Dimers corannulenes

The explanation offered for this unusual spectroscopic behavior is the existence of stable, dimeric aggregates. Due to the symmetry of l-/ -corannulenes, face-to-face association results in two topologically different dimers (dl and meso). Moreover, the lack of any signal coalescence in either or NMR up to room temperature suggests that these dimers are unusually stable, and do not dissociate on the NMR time scale. ... 

The two Li NMR peaks coalesce into a single peak at room temperature (T. = -8 °C), indicating that the interior and exterior cations are able to exchange places without dissociation of the dimer The Li NMR spectra of both 1-i-Pr and 1-r-Bu corannulene "/4Li exhibit quite analogous behavior. Moreover, tetraanion solutions generated from mixtures of corannulene and r-Bu-corannulene show the presence of mixed dimers in the H NMR spectrum. ... 

With this model, it is necessary to assume fast inversion of the two corannulene units within the dimer (or planarity) otherwise the top and bottom bowls would not be equivalent in the NMR spectra. MNDO calculations predict an activation enthalpy of approximately 13 kcal/mol for the concerted inversion process, and this value is inconsistent with the NMR behavior that shows no evidence of freezing out such a dynamic process. These calculations, however, do not take solvation of the exterior lithiums into account, and so the exterior lithiums were removed, and the system was calculated under conditions that would roughly model solvent separation of the lithium cations. Accordingly, the approximate barrier for inversion was lowered to approximately 1 kcal/mol, suggesting that solvation of the exterior lithium cations can indeed greatly reduce the inversion barrier. ... 

A similar concave-to-convex arrangement was recently proposed for the adduct of the radical cation dimer of corannulene with neutral corannulene, formed in a gas-phase. Becker, H. Javabery, G. Petrie, S. Cheng, P-C. Schwartz, H. Scott, L. T. Bohme, D. K. J. Am. Chem. Soc. 1993,115, 11636. 

Corannulene tetraanion Self-assembly and formation of a dimer.. . 498... 

The first evidence for the self-assembly of corannulene tetraanions into a supramolecu-lar dimer, 44, was provided by studies on derivatives of 4371. Owing to their lower symmetry, dimers of monosubstituted corannulene tetraanions are expected to exhibit supramolec-ular stereochemistry, and thus exist in meso and/or as dl dimeric forms (Figure 14a, and b respectively). Reduction of tert-butylcorannulene (45) with excess lithium metal in THF-d i leads to two sets of alkyl groups in almost equal abundance, thus pointing to the presence of tightly bound dimers. Compelling evidence for dimerization comes from the successful detection of a mixed dimer between 434 and 454. In addition, diffusion... 

FIGURE 14. The meso (a) and dl (b) dimers of monosubstitute (d) corannulene tetraanions. Reproduced by permission of Science from Reference 71... 

Although the geometries of the corannulene moieties in these dimers could not be obtained from NMR experiments, semiempirical MNDO molecular orbital calculations clearly favor a stacked-bowl geometry (convex face to concave face). The equivalence of the four external lithium cations is explained in terms of rapid intermolecular exchange. Moreover, the equivalence of the two corannulene units is accounted for by a rapid bowl-to-bowl inversion of both corannulene decks. The validity of this proposal was supported both theoretically71 and experimentally73. 

Corannulene cyclophane 105 was prepared by treating 36 with a mixture of 1,4-bis (mereaptomethyl)benzene and potassium tert-butoxide (Scheme 31) [32]. Similarly, l,5-pentadithiol/l,6-bis(bromomethyl)corannulene cyclophane (106) was prepared from 37 and 1,5-pentadithiol in 75% yield [129]. Synthesis of [3,3]seleno/l,6-dimethylcorannulene cyclophane (107), a cyclophane from two corannulenes, was not straightforward [129]. The key step is the in situ transformation of 37 to the bis(selenide anion), which reacts with the additional 37 to afford the seleno-bridged dimer 107. 

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. 

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. 

Following the dimerization of corannulene, a system of two tethered corannulene units, 1,8-dicorannulenyloctane (38) [118], was reduced with alkali metals to study the formation of an intramolecular sandwich [8]. The reduction was done with Li, Na, K, Rb and Cs, and the two diamagnetic anions tetraanion and octa-... 

The second diamagnetic anion is the octaanion 38, which consists of two sets of H NMR peaks. One set is between = 6.7 and 7.2 ppm, in the same region as the signal of the 34 itself, therefore they were assigned to the protons of the corannulene fragments. The second set contained four broad peaks in the range = 2.2 to 3.2 ppm assigned to the protons of the tether. The Li NMR spectmm of 38 is very similar to that of 34 (dimer of 2 ). The chemical shifts of the two... 

The PAHs indenocorannulene (39), dibenzo[a,g]corannulene (40), and diben-zo[o,g]cyclopenta[h,I]corannulene (41) are three examples of extended corannulenes, which contain a central corannulene system fused to five- and six-membered rings. The reduction of these systems [119, 120] focused on the following issues. First, what is the aromaticity of these curved PAHs anions, will they behave like large polycyclic systems, or have aimulenic character Secondly, what is the possibility of aggregation and dimerization in these systems The third subject of interest was the effect of different alkali metals on the reduction process. 

Compounds 40 and 41 do not contain the dibenzofulvene subunit, which is responsible for the dimerization process in 39 therefore, their reduction produces only monomeric anions. The LUMO level in 40 and 41 is not doubly degenerate like that of corannulene, but there is rather a small energy gap between the LUMO and the NLUMO, which indicates that the formation of tetraanions of 40 and 41 should be possible [120]. 

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