Sigmatropic Rearrangements
In a sigmatropic rearrangement one bond is being broken while another bond is being formed across a π system. Processes with 2- and 6-electron (Huckel, 4n+2) are allowed suprafacially, 4-electron processes must occur antarafacially (Moebius, 4n). The numbering system [m,n] gives the number of atoms between the bond being broken and the bond being formed in the two directions. In the FMO analyses below the transition state was treated as dipolar. In each case the same result is obtained if the transition state is treated as radical in nature, or if the reverse polarity is used.
