Anions Annulenes

Potentialselektiv gelingt es, [ 16]-Annulen zum Radikal-Anion bzw. Dianion zu reduzie-ren2. 

State, as shown in Fig. 10.8 [65]. The "trefoil bonding state, previously proposed and predicted to exist in aromatic hydrocarbon annulene molecules, is finally encountered, albeit in an extended intermetallic network [65]. This also highlights the important role of interactions between incompletely filled lone pairs in the stabilization of low-dimensional anion structures. 

It is straightforward in the charging of a layered electrophore to increase the interplane distance of the -layers and thus to relax the resulting Coulombic strain this approach is discussed for the doubly layered annulenes [14], In this particular case, a stable tetra-anion is available which can be characterized by a highly resolved H-nmr spectrum (Irmen et al., 1984). 

The homoaromatic interaction in other bridged annulenes has also been examined. The dications of several bridged annulenes were prepared and also studied theoretically and by NMR spectroscopy (Mullen et al., 1987 Wallraff et al., 1988). Once again homoaromatic interactions were deemed to be most important in determining the properties of these systems. Another cationic polycyclic potential homoaromatic system was investigated by Murata and Nakasuji (1980). They concluded, from NMR studies, that homoaromaticity was unimportant in the homophenalenyl cations [66], [67] and [68], (They reached the same conclusion for the corresponding anions.)... 

Thus, the enolates 143b and c (and probably also 143a) are intermediate in behavior between true enolates like 62 and the acyl-[9]annulene anions 61 (in solvent separated ion pairs). This order is well documented by MNDO calculations. 

Reductive Generation of Anionic Species 1101 Tab. 2 Structure formula of annulenes and cyclic conjugated systems (1-17)... 

There have been several developments in this area since this manuscript was prepared. The heat of combustion of corannulene was determined by microbomb combustion calorimetry and its gas-phase enthalpy of formation was estimated at 110.8 kcal/mol. All anionic oxidation states of corannulene were observed by optical absorption, EPR, and NMR spectroscopies. More support for the an-nulene-within-annulene model of the corannulene tetraanion was presented. An alternative pyrolysis route to corannulene was reported, as well as some attempts toward the synthesis of bowl-shaped subunits of fullerenes. And in contrast with previous semiempirical studies," ab initio calculations predicted a general concave preference for the metal cation binding to semibuckminsterfullerene 2%. ... 

Our definition does not take account of lone pairs on heteroatoms in the ring which do not contribute to the -system we are normally concerned here with pyridinoid nitrogen atoms, and their influence on aromaticity is discussed in Section V,B. It is also noteworthy that for molecules possessing an unsubstituted pyrrole-type nitrogen atom, the aromaticity of the anion produced on deprotonation can be significantly greater than that of the protonated form. This has been found in a number of cases with aromatic annulenes,15-18 and we will meet similar results with certain mesoionic azapentalenes (Section IV,C,l,a). 

Cycl[3,2,2]azine (I) is related to annulenes as well as to nonalternant hydrocarbons 45 that is, it can be considered as an amino-substituted [10] annulene and at the same time as isoelectronic with the aceindenyl anion (41).46... 

A simple model which correlates the HOMO and LUMO of cycl-[3,3,3]azine with the degenerate nonbonding orbitals of [12]annulene accounts for the ESR spectra of the radical cations and anions of 2a and some derivatives. This model also predicts the almost identical rc-spin population observed for the 2a-cation and the isoelectronic phenalenyl... 

Unlike [3.3.3]cyclazine (230), the cyclopentacyclazines (285) are unequivocally diatropic compounds (see Table 11). The five-membered ring double bonds have been shown to be delocalized on the basis of the vicinal coupling constants of the corresponding protons. These observations suggest that the cyclazines (285) may be considered as [13]annulene anions weakly coupled to a localized azomethinium cross-link represented as in (286). The mutual interaction between the two closed shell systems may be small since the bonding MOs of the peripheral [13]annulene can interact only weakly with the antibonding MO of the azomethine group by reason of symmetry. 

We start with an examination of some examples of acyclic systems in which there is evidence or the possibility of cyclopropyl homoconjugation. We then move on to a broader examination of homoaromatic systems, treating cationic, neutral and anionic systems in separate sections. The results of experimental work and theoretical examinations are integrated so as to provide a cohesive overview of each system. In order to limit the size of the chapter, we refrain from reviewing in detail systems such as the bridged annulenes and radical species. The chapter concludes with a reflective section that seeks to draw together theory with experiment and point out new directions for future work. 

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