Fulvene and Spiroannelated Cyclopentadiene Derivatives

The remarkable stabilizing effect of the cydopropyl substituents is also illustrated by the fact that the fulvene 18, upon protonation with tetrafluoroboric acid in dichloromethane, yields the fulvenyl tetrafluoroborate 22, which is completely stable in solution at —50 °C. Compound 22 could be characterized by NMR spectroscopy (1H and 13C) and survives at ambient temperature for several days. 

As far as purely five-membered carbocyclic systems are concerned, spiro[4.4]nonatetraene (31) is the prototypical example of two -systems experiencing electronic interaction through their common central carbon atom. Based on symmetry considerations, it was proposed that spiroconjugation in molecules such as 31 should result in a splitting of the cyclopentadienyl HOMOs, whereas the LUMOs should be degenerate [22, 23]. As a consequence, the first electronic absorption (EA) band of cyclopentadiene in 31 was predicted to be split into a lower and a higher lying component of quite different absorptivities. 

Rather recently, the electronic structure of spiro[4.4]nonatetraene (31), as well as those of its radical anion and radical cation, respectively, have been probed by a variety of spectroscopic methods (electron transmission and energy loss in the gas phase, electronic absorption in cryogenic matrices) [24]. The extension of this concept of spiroconjugation to higher analogues of spiro[4.4]nonatetraene 31 would lead to the hydrocarbons 32 and 33. However, no attempts to prepare these compounds have yet been reported. 

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