The Higher Annulenes

Most of the synthetic work directed toward the higher annulenes was earned out by Franz Sondheimer and his students first at Israel s Weizmann Institute and later at the University of London Sondheimer s research systematically explored the chemistry of these hydrocarbons and provided experimental venfication of Huckel s rule... 

Among the higher annulenes, a mono-charged analogue has been reported in one case, that of the seventeen-carbon macrocycle 63 (18 tu 17 C) (see Table 13) 205). This anion is a counterpart of the tridehydro-[18]annulenes, and contains 18 out-of-plane 7t-electrons. Like the neutral higher annulenes, it possesses both inner and outer protons, the reson-... 

Monohomoaromatic neutral species 296 Bishomoaromatic neutral systems 299 Trishomoaromatic neutral systems 308 Higher homoaromatic neutral systems 311 Homoaromaticity in the bridged annulenes 312 Other neutral homoaromatic systems 313... 

The redox behavior of a number of higher annulenes than (1) has been studied during the past twenty-five years. The evaluation of their electronic properties has attracted much interest because it offers a good comparison of the fundamental differences between 4n and... 

Because of the enhanced rigidity of the higher-membered heteroannulenes, e.g. (36), imparted chiefly by the presence of properly positioned trans double bonds, these frames are generally less thermally labile than their heteronin counterparts although here too aromatic derivatives are substantially more heat resistant than their polyenic relatives. It was noted, for example (75PAC(44)69l), that while parent aza[13]annulene (37a) is thermally stable at 56 °C, its polyenic acetamide (36a) readily rearranges ( i/2< 1 h) under these conditions to what is believed to be a tricyclic isomer (96). 

The annulenes are that series of monocyclic polyolefins (C H ) containing a complete system of contiguous double bonds. While benzene (the best known member of this class of compounds) has been in evidence for some time it is only of late that interest in the higher members has become apparent. This interest has its origins in the LCAO-MO theory of re-elec-tron systems as formulated by E. Hiickel (in particular the "Hiickel rule relating aromatic stability to structure). Although the non-classical chemistry of the benzenoid hydrocarbons had previously been the subject of some conjecture, Httckel s theoretical studies provided the first satisfactory explanation of the peculiar stability of this class of compounds and, incidently, the elusiveness of cyclobutadiene. 

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