Reduction of Corannulene

V. REDUCTION OF POLYCYCLIC BOWLS-THE CASE OF CORANNULENE A. Reduction of Corannulene... 

Reduction of Corannulene with Different Alkali Metals... 

An early electrochemical study of corannulene revealed the presence of two well-defined polarographic waves with half-wave potentials of-1.88 and -2.36 V (r-butylammonium perchlorate in acetonitrile). The first wave represented a reversible, one-electron reduction leading to radical anion formation (emerald green solution) further characterized by UV-VIS and ESR. The second wave was reported to be associated with the formation of a bright red species which is not paramagnetic, but it is not believed to be the dianion, but rather some decay product of it. Treatment of THF solutions of 8 with sodium and potassium metals also led to the formation of the same species. ... 

Recently, an NMR study of the diamagnetic species resulting from lithium reduction of 8 was reported. The THF solution revealed a single H NMR peak at 6.95 ppm, and three C NMR lines at 86.8, 95.1, and 112.4 ppm, indicating that the reduced species retains the high symmetry of neutral corannulene. Moreover, consideration of the C NMR chemical shift, as compared to neutral 8, provides a... 

The reduction of several annelated corannulene derivatives has been performed using lithium and potassium metals.7 It has been found that annelation affects the annulenic character of corannulene by changing its charge distribution the dianions of derivatives that are annelated with six-membered rings have less annulenic character and are less paratropic than corannulene dianion. 

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... 

In the meantime, it was found that addition of corannulene, which serves as an efficient electron shuttle between the lithium metal and the solid fullerenes, facilitates the reduction of the fullerenes to their hexaanions (Figure 19). This finding was very crucial for the continuing study of charged fullerenes40b. 

FIGURE 19. Corannulene as an electron shuttle in the reduction of fullerenes... 

Corannulene (1) displays interesting electrochemical properties. The reduction states of 1 lie between those of the neutral hydrocarbon and the tetraanion (1 740 ). Reduction of 1 at —78 C with excess lithium metal in [Dg]-THF over a period of several days led to a series of three color changes, first to green, then to purple, and finally to brownish-red [76]. Quenching this solution with water gave tetrahydrocorannulene as the major product accompanied by small amounts of dihydrocorannulene and 1. More recently, the structure of [Na(DME)3] [l ] was analyzed by X-ray crystallography, and its bowl depth (0.85 A) was found to be slightly shallower than the parent 1 [77]. 

The first and second reduction potentials and the first oxidation potential of corannulene measured by CV (cyclic voltanunetry) strongly depend on measured conditions, such as temperature, solvent, and electrolytes [34, 68, 78]. The third reduction potential was observed when the experiment was conducted with a suitable combination of solvent and electrolyte at low temperature (below 213 K) [68]. The reduction potentials of corannulene and their comparisons with other functionalized derivatives are presented in Table 3 (cf. Sect. 2.2.2). 

Table 3 Reduction potentials of corannulene measured by cyclic voltammetry ...

It also appears that the addition of alkali metals may have an effect on the bowl-to-bowl inversion of 1. An interesting study of the alkali metal reduction of S9, in which two corannulene moieties are tethered by an eight carbon atom chain, allowed for the estimation of inversion barriers in the corannulene dianion [49]. The barrier for the doubly charged corannulene moieties (8.8 and 9.2 kcal mol at 186 and 196 K for K and Cs salts, respectively) is significantly lower than for neutral corannulene (ca. 11 kcal mol ), and closely agrees with our theoretical prediction (7.9-9.2 kcal mol [48]) published earlier. 

The new carbon allotropes, fullerenes [14] and nanotubes [15], can accept a large number of electrons to their -system however, the neutral compounds are hardly soluble, especially in THF. Because of the low solubility of fullerenes, their complete reduction to hexaanions with Li is possible only under extreme conditions, such as the use of an ultrasound bath [16], When the reduction of fullerenes is performed in the presence of a small amount of 2, moderate conditions are required, e.g., low temperature and without sonication [17]. Corannulene, which is a fragment of fullerenes, is highly soluble in THF and is easily reduced. The resulting corannulene anion serves as an efficient electron shuttle, which assists in complete reduction of the practically insoluble fullerenes. In a similar way, single walled nanotubes were reduced by charge transfer by the use of small aromatic compounds, such as naphthalene, fluorenone and anthraquinone [18]. 

The reduction of two types of nonplanar PAHs is discussed here. We first describe the reduction of curved PAHs that contain five-membered rings (5MR), which cause deviation from planarity. This group is represented by corannulene (2) and its derivatives, as 2 is the smallest curved subunit of buckminsterfullerene. 

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 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]. 

For the reduction of fullerenes a small amount (0.5-1 mg) of corannulene is added to the sample. 

Low valent titanium reagents have been used in the first low temperature synthesis of the fullerene fragment dimethyl corannulene (53) from the 1,6-bis(bromomethyl)-7,10-bis(l-bromoethyl)fluoranthene (54, see Scheme 24 [107]). Obviously, the high thermodynamic driving forces for the reductive elimination of bromine as well as for the aromatization with 2,3-dichloro-5,6-dicyanobenzoquinone (DDQ) in the course of the reaction via the relatively unstrained a s/frans-dimethyltetrahydrocorannulenes enabled the formation of... 

Siegel and coworkers originally introduced the key step, i.e.. Method A, on the basis of reductive benzylic coupling chemistiy pioneered by Prakash and Olah [33] (Scheme 3). The synthetic approach started with 7,10-diethyl-l,6-dimethyl-fluoranthene (11), which was prepared according to Scheme 2 [32]. Bromination of 11 efficiently generated tetrabromide 12. Under titanium-mediated conditions, 12 was converted to tetrahydrocorannulene 14, oxidation of which furnished the desired 2,5-dimethylcorannulene (13). In addition to providing access to corannulene, this synthesis showed that solution phase methods could introduce substituents regioselectively, as in 13,15, and 16, which was not possible by means of pyrolysis methods [34]. 

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. 

---