The Quest for Bridgehead Bridgehead Dications with Bicyclo[2.2.2]octyl Skeletons

It is predicted by MINDO/3 calculations that the 1,5-trishomobarrelenediyl dication 4 would be as much stabilized over the bicyclo2.2.2]octanediyl dication 3 as the monocation 7 -H is energetically favored over the hypothetical 1-bicyclo2.2.2]octyl 10 -H. In spite of this, the bridgehead cations generated from the 1,5-dihalo-trishomobarrelenes 6 -F, 6 -Cl, 6 -Br, and 6 -I and from the 1,5-diol 6 -OH under long-lived ion conditions were only the 5-substituted monocations 7 -F, 7 -Cl, 7 -Br, 7 -I, and 7 -OH, respectively, unequivocally identified by their ¹H- and ¹³C-NMR spectra as well as quenching products. Although there is efficient charge delocalization in 7 -X, as revealed by the ¹³C-chemical shifts, the lack of formation of the bridgehead bridgehead dication 4 is not due to an unforeseen destabilization by the three α-annellated cyclopropyl groups. Even the 1,5-dichlorotetracyclo3.3.2.0^2,4.0^6,8]decanes 17 -Cl₂ and 19 -Cl₂ and 1,5-dichlorotricyclo3.2.2.0^2,4]nonane 12 -Cl₂ with only two and one α-cyclopropyl groups, respectively, gave the bridgehead monocations 18 -Cl, 20 -Cl, and 13 -Cl, respectively.