Recent Studies of Persistent Carbodications

This chapter presents various aspects of persistent carbodications. Persistent carbodications, bearing two positive charges in the same molecule, show unusual behavior in their reactivity and spectral characteristics. Carbodications show uniquely high reactivities in electrophilic substitution reactions, and thus superelectrophilic activation of carbomonocations in suitable cases have direct synthetic applications. The prototypical pentacoordinated carbocation, the parent nonclassical carbocation, CH ⁺ ₅ , 1, has been extensively studied in the gas phase, as well as by theoretical calculations. The electronic structures and energies of superelectrophilic dications derived from protonation of methyl and dimethylmethyleniminium ions were also calculated at the ab initio level and it was found that the ionization of 1,6-dihalodiamantane (71) in SbF ₅ /SO ₂ CIF or other superacids under various conditions gave only the monocation monodonor–acceptor complex and the 1,6-dication, 73, was not observed. The formation of the dication involves protonation of one of the sulfur atoms forming the sulfonium ion and subsequent elimination of H ₂ S. A single-crystal X-ray diffraction of the dication showed that the two tropylium rings are twisted with each other by an angle of 44°, and the tropylium rings are slightly bent into boat conformations.