Pyracylene

In the C60 fragment all the carbon atoms are tricoordinated, except Cl and C2, which, being coordinated to an oxygen atom of the metal fragment, are tetracoordinated. As shown in Scheme 1, the sphere of carbon atoms, which has an approximate radius of 3.51 A, is composed of 20 six-membered rings fused with 12 five-membered rings with a repetition of the pyracylene building unit. 

In fact, it is just the unsaturated central double bond of the pyracylene unit which commonly becomes the reactive site for coordination to metal fragments. This is the case not only for [( 2-C6O) (K20,0)-Os(O4)(4-terf-butylpyridine) ], but for example also for C60[Pt(PPh3)2]25 and [Ir(CO)( 2-C6o)Cl(PPh3)2].26 In both compounds the C1-C2 bond length ( 1.50 A) is considerably longer than the value (1.38 A) that the 6 6 C = C bond should have in the absence of coordination (in the strictly related [Pd( 2-C60)(PPh3)2] the C1-C2 distance is 1.45 A27). At variance with [O72-C6o) (K20,0)-Os(O4)(4-terr-butylpyridine) ], in the latter cases the metal atom is directly linked to the fullerene unit. 

Higher fullerenes are also able to coordinate one or more metal fragments, as is the case of the monoadducts [Ir(CO)(7/2-C7o)Cl(PPh3)2], [Ir(CO)((/2-C84)Cl(PPh3)2],33 34 and the diadduct [(ij2-C70) Ir(CO)Cl (PMe2Ph)2 2].35 Also in this case the coordination to the metal fragment occurs through the double bond of the pyracylene unit. 

Figure 3.1 Electron densities (Mulliken charges) of the affected pyracylene unit of the intermediate BuQq obtained from AMI calculations (a), and numbering of the carbon atoms (b).

The chiral fullerene C75 was also asymmetrically osmylated using the chiral ligands and (Scheme 8.8) [63]. In this way an optically active allotrope of a pure element was prepared. C75 contains 15 different types of [6,6] bonds. The pronounced regioselectivity of C7Q towards osmylation [58] suggests that specific bonds in C75 may be favored for an attack by OSO4. An analysis of the ab initio calculated curvature of 75 shows that two of the five pyracylene-type bonds are particularly distorted, which could enhance their reactivity [64]. Indeed HPLC analysis of C75[Os04L ] shows that two regioisomers are predominantly formed upon osmylation of C75 [63]. 

For example, the question of a paramagnetic ring current in pyracylene 1C. A. Coulson and R. B. Mallion, J. Am. Chem. Soc. 98, 592 (1976)1, which is closely related to hydrazino-bridged [ 12]annulenes, treated in Section VIII,A. 

Three structures may be proposed for diazapyracylene an A/,N-bipyrryl core with two vinyl bridges (307a), a cyclododecahexaene dianion with an internal cross-linked N=N unit (307b) and a peripheral [12]annulene loosely linked to an internal hydrazine unit (307c). Structure (307c) does not correlate well with the isoelectronic pyracylene dianion (308). 

Kikuchi et al. 1992 Taylor et al. 1992) indicate variable proportions of the isomers D3, C2v and C2v-. The spectra expected for each isomer can be predicted by enumerating the number of symmetrically distinct carbon atoms in the asymmetric unit and noting their local environment (Fowler et al. 1991). The deltahedral facets composing the asymmetric units of each A78 isomer in figure 7 could be labelled as in figure 4, distinguishing the pyracylene, corrannulene and pyrene sites. 

As the enantiomers of D2-C84 can formally be interconverted by Stone-Wales pyracylene rearrangements91,92 via the achiral D2d-Cs4, they were ideal candidates to study the activation barrier of this transformation. However, taking into account the loss of material through decomposition, neither heating (600/700°C) nor irradiation (X = 193 nm) led to a significant loss of optical activity in samples of enantiomerically enriched D2-Cm or D2-C76. This shows that the activation barrier amounts to at least 83 kcalmol-1 for a potential Stone-Wales rearrangement.5... 

C70 is a higher fullerene that, in contrast to C60, contains eight different types of carbon-carbon bond. It is worthy of note that its osmylation shows a greater kinetic reactivity at sites of greater local curvature, rather than at sites of greater bond order. Thus, addition of Os04 occurs selectively at pyracylene-type positions within the corannulene apex (major isomer) and at the edge of the corannulene apex (minor isomer)112. 

In contrast to the aforementioned fullerenes, C76 is a chiral molecule containing 30 different types of carbon-carbon bond. In this molecule five different pyracylene-type carbon-carbon bonds repeat to form chrysene-shaped units. Kinetic resolution of this fullerene has been achieved via asymmetric osmylation in the presence of a cinchona based chiral ligand (see Section 4.4.4.1.1., ligand 1 d/2 d, Table 5). The calculated enantiomeric excess of the recovered material (after 95% conversion) is >97%, whereas the regenerated C76, formed by tin(II) chloride reduction of the osmylated material (after 33 % conversion), is enriched in the opposite enantiomer. Analysis of the local curvature of the C76 molecule indicates that Os04 should selectively add to two of the 30 types of bonds86. 

Both pyracyclene (133) (which because of strain is stable only in solution) and dipleiadiene (134) are paratropic, as shown by NMR spectra. These molecules might have been expected to behave like naphthalenes with outer bridges, but the outer n frameworks (12 and 16 electrons, respectively) constitute antiaromatic systems with an extra central double bond. With respect to 133, the 4n + 2 rule predicts pyracylene to be aromatic if it is regarded as a 10-7i-electron naphthalene unit connected to two 2-7t-electron etheno systems, but antiaromatic if it is viewed as a 12-7t-electron cyclododecahexaene periphery perturbed by an internal cross-linked etheno unit. Recent studies have concluded on energetic grounds that 133 is a borderline case, in terms of aromaticity-antiaromaticity charac-... 

Fig. 5. Interconversion between the C2,.- and Dag-isomers of C78 through the 4e pyracylene rearrangement.

The fullerenes, parttCulabiy Qq, have been well characterized (1-3), but few reports about their chemical reactivity appeared 4-6). It has been proposed that the relatively high electronegativity of this carbon cluster was due to its pyracylene character (2). Further, the inter-five-membered ring bonds are fulvenoid and are potent electrophilic as well as dienophilic and dipolarophilic sites 5, 7). 

The analogous structure 97a may be the most adequate representation for 8b,8c-diazapyracylenes since the interaction energy of the 14 r-periphery could hardly be sufficient to overcome the energy required for charge separation in 97b. The properties of diazapyracylenes (92) might be affected by a remarkable strain energy which has been calculated to be 52.6 kcal mol for the closely related pyracylene" 92a has been shown to be planar by X-ray analysis. "... 

Figure 1.19. The pyracylene or Stone-Wales (SW) rearrangement in Ceo. Top schematic view of the atoms in the SW patch. Bottom pathway calculated with the density-functional tight-binding potential described in the text.

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