Fragments of fullerenes: Novel syntheses, structures and reactions

Abstract

Abstract. The utility of high-temperature gas-phase cyclization reactions for constructing bowl-shaped polycyclic aromatic hydrocarbons related to C6o was first demonstrated by our simple synthesis of corannulene, 1. This strategy has subsequently been extended to the preparation of larger fullerene fragments and promises to play a central role in future syntheses of even more spectacular molecular bowls, baskets and tubes. Isomerization reactions of benzenoid hydrocarbons at high temperatures in the gas phase have also been studied and are now known to occur by reversible benzene ring contractions to fulvene intermediates. The relationship of these small molecule rearrangements to the mechanisms of fullerene production and isomerization ("annealing") is discussed. Finally, the conversion of simple benzenoid hydrocarbons to fragments of fullerenes containing fivemembered rings with two hydrogens fewer is shown to be a common high-temperature reaction of bay region polycyclic aromatic compounds, and a preliminary mechanistic proposal is offered along with supporting experimental evidence.The landmark synthesis of corannulene, 1, by Barth and Lawton was first announced in 1966 (l), more than two decades prior to the earliest preparative synthesis of C a and the higher fullerenes (2). This marvelous bowl-shaped C20H10 hydrocarbon represents the minimal structural subunit of a fullerene that retains a curved network of trigonal carbon atoms, and recent years have witnessed a resurgence of interest in corannulene and the larger fragments of fullerenes (3,4). My coworkers and I have been actively engaged in this area of research since 1990, and the first section of this paper presents an overview of our syntheses and selected transformations of bowl-shaped polycyclic aromatic hydrocarbons (PAHs). The latter portion of the paper describes some of our findings on the high temperature chemistry of PAHs that we believe provide insight into the mechanism of formation and isomerization of fullerenes.Buckminsterfullerene C, Corannulene 1 Strategies for Constructing Bowl-Shaped Fragments of FullerenesHow can one fashion a curved network of trigonal carbon atoms when all the constituent atoms have a natural tendency to remain planar? Any rational synthesis of a bowl-shaped fullerene fragment must overcome the angle strain associated with pyramidalization of numerous sp2 hybridized carbon atoms. Barth and Lawton solved this problem by first putting together the entire carbon skeleton of corannulene with very few centers of unsaturation and then aromatizing the rings in the final step (1,5). Our approach, on the other hand, is completely different. Inspired by the recognition that fullerenes, the most strikingly curved networks of trigonal carbon atoms, can be formed under high energy conditions in the gas phase (2, 6), we reasoned that it should be possible to capitalize on the principles at work there for a new synthesis of corannulene and related fragments of fullerenes. Toward this end, we prepared 7,1O-diethynylfluoranthene...

Chemical mimicry of viral capsid self-assembly

“…For example, the systematic conversion of 1 into a variety of sympentasubstituted corannulene derivatives can lead to the appropriate placement of the desired functional groups around the rim of the bowl with fivefold symmetry (18)(19)(20). A convenient entry to such compounds is sym-pentachlorocorannulene, 2, which is obtained by chlorination of 1 with iodine monochloride (21,22). The various examples of potential tiles illustrated in Fig.…”

The INV24 test set: how well do quantum-chemical methods describe inversion and racemization barriers?

“…To determine whether low-level methods also capture this effect, we have decided to include it in INV24 (molecule 21). Recent works have also shown interest in inversion of bowl-shaped and warped BN systems, which is why we also in- 35,36 These fragments have also been studied with the B3LYP density functional approximation in an early computational study of bowl-to-bowl inversion. 61 In summary, the new INV24 set covers different inversion barriers in a wide variety of systems that represent those that both experimentalists and computational chemists have focused on in the past and present.…”

“…22,23 More recently, the experimental and computational interest in inversion barriers has moved to bowl-shaped molecules, such as corannulene, [24][25][26][27][28][29] sumanene and its derivatives, [29][30][31][32] and other bowl-shaped hydrocarbons, including some derived from fullerene fragments. [33][34][35][36] Also, bowl-shaped and warped ("pringleshaped") boron nitride (BN) and aluminum nitride analogues of those hydrocarbons have been studied. 37 The significance of understanding inversion in such systems has been demonstrated by Karton, who investigated the catalytic potential of graphene sheets on bowl-to-bowl inversion.…”

Gas phase adiabatic electron affinities of cyclopenta-fused polycyclic aromatic hydrocarbons

“…The computed AEAs show that CP-PAHs represent an interesting class of electron acceptors. Since CPPAHs are readily accessible [1,2,9,10,18] and their AEAs can be enhanced further by substitution of the perimeter hydrogens [32,33], (functionalized) CP-PAHs are timely synthetic targets for novel n-type materials, viz. strong oxidants.…”

“…This stems from their identity as sub-structures of fullerenes and carbon nanotubes [1,2], their potential to show unusual (photo)physical properties, e.g. facile (consecutive) one-electron reduction processes [3], anomalous fluorescence [4] and deviant aromatic character [5], and, their alleged presence (in charged form) in the interstellar medium [6].…”