Cyclononatetraenide anion

The cyclononatetraenide anion is generated by treatment of the halide 5 with lithium metal ... 

A very characteristic feature of cyclononatetraene derivatives is the collapse of the monocyclic nine-membered ring to a f)icyclo [4.3.0]triene, i.e. a dihydroindene, and this is a frequent result of reactions of the cyclononatetraenide anions with electrophiles. Thus reaction of potassium all-cfs cyclononatetraenide with carbon dioxide gives a dihydroindenecarboxylic acid and with methyl iodide gives a methyldihydroindene [52] ... 

The all-c is cyclononatetraenide anion extracts a proton from cyclopentadiene to give the cyclopentadicnide anion [61,66]. Ilquili-bration of the cyclononatetraenide ion with cyclopentadiene suggests that it is more thermodynamically stable than the cyclopentadicnide anion, and that the pK of the cyclononatetraene lies between 16 and 21, since no proton exchange takes place with indene [66]. 

VI) represents a compromise between (V) and (VII). Experimentally isomers (V) and (VI) have been detected but not (VII), but both try to escape from angle strain by becoming non-planar. In contrast to the case of the cyclononatetraenide anions, discussed in the previous chapter, the energetic gain from planarity and delocalisation of the 7r-electrons in this case appears to be insufficient to outweigh the steric disadvantages of planarity. 

Acylation of either the all-cte [71,72] or mano-tvcms [73] cyclononatetraenide anions at low temperatures gives acyl-c-ie-cyclo-nonatetraenes, but at room temperature these rearrange to give acyl-dihydroindenes ... 

A5 = — 10.4e.u. Furthermore, protonation of the cyclononatetraenide anion, presumably giving the all cis-cyclic tetraene results in cw-dihydroindene at room temperature consistent with a disrotatory electrocyclization of the all c/ -tetraene. " However, it should be recognized that the cis-6A.O triene cannot give the all cis-tetraene in a concerted retro electrocyclization, so a biradical pathway must be accessible for this process (Scheme 10.12). 

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