Corannulene hydrogenation

The partially hydrogenated ring of dihydtocorannulene constitutes a 1,3-cyclo-hexadiene ring, a system that has been well-studied with respect to its geometry and the conformational preferences of substituents. However, the curvature of the corannulene surface introduces an additional stereochemical factor that makes the conformational analysis especially interesting. 1,3-Cyclohexadiene (23) and 9,10-dihydrophenanthrene (24) serve as models they are structurally similar systems, and their stereochemistry and conformational preferences are well documented in the literature. In both cases, the reduced ring adopts a nonplanar, semi-chair conformation of symmetry. 

According to the epikernel principle, the symmetry of the stable minima of the corannulene ion is Cs. If corannulene monoanion is a static JT system, the 11 peaks of the hyperhne structure with intensity ratio 1 10 45 120 210 252 210 120 45 10 1 due to the 10 equivalent hydrogens can be split into 3s = 243 peaks. 

However, the ESR spectra for coronene monoanion did not exhibit such complicated peaks as corannulene monoanion. This suggests that AE in the coronene ion is smaller than that of the corannulene ion. In fact, the calculated energy barrier AE of one of the Dlh saddle points between the stable Clh structures is 0.2 meV, which is smaller than that of corannulene as mentioned previously. Therefore, the pseudorotation about the D6h JT crossing makes the HFC constant averaged and all the peripheral hydrogen atoms equivalent even at low temperatures. 

In a further refinement to the synthesis, Sygula and Rabideau discovered an appealing alternative method (Method C, Scheme 4) to prepare 1,2,5,6-tetrabromo-corannulene 18 from 17 by the use of sodium hydroxide to deprotonate the remaining benzylic hydrogen and initiate carbon-carbon bond formation [36-38], The desired molecule 1 was formed (90%) by treatment of 18 with zinc and potassium iodide [37, 38],... 

As the hydrogens of the peri positions in 1 are replaced by larger moieties, the repulsion energy increases and AG inv decreases relative to 1 (Table 1) [57, 62]. The order of barrier heights of some 2,3-disubstituted corannulenes determined experimentally follows oxygen (35, 9.9 kcal/mol) > phenyl (32, 9.4 kcal/mol) > bromomethyl (34, 9.1 kcal/mol), and aU these examples exhibit lower barriers than other non-peri disubstituted corannulene derivatives, such as 36 (9.9 kcal/mol) and 37 (10.4 kcal/mol). The lower barriers found for the peri-substituted compounds compared to that found for the same substituents in isolated positions shows the special contribution from peri X/X repulsion. Substitution at the peri positions as well as the 1,6-positions leads to a further reduction in the barrier, for example compound 38 (8.7 kcal/mol). From an assumption of additivity in steric bulk, one can assess the steric size of a peri substituent as being roughly OR < Ph=Cl < Me. 

Scheme 46 presents an unexpected formation of buckybowl 149 from corannulene derivative 148, which was prepared by a protocol similar to Method C in Scheme 5 [25]. Under FVP conditions, 148 underwent cyclization to give hydrocarbon 149 through the formation of three C-C bonds. The key step in the cyclization should be 1,2-shift of the hydrogen atom of the rim radical, generated by the rupture of C—Br bond one at a time, for the formation of five-membered rings.

Dinadayalane, T. G., Deepa, S., Sastry, G. N. (2003). Is peri hydrogen repulsion responsible for flattening buckybowls The effect of ring annelation to the rim of corannulene. Tetrahedron Letters, 44, 4527-4529. 

Fig. 14.14. The best P2 c crystal structure for dicarbonyl-corannulene, 3. In spite of the sbghtly different viewing, the structure is similar to that of the chloroderivative in Fig. 14.11, with alternating up and down molecular columns. Inter-column contacts involve favorable coulombic interactions between oxygens and hydrogens (dark and white calottes, respectively).

Fig. 14.15. Lattice energies of computational crystal structures for corannulene carboxylic acid. Each point in the graph corresponds to a separately optimized crystal structure. Volumes in and energies in kJ mol . Structures in the upper cluster with energies above—165 kJ mol are closely packed with aromatic ring stacking but are without hydrogen bonds.

Fig. 14.16. Packing diagrams for computational crystal structures of corannulene carboxylic acid, 4. In the PI structure the hydrogen bond cannot be seen in the other diagrams, oxygen...

A quite different approach to the complexation of corannulene derivatives with transition metals was applied by Chin [65]. His group hydrogenated 1 to its octahy-dro derivative 67 which was subsequently deprotonated by BuLi to the respective fluorene-type anion 68. This species was then used for the formation of two complexes with (CpZrCh) and [Re(CO)3]+. In both cases X-ray structure determination showed // coordination of the metal to the central ring on the exo (convex) side of the very shallow bowl of 68. However, these complexes relate more to the fluorene anion chemistry than to buckybowls. 

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