Aromatic hydrocarbons, annelated

Condensed Aromatic Hydrocarbons Annelated with Bicyclo[2.2.2]octene Frameworks... 

In connection with the chemistry of the reactive transient species, nitrene, the chemistry of azepines is well documented u. Also, the chemistry of oxepins has been widely developed due to the recent interest in the valence isomerization between benzene oxide and oxepin and in the metabolism of aromatic hydrocarbons 2). In sharp contrast to these two heteropins, the chemistry of thiepins still remains an unexplored field because of the pronounced thermal instability of the thiepin ring due to ready sulfur extrusion. Although several thiepin derivatives annelated with aromatic ring(s) have been synthesized, the parent thiepin has never been characterized even as a transient species3). 

In the aromatic-ring-annelated oxepin series the resonance effect is clearly the major influence dominating other factors (e.g. temperature, solvent, etc.) which affect the oxepin-arene oxide equilibrium. It is however very difficult to exclude the presence of a minor (spectroscopically undetectable) contribution from either tautomer at equilibrium. This problem has been investigated by the synthesis of chiral arene oxides from polycyclic aromatic hydrocarbons (PAHs). The presence of oxepin (26) in equilibrium with naphthalene 1,2-oxide has been excluded by the synthesis of the optically active arene oxide which showed no evidence of racemization in solution at ambient temperature via the achiral oxepin (26) <79JCS(Pl)2437>. 

Double and triple helicenes are Jt-conjugated molecules, which consist of two or three annelated helicene-like subunits. Such structures may also be considered as examples of chiral polycyclic aromatic hydrocarbons (PAHs). 

Figure 2.8. Wavelengths of Lb, L and bands of condensed aromatic hydrocarbons plotted versus the number of benzene rings a) linearly annelated aromatics (acenes) b) angularly annelated aromatics (phenes) (by permission from Badger. 1954).

The Diels-Alder reaction is a very valuable approach in classical annelation sequences of PAHs and is intensely documented in the reviews on the synthesis of aromatic hydrocarbons [26]. Therefore, we will describe herein only some few, selected, modern applications of the Diels-Alder reaction that caught our interest. To avoid any misunderstanding, the authors will absolutely not pass any judgement on a specific synthetic method by using the terms classical and modern . 

Wegner, H.A., Scott, L.T. and de Meijere, A. (2003) A new Suzuki-Heck-type coupling cascade indeno[l,2,3]-annelation of polycycUc aromatic hydrocarbons. J. Org. Chem., 68, 883-7. 

Under palladium catalysis, o-bromobenzeneboronic acid (335) can be coupled with 1-bromonaphthalene (334) (Scheme 8.71) or other oligocyclic bromoarenes containing peri-positioned hydrogens to furnish indeno-annelated polycyclic aromatic hydrocarbons 336 in a single operation in moderate to very good yields [406]. 

The 1930s marked a significant increase in new and larger polycydic aromatic hydrocarbons with the first appearance of hexa-annelated systems and initial work on benzenoid isomers and derivatives of anthanthrene 29, tetraphene 30, pyrene, and perylene (Fig. 1.3a). In addition to continued research from distinguished chemists such as Scholl, the pioneering efforts of Erich Clar and James W. Cook were a key factor for this development. Mullen and Wu discuss recent advances in polycyclic aromatic hydrocarbons in Chapter 3. 

Numerous aromatic compounds including several bowl-shaped fullerene fragments have been prepared by this method, e.g. cyclopenta[z/]fluoranthene (22, Scheme8, [54d,56a]), cyclopenta[bc]corannulene (23, [55a]) and diace-naphtho[3,2,l,8-cdejg 3, 2, l, 8 -Zmnop]chrysene (24, see Scheme 9, [55b,c]). Of special importance is this approach for the synthesis of cyclopenta-annelated PAHs, e.g. and the three isomeric dicyclopentapyrenes (25-27, Scheme 9, [54e, 56b]). Using these reference samples, several cyclopenta-annelated PAHs could be identified as byproducts formed in the incomplete oxidation of hydrocarbons in fuel rich flames [57]. 

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