Cyclononatetraenyl systems

The An+ 2 cyclononatetraenyl system is the anion which is prepared by the reaction of chlorobicyclo-[6.1. OJnonatetraene with potassium. The planar Dgh structure has been confirmed by low-temperature H NMR spectroscopy. This is remarkable in view of the 140 °C—C—C bond angles, which lead to consid-... 

In cyclononatetraenyl(dipropyl)borane 6 the extended system of conjugated double bonds together with flexibility of the cycle lead to plethora of intramolecular rearrangements, many of which do in fact take place. 

Four compounds, 89 240>, 732il 742i2 > and 75 242) in the nine membered IOtt heteroannulene series have now been reported these compounds are isoelectronic with the cyclononatetraenyl anion, 60, a planar aromatic system. Of the three heteronin systems (N, O, S) reported however, only 1-H azonin, 89, appears to possess aromatic character, as judged by the n.m.r. spectra 240>. 

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. 

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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