DYNAMIC MATCHING - THE CAUSE OF INVERSION OF CONFIGURATION IN THE [1,3] SIGMATROPIC MIGRATION

Direct dynamics calculations using the PM3 semiempirical molecular orb ital method are carried out on the formal [1,3] sigmatropic migration that interconverts bicyclo[3.2.0]hept-2-ene and norbornene. Methylated analogs are also investigated. It is shown that, despite the existenc e of a 5 kcal/mol local minimum corresponding to a biradical intermedi ate on the PM3 potential energy surface for the reaction, the transfor mation still occurs with a preference for inversion of configuration a t the migrating carbon. This stereoselectivity is dynamic in origin an d has nothing to do with conservation of orbital symmetry. It can be o vercome with a suitably placed methyl substituent. asis observed exper imentally. The calculations result in movies that the reader can find on the World Wide Web ttp://www.chem.cornell.edu/bkc1/Sigmatrophy.html ). The implications of the calculations for the mechanisms of nominal [1,n] sigmatropic shifts of carbon are discussed.