Sigmatropic Rearrangements: Stereochemistry of [n,m] Sigmatropic Rearrangements

Basically, [n,m] sigmatropic rearrangements can proceed through a chair or boat transition state. Only the chair transition state has been observed experimentally though both are suprafacial and are allowed in 4n+2 electron systems.

2D Animation of the Cope rearrangement

3D Animation of the Cope rearrangement

Large substituents similar to their behavior in chair conformations of cyclohexane rings prefer an equatorial configuration in the transition state of [3,3] sigmatropic reactions. Heating S,S-3,4-dimethyl-1,5-hexadiene to approximately 200°C yields in 90% a product derived from a chair transition state with equatorial methyl groups. The product arising from a diaxial conformation is formed in only 10% yield. Obviously, the reaction does not proceed through the boat transition state.