Polycyclic bowls

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

Flash-vacuum pyrolysis has been utilized to synthesize complex thiophene-containing polycyclic hydrocarbons from alkynyl-substituted thiophenes and chlorovinyl-substituted thiophenes <99TL2789>. For example, the bowl-shaped heteroaromatic thiophene 92 was prepared by flash-vacuum pyrolysis of benzotrithiophene 91 <99CC1859>. 

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. 

In 2011, Wu [43] applied intramolecular C-H arylation to the synthesis of highly curved buckybowls, which contain corannulene and sumanene fragments. Several polycyclic aromatic hydrocarbons (PAHs) to prepare less strained bowls have aheady been synthesized using palladium-catalyzed intramolecular arylation reactions [45]. 

The last part of this section concentrates on the synthesis of fuUerene fragments possessing fulvalene it-systems. Recent advances in fuUerene chemistry have attracted much interest in bowl-Uke polycyclic aromatic hydrocarbons (PAHs)... 

The existence of fullerenes, consisting of sp -hybridized carbon networks of six-and five-membered rings, renewed an interest in the chemistry and physics of carbon-rich polycyclic aromatic hydrocarbons (PAHs) in general, and especially in the subclass of PAHs that have become known as fullerene fragments or bucky-bowls. The latter curved-surface PAHs have carbon frameworks that can be identified on the buckminsterfiillerene (Ceo) surface. The simplest example is corannu-lene and it has been known for decades all of the other known buckybowls have been prepared since the early 1990s. 

In this chapter we concentrate on reduction processes of carbon-rich systems. The formation of anions and radical anions of -conjugated monocyclic systems, cyclophanes, bowls and fullerenes is described. Carbon-rich compounds can be reduced directly by contact with alkali metals Li, Na, K, Rb and Cs, which have a low reduction potential. Proton, carbon and lithium NMR and EPR spectroscopies are the main methods used to gain a better understanding of the mono- and polycyclic systems in solution. Special attention will be given to modes of electron delocalization, aromaticity, anti-aromaticity, as well as aggregation, bond formation and bond cleavage processes of diamagnetic electron transfer products. Electrochemical reductions will be briefly discussed. 

Corannulenes (Figure 2.2) were first obtained in 1966, and described as bowl-shaped polycyclic aromatic hydrocarbons (PAHs). Almost simultaneously fullerenes were discovered in graphite vapour (Figure 2.2) [162], Fullerenes comprise 20 six-membered and 12 five-membered rings. The five-memhered rings (corannulene fragment) are bent. In 1991 nanotubes were discovered. Corannulene is a potential candidate for the synthesis of nanotuhes and fullerenes and was first synthesized in 17 steps [162]. 

Synthesis of Bowl-Shaped Polycyclic Aromatic Hydrocarbons via Palladium-Catalyzed Intramolecular Arylation Reactions... 

Two intramolecular substitutional arylation reactions are catalyzed by Pd(0) and lead to the spherical polyaromatic hydrocarbon 3.616. X-ray diffraction study of 3.616 showed significant distortions in the structure. Such bowl-shaped polycyclic aromatic hydrocarbons are the building blocks for preparing fullerene structures [162, 291]. The first syntheses of the fullerene precursors were carried out by vacuum pyrolysis. Corannulene 3.617, diindeno[l,2,3,4-d /g l, 2, 3, 4 -mnop]chrysene 3.618, and semibuckminsterfullerene 3.619 were obtained, which creates a basis for the development of the synthesis of Cgo molecules (Figure 3.5) [300, 162]. 

Scheme 3.51 Synthesis of bowl-shaped polycyclic aromatic hydrocarbons via palladium-catalyzed intramolecular arylation reactions.

The main use of this reaction is for the synthesis of bowl-shaped polycyclic aromatic hydrocarbons and fullerenes [325], Exploring the boundaries of these reactions resulted in the synthesis of compounds of different geometry, tt-electron structure, aromaticity, and with the presence of heteroatoms [324]. Various cross-conjugated enediynes exert anionic cycloaromatization to form fulvene and fulvalene anions and even heterocycles. In accordance with the concept of aromaticity, the anionic Bergman type cyclization is preferred to the classical Bergman cycloaromatization of linear enediynes. This anionic cyclization differs considerably from the classical Bergman cyclization and related cycliza-tions, as well as from other dianionic cyclizations [326]. 

Kim, D., Petersen, J.L. and Wang, K.K. (2006) Synthesis of bowl-shaped polycyclic aromatic hydrocarbons via palladium-catalyzed intramolecular arylation reactions. Organic Letters, 8(11), 2313-2316. 

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