Fluorinated and Trifluoromethylated Corannulenes

Abstract

The syntheses and properties of corannulenes carrying electron-withdrawing groups (F, CF3 , C6 F5 ) are reported. Direct fluorination of corannulene (C20 H10 ) was carried out with xenon difluoride, and the crystal structure of the product was confirmed by the X-ray analysis. Novel trifluoromethylated corannulenes, including the versatile 4,9-dibromo-1,2-bis(trifluoromethyl)corannulene, were obtained by various established ring-closing reactions. Besides the use of hexafluorobutyne for the construction of fluoranthenes by Diels-Alder reaction as precursor molecules to form 1,2-disubstituted corannulenes, bis(pentafluorophenyl)acetylene was employed as dienophile. The molecular structure and crystal packing of a trifluoromethylated corannulene was determined by single-crystal X-ray analysis and compared with those known brominated and trifluoromethylated corannulenes. The general electron-acceptor properties of corannulenes bearing substituents introduced in particular positions by liquid-phase synthesis are discussed together with published computational results.

Synthesis, Structures, and Physical Properties of Aromatic Molecular-Bowl Hydrocarbons

“…The halogen substituent in monohalocorannulenes, such as 28-F and 28-Cl, does not change the reduction potential of corannulene significantly. Strong electron-withdrawing substituents, trifluoromethyl groups, cause the reduction potential of trifluoromethyl-substituted corannulene derivatives to make a large positive shift, compared to 1 [40,41,81]. Pentakis(trifluoromethyl)corannulene (45-CF 3 ) is a representative example.…”

“…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 still 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 N-iodosuccinimide (NIS) and N-chlorosuccinimide (NCS), respectively, look promising as a methods for pure monohalo derivatives.…”

“…In contrast to other methods presented above, this protocol provides higher tolerance of functional groups, especially esters. In the absence of ester moiety, corannulene derivatives such as 23c and 23d were obtained in low yield [40,41]. Closure of two six-membered rings leads to completely debrominated products 23.…”

“…Extension of the alkyl substituents (for example to ethyl) at the 1,6,7,10 positions also limits the degree of bromination. Nonetheless, ring closure of hexabromides 20 regioselectively to dibromocorannulenes 21 can be achieved by the base promoted procedure (Method C, Scheme 5) [25,39,40 Although methods A-C are efficient in generating corannulene derivatives, they would be problematic for sensitive functionality like the ester moiety. For example, cyclization of precursors 22 (R 1 ¼CO 2 Me) is doomed to failure under the reduction conditions of Methods A and B or in the presence of aqueous base as in Method C. A protocol developed by Sygula et al formed the corannulene core by a nickelmediated intramolecular coupling of benzyl and benzylidene bromides (Method D, Scheme 6) [26,27].…”

“…Generally, they have smaller or comparable values to corannulene. The representative examples include 28-F (0.87 Å) [40], 28-Cl (0.86 Å) [40], 21c (0.80 Å) [40], and 23c (0.82 Å) [41]. 1,2,5,6-Tetrabromocorannulene 18 is an exception; its bowl depth was determined to be 0.91 Å [39].…”