Migratory tendencies for 1,5-sigmatropic shifts in the 1,3-dimethylindene system

Abstract

The tendency for several groups X to undergo a 1,5-sigmatropic shift in which 1 is converted into 2 has been tested. Several of the groups studied are based on the carbon–nitrogen double bond [HCNBu^t HCNPh, HCNNMe₂, 2-pyridyl and 1-methylpyridin-2-io], some were expected to be fast migrators (HC)CHNO₂, COCOPh, COSPh, C(S)NMe₂, and others (1- and 2-naphthyl, 2-furyl, 2-thienyl and 1-propylpyrrol-2-yl), showed a variation in aromatic character. The conjugative electronwithdrawing ability of a group and the availability of a low-energy vacant orbital are linked to good migratory ability but steric, conformational and secondary orbital interaction effects can mask the effect, e.g. the 1- and 2-naphthyl groups migrate slowly despite very low π* energies. All the aromatic groups migrate slowly and at similar rates. The results provide further evidence against biradical intermediates or transition states of type 6 in these rearrangements.