Acenes, reactivity

Generally, benzene and naphthalene derivatives show only little reactivity as dienes in Diels-Alder synthesis, contrary to anthracene and the higher acene derivatives which are frequently used as dienes. Exceptions are the reactions of benzene and naphthalene derivatives with highly reactive dienophiles such as dicyanoacetylene (DCA), which... 

The present section is organized as follows. Firstly, the reactivity and aromaticity of the different rings that compose an acene system as a reactant is analyzed, and secondly, the aromaticity of the TS structures of pericyclic and pseudopericyclic reactions is discussed. 

One way to increase the persistence of reactive molecules is the introduction of bulky groups that can exert kinetic stabilization by increasing the barriers for decomposition reactions. In the case of the acenes, the most important of these reactions are oxidation via endoperoxide formation and dimerization. The introduction of silylethynyl groups was shown to be advantageous in pentacene... 

The acene could still be detected in solutions that were exposed to air for 41 h [56], The oxidation of the heptacene results in a change of color of the solution from brown to orange. Based on UVAis, H-NMR, and FAB-MS data, the authors suggested that the products of oxidation are endoperoxides, resulting from addition to the 6,17 or 8,15 positions (Fig. 23). These are the most reactive sites next to the 7,16 positions that are protected by the silylethynyl groups. Remarkably, dimerization of heptacene 44 was not observed [56], This indicates that the four additional phenyl rings are sufficiently bulky to prevent dimerization. Thus, a combination of... 

Most benzenoid PHs actually can be characterized by a closed-shell electronic configuration accommodating their % electrons only in bonding orbitals. However, researchers faced difficulties for certain types of PHs due to their high reactivity. An important work by Bendikov et al. came into spotlight in 2004 when their computational study on oligoacenes supported that the longer acenes... 

Ladder structures that are composed exclusively of aromatic carbon centers have been the focus of theoretical calculations and diverse synthetic efforts for a long time. Polyacenes are a major focus in this interest [40,41]. Polyacenes are band molecules composed exclusively of annelated six-member carbon rings. Linear polyacenes, poly(n)acenes, are, as polymers with low band gap energy, poorly accessible as a result of their high reactivity (oxidation, dimerization) [42-44]. Hitherto only oligo-( )acenes up to hexa- and heptacene [45,46] have been known and characterized. A synthetic entry to the corresponding polymer does not yet exist, although there are... 

Besides the shape, another important factor that affects the electronic properties and chemical reactivity of PAHs is the nature of the periphery. According to Clar s classification, the graphitic molecules with armchair and cove peripheries shown in Fig. 3.14 (A and B) are all-benzenoid PAHs. In addition to these linear topologies, Stein and Brown considered two other peripheral structures, i.e. acene-like (C) and quinoidaT (D) structures, which lie in a higher energy state and thus show higher chemical reactivity [62]. 

For the acene and phene series, the UV-vis and fluorescence spectra shift dramatically when the number of the phenyl rings increases. In contrast, the shifts for all-benzenoid PAHs with either armchair or cove -type edges are small and they show a high chemical stability. Very recently, graphitic molecules with partial zig-zag periphery such as 68a, 68b and 73 (Scheme 3.19) were synthesized. It was found that the introduction of two or six extra re-centers onto the all-benzenoid graphitic molecules dramatically influences their electronic properties, chemical reactivity and two- and three-dimensional self-assembly [64]. 

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