Cyclononatetraenyl cation

Yakali, E. Genesis and bond relocation of the cyclononatetraenyl cation and related compounds, Dissertation, Syracuse University, 1973. 

One of the explanations of the results from solvolysis of 9-chlorobicyclo-[6.1.0]nonatriene [406] is that the reaction occurs via the cyclononatetraenyl cations [407] (Barborak et al., 1971). Reaction of [406] in aqueous acetone... 

Chloro-9-deuterio-cis-4-cyclononatetraene dissolves in liquid SO2 at — 66 °C to give (after replacement of the solvent by methylene chloride and trapping of the intermediate with 4-phenyl-1,2,4-triazoline-3,5-dione) compound (309) in which the D-label is completely scrambled this is interpreted in terms of the formation of cyclononatetraenyl cation. If (310) is dissolved in liquid SO2 at — 66 to — 20 "C, the D-label is found exclusively at C-1 and C-3 in (309), whereas the 9-chloro-9-deuterio-compound rearranges in CDCI3 to (310) with no scrambling of the label. 

The An cyclononatetraenyl cation was studied by Anastassiou and Yakali via the treatment of deuterium-labeled 9-chlorocyclononatetraene with liquid SO2 (an ionizing solvent), and they found that the deuterium became statistically distributed, presumably via the cation. They wondered why this An system could be so easily formed and proposed a helical geometry. Subsequent calculations by Schley-er et al. found that the lowest energy conformation of the ion did have a structure of this type, which leads to a Mobius 7r-electron system in which there is one inversion of phase. Compounds of this type were predicted by Heilbronner to have aromatic character, and the NICS value for this ion is in accord with this expectation. Calculations for other conformations of this ion have been reported. ... 

The data in Tables 5 and 6 provide Information on the stability of the cation. The reaction of cyclononatetraene with isopropyl cation is calculated to be exothermic by —36 kcal/mol, which may be compared with the reaction of cyclopentadiene which is endothermic by 14 kcal/mol. In fact, the reaction of cyclononatetraene is only 20 kcal/mol less exothermic than that of cycloheptratriene despite the latter being a 4n + 2 system. It appears that the cyclononatetraenyl cation should be considered to be a strained aromatic system. 

The cyclononatetraenyl cation adopts a Mobius geometry, see text "The r-stabilization is reduced by a significant strain energy. 

Which would you expect to be most stable, cyclononatetraenyl radical, cation, or anion ... 

Cyclononatetraenyl anion has Dzh symmetry, which is reduced to Cz in the triplet state. However, this triplet is calculated to have a Mobius r-system which leads to some stabilization. Here, the calculated singlet-triplet gap is 41 kcal/mol, significantly smaller than for the other 4n -I- 2 systems. As noted above, the cation with Cz symmetry has a Mobius r-system and the triplet has Cz symmetry. The singlet—triplet gap is 16 kcal/mol. 

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