Fulleroids Bridged Adducts with Open Transannular Bonds

The simplest cluster opened fullerenes are represented by the fulleroids (l(6)a-homo[60]fullerenes) [4], the azafulleroids (la-aza-l(6)a-homo[60]fullerenes) [4] and the oxafulleroids (la-oxa-l(6)a-homo[60]fullerenes) [4] (Chapter 4). The 

In general, open structures II are energetically preferred over the closed forms I [8, 9], In the ring closed isomers I two unfavorable double bonds within five-membered rings would be required. No monoadduct with such a structure has been observed. Fulleroids such as 1-3 are usually formed via rearrangement of their pyrazoline-, triazoline- or ozonide [6,6]-precursor adducts accompanied by extrusion of N2 or O2 (see Chapter 4) [7,10-12]. 

As outlined in Chapter 10, three types of bisazafulleroids, 4 [13], 5 [14-16] and 6 [17], derived from Cjq involving opened 11-, 13- and 14-membered rings, respectively, have been synthesized. 

The possible opening and closure of transannular cis-1 bonds in bisimino-bridged fullerenes such as 6 and 7 is governed by the following factors [17, 18]  

Again it has to be noted that the frontier orbitals participating in such a valence isomerization are delocalized over the whole molecule [22]. This has consequences for the orbital symmetry and, thereby, a prior analogy with comparable processes involving 6 t-electrons only is not given. However, compared with smaller Jt-systems the selection rules for orbital symmetry controlled processes in fullerenes seem to be less restrictive, since a large number of tt-orbitals with small energy separation are available. Calculations at the AM 1 and PM3 level show that the photocycKzation 

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Fulleroids Bridged Adducts with Open Transannular Bonds... 

Formation of cluster opened methano- and imino[60[fullerenes (fulleroids and azafUlleroids) Thermal [3+2]-cycloadditions ofdiazo compounds or azides leads to the formation of fulleropyrazolines or fullerotriazolines. Thermolysis of such adducts after extrusion of N2 affords as kinetic products the corresponding [5,6]-bridged methano and iminofullerenes with an intact 60 re-electron system and an open transannular bond (see Chapter 4) [88-91]. The corresponding [6,6]-bridged structures with 58 re-electrons and a closed transannular bond are formed only in traces. 

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