Buckybowl

P. W. Rabideau, A. Sygula, Buckybowls Polynuclear Aromatic Hydrocarbons Related to the Buckminsterfullerene Surface , Acc. Chem. Res. 1996,29, 235-242. 

As we prcx eed further with the process of generating hydrocarbons from carbon frameworks represented on the buckminsterfullerene surface, we encounter a new class of compounds known as semibuckminsterfullerenes. These Cjq hydrocarbons (Figure 5), which we will also refer to as buckybowls , represent one-half of the buckminsterfullerene Cgo surface, and share several common characteristics including 30 sp carbon frameworks consisting of multiple fused five- and six-membered rings and bowl-shaped geometries. 

Metallacycles (see Metallacycle) are readily prepared via C-C bond activation, particularly in a strained ring. Insertion of Pt(0) into biphenylene leads to the formation of biphenyl complex. Following this strategy, the reaction of semibuckminsterfullerene (C30H12) with Pt(0) yields a buckybowl complex (Scheme 21). Similar ring expansion... 

Fullerene derivatization is also possible by structural modification of the core. The simplest case consists in the insertion of a carbon atom or a heteroatom into a C-C bond and is encountered in homo[60]- [28-32, 75] and -[70] fullerenes [27,75-77]. Only a few deeper modifications, creating small holes in the fullerene shell are known of Cgo [31,75] and C70 [75,78]. Looking at structurally modified fullerenes in a broader sense and from the viewpoint not of cage degradation but assembly, it is clear that much more work - mostly aimed at buckybowls [79] or the total synthesis of buckminsterfuUerene [80] - has been dedicated to this topic. 

Furan has also been employed by Sygula et al. to trap the first buckybowl aryne, corannulyne 118 [67]. Treatment of bromocorannulene 117 with strong bases in the presence of furan led to the formation of cycloadduct 119 in an 80% yield (Scheme 12.36). 

R142 D. Eisenberg, R. Shenhar and M. Rabinovitz, Anions in Buckybowls , in Fragments of Fullerenes and Carbon Nanotubes Designed Synthesis, Unusual Reactions, and Coordination Chemistry, eds. M. A. Petrukhina and L. T. Scott, John Wiley Sons, Inc., Hoboken, N. J., 2012, p. 63. 

Abstract This chapter summarizes the synthesis, physical properties, structure, and crystal packing of buckybowls. Buckybowls exemplify an intermediate class of polynuclear aromatic compounds between the closed-shell fullerenes and the flat extended arrays of graphene. These warped sheets can be seen as fragments of fullerenes or the end cap of single-waUed carlxMi nanotubes and, their curvature endows them with physical properties distinct from flat polynuclear hydrocarbons, which opens up unique possibilities for molecular bowls in various organic materials applications. 

Keywords Buckybowl Carbon nanotube Corannulene Sumanene... 

Compared to planar polycyclic aromatic hydrocarbons (PAHs), the curved structure of buckybowls endows them with additional interesting physical properties. For example, a bowl-shaped molecule has a dipole moment and a self complimentary shape that could lead to the formation of polar crystals. Moreover, buckyballs and carbon nanotubes are well known for their (potential) applications as electro-optical organic materials. Studies of buckybowls can provide fundamental information on buckyballs and carbon nanotubes. 

The chemistry of corannulene-based [4—12] and sumanene-based buckybowls [4, 12-15] has been described in several review articles. In 2006 we published a review article focusing on the solution-phase synthesis of buckybowls, and the structures and physical properties of compounds thus prepared [7]. Since then, this research field has expanded dramatically and many important results have appeared we update the contents herein, and introduce some important compotmds, which are not famished by solution-phase synthesis. Due to page limitations, some... 

The curvature and rigidity of a buckybowl can be characterized by the bowl depth, POAV (jt-orbital axis vector) pyramidalization angle [58-60], and bowl-to-bowl inversion barrier (AG i v). The experimental data of the first two are available by single-crystal X-ray crystallography of the desired bowl molecule. Because the barriers of many corannulene derivatives lie in the region of 7-20 kcal/mol, the can be conveniently analyzed by variable temperature NMR study of a suitably derivatized molecule. 

Stabilization of neutral radicals is another interesting property of bowl-shaped corannulene. The buckybowl neutral radicals 39 [52] and 40 [79] are stable as solids in air or as solutions in degassed toluene for long periods. Experimental results suggest spin delocalization of the radical species onto the corannulene moiety in 40 is more significant than 39. 

On the basis of crystallographic analysis, the corannulene cores in dibenzo[u,g]-corannulene (88) and A-octyldibenzo[ Z,m]-l,2-corannulimide (91) are shallow with a bowl depth of 0.83 and 0.65 A, respectively [120, 123]. The cyclopenta-annulated buckybowl 90 is much deeper, and its bowl depth was determined as 1.03 A. The maximum POAV pyramidalization angle of 90 was found to be 10.7°, which is very close to that of acecorannulene (69) [119]. However, the bowl inversion barrier for 90 was experimentally estimated as 23.5-23.6 kcal/mol, which is much smaller than that of acecorannulene (69, 27.6 kcal/mol). 

The enantioselective synthesis of a chiral buckybowl was reported by Sakurai and Higashibayashi in 2008 [145]. As in the synthesis of sumanene, C3 symmetric iyn-tris(norbomeno)-benzene 126 is the key intermediate in this synthetic approach (Scheme 42). The synthesis started with enantiopure iodonorbomanone. The... 

A buckybowl containing both corannulenyl and sumanenyl fragments is described as a mixed buckybowl in this context. Pentaindenocorannulene (104) is one such example. 

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.

Buckybowl 151 (R=H) was first prepared by an inefficient route in which 7,12-bis-(2-bromophenyl)benzo[ ]fluoranthene was cyclized under FVP conditions at 1,100 °C [161]. Later on, a new synthetic approach in solution phase made 151 easily accessible. The first step of the synthesis was Pd-catalyzed annellation of l,8-bis(arylethynyl)naphthalene 87 (R=H) with iodobenzene to give benzo[ ] fluoranthene 150 (Scheme 47) [28]. Cyclization of 150 was conducted with a mixture of DBU and Pd(PCy3)2Cl2 to generate 151 (R=H) in 31% yield [162]. 

Scheme 52 Synthesis and bowl-to-bowl inversion of buckybowl 166 [177]...

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