Substituted Corannulenes

The monosubstituted corannulene derivatives tert-butylcorannulene (3S) and iso-propylcorannulene (36) have lower symmetry than corannulene therefore their reduction provided evidence for the formation of the dimers of corannulene tet-raanion. The dimers of monosubstituted coraimulene tetraanions are expected to exhibit supramolecular stereochemistry, existing in meso and d,l dimeric forms. 

Reduction of 35 with Li led to two sets of alkyl groups in almost equal abundance, thus indicating the presence of tightly bound dimers. In the reduction of 36 the two methyl groups in each isopropyl substituent becomes diasteriotopic, so each dimer gives rise to two methyl hydrogen doublets in the NMR spectrum, even at room temperature [6]. 

Compelling evidence for dimerization came from the successful detection of a mixed dimer between 2 /4Li+ and 3S /41A [6]. In this experiment, a 5 1 mixture of 2 and 35 was reduced and detected by NMR. The NMR spectra contained the dominant singlet of self-dimer of 2 (34), which was stoichiometrically favored. On the other hand, the signals of the self-dimers of 35 , the stoichiometrically unfavorable species, hardly appeared. In addition, a new set of signals also appeared in the spectra that could not be attributed to the self-dimer of either of the tetraanions. The new peaks were assigned as the signals of the mixed dimer, 2 -/35 78Li+. 

In the reduction of 35 to its radical anion 35 the chromophoric properties were not changed significantly (g = 2.0027), and the ENDOR measurements allowed resolution of at least eight different proton hyperfine couplings [9]. 

The chiral penta-substituted corannulene, 1,3,5,7,9-penta-terfbutylcorannulene (37), which shows anomalous dynamic behavior [116], was reduced with Li and four reduction stages were observed [117]. The behavior of the anti-aromatic dianion and the aromatic tetraanion resemble that of the parent corannulene. In the final stage of the reduction, three distinct tetraanionic species could be detected. Two of the species are sandwich-type dimers, as in the case of corannulene, and are diastereomers, due to the chirality of 37 (RR/SS and meso - RS/SR). The third species was assigned as a tetraanionic monomer, which slowly disappears. 

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Scheme 5 Synthesis of dibromo-substituted corannulenes derivatives by Method C [25, 39, 40]...

Scheme 7 Preparation of mono-substituted corannulene derivatives directly from corannulene [46-48]...

For preparing mono-substituted corannulenes, functionalization of halo-corannulene is ideal. Bromocorannulene (28-Br) was obtained in a good yield by the reaction of corannulene with bromine in the presence of a Lewis-acid catalyst (Scheme 8) [49], albeit stUl an issue to obtain really pure samples of 28-Br. The reaction of corannulene with xenon difluoride gave fluorocorannulene (28-F), but the product has to be purified by reverse-phase HPLC [40]. Recent reports of Au(III)-catalyzed iodination [50] and chlorination [40] of corannulene using A-iodosuccinimide (NIS) and A-chlorosuccinimide (NCS), respectively, look promising as a methods for pure monohalo derivatives. 

Scheme 8 Synthesis of mono-substituted corannulenes from bromocorannulene [40,49-54]...

Highly substituted corannulenes are not only important building blocks for new organic materials, they also provide the possibility to extend the aromatic system of the corannulene core. Direct multiple electrophilic aromatic substitution oti corannulene has shown limited but important successes. In contrast, functiOTial-ization of multihalocorannulenes is a general route. 

Compared to metal-catalyzed functionalization of 52, decaaryl-substituted corannulenes 57 were prepared by repetitive palladium-catalyzed arylation of corannulene with arylboroxin through C-H activation (Scheme 15) [74]. It was... 

Decasubstituted corannulenes can also be generated by functionalization of pentasubstituted corannulenes. The alkoxy groups on 48 increase electron density on corannulene to offer opportunities for halogenation to produce decahetero-substituted corannulenes 59 with two different substituents under mild conditions (Scheme 17). The treatment of pentaalkoxycorannulenes 48 with excess Bt2 or excess thionyl chloride afforded 59-Br and 59-Cl, respectively [93]. 

The structures and properties of highly substituted corannulenes strongly depend on the number and type of substituents. In the subclass of alkyl-substituted corannulenes, decamethylcorannulene (54) is much shallower than pentamethyl-corannulene 45-Me (Table 1). The calculated AG i v and bowl depth for 54 are only... 

Table 2 Photophysical properties of multiethynyl substituted corannulene derivatives [39]...

The synthesis of a plethora of sym-pentakis substituted corannulenes was recently published by Siegel et al. [37]. The authors introduced various substituents, including sp (37), sp (38) and sp hybridized carbon atoms (39) directly connected to 1 as well as some -SR (40) and -OR (41) derivatives [37]. The UV/vis absorption and emission properties of the derivatives were also studied, as well as the inversion barriers of some of the derivatives vide infra). The Stokes shifts were found to be in the range of 130 nm in all derivatives investigated [37]. 

Bowl-to Bou/I Inversion in Buckybowls 551 Table 12.1. Barriers for ring inversion in substituted corannulenes. 

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