Aromaticity polycyclic systems

The organic chemical structural types believed to be characteristic of coals include complex polycyclic aromatic ring systems with connecting bridges and varied oxygen-, sulfur-, and nitrogen-containing functionalities. 

This type of addition process is particularly likely to be observed when the electrophile attacks a position that is already substituted, since facile rearomatization by deprotonation is then blocked. Reaction at a substituted position is called ipso attack. Addition products have also been isolated, however, when initial electrophilic attack has occurred at an unsubstituted position. The extent of addition in competition with substitution tends to increase on going to naphthalene and the larger polycyclic aromatic ring systems. ... 

Molecules that are involved in CL reactions are generally reduced species that can be easily oxidized, such as molecules containing amino and hydroxy groups and polycyclic aromatic ring systems. The solvent in which the experiment is carried out has a dramatic effect on the efficiency of the reaction. Solva-... 

Some indices aim to measure the local aromaticity of an individual ring in a polycyclic system, while others measure the global aromaticity of the whole molecule. 

Bossert, I.D. and Bartha, R. Structure-biodegradability relationships of polycyclic aromatic-hydrocarbon systems in soil. Bull... 

Polynitro derivatives of monocychc aromatic systems (trinitrobenzene, trinitrotoluene, tetranitro-iV-methylaniline, trinitrophenol, etc.) have long been used as explosives [1]. It has been found that a series of polynitroderivatives of biphenyl, diphenylmethane and 1,2-diphenylethylene (stilbene) are explosives liable to detonate on grinding or impact [2]. The same may be true of other polynitro derivatives of polycyclic systems not normally used as explosives (e.g. polynitro-fluorenones, -carbazoles, etc. Penta- and hexa-nitrobenzophenones are also high-energy explosives [3]. The thermal stability of 33 polynitroaromatics was studied by DTA [4]. Two empirical equations relating the heat of decomposition to the heat of detonation have been developed and used to calculate the heats of detonation for 47 polynitroaryl compoimds [5]. 

DeAngelis, G.G. and Wildman, W.C., Circular dichroism studies. 1. A quadrant rule for the optically active aromatic chromophore in rigid polycyclic systems. Tetrahedron, 25, 5099, 1969. 

Benzene (1) is the simplest aromatic hydrocarbon upon which our knowledge of aromatic chemistry is based. This hydrocarbon, the alkylbenzenes (2), the arylmethanes [e.g. diphenylmethane (3)], the biphenyls [e.g. biphenyl (4)] and the condensed polycyclic systems [e.g. naphthalene (5) and anthracene (6)] all exhibit chemical reactivity and spectroscopic features which are markedly different from their aliphatic and alicyclic hydrocarbon counterparts. Indeed the term aromatic character was introduced to specify the chemistry of this group of hydrocarbons and their substituted functional derivatives, and it was soon used to summarise the properties of certain groups of heterocyclic compounds having five- and six-membered ring systems and the associated condensed polycyclic analogues (Chapter 8). 

Polycyclic aromatic Respiratory system, Furnace, foundry, shale, and gas... 

Benzo(a)pyrene (Figure 1.14) is the most studied of the polycyclic aromatic hydrocarbons (PAHs), which are characterized by condensed ring systems ( chicken wire structures). These compounds are formed by the incomplete combustion of other hydrocarbons, a process that consumes hydrogen in preference to carbon. The carbon residue is left in the thermodynamically favored condensed aromatic ring system of the PAH compounds. 

Many aromatic compounds have considerable resonance stabilization but do not possess a benzene nucleus, or in the case of a fused polycyclic system, the molecular skeleton contains at least one ring that is not a benzene ring. The cyclopentadienyl anion C5HJ, the cycloheptatrienyl cation C7H+, the aromatic annulenes (except for [6]annulene, which is benzene), azulene, biphenylene and acenaphthylene (see Fig. 14.2.2(b)) are common examples of non-benzenoid aromatic hydrocarbons. The cyclic oxocarbon dianions C Of (n = 3,4,5,6) constitute a class of non-benzenoid aromatic compounds stabilized by two delocalized n electrons. Further details are given in Section 20.4.4. 

Fig. (a) Cydopentadienyl anion (b) cycloheptatrienyl cation. Bicyclic and polycyclic systems can also be aromatic. 

A new synthetic approach to polycyclic aromatic compounds has been developed based on double Suzuki coupling of polycyclic aromatic hydrocarbon bis(boronic acid) derivatives with o-bromoaryl aldehydes to furnish aryl dialdehydes. These are then converted to larger polycyclic aromatic ring systems by either (a) conversion to diolefins by Wittig reaction followed by photocyclization, or (b) reductive cyclization with trifluoromethanesulfonic acid and 1,3-propanediol (Eq. (12)) [30]. 

Arene activation by metal complexation is not always necessary for the cyclisation of samarium ketyls on to aromatic systems. Studies carried out by Reissig demonstrated the utility of these reactions for the synthesis of a wide variety of polycyclic systems, although yields and stereoselectivities depend highly on the substrate structure.68 One well-behaved system is represented by the cyclic y-naphthyl ketones 50-54, which cyclise with excellent diaster-eocontrol, thereby incorporating four- to eight-membered rings into the tetracyclic structures 55-59 (Scheme 5.38).68 Tetracyclic product 56 (n = 1) is of particular interest because of its steroid-like structure. 

In terms of functional group compatibility, ethers, alcohols, tertiary amines, acetals, esters, amides and heterocycles are compatible with the Pauson-Khand reaction. In the intramolecular version, relatively few carbon skeletons undergo the cyclization. Most intramolecular PKRs use systems derived from hept-l-en-6-yne (6) or propargyl allyl ethers (7) or amines (8). Other interesting and more recent substrates are enynes connected through aromatic rings like 9-11, which have allowed us and other groups to obtain aromatic polycycles (Fig. 1) [28-31]. 

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