Spiro-heptatriene

H. Durr, W. Gombler, andR. Sergio, Photofragmentierung von Spiro-pyrazolen stabile Norcaradiene und Spiro-heptatriene, Angew. Chem. 84, 215-221 (1972). 

Spiro-aromaticity (Section 1.5.5) is barely detectable. Explain why the spiro cation 1.54, might have more tt stabilisation by spiro conjugation than spiro-heptatriene 1.30. 

Common to these molecules with their cyclopentadiene moieties is the so-called fulvene subunit 27. The first fulvenes, 6,6-dialkylfulvenes, were prepared as early as 1906 by Thiele et al. from sodium cydopentadienide and ketones [16]. The parent hydrocarbon 27 and many other derivatives have been thoroughly studied since the 1960s [17-19]. Diazocyclo-pentadiene (28), which is also easily prepared from cydopentadienide, is a heteroanalogue of fulvene. It has frequently been used as a precursor to other theoretically interesting molecules containing annelated cydopentadiene moieties, because its irradiation readily generates the cyclopentadienylidene 29. This carbene has, for example, been trapped with alkynes to form spiro-annelated cydopentadiene derivatives 30 (Scheme 5) [20]. It has been proved by UV spectroscopy [21] and supported by calculations [22] that these spiro[2.4]heptatrienes (so-called [1.2]spirenes) 30 experience a spedal kind of electronic... 

The cycloaddition of 4-methyl-3//-l,2,4-triazole-3,5(4/f)-dione to spiro[fluorenyl-l,3,5-cyclo-heptatriene] at — 10°C gives the norcaradiene-type cycloadduct, monitored by NMR spectroscopy, which by warming at room temperature is converted to an isomeric triazolidinedione, whose structure is confirmed by X-ray analysis22. 

By taking the example of spiro[2.4]heptatriene, explain the concept of spiroaromaticity. 

The relatively low first ionization potential of 6 compared to fulvene can be rationalized by assuming a charge transfer due to the interaction of the symmetric le" orbital of the cyclopropane ring with the corresponding a orbital of the five-membered ring as discussed in case of spiro[4.2)heptatriene... 

Vinyl carbenes (114), generated by photolysis of the corresponding spiro[4,2]-heptatrienes, undergo intramolecular cycloaddition to generate norcaradienes, which... 

Few examples of 1,3-dipolar cycloadditions to highly fluorinated acetylenes are known, which enhances the significance of recent studies of the reactions between hexafluorobut-2-yne and a series of phenylated and annelated diazocyclopentadienes. For example, diazofluorene is converted into the spiro-adduct (165) which, although stable at -30 °C in the dark, is photo-lysed readily with loss of nitrogen, yielding the spiro[2,4]heptatriene (166). ... 

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