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Monday, 30 July 2012

Stereochemistry of [n,m] 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.

Chair and boat transition states in [n,m] sigmatropic rearrangements


Examples for sigmatropic rearrangements with a chair transition state

 Fig.3Cope rearrangement


Fig.4[3,3] Sigmatropic

                        6 Electrons
                        Hückel aromatic

 view animation, 2D Animation of the Cope rearrangement

try copy paste of link below, do not miss out on a beautiful animation


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.

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