Helga Thies

Carbon Skeletal Rearrangements Via Pyramidal Carbocations

The scrambling of carbon atoms in gaseous carbocations is readily explicable by invoking transition states or intermediates of pyramidal structure, which are now be seen as the logical bridge between organic and organometallic chemistry. The electronic structure of this species is treated with respect to some of their physical and chemical properties (energy, charge distribution, geometry). Specific examples discussed include the following hydrocarbon ions, the gas phase chemistry of which proceeds via pyramidal intermediates: C5H5+ C5H9+ C5H11+, C5H5+. Reasons are provided why, in contrast, the carbon scrambling in some other systems, as for example tropylium ion ⇄ benzyl cation, does not involve pyramidal cations but proceeds via a sequence of orbital symmetry allowed isomerization, or why in the case of simple saturated hydrocarbon ions, as for example C4H9+, the C-skeleton reorganization is achieved by the well-known Wagner/Meerwein type rearrangement. The carbon scrambling in ionised cyclopentadiene, which precedes the formation of aliene cation radical and acetylene, is due to the intermediacy of ionised bicyclo[2.1. 0]pent-2-ene, a species which is also involved in the phototransposition of carbon atoms in neutral cyclopentadiene. Pyramidal-like structures are much too high in energy to play a role in the degenerate isomer ization of C5H6+.