this kind of rearrangement is:
Atom Y may besulfur,selenium, ornitrogen. If Y is nitrogen, the reaction is referred to as theSommelet–Hauser rearrangement if a quaternary ammonium salt is involved or theaza-Wittig reaction if an alpha-metalated tertiary amine is involved; if Y is oxygen, then it is called a2,3-Wittig rearrangement (not to be confused with the well-knownWittig reaction, which involves a phosphoniumylide). If Y is sulfur, the product can be treated with a thiophil to generate an allylicalcohol in what is known as theMislow–Evans rearrangement.
A [2,3]-rearrangement may result in carbon-carbon bond formation. It can also be used as aring-expansion reaction.[2]
2,3-sigmatropic rearrangements can offer highstereoselectivity. At the newly formeddouble bond there is a strong preference for formation of the E-alkene ortrans isomer product. Thestereochemistry of the newly formed C-C bond is harder to predict. It can be inferred from the five-membered ringtransition state. Generally, the E-alkene will favor the formation ofanti product, while Z-alkene will favor formation ofsyn product.
Diastereoselectivity can be high for Z-alkene with alkynyl, alkenyl, oraryl anion-stabilizing group. Diastereoselectivity is usually lower with E-alkenes. Hydrocarbon groups will preferexo orientation in the envelope-like transition state. Anion-stabilizing group will prefer theendo orientation in transition state.